ICES Journal of Marine Science: Journal du Conseil

Bekkby, Trine; Rinde, Eli; Erikstad, Lars; Bakkestuen, Vegar; Longva, Oddvar; Christensen, Ole; Isus, Martin; Isachsen, Pl Erik

doi: 10.1093/icesjms/fsn095pmid: N/A

Bekkby, T., Rinde, E., Erikstad, L., Bakkestuen, V., Longva, O., Christensen, O., Isus, M., and Isachsen, P. E. 2008. Spatial probability modelling of eelgrass (Zostera marina) distribution on the west coast of Norway. ICES Journal of Marine Science, 65: 10931101.Based on modelled and measured geophysical variables and presence/absence data of eelgrass Zostera marina, we developed a spatial predictive probability model for Z. marina. Our analyses confirm previous reports and show that the probability of finding Z. marina is at its highest in shallow, gently sloping, and sheltered areas. We integrated the empirical knowledge from field samples in GIS and developed a model-based map of the probability of finding Z. marina using the model-selection approach Akaike Information Criterion (AIC) and the spatial probability modelling extension GRASP in S-Plus. Spatial predictive probability models contribute to a better understanding of the factors and processes structuring the distribution of marine habitats. Additionally, such models provide a useful tool for management and research, because they are quantitative and defined objectively, extrapolate knowledge from sampled to unsurveyed areas, and result in a probability map that is easy to understand and disseminate to stakeholders.

Sellanes, J.; Quiroga, E.; Neira, C.

doi: 10.1093/icesjms/fsn099pmid: N/A

Abstract Sellanes, J., Quiroga, E., and Neira, C. 2008. Megafauna community structure and trophic relationships at the recently discovered Concepción Methane Seep Area, Chile, ∼36°S. – ICES Journal of Marine Science, 65: 1102–1111. The fauna, community composition, and trophic support of the newly discovered Concepción Methane Seep Area (CMSA) are compared with those at a nearby non-seep control. The assemblage of chemosymbiotic bivalves is defined by eight species, including the families Lucinidae, Thyasiridae, Solemyidae, and Vesicomyidae. Seep polychaetes are represented by Lamellibrachia sp. and two commensal species of the vesicomyid Calyptogena gallardoi. Although taxonomic analysis is still under way, most of the chemosymbiotic species seem to be endemics. The CMSA is a hotspot for non-seep benthic megafauna too; 101 taxa were present, but most of them are colonists or vagrants (i.e. not endemics of methane seeps). Isotope analysis supported the belief that non-symbiont-bearing species utilize photosynthetically fixed carbon, because they were isotopically distinct from the chemosymbiotic bivalve species present. It is our opinion that, at this site, which underlies one of the most productive coastal upwelling regions of the world, spatial heterogeneity and the availability of hard substratum, generated by the presence of authigenic carbonate crusts, are more important factors in attracting non-seep fauna than the availability of locally produced chemosynthetic food. Introduction The discovery of deep-water chemosynthetically fueled systems has been one of the most fascinating scientific findings of the past century (Spiess et al., 1980). Although only discovered two decades ago (Paull et al., 1984; Kennicut et al., 1985), cold-seep communities, those fueled by the emission of methane, have been identified at nearly 100 deep-water sites worldwide, on both active and passive continental margins, in the Atlantic, Pacific, and Indian Oceans, and in the Mediterranean Sea (Sibuet and Olu-Le Roy, 2002; Mazurenko and Soloviev, 2003; Levin, 2005), and most recently, at the shelf below the ice cap near the Antarctic Peninsula (Domack et al., 2005). However, just a small fraction of these sites have been characterized in terms of faunal communities (Levin, 2005). A key area to increase our understanding of seep biogeography is the SE Pacific coast off South America, because to date no studies have described the fauna comprehensively. An area of special interest is the Chilean margin, where, other than the fauna of the oxygen minimum zone (OMZ), little is known about the reducing ecosystems present. The OMZ benthos consists mainly of a community formed by the giant, sulphide-oxidizing bacteria Thioploca (Gallardo, 1977), which carpets the shelf sediments that are impinged by the SE Pacific oxygen minimum layer. However, the realm beyond the shelf break off the Chilean margin has remained mostly unexplored for chemosynthetic systems. Indeed, there have been only a few reports of chemosynthetic taxa off South American coasts: off NE Brazil (e.g. Calyptogena birmani; Domaneschi and Lopes, 1990), off Uruguay (e.g. Lamellibrachia victori; Mañé-Garzón and Montero, 1986), off Chile (e.g. Ectenagena australis; Stuardo and Valdovinos, 1988), and the Yaquina Basin methane seeps off Peru (Olu et al., 1996) are the only ones with detailed biological characterization to date. Several aspects of the Chilean margin suggest the existence of extensive methane-seep communities: (i) the report of vast gas hydrate fields extending from 35°S to 45°S (Morales, 2003); (ii) the presence of an active subduction front with a well-developed accretionary prism and suggesting fluid expulsion from the sediment (Lagabrielle et al., 2004); and (iii) the early description of the chemosymbiotic clam, E. australis, from ∼1400 m after incidental collection of two specimens by bottom longliners (Stuardo and Valdovinos, 1988). Based on dredged chemosymbiotic clam fragments and carbonate blocks, the first active seep site off the Chilean coast was recently discovered, the Concepción Methane Seep Area (CMSA; ∼36°S; Sellanes et al., 2004). This study describes the fauna of this southernmost South American methane-seep area, and compares the roles of chemosynthetic and photosynthetic nutrition in a deep-water seep that underlies one of the most productive photic zones in the world. In this context, we aim to characterize the taxonomic composition of megafaunal communities, including both chemosymbiotic and accompanying non-chemosymbiotic species, and to assess the utilization of chemosynthesis-based food sources by members of the cold-seep community based on C and N stable isotope analysis. Material and methods Study area The CMSA is located 72 km northwest of Concepción Bay (36°22′S 73°73′W), on the mid-slope (740–870 m water depth), and on one side of two adjacent mounds separated by a shallow depression (Figure 1). Previous piston-core deployments have found large gas hydrate deposits with carbon isotopic values of −62.8 ± 1.0‰ for both the hydrates themselves and sedimentary porewater (Coffin et al., 2006); this value is indicative of a biogenic origin of the methane (Ussler et al., 2003). There are abundant blocks formed by carbonate-cemented mud, i.e. mud breccia, containing shell fragments of at least two species of clam known to harbour chemosymbiotic endosymbionts, e.g. a vesicomyid and a solemyid (Figure 2; Sellanes et al., 2004). Previous studies have also reported dead vesicomyids and other species of chemosymbiotic clam (e.g. Lucinoma anemiophila, Thyasira methanophila, and Conchocele sp.; Sellanes and Krylova, 2005). Figure 1. Open in new tabDownload slide Study area off Concepción Bay, Central Chile. The triangle indicates trawls where evidence of active methane seepage was collected (carbonate blocks, live chemosymbiotic clams, and shell fragments). The star indicates the position in which shallow subsurface gas hydrates have been observed. Figure 1. Open in new tabDownload slide Study area off Concepción Bay, Central Chile. The triangle indicates trawls where evidence of active methane seepage was collected (carbonate blocks, live chemosymbiotic clams, and shell fragments). The star indicates the position in which shallow subsurface gas hydrates have been observed. Figure 2. Open in new tabDownload slide Typical carbonate block collected at the study area. Cemented valves of vesicomyids are also visible. Scale bar = 5 cm. Figure 2. Open in new tabDownload slide Typical carbonate block collected at the study area. Cemented valves of vesicomyids are also visible. Scale bar = 5 cm. A control non-seep site was also sampled to compare the megafaunal composition of the CMSA with the typical bathyal fauna at a similar depth. This site, situated at 36°32.54′S 73°40.05′W, 798 m deep, and 27 km south of the CMSA, was chosen based on faunal information gathered during a RV “Sonne” cruise (site GeoB 7162; Hebbeln et al., 2001). An underwater video transect, a piston-corer deployment, and trawled megafauna did not show any evidence of methane-seep activity in the area. Indeed, analysis of the piston corer showed that the upper 345 cm of the sediment at this non-seep site consisted largely of hemipelagic clays (Hebbeln et al., 2001). Sample collection and analysis Sampling was conducted on board the Chilean Navy’s RV “Vidal Gormáz” during October 2004 (VG–04 cruise) and September 2006 (SeepOx cruise), and on the RV “Sonne” (a few samples; Table 1). Samples were collected by an Agassiz trawl (mouth opening 1.5 × 0.5 m, mesh size 10 × 10 mm in the codend), in 20-min hauls. Animals were sorted from the non-biological material and preserved on board using appropriate methods for later analysis (e.g. frozen, buffered 10% seawater formalin, glutaraldehyde, and absolute ethanol). Only trawls in which evidence of chemosynthetic activity was observed (e.g. carbonates, shell fragments, or living chemosymbiotic clams) were used for the analysis. No estimates of biomass or abundance were attempted because of the hardness of the substratum and the consequent inadequacy of the collection method for quantitative calculations. Additionally, the collection of large volumes of carbonate blocks (500–1000 kg at each haul) made complete sorting of the samples impractical. However, an indication of the relative frequency of occurrence of each species, based on its relative abundance, was recorded as follows: (i) abundant, present in all hauls and in considerable quantities (e.g. >10 specimens), (ii) common, present in 50% or more of the hauls and sometimes in considerable numbers, (iii) occasional, present in <50% of hauls, in general in small quantities, and (iv) rare, just a few specimens collected in one or two hauls. Table 1. Initial position of trawls (Agassiz trawls, AGT) during VG–04, SeepOx, and SO–156 cruises at the CMSA and the control non-seep site. Observations on the occurrence of chemosymbiotic fauna for AGTs at the seep site are also provided. Cruise . Gear . Latitude S . Longitude W . Depth (m) . Observations* . VG–04 AGT 6 36°21.75′ 73°43.55′ 726–865 Cgl,d, V1d, Ccd AGT 7 36°21.64′ 73°43.57′ 865–926 Cgl,d, V2l, Cxd, Axl, Tml, Lbd AGT 8 36°21.80′ 73°43.10′ 708–854 Cgd, V1d, Tml,d AGT 9 36°21.90′ 73°43.21′ 713–850 Cgl,d, V1l,d, Lbd AGT 10 36°22.68′ 73°42.46′ 708–709 Axd, Lad AGT 13 36°21.91′ 73°43.21′ 728–843 Cgl,d, V1d, Axd, Lad, Tml,d SeepOx AGT 6–3 36°21.18′ 73°43.89′ 919–891 Cgl,d, V2d, Tmd AGT 6–5 36°21.60′ 73°43.88′ 728–885 Cgd, V1d, Tmd AGT 6–8 36°21.90′ 73°43.21′ 710–870 Cgl,d, V1d, Lbl,d AGT 7–1 36°32.05′ 73°45.02′ 879–880 Control, non-seep site AGT 7–2 36°32.54′ 73°40.52′ 817–820 Control, non-seep site SO–156 AGT 7162 36°32.54′ 73°45.72′ 798–782 Control non-seep site Cruise . Gear . Latitude S . Longitude W . Depth (m) . Observations* . VG–04 AGT 6 36°21.75′ 73°43.55′ 726–865 Cgl,d, V1d, Ccd AGT 7 36°21.64′ 73°43.57′ 865–926 Cgl,d, V2l, Cxd, Axl, Tml, Lbd AGT 8 36°21.80′ 73°43.10′ 708–854 Cgd, V1d, Tml,d AGT 9 36°21.90′ 73°43.21′ 713–850 Cgl,d, V1l,d, Lbd AGT 10 36°22.68′ 73°42.46′ 708–709 Axd, Lad AGT 13 36°21.91′ 73°43.21′ 728–843 Cgl,d, V1d, Axd, Lad, Tml,d SeepOx AGT 6–3 36°21.18′ 73°43.89′ 919–891 Cgl,d, V2d, Tmd AGT 6–5 36°21.60′ 73°43.88′ 728–885 Cgd, V1d, Tmd AGT 6–8 36°21.90′ 73°43.21′ 710–870 Cgl,d, V1d, Lbl,d AGT 7–1 36°32.05′ 73°45.02′ 879–880 Control, non-seep site AGT 7–2 36°32.54′ 73°40.52′ 817–820 Control, non-seep site SO–156 AGT 7162 36°32.54′ 73°45.72′ 798–782 Control non-seep site *Cg, Calyptogena gallardoi; V1, vesicomyid gen. sp. 1; V2, vesicomyid gen. sp. 2; Cx, Calyptogena sp.; Ax, Acharax sp.; Tm, Thyasira methanophila; Cc, Conchocele sp.; La, Lucinoma anemiophila; Lb, Lamellibrachia sp.; lliving specimens; ddead or shell fragments. Open in new tab Table 1. Initial position of trawls (Agassiz trawls, AGT) during VG–04, SeepOx, and SO–156 cruises at the CMSA and the control non-seep site. Observations on the occurrence of chemosymbiotic fauna for AGTs at the seep site are also provided. Cruise . Gear . Latitude S . Longitude W . Depth (m) . Observations* . VG–04 AGT 6 36°21.75′ 73°43.55′ 726–865 Cgl,d, V1d, Ccd AGT 7 36°21.64′ 73°43.57′ 865–926 Cgl,d, V2l, Cxd, Axl, Tml, Lbd AGT 8 36°21.80′ 73°43.10′ 708–854 Cgd, V1d, Tml,d AGT 9 36°21.90′ 73°43.21′ 713–850 Cgl,d, V1l,d, Lbd AGT 10 36°22.68′ 73°42.46′ 708–709 Axd, Lad AGT 13 36°21.91′ 73°43.21′ 728–843 Cgl,d, V1d, Axd, Lad, Tml,d SeepOx AGT 6–3 36°21.18′ 73°43.89′ 919–891 Cgl,d, V2d, Tmd AGT 6–5 36°21.60′ 73°43.88′ 728–885 Cgd, V1d, Tmd AGT 6–8 36°21.90′ 73°43.21′ 710–870 Cgl,d, V1d, Lbl,d AGT 7–1 36°32.05′ 73°45.02′ 879–880 Control, non-seep site AGT 7–2 36°32.54′ 73°40.52′ 817–820 Control, non-seep site SO–156 AGT 7162 36°32.54′ 73°45.72′ 798–782 Control non-seep site Cruise . Gear . Latitude S . Longitude W . Depth (m) . Observations* . VG–04 AGT 6 36°21.75′ 73°43.55′ 726–865 Cgl,d, V1d, Ccd AGT 7 36°21.64′ 73°43.57′ 865–926 Cgl,d, V2l, Cxd, Axl, Tml, Lbd AGT 8 36°21.80′ 73°43.10′ 708–854 Cgd, V1d, Tml,d AGT 9 36°21.90′ 73°43.21′ 713–850 Cgl,d, V1l,d, Lbd AGT 10 36°22.68′ 73°42.46′ 708–709 Axd, Lad AGT 13 36°21.91′ 73°43.21′ 728–843 Cgl,d, V1d, Axd, Lad, Tml,d SeepOx AGT 6–3 36°21.18′ 73°43.89′ 919–891 Cgl,d, V2d, Tmd AGT 6–5 36°21.60′ 73°43.88′ 728–885 Cgd, V1d, Tmd AGT 6–8 36°21.90′ 73°43.21′ 710–870 Cgl,d, V1d, Lbl,d AGT 7–1 36°32.05′ 73°45.02′ 879–880 Control, non-seep site AGT 7–2 36°32.54′ 73°40.52′ 817–820 Control, non-seep site SO–156 AGT 7162 36°32.54′ 73°45.72′ 798–782 Control non-seep site *Cg, Calyptogena gallardoi; V1, vesicomyid gen. sp. 1; V2, vesicomyid gen. sp. 2; Cx, Calyptogena sp.; Ax, Acharax sp.; Tm, Thyasira methanophila; Cc, Conchocele sp.; La, Lucinoma anemiophila; Lb, Lamellibrachia sp.; lliving specimens; ddead or shell fragments. Open in new tab Samples for bottom water particulate organic matter (POM) were collected at the CMSA using a Rosette with 12 × 8 l Niskin bottles. For each sample, ∼2 l of water was pre-sieved through a 63-µm mesh to remove zooplankton and large detrital particles, then filtered onto pre-combusted (500°C for 4 h) Whatman GF/F filters (nominal 0.7 µm pore size). Stable isotope analysis Stable C and N isotope signatures were analysed for animals, sedimentary organic matter (SOM), bottom-water-suspended POM, and randomly collected potential food sources (e.g. remains of macroalgae, probably Macrocystis pyrifera, at the control site). Additional muscle tissue samples of Patagonian toothfish (Dissostichus eleginoides), a large predatory fish known to be present at the seep site but difficult to collect by trawling, were obtained from fish caught by artisanal fishers operating at the CMSA. Samples were frozen (−20°C) and later dried at 60°C overnight. After being ground to a fine powder using an agate mortar, samples were treated with a 1% solution of PtCl2 to remove inorganic carbon. Because of the elevated lipid content of D. eleginoides, small pieces of tissue (<1 g) were rinsed with distilled water, air-dried, soaked in a 1:1 chloroform:methanol solution three times, then rinsed with distilled water to remove lipids before stable isotope analyses (Beaudoin et al., 2001). This lipid removal does not produce significant δ13C and δ15N shifts in lipid-free samples (Sotiropoulos et al., 2004). Isotope composition was analysed in the laboratory of R. Lee (School of Biological Sciences, Washington State University) by a Eurovector elemental analyser (Milan, Italy) coupled to a Micromass Isoprime isotope ratio mass spectrometer (Manchester, UK). Stable isotope ratios are reported in the δ notation as the deviation from standards (Pee Dee Belemnite for δ13C and atmospheric N for δ15N), so δ13C or δ15N = [(R sample/R standard) − 1] × 103, where R is 13C/12C or 15N/14N, respectively. Typical precision of the analyses was ±0.5‰ for δ15N and ±0.2‰ for δ13C. Results Chemosymbiotic fauna The chemosymbiotic assemblage of the CMSA was dominated by eight species of bivalve (Figure 3). Of the Vesicomyidae, Calyptogena gallardoi was the most frequently collected. Three other unresolved vesicomyids were present too: a large and slender species (gen. sp. 1), measuring up to 180-mm long, a species of medium size, with an elliptical outline and adherent periostracum (gen. sp. 2), and another species of the genus Calyptogena, similar to C. gallardoi in appearance, but with a subcircular shell outline (Figure 3). Other chemosymbiotic bivalve families included the Lucinidae (L. anemiophila; Holmes et al., 2005), Thyasiridae (T. methanophila; Oliver and Sellanes, 2005, and Conchocele sp.), and Solemyidae (Acharax sp.). Although more living specimens of Conchocele sp. and Acharax sp. are needed for proper taxonomic studies, upon initial examination they do not correspond to species previously described. Indeed, it is the first time that the genus Conchocele has been reported in the SE Pacific (Oliver and Sellanes, 2005). Figure 3. Open in new tabDownload slide Chemosymbiotic bivalve assemblage of the CMSA: (a) vesicomyid gen. sp. 1, (b) vesicomyid gen. sp. 2, (c) Calyptogena gallardoi, (d) Calyptogena sp., (e) Thyasira methanophila, (f) Conchocele sp., (g) Lucinoma anemiophila, and (h) Acharax sp. Scale bar = 2 cm. Figure 3. Open in new tabDownload slide Chemosymbiotic bivalve assemblage of the CMSA: (a) vesicomyid gen. sp. 1, (b) vesicomyid gen. sp. 2, (c) Calyptogena gallardoi, (d) Calyptogena sp., (e) Thyasira methanophila, (f) Conchocele sp., (g) Lucinoma anemiophila, and (h) Acharax sp. Scale bar = 2 cm. Three polychaete species were sampled successfully, including one chemosymbiotic and two commensal taxa. Six living Lamellibrachia spp. (Siboglinidae), and several unoccupied tubes up to 130-cm long, were collected. A few C. gallardoi (∼10%) hosted commensal polychaetes belonging to two different families, one to the Nautiliniellidae (Shinkai sp.) and the other to the Antonbruunidae (Antonbruunia sp.). To our knowledge, this is the first time that two species of commensal polychaete have been reported for a single vesicomyid species. These species are novel and are currently being described. Heterotrophic fauna Of the 101 non-chemosymbiotic megafaunal species observed at the CMSA (Table 2), an onuphid polychaete (Hyalinoecia sp.), a solenocerid shrimp (Haliporoides diomedeae), and many species of cnidarians, gastropods (Bathybembix, Miomelon, and Homalopoma), annelids, crustaceans, and echinoderms were particularly abundant. Macrourids (grenadiers or rattails) were also common (Table 2). All 24 species collected at the control site were also present at the CMSA, but none was as abundant at the control site as at the CMSA. Hyalinoecia sp., H. diomedeae, and the elasmobranchs Halaelurus canescens and Bathyraja sp., as well as the rattail Coelorinchus fasciatus, were the most conspicuous species at the control site. Table 2. Species collected at the CMSA and control non-seep sites. Taxa . Seep endemic . CMSA . Non-seep . PORIFERA    gen. sp. Y/N R – CNIDARIA Anthozoa  Alcyonaria   Gorgonacea    Paragorgia sp. N A –    Callogorgia sp. N C –    Swiftia sp. N O –    gen. sp. N R –  Zoantharia   Actinaria    Actinostola sp. N R –    Coralliomorphus sp. N O –    Hormathia sp. N O –    gen. sp. N R –   Scleractinia   Cariophyllidae    Bathycyatus chilensis N R –    Caryophyllia huinayensis N C –   Flabellidae    Flabellum apertum N R – MOLLUSCA Polyplacophora   Leptochitonidae    Leptochiton americanus N O –   Ischnochitonidae    Stenosemus exaratus N R –   Mopaliidae    Placiphorella atlantica N R – Gastropoda   Neolepetopsidae    Bathylepeta sp. Y/N C –   Fissurellidae    Puncturella sp. 1 Y/N C –    Puncturella sp. 2 N C R   Trochidae    Bathybembix macdonaldi N A R    Margarites huloti Y/N R –    Zetela alphonsi N O –    Calliotropis sp. N R –   Calliostomatidae    Calliostoma chilena N A    Calliostoma crustulum N R –   Turbinidae    Homalopoma panamense N C –   Naticidae    Natica sp. N R R   Ranellidae    Fusitriton magellanicus N R –   Muricidae    Coronium cf. wilhelmense N R –    Pagodula concepcionensis N R –    Trophon ceciliae N R –    Trophon condei N R –   Buccinidae    Kryptos explorator N O –   Volutidae    Miomelon philippiana N O R   Turridae    Aforia cf. goniodes N A R    gen. sp. 1 N R –    gen. sp. 2 N R – Bivalvia   Nuculidae    Ennucula grayi N R R   Solemyidae    Acharax sp. Y R –   Limopsidae    Limopsis ruizana N R –   Lucinidae    Lucinoma anemiophila Y R –   Thyasiridae    Thyasira methanophila Y O –    Conchocele sp. Y R –   Vesicomyidae    Calyptogena gallardoi Y C –    Calyptogena sp. Y O –    gen. sp. 1 Y O –    gen. sp. 2 Y R – Scaphopoda   Dentalidae    Fissidentalium majorinum N O R Cephalopoda   Octopodidae    Benthoctopus sp. N R –   Sepiolidae    Semirossia patagonica N R – ANNELIDA Polychaeta   Onuphidae    Hyalinoecia sp. N A C   Eunicidae    Eunice cf. magellanica N R R    Eunice sp. N O –   Aphroditidae    Aphrodite longirostris N R –   Sabellidae    gen. sp. N R –   Lumbrineridae    gen. sp. N R –   Antobruunidae    Antonbruunia sp. Y R –   Nautiliniellidae    Shinkai sp. Y R –   Siboglinidae    Lamellibrachia sp. Y R – CRUSTACEA Cirripedia  Thoracica   Scalpellidae    Arcoscalpellum sp. N R –    Scalpellum projectum N R – Decapoda  Dendrobranchiata   Solenoceridae    Haliporoides diomedeae N A C  Pleocyemata   Oplophoridae    Acantephyra pelagica N O –    Oplophorus novaezeelandiae N R –   Campylonotidae    Campylonotus semistriatus N C O   Crangonidae    Paracrangon areolata N R –    Sclerocrangon atrox N R –   Polychelidae    Stereomastis sculpta N R R   Galatheidae    Munidopsis quadrata N R –    Munidopsis trifida N C –    Munida curvipes N R –    Munida propinqua N R –   Atelecyclidae    Trichopeltarion corallinus N R –    Trichopeltarion hystricosus N R –   Lithodidae    Lithodes turkayi N R –    Paralomis sp. N R – Isopoda   Cirolanidae    Cirolana sp. N R –    Aega sp. N R – BRACHIOPODA    Liothyrella cf. scotti N O –    gen. sp. N O – ECHINODERMATA Crinoidea    Solanometra sp. N O – Asteroidea   Ctenodiscididae    Ctenodiscus australis N C –   Pterasteridae    Hymenaster sp. N R –   Solasteridae    Solaster regularis N R –   Zoroasteridae    Doraster qawashqari N O O   Goniasteridae    Ceramaster patagonicus N O –    Hippasteria hyadesi N C –    gen. sp. 1 N R R    gen. sp. 2 N R – Ophiuroidea   Gorgonocephalidae    Gorgonocephalus chilensis N C –   Asteronychidae    Asteronyx loveni N R –    Astrodia tenuispina N C –    Astrodia sp. N O –   Ophiuridae    Ophiura carinata N R R    Stegophiura sp. N A –   Ophiolepididae    Ophiomusium biporicum N C –    Ophiomusium lymani N R – Echinoidea   Schizasteridae    Tripylaster sp. N R R   Phymosomatidae    Phormosoma sp. N R R Holothuroidea    gen. sp. N O – SIPUNCULIDA    gen. sp. N R R CHORDATA Chondricthyes   Dalatiidae    Centroscyllium granulatum N C –   Scyliorhinidae    Halaelurus canescens N O C   Rajidae    Bathyraja sp. N R C Actinopterygii   Psychrolutidae    Psychrolutes sio N R –   Macruridae    Coryphaenoides ariommus N C –    Coelorinchus fasciatus N C C    Coelorinchus chilensis N A –   Moridae    Antimora rostrata N O O   Zoarcidae    Bothrocara alalongum N R R   Notocanthidae    Notacanthus sexspinis N R –   Alepocephalidae    gen. sp. N R – Taxa . Seep endemic . CMSA . Non-seep . PORIFERA    gen. sp. Y/N R – CNIDARIA Anthozoa  Alcyonaria   Gorgonacea    Paragorgia sp. N A –    Callogorgia sp. N C –    Swiftia sp. N O –    gen. sp. N R –  Zoantharia   Actinaria    Actinostola sp. N R –    Coralliomorphus sp. N O –    Hormathia sp. N O –    gen. sp. N R –   Scleractinia   Cariophyllidae    Bathycyatus chilensis N R –    Caryophyllia huinayensis N C –   Flabellidae    Flabellum apertum N R – MOLLUSCA Polyplacophora   Leptochitonidae    Leptochiton americanus N O –   Ischnochitonidae    Stenosemus exaratus N R –   Mopaliidae    Placiphorella atlantica N R – Gastropoda   Neolepetopsidae    Bathylepeta sp. Y/N C –   Fissurellidae    Puncturella sp. 1 Y/N C –    Puncturella sp. 2 N C R   Trochidae    Bathybembix macdonaldi N A R    Margarites huloti Y/N R –    Zetela alphonsi N O –    Calliotropis sp. N R –   Calliostomatidae    Calliostoma chilena N A    Calliostoma crustulum N R –   Turbinidae    Homalopoma panamense N C –   Naticidae    Natica sp. N R R   Ranellidae    Fusitriton magellanicus N R –   Muricidae    Coronium cf. wilhelmense N R –    Pagodula concepcionensis N R –    Trophon ceciliae N R –    Trophon condei N R –   Buccinidae    Kryptos explorator N O –   Volutidae    Miomelon philippiana N O R   Turridae    Aforia cf. goniodes N A R    gen. sp. 1 N R –    gen. sp. 2 N R – Bivalvia   Nuculidae    Ennucula grayi N R R   Solemyidae    Acharax sp. Y R –   Limopsidae    Limopsis ruizana N R –   Lucinidae    Lucinoma anemiophila Y R –   Thyasiridae    Thyasira methanophila Y O –    Conchocele sp. Y R –   Vesicomyidae    Calyptogena gallardoi Y C –    Calyptogena sp. Y O –    gen. sp. 1 Y O –    gen. sp. 2 Y R – Scaphopoda   Dentalidae    Fissidentalium majorinum N O R Cephalopoda   Octopodidae    Benthoctopus sp. N R –   Sepiolidae    Semirossia patagonica N R – ANNELIDA Polychaeta   Onuphidae    Hyalinoecia sp. N A C   Eunicidae    Eunice cf. magellanica N R R    Eunice sp. N O –   Aphroditidae    Aphrodite longirostris N R –   Sabellidae    gen. sp. N R –   Lumbrineridae    gen. sp. N R –   Antobruunidae    Antonbruunia sp. Y R –   Nautiliniellidae    Shinkai sp. Y R –   Siboglinidae    Lamellibrachia sp. Y R – CRUSTACEA Cirripedia  Thoracica   Scalpellidae    Arcoscalpellum sp. N R –    Scalpellum projectum N R – Decapoda  Dendrobranchiata   Solenoceridae    Haliporoides diomedeae N A C  Pleocyemata   Oplophoridae    Acantephyra pelagica N O –    Oplophorus novaezeelandiae N R –   Campylonotidae    Campylonotus semistriatus N C O   Crangonidae    Paracrangon areolata N R –    Sclerocrangon atrox N R –   Polychelidae    Stereomastis sculpta N R R   Galatheidae    Munidopsis quadrata N R –    Munidopsis trifida N C –    Munida curvipes N R –    Munida propinqua N R –   Atelecyclidae    Trichopeltarion corallinus N R –    Trichopeltarion hystricosus N R –   Lithodidae    Lithodes turkayi N R –    Paralomis sp. N R – Isopoda   Cirolanidae    Cirolana sp. N R –    Aega sp. N R – BRACHIOPODA    Liothyrella cf. scotti N O –    gen. sp. N O – ECHINODERMATA Crinoidea    Solanometra sp. N O – Asteroidea   Ctenodiscididae    Ctenodiscus australis N C –   Pterasteridae    Hymenaster sp. N R –   Solasteridae    Solaster regularis N R –   Zoroasteridae    Doraster qawashqari N O O   Goniasteridae    Ceramaster patagonicus N O –    Hippasteria hyadesi N C –    gen. sp. 1 N R R    gen. sp. 2 N R – Ophiuroidea   Gorgonocephalidae    Gorgonocephalus chilensis N C –   Asteronychidae    Asteronyx loveni N R –    Astrodia tenuispina N C –    Astrodia sp. N O –   Ophiuridae    Ophiura carinata N R R    Stegophiura sp. N A –   Ophiolepididae    Ophiomusium biporicum N C –    Ophiomusium lymani N R – Echinoidea   Schizasteridae    Tripylaster sp. N R R   Phymosomatidae    Phormosoma sp. N R R Holothuroidea    gen. sp. N O – SIPUNCULIDA    gen. sp. N R R CHORDATA Chondricthyes   Dalatiidae    Centroscyllium granulatum N C –   Scyliorhinidae    Halaelurus canescens N O C   Rajidae    Bathyraja sp. N R C Actinopterygii   Psychrolutidae    Psychrolutes sio N R –   Macruridae    Coryphaenoides ariommus N C –    Coelorinchus fasciatus N C C    Coelorinchus chilensis N A –   Moridae    Antimora rostrata N O O   Zoarcidae    Bothrocara alalongum N R R   Notocanthidae    Notacanthus sexspinis N R –   Alepocephalidae    gen. sp. N R – Organisms endemic to seeps (Y) or not (N), and relative abundance: A, abundant; C, common; O, occasional; R, rare; –, not present. Open in new tab Table 2. Species collected at the CMSA and control non-seep sites. Taxa . Seep endemic . CMSA . Non-seep . PORIFERA    gen. sp. Y/N R – CNIDARIA Anthozoa  Alcyonaria   Gorgonacea    Paragorgia sp. N A –    Callogorgia sp. N C –    Swiftia sp. N O –    gen. sp. N R –  Zoantharia   Actinaria    Actinostola sp. N R –    Coralliomorphus sp. N O –    Hormathia sp. N O –    gen. sp. N R –   Scleractinia   Cariophyllidae    Bathycyatus chilensis N R –    Caryophyllia huinayensis N C –   Flabellidae    Flabellum apertum N R – MOLLUSCA Polyplacophora   Leptochitonidae    Leptochiton americanus N O –   Ischnochitonidae    Stenosemus exaratus N R –   Mopaliidae    Placiphorella atlantica N R – Gastropoda   Neolepetopsidae    Bathylepeta sp. Y/N C –   Fissurellidae    Puncturella sp. 1 Y/N C –    Puncturella sp. 2 N C R   Trochidae    Bathybembix macdonaldi N A R    Margarites huloti Y/N R –    Zetela alphonsi N O –    Calliotropis sp. N R –   Calliostomatidae    Calliostoma chilena N A    Calliostoma crustulum N R –   Turbinidae    Homalopoma panamense N C –   Naticidae    Natica sp. N R R   Ranellidae    Fusitriton magellanicus N R –   Muricidae    Coronium cf. wilhelmense N R –    Pagodula concepcionensis N R –    Trophon ceciliae N R –    Trophon condei N R –   Buccinidae    Kryptos explorator N O –   Volutidae    Miomelon philippiana N O R   Turridae    Aforia cf. goniodes N A R    gen. sp. 1 N R –    gen. sp. 2 N R – Bivalvia   Nuculidae    Ennucula grayi N R R   Solemyidae    Acharax sp. Y R –   Limopsidae    Limopsis ruizana N R –   Lucinidae    Lucinoma anemiophila Y R –   Thyasiridae    Thyasira methanophila Y O –    Conchocele sp. Y R –   Vesicomyidae    Calyptogena gallardoi Y C –    Calyptogena sp. Y O –    gen. sp. 1 Y O –    gen. sp. 2 Y R – Scaphopoda   Dentalidae    Fissidentalium majorinum N O R Cephalopoda   Octopodidae    Benthoctopus sp. N R –   Sepiolidae    Semirossia patagonica N R – ANNELIDA Polychaeta   Onuphidae    Hyalinoecia sp. N A C   Eunicidae    Eunice cf. magellanica N R R    Eunice sp. N O –   Aphroditidae    Aphrodite longirostris N R –   Sabellidae    gen. sp. N R –   Lumbrineridae    gen. sp. N R –   Antobruunidae    Antonbruunia sp. Y R –   Nautiliniellidae    Shinkai sp. Y R –   Siboglinidae    Lamellibrachia sp. Y R – CRUSTACEA Cirripedia  Thoracica   Scalpellidae    Arcoscalpellum sp. N R –    Scalpellum projectum N R – Decapoda  Dendrobranchiata   Solenoceridae    Haliporoides diomedeae N A C  Pleocyemata   Oplophoridae    Acantephyra pelagica N O –    Oplophorus novaezeelandiae N R –   Campylonotidae    Campylonotus semistriatus N C O   Crangonidae    Paracrangon areolata N R –    Sclerocrangon atrox N R –   Polychelidae    Stereomastis sculpta N R R   Galatheidae    Munidopsis quadrata N R –    Munidopsis trifida N C –    Munida curvipes N R –    Munida propinqua N R –   Atelecyclidae    Trichopeltarion corallinus N R –    Trichopeltarion hystricosus N R –   Lithodidae    Lithodes turkayi N R –    Paralomis sp. N R – Isopoda   Cirolanidae    Cirolana sp. N R –    Aega sp. N R – BRACHIOPODA    Liothyrella cf. scotti N O –    gen. sp. N O – ECHINODERMATA Crinoidea    Solanometra sp. N O – Asteroidea   Ctenodiscididae    Ctenodiscus australis N C –   Pterasteridae    Hymenaster sp. N R –   Solasteridae    Solaster regularis N R –   Zoroasteridae    Doraster qawashqari N O O   Goniasteridae    Ceramaster patagonicus N O –    Hippasteria hyadesi N C –    gen. sp. 1 N R R    gen. sp. 2 N R – Ophiuroidea   Gorgonocephalidae    Gorgonocephalus chilensis N C –   Asteronychidae    Asteronyx loveni N R –    Astrodia tenuispina N C –    Astrodia sp. N O –   Ophiuridae    Ophiura carinata N R R    Stegophiura sp. N A –   Ophiolepididae    Ophiomusium biporicum N C –    Ophiomusium lymani N R – Echinoidea   Schizasteridae    Tripylaster sp. N R R   Phymosomatidae    Phormosoma sp. N R R Holothuroidea    gen. sp. N O – SIPUNCULIDA    gen. sp. N R R CHORDATA Chondricthyes   Dalatiidae    Centroscyllium granulatum N C –   Scyliorhinidae    Halaelurus canescens N O C   Rajidae    Bathyraja sp. N R C Actinopterygii   Psychrolutidae    Psychrolutes sio N R –   Macruridae    Coryphaenoides ariommus N C –    Coelorinchus fasciatus N C C    Coelorinchus chilensis N A –   Moridae    Antimora rostrata N O O   Zoarcidae    Bothrocara alalongum N R R   Notocanthidae    Notacanthus sexspinis N R –   Alepocephalidae    gen. sp. N R – Taxa . Seep endemic . CMSA . Non-seep . PORIFERA    gen. sp. Y/N R – CNIDARIA Anthozoa  Alcyonaria   Gorgonacea    Paragorgia sp. N A –    Callogorgia sp. N C –    Swiftia sp. N O –    gen. sp. N R –  Zoantharia   Actinaria    Actinostola sp. N R –    Coralliomorphus sp. N O –    Hormathia sp. N O –    gen. sp. N R –   Scleractinia   Cariophyllidae    Bathycyatus chilensis N R –    Caryophyllia huinayensis N C –   Flabellidae    Flabellum apertum N R – MOLLUSCA Polyplacophora   Leptochitonidae    Leptochiton americanus N O –   Ischnochitonidae    Stenosemus exaratus N R –   Mopaliidae    Placiphorella atlantica N R – Gastropoda   Neolepetopsidae    Bathylepeta sp. Y/N C –   Fissurellidae    Puncturella sp. 1 Y/N C –    Puncturella sp. 2 N C R   Trochidae    Bathybembix macdonaldi N A R    Margarites huloti Y/N R –    Zetela alphonsi N O –    Calliotropis sp. N R –   Calliostomatidae    Calliostoma chilena N A    Calliostoma crustulum N R –   Turbinidae    Homalopoma panamense N C –   Naticidae    Natica sp. N R R   Ranellidae    Fusitriton magellanicus N R –   Muricidae    Coronium cf. wilhelmense N R –    Pagodula concepcionensis N R –    Trophon ceciliae N R –    Trophon condei N R –   Buccinidae    Kryptos explorator N O –   Volutidae    Miomelon philippiana N O R   Turridae    Aforia cf. goniodes N A R    gen. sp. 1 N R –    gen. sp. 2 N R – Bivalvia   Nuculidae    Ennucula grayi N R R   Solemyidae    Acharax sp. Y R –   Limopsidae    Limopsis ruizana N R –   Lucinidae    Lucinoma anemiophila Y R –   Thyasiridae    Thyasira methanophila Y O –    Conchocele sp. Y R –   Vesicomyidae    Calyptogena gallardoi Y C –    Calyptogena sp. Y O –    gen. sp. 1 Y O –    gen. sp. 2 Y R – Scaphopoda   Dentalidae    Fissidentalium majorinum N O R Cephalopoda   Octopodidae    Benthoctopus sp. N R –   Sepiolidae    Semirossia patagonica N R – ANNELIDA Polychaeta   Onuphidae    Hyalinoecia sp. N A C   Eunicidae    Eunice cf. magellanica N R R    Eunice sp. N O –   Aphroditidae    Aphrodite longirostris N R –   Sabellidae    gen. sp. N R –   Lumbrineridae    gen. sp. N R –   Antobruunidae    Antonbruunia sp. Y R –   Nautiliniellidae    Shinkai sp. Y R –   Siboglinidae    Lamellibrachia sp. Y R – CRUSTACEA Cirripedia  Thoracica   Scalpellidae    Arcoscalpellum sp. N R –    Scalpellum projectum N R – Decapoda  Dendrobranchiata   Solenoceridae    Haliporoides diomedeae N A C  Pleocyemata   Oplophoridae    Acantephyra pelagica N O –    Oplophorus novaezeelandiae N R –   Campylonotidae    Campylonotus semistriatus N C O   Crangonidae    Paracrangon areolata N R –    Sclerocrangon atrox N R –   Polychelidae    Stereomastis sculpta N R R   Galatheidae    Munidopsis quadrata N R –    Munidopsis trifida N C –    Munida curvipes N R –    Munida propinqua N R –   Atelecyclidae    Trichopeltarion corallinus N R –    Trichopeltarion hystricosus N R –   Lithodidae    Lithodes turkayi N R –    Paralomis sp. N R – Isopoda   Cirolanidae    Cirolana sp. N R –    Aega sp. N R – BRACHIOPODA    Liothyrella cf. scotti N O –    gen. sp. N O – ECHINODERMATA Crinoidea    Solanometra sp. N O – Asteroidea   Ctenodiscididae    Ctenodiscus australis N C –   Pterasteridae    Hymenaster sp. N R –   Solasteridae    Solaster regularis N R –   Zoroasteridae    Doraster qawashqari N O O   Goniasteridae    Ceramaster patagonicus N O –    Hippasteria hyadesi N C –    gen. sp. 1 N R R    gen. sp. 2 N R – Ophiuroidea   Gorgonocephalidae    Gorgonocephalus chilensis N C –   Asteronychidae    Asteronyx loveni N R –    Astrodia tenuispina N C –    Astrodia sp. N O –   Ophiuridae    Ophiura carinata N R R    Stegophiura sp. N A –   Ophiolepididae    Ophiomusium biporicum N C –    Ophiomusium lymani N R – Echinoidea   Schizasteridae    Tripylaster sp. N R R   Phymosomatidae    Phormosoma sp. N R R Holothuroidea    gen. sp. N O – SIPUNCULIDA    gen. sp. N R R CHORDATA Chondricthyes   Dalatiidae    Centroscyllium granulatum N C –   Scyliorhinidae    Halaelurus canescens N O C   Rajidae    Bathyraja sp. N R C Actinopterygii   Psychrolutidae    Psychrolutes sio N R –   Macruridae    Coryphaenoides ariommus N C –    Coelorinchus fasciatus N C C    Coelorinchus chilensis N A –   Moridae    Antimora rostrata N O O   Zoarcidae    Bothrocara alalongum N R R   Notocanthidae    Notacanthus sexspinis N R –   Alepocephalidae    gen. sp. N R – Organisms endemic to seeps (Y) or not (N), and relative abundance: A, abundant; C, common; O, occasional; R, rare; –, not present. Open in new tab Stable isotope signatures Usually, chemosymbiotic organisms were isotopically distinct from the heterotrophic fauna (Figure 4, Appendix). Vesicomyids had the lowest δ13C and δ15N signatures, ranging from −36.2 to −35.4‰ for δ13C, and from 2.9 to 4.8‰ for δ15N, whereas values for 15N of Thyasira methanophila (δ15N = 10.0‰) and for δ13C of Lamellibrachia sp. (δ13C = −22.9‰) were close to the range of POM and SOM. Figure 4. Open in new tabDownload slide Dual isotope plot of δ13C and δ15N (mean ± s.d.) of chemosynthetic invertebrates, non-chemosynthetic food sources, and secondary consumers (invertebrates and fish) at the CMSA (white dots) and control non-seep sites (black dots). *See the text for explanation on the comparatively heavy 13C values of Lamellibrachia sp. Figure 4. Open in new tabDownload slide Dual isotope plot of δ13C and δ15N (mean ± s.d.) of chemosynthetic invertebrates, non-chemosynthetic food sources, and secondary consumers (invertebrates and fish) at the CMSA (white dots) and control non-seep sites (black dots). *See the text for explanation on the comparatively heavy 13C values of Lamellibrachia sp. Isotopic signatures did not differentiate the non-symbiont-bearing fauna between the control and CMSA sites (Figure 4). At the CMSA, heterotrophic fauna δ13C ranged from −19.8 to −11.0‰, and δ15N from 12.6 to 23.5‰. Isotopic signatures of the heterotrophic fauna at the control site were within this range (Appendix). This suggests that at both CMSA and control sites, the primary organic food sources are basically the same, most probably POM and SOM. However, some top predators at the CMSA displayed slightly lighter δ13C values than lower trophic level consumers (Appendix). These include the Patagonian toothfish (δ13C = −18.6‰), the octopus Benthoctopus sp. (δ13C = −17.8‰), and the morid Antimora rostrata (δ13C = −17.4‰), the latter also with a relatively low δ15N (15.0‰). Antimorarostrata collected at the control site displayed heavier isotopic signatures for both isotopes (δ13C = −16.0‰ and δ15N = 20.1‰). Discussion Chemosymbiotic faunal composition As cold-seep exploration at the SE Pacific margin is just starting, and more seep sites are expected to be discovered there, it is still too early to discuss the degree of endemicity of the CMSA chemosymbiotic fauna, or to consider patterns of latitudinal or depth zonation. However, none of the chemosynthetic species collected here is apparently shared with similar communities off Peru (5–6°S), the closest seep area so far described (∼3500 km to the north). Peruvian seeps have been reported from 2630 to 5140 m deep, with their fauna distributed as a function of depth. Their chemosymbiotic fauna are constituted by three vesicomyids, a solemyid bivalve of the genus Acharax, and a “pogonophoran” (Olu et al., 1996). The vesicomyid Calyptogena goffrediae was recently described, and it resembles C. gallardoi in shape and size, but differs in having a shallower escutcheon, a more curved beak, and a more expanded anterodorsal shell region (Krylova and Sahling, 2006). The other two vesicomyids observed still remain undescribed. Our work has produced the first record of living siboglinids for the Chilean margin. Tubeworms associated with seep sites have been also reported for the Peruvian margin (Olu et al., 1996), but no detail of their taxonomic status was given. The only species of siboglinid reported for the Eastern Pacific is Lamellibrachiabarhami from the continental slope of southern California and Oregon, the median valley of the Juan de Fuca Ridge, San Clemente Basin, and Monterey Bay, at depths of 600–2400 m (Schulze, 2003), and from the Costa Rica margin, ∼8 to 10°N at depths of 1000–2400 m (Mau et al., 2006). However, according to a Cytochrome C Oxidase Subunit 1 (CO1) analysis of the CMSA species, and comparison with other representatives of the genus, the Chilean species is closer to Lamellibrachia luymesi. This last species, which inhabits the upper slope of the Gulf of Mexico (<1000 m), has a 3% sequence divergence with the CMSA species, suggesting that it is indeed different (C. Fisher and K. Nelson, pers. comm.). Our limited knowledge of the Chilean margin seep fauna also suggests that its bivalve-dominated chemosymbiotic community structure is similar to the better-studied counterpart at Monterey Bay on the Northeast Pacific upper slope (∼39°N, 500–1000 m). As an example, the most common CMSA vesicomyids, C. gallardoi and vesicomyid gen. sp. 1, morphologically resemble Calyptogenapacifica and Calyptogenakilmeri, respectively, which are the dominant species at Monterey seep habitats (Goffredi et al., 2004). Three additional vesicomyids have been reported for Monterey Bay, but in lesser abundance, and two less abundant vesicomyids were also observed at the CMSA. Moreover, as well as the fauna reported in Mediterranean mud volcanoes (Olu-Le Roy et al., 2004), the fauna of the CMSA seems to include symbiotic species that are not restricted to seeps but adapted to organic rich environments (e.g. oxygen-deficient settings), such as thyasirids and lucinids. The proximity of an intense OMZ, where lucinid and thyasirid bivalves are often common (Levin, 2003), may contribute to the adaptation, diversity, and evolution of these groups within seeps. We do not know yet whether all the habitat patch types typical of the Alaska, Oregon, California, Costa Rica, and Peru seeps (mats of filamentous sulphur bacteria, Calyptogena, Acharax beds, serpulid beds, vestimentiferan aggregations, and siboglinid fields) are present off Chile, or whether there are novel habitat configurations. Finally, it is noteworthy that, of all the known eastern Pacific seeps, only those off Costa Rica support mussel beds (Mau et al., 2006), despite mytilids being common at eastern Pacific vent sites, such as the Galapagos Spreading Centre and the East Pacific Rise (Van Dover, 2000). Vesicomyids appear to replace mussels as the dominant biomass at the well-studied eastern Pacific seeps (Levin, 2005). Megafaunal community structure at the CMSA vs. the control non-seep site Although we did not attempt to calculate faunal densities, owing to the roughness of the seep-site terrain, it was evident that our site was clearly richer in species and that fauna was more abundant than at the control non-seep site. Only 24 megafauna species were observed at the non-seep site, and all were also present at the CMSA (Table 2). At the control site, the solenocerid shrimp H. diomedeae was dominant, along with the polychaete Hyalinoecia sp. and macrourids, but always in lesser numbers than at the seep site. The reduced species richness of the control site can in part be explained by the absence of hard substratum, which resulted in a virtual absence of sessile fauna, mainly cnidarians (e.g. gorgonians) and their associated fauna (e.g. brittlestars and galatheid crustaceans). The role of deep-water corals in structuring benthic communities is widely recognized as providing a food source, a perch for suspension-feeders, and protection from predation (Krieger and Wing, 2002). When compared with other seep sites worldwide, the CMSA epibenthos species number ranks among the highest reported. Just 25 species have been reported for Mediterranean mud volcanoes (Olu-Le Roy et al., 2004), 83 taxa for the clam-bed and microbial-mat habitats on the northern California slope (Levin et al., 2003), 86 taxa for the San Clemente cold seep (Baco and Smith, 2003), and 66 species associated with vestimentiferan aggregations in the Gulf of Mexico (Bergquist et al., 2003). However, due to each of these studies only examining a subset of the total fauna (i.e. some were more focused on the macrofauna and others on the megafauna), and methodological approaches differing, comparisons should be made with caution. Nevertheless, ecosystem-wide species counts are likely to be much higher, because it is expected that the CMSA also hosts a rich meiofaunal and macrofaunal assemblage. Once assessed, these groups will probably increase the species number of animals associated with our site considerably. According to Carney (1994), colonists are heterotrophic species attracted to vent or seep sites by the aggregation of chemo-autotrophically derived organic matter, endemics are species never found outside reducing environments, and vagrants occur uniformly within and outside vents and seeps. The very limited knowledge of Chilean background bathyal fauna prevents us evaluating whether the non-chemosymbiotic fauna we found are seep endemics, although most of them seem to be colonists or vagrants. Many of the species we collected, like Pagodula concepcionensis, Otukaia crustulum, and Margarites huloti, are new to science (Houart and Sellanes, 2006; Vilvens and Sellanes, 2006). Representatives of some of these genera have been reported for seep sites off Japan (e.g. Margarites shinkai). However, excluding the antonbruunid and nautiliniellid polychaetes, commensal of C. gallardoi, no other species of non-chemosymbiotic fauna endemic of seeps (e.g. provannid gastropods, alvinocarid shrimps), were found. Hence, the CMSA assemblage shares some characteristics of the shallow-water methane seeps off California (Levin et al., 2000) and the North Sea (Dando et al., 1991), which have dense faunal populations but few seep endemics. This contrasts with observations in the Gulf of Mexico, where relatively high levels of faunal endemism are tightly associated with vestimentiferan aggregations (MacAvoy et al., 2005). Primary sources driving the heterotrophic foodweb Except Lamellibrachia, chemosymbiotic animal signatures did not overlap with photosynthetic or sediment organic carbon isotopic signatures. Sediment and suspended matter organic carbon isotopic values were nearly in the range for photosynthetically fixed material (δ13C from −20.2 to −23.47‰; Appendix). There were also relatively high organic carbon (2.6%) and chlorophyll a (4.07 µg g−1) contents in the control-site sediments (Quiroga et al., in press). This information, along with sediment isotopic signatures, suggests that a large fraction of the partially undegraded phytodetritus reaches the mid-slope seabed off Concepción. Moreover, trawled fragments of Macrocystis at the control site also indicate that inputs of other photosynthetic sources are present (Appendix). Chemosymbiotic animals showed δ13C values lighter than −35‰, except Lamellibrachia sp., with a δ13C of −22.8‰. Similar 13C-enriched values have been reported for other siboglinids (e.g. −20.1‰ for L. luymesi in the Gulf of Mexico; MacAvoy et al., 2005). This enrichment relative to other cohabiting chemosymbiotic fauna (e.g. vesicomyids) is likely a consequence of metabolic and morphological differences. Symbionts in the two groups use different forms of ribulosebiphosphate carboxylase–oxygenase, which fractionate carbon to different extents (Fisher et al., 1990). It has also been suggested that the different groups take up dissolved inorganic carbon (DIC) that is quite dissimilar in carbon isotopic signature. Although bivalves take up DIC at the sediment surface, which is low in 13C, tubeworms grow with their plumes well above the sediment and hence are able to take up DIC with signatures more typical of seawater (MacAvoy et al., 2005). The similarity between 13C signatures of Lamellibrachia sp. and other photosynthetic food sources (Figure 4) make it difficult to discriminate whether heterotrophs also consume Lamellibrachia. However, siboglinids (e.g. L. luymesi) seem to be unpalatable for predators, probably because they contain chemical compounds that deter consumption (Kicklighter et al., 2004). The general distribution of stable isotope signatures of the fauna at both the CMSA and control site indicates that primary organic food sources are the same, and mainly of photosynthetic origin (e.g. phytodetritus and SOM). Because the CMSA is located beneath highly productive waters, photosynthetically originated C is expected not to be a limiting factor even at such depths, overriding the potential significance of other locally fixed carbon sources for heterotrophic consumers. However, it is interesting that δ13C values of top predators such as A. rostrata (−17.4 ± 0.1‰), Benthoctopus sp. (−18.9 ± 1.5‰), and even D. eleginoides (−18.6 ± 2.2‰) are more depleted than their expected prey (i.e. background fauna δ13C = –14.3 ± 1.3‰ on average), suggesting partial or occasional inputs from lighter sources (Figure 4). However, although high 15N values for D. eleginoides and Benthoctopus sp. do not really support chemosynthetic food sources, lighter values of A. rostrata at the seep site (Appendix) suggest that this species at least could ingest some chemosynthetic production. Mechanisms promoting faunal aggregation at the CMSA Levin and Michener (2002) hypothesized that as food becomes limiting, seep-related resources should comprise a larger part of the diet of non-seep vagrants. For example, at the Oregon margin seeps, also located beneath a highly productive eastern boundary system, the isotopic signatures of mobile sea urchins and crabs closely resemble non-seep production, whereas there is significant incorporation of chemosynthetic material into the benthic foodweb from methane-based communities underlying the oligotrophic waters of the Gulf of Mexico (Levin and Michener, 2002; MacAvoy et al., 2003). At the CMSA, large predatory fish are frequent, and this site seems to be a preferred fishing ground. This is evidenced by abundant lost fishing gear (hooks and weights) in many of the trawls which collected living chemosymbiotic fauna (Sellanes and Krylova, 2005). However, the overall increase in abundance, biomass, and diversity of the megafaunal communities, including those top predators, is not a function of increased local primary production, because there is no reliance on in situ production. Instead, we suggest that the presence of methane-derived authigenic carbonates provides a suitable habitat for sessile organisms and associated fauna. This hard substratum may provide a rich feeding ground for species such as D. eleginoides, because much of their prey (e.g. rattails, cephalopods, and crustaceans; Oyarzún et al., 2001) are present in large quantities there. This has been suggested for the Gorda Escarpment off northern California, where multispecies aggregations of octopus (Benthoctopus sp. and Graneledone sp.) and blob sculpins (Psychrolutes phrictus) brood at seep sites. This preference has been ascribed to the interaction of local topography, physical, and geological settings (Drazen et al., 2003). Females of a local species of blob sculpin (Psychrolutes sio), in an advanced reproductive stage, have been also caught at the CMSA (Table 2), suggesting similar behaviour to their NE Pacific counterparts. The Chilean margin seep environments seem to act as nuclei for increased diversity and abundance for invertebrates and fish, including commercial species. Damage to these environments by anthropogenic activities may affect the populations of associated species, some of which have been found exclusively at this site, at least up to now. Future studies should describe the extent of seeps across and along the Chilean margin to clarify the species associations and interactions within them and the surrounding non-seep environment, including nearby OMZ. In addition, further studies to elucidate patterns of energy transfer within the system need to include unresolved constituents of the foodweb such as mat-forming bacteria, infauna, and fish. The whole information should facilitate better comparison with other seep settings along the Pacific margin and contribute to improve our understanding of endemicity, biogeographic, bathymetric, and latitudinal zonation patterns of these particular systems. Acknowledgements We thank the captain and crew of the Chilean Navy’s RV “Vidal Gormáz” for support at sea, and R. Coffin and J. Díaz-Naveas, who acted as co-chief scientists in the first expedition (VG-04 cruise). Special thanks for help during SeepOx cruise go to V. A. Gallardo, G. Guzmán, L. Dezileau, W. Alarcón, L. Cárdenas, S. Fuentes, J. González, J. Inostroza, P. Inostroza, L. Muñoz, J. Maturana, and M. Silva. Elena Krylova is acknowledged for her help with the taxonomy of vesicomyids, and we are also grateful to L. A. Levin and A. Thurber (Scripps Institution of Oceanography), D. Desbruyères (IFREMER), and two anonymous referees for providing valuable comments on earlier versions of the manuscript. The work was funded by FONDECYT project No. 1061217 to JS, the Research Direction of the University of Concepción and the Centre of Oceanographic Research in the Eastern-South Pacific (COPAS) of the University of Concepción. Additional support was provided by FONDECYT project No. 1061214 to Práxedes Muñoz, Scripps Institution of Oceanography through NOAA Ocean Exploration program Grant # NOAA NA17RJ1231 to L. A. Levin (for ship’s time and cruise participation of CN and J. González). References Baco A. R. , Smith C. R. . 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Psychrolutes sio 23.5 0.8 −14.5 0.4 2 Dissostichus eleginoides (>85 cm) 22.1 1.1 −18.6 2.2 5 Centroscyllium granulatum 21.8 −14.7 1 Coelorinchus fasciatus 21.8 −14.8 1 Coryphaenoides ariommus 21.2 1.0 −15.7 0.7 2 Stegophiura sp. 20.7 −15.0 1 Calliostoma chilena 20.6 1.1 −15.0 0.6 4 Miomelon philippiana 20.3 −13.9 1 Coralliomorphus sp. 19.9 −18.5 1 Munidopsis trifida 19.4 0.3 −15.4 0.2 3 Homalopoma panamense 19.3 0.2 −14.3 0.7 4 Hyalinoecia sp. 19.1 0.5 −14.8 0.7 3 Astrodia tenuispina 18.9 −15.5 1 Asteronyx loveni 18.8 −13.5 1 Ctenodiscus australis 18.7 2.0 −11.0 1.1 3 Bathybembix macdonaldi 18.7 0.4 −14.6 0.2 4 Campylonotus semistriatus 18.3 0.2 −14.0 0.3 2 Haliporoides diomedeae 18.2 0.4 −15.1 0.3 3 Benthoctopus sp. 18.1 2.7 −17.8 1.6 4 Limopsis sp. 17.8 0.2 −15.2 0.3 2 Aforia cf. goniodes 17.4 0.4 −13.6 0.1 3 Callogorgia sp. 16.5 −19.4 1 Paragorgia sp. 16.5 −19.7 1 Gorgonocephalus chilensis 16.1 2.3 −14.5 1.4 3 Turridae 16.0 −14.3 1 Ophiura carinata 15.3 1.4 −18.1 2 Antimora rostrata 15.0 0.8 −17.4 0.1 3 Ophiomusium biporicum 12.6 −17.0 1 Thyasira methanophila 10.2 −35.4 1 POM 9.8 1.1 −23.5 0.0 2 SOM 8.8 −20.18 1 Lamellibrachia sp. 7.6 −22.8 1 Calyptogena gallardoi 4.8 0.8 −36.4 0.6 5 Vesicomyidae gen. sp. 1 2.9 3.7 −36.2 1.3 2 Control site taxa and parameters Bathyraja sp. 21.9 −14.9 1 Bathybembix macdonaldi 21.4 −14.5 1 Halaelurus canescens 21.4 −15.2 1 Coelorinchus fasciatus 21.3 0.6 −14.8 0.2 2 Antimora rostrata 20.1 −16.0 1 Fissidentalium majorinum 19.4 −17.2 1 Stereomastis sculpta 19.3 −16.8 1 Haliporoides diomedea 19.1 −15.1 1 Puncturella sp. 18.1 −14.3 1 +Benthoctopus sp. 18.0 −16.9 1 Ennucula grayi 15.1 −15.7 1 Macrocystis sp. debris 12.0 −14.2 1 SOM 8.0 −20.1 1 Seep site taxa and parameters . δ15N (‰) . δ13C (‰) . n . . Mean . s.d. . Mean . s.d. . . Psychrolutes sio 23.5 0.8 −14.5 0.4 2 Dissostichus eleginoides (>85 cm) 22.1 1.1 −18.6 2.2 5 Centroscyllium granulatum 21.8 −14.7 1 Coelorinchus fasciatus 21.8 −14.8 1 Coryphaenoides ariommus 21.2 1.0 −15.7 0.7 2 Stegophiura sp. 20.7 −15.0 1 Calliostoma chilena 20.6 1.1 −15.0 0.6 4 Miomelon philippiana 20.3 −13.9 1 Coralliomorphus sp. 19.9 −18.5 1 Munidopsis trifida 19.4 0.3 −15.4 0.2 3 Homalopoma panamense 19.3 0.2 −14.3 0.7 4 Hyalinoecia sp. 19.1 0.5 −14.8 0.7 3 Astrodia tenuispina 18.9 −15.5 1 Asteronyx loveni 18.8 −13.5 1 Ctenodiscus australis 18.7 2.0 −11.0 1.1 3 Bathybembix macdonaldi 18.7 0.4 −14.6 0.2 4 Campylonotus semistriatus 18.3 0.2 −14.0 0.3 2 Haliporoides diomedeae 18.2 0.4 −15.1 0.3 3 Benthoctopus sp. 18.1 2.7 −17.8 1.6 4 Limopsis sp. 17.8 0.2 −15.2 0.3 2 Aforia cf. goniodes 17.4 0.4 −13.6 0.1 3 Callogorgia sp. 16.5 −19.4 1 Paragorgia sp. 16.5 −19.7 1 Gorgonocephalus chilensis 16.1 2.3 −14.5 1.4 3 Turridae 16.0 −14.3 1 Ophiura carinata 15.3 1.4 −18.1 2 Antimora rostrata 15.0 0.8 −17.4 0.1 3 Ophiomusium biporicum 12.6 −17.0 1 Thyasira methanophila 10.2 −35.4 1 POM 9.8 1.1 −23.5 0.0 2 SOM 8.8 −20.18 1 Lamellibrachia sp. 7.6 −22.8 1 Calyptogena gallardoi 4.8 0.8 −36.4 0.6 5 Vesicomyidae gen. sp. 1 2.9 3.7 −36.2 1.3 2 Control site taxa and parameters Bathyraja sp. 21.9 −14.9 1 Bathybembix macdonaldi 21.4 −14.5 1 Halaelurus canescens 21.4 −15.2 1 Coelorinchus fasciatus 21.3 0.6 −14.8 0.2 2 Antimora rostrata 20.1 −16.0 1 Fissidentalium majorinum 19.4 −17.2 1 Stereomastis sculpta 19.3 −16.8 1 Haliporoides diomedea 19.1 −15.1 1 Puncturella sp. 18.1 −14.3 1 +Benthoctopus sp. 18.0 −16.9 1 Ennucula grayi 15.1 −15.7 1 Macrocystis sp. debris 12.0 −14.2 1 SOM 8.0 −20.1 1 s.d., standard deviation; n, number. Open in new tab © 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: [email protected] © 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: [email protected]

Madsen, Matias L.; Gaard, Eilif; Hansen, Benni W.

doi: 10.1093/icesjms/fsn097pmid: N/A

Madsen, M. L., Gaard, E., and Hansen, B. W. 2008. Wax-ester mobilization by female Calanus finmarchicus (Gunnerus) during spring ascendance and advection to the Faroe Shelf. ICES Journal of Marine Science, 65: 11121121.Time of ascendance and initiation of reproduction in Calanus finmarchicus is closely correlated with the phytoplankton spring bloom. However, significant egg production can take place before the bloom, fuelled by mobilizing stored wax-ester lipids. Calanus finmarchicus from stations on the Faroe Shelf were compared with specimens collected off the shelf. Biological parameters such as gut contents, egg production, developmental stage, and lipid content were determined and correlated with phytoplankton concentration and spatial distribution along two transects in late April 2003 across the shelf northeast and southwest of the Faroe Islands. Grazing by C. finmarchicus on phytoplankton was significantly lower northeast of the Faroe plateau. However, the egg production was generally high for a pre-bloom situation, with significantly higher rates of egg production on the shelf than off it, along both transects. Wax-ester content of female C. finmarchicus was significantly higher and more variable at off-shelf stations than on the shelf (<2 g female1). From this, we suggest that off-shelf C. finmarchicus had recently emerged from overwintering depths, in contrast to individuals from stations on the shelf, which had been in the upper water masses for some time. Females (from off-shelf stations) most likely supported the initial egg production from their wax-ester reserves.

Mayfield, Stephen; McGarvey, Richard; Carlson, Ian J.; Dixon, Cameron

doi: 10.1093/icesjms/fsn105pmid: N/A

Mayfield, S., McGarvey, R., Carlson, I. J., and Dixon, C. 2008. Integrating commercial and research surveys to estimate the harvestable biomass, and establish a quota, for an unexploited abalone population. ICES Journal of Marine Science, 65: 11221130.A key challenge facing many fisheries managers is the absence of information on the level of harvestable biomass. We describe an integrated, two-stage survey approach that was used to measure the spatial distribution and harvestable biomass of a largely unexploited metapopulation of greenlip abalone (Haliotis laevigata) over a large area of northwestern Spencer Gulf, South Australia. In stage 1, commercial fishers conducted systematic surveys to identify subareas with abalone at harvestable densities. Cpue measures from these surveys were used to map and stratify a bounded survey subregion, within which leaded-line, research-diver surveys measured absolute density and harvestable biomass (stage 2). Decision tables, showing minimum biomass at various probabilities vs. harvest fraction, were developed to provide a risk-assessment framework for quota setting. Within two years, our approach allowed, first, the mapping of the broad-scale, spatial distribution and abundance of greenlip abalone in an area of 1143 km2, second, the estimation of harvestable biomass in a smaller (16.9 km2) area, and finally, the allocation by State fishery managers of an additional quota inside a newly defined management subzone. The collaborative approach we describe for providing estimates of absolute biomass over large spatial scales affords multiple advantages for the assessment and management of invertebrate dive fisheries.

Len, Roxana; Castro, Leonardo R.; Cceres, Mario

doi: 10.1093/icesjms/fsn093pmid: N/A

Len, R., Castro, L. R., and Cceres, M. 2008. Dispersal of Munida gregaria (Decapoda: Galatheidae) larvae in Patagonian channels of southern Chile. ICES Journal of Marine Science, 65: 11311143.The dispersal of Munida gregaria larvae in Chilean Patagonian channels was assessed in spring 2002 and 2003, and winter 2003. In winter 2003, zoea I was the most abundant stage in all channels and there were no larval stages older than zoea IV. In spring 2002 and 2003, there were six larval stages in all channels, and the greater abundance of older larvae suggested that reproduction takes place in winter and larval development in spring. Further, analysis of spatial distribution by stage revealed that early zoeae moved seawards. Generalized Additive Models analyses showed that most larval stages were temperature-dependent, and that the salinity range of the youngest zoea was wider than that of older larvae and post-larvae, coinciding with an ontogenetic distribution change from estuary to shelf. Residual flows determined with an acoustic Doppler current profiler revealed two layers of opposite flow: the shallowest layer moved seawards and the deeper layer onshore. The surface layer was wider in spring than in winter because of seasonal increase in fresh-water input. The dispersal pattern of M. gregaria consisted of an inner channel larval release in winter, followed by an along-channel larval drift and subsequent export to the shelf in spring. The mechanism by which juveniles return to the channels seemed to be associated with the onshore subsurface flow.

Duarte-Neto, Paulo; Lessa, Rosngela; Stosic, Borko; Morize, Eric

doi: 10.1093/icesjms/fsn090pmid: N/A

Duarte-Neto, P., Lessa, R., Stosic, B., and Morize, E. 2008. The use of sagittal otoliths in discriminating stocks of common dolphinfish (Coryphaena hippurus) off northeastern Brazil using multishape descriptors. ICES Journal of Marine Science, 65: 11441152.The shape of sagittal otoliths from the dolphinfish (Coryphaena hippurus) was studied to test the hypothesis that two stocks exist off the northeast coast of Brazil. In all, 82 sagittal otoliths were collected from fish caught by the artisanal fleet in two coastal regions of northeastern Brazil in December 2003 and April/May 2004. Several shape descriptors were determined [area, perimeter, rectangularity, circularity, eccentricity, fractal dimension (FD), and Fourier coefficients (FCs)] to evaluate the degree of similarity in the otoliths between regions. A three-morphotype pattern was revealed through cluster and principal component analyses based on FCs of the 30th harmonics. Apparently, this pattern is not influenced by clinal factors. Despite the great variability between otolith shapes, separation of the samples from two regions was suggested using multivariate and univariate analyses of variance for all shape descriptors and using canonical discriminant analysis. The methods successfully classified 57.1 and 69.6% of otoliths from the Maranho and Rio Grande do Norte regions, respectively. The FD was a powerful descriptor in discriminating the two stocks. Differences in the shapes of sagittal otoliths may be related to different growth rates of the species and lend credence to the belief that there are two stocks along the Brazilian coast.

Fernandez-Jover, Damian; Sanchez-Jerez, Pablo; Bayle-Sempere, Just Toms; Valle, Carlos; Dempster, Tim

doi: 10.1093/icesjms/fsn091pmid: N/A

Fernandez-Jover, D., Sanchez-Jerez, P., Bayle-Sempere, J. T., Valle, C., and Dempster, T. 2008. Seasonal patterns and diets of wild fish assemblages associated with Mediterranean coastal fish farms. ICES Journal of Marine Science, 65: 11531160.Fish are attracted to floating structures, including coastal cage fish farms, sometimes in dense aggregations. To understand better the influence of aquaculture on wild fish stocks, we carried out seasonal visual censuses around three southwestern Mediterranean farms over 2 years to assess the temporal patterns of the aggregated fish assemblage. In addition, we analysed the diet of the five most abundant species. Aggregations around all farms were large throughout the year, although species composition and abundance differed among farms and seasons. Fish farms are attractive habitats for certain species of wild fish in specific seasons. Adult fish of reproductive size dominated the assemblages, and stomach content analysis revealed that 6689 of fish of the five most abundant taxa had consumed food pellets lost from the cages. We estimated that wild fish consume up to 10 of the pellets used at farms, indicating that food is a key attractant. Regional monitoring of farm-associated wild fish assemblages could aid management of the interaction of aquaculture and wild fish resources, because changes in feeding behaviour may have consequences for fish populations and local fisheries.

Williams, Tom; Helle, Kristin; Aschan, Michaela

doi: 10.1093/icesjms/fsn103pmid: N/A

Williams, T., Helle, K., and Aschan, M. 2008. The distribution of chondrichthyans along the northern coast of Norway. ICES Journal of Marine Science, 65: 11611174.The relationship between temperature, latitude, and depth and the distribution and relative abundance of chondrichthyans along the northern coast of Norway was examined based on catches made in scientific trawls north of 62N from 1992 to 2005. It appears that Chimaera monstrosa, Etmopterus spinax, Squalus acanthias, and Galeus melastomus were more abundant in the south, and Amblyraja radiata more common in the north. Between 1992 and 2005, the distribution and relative abundance did not appear to change significantly, although average water temperatures rose during the period. Current fishing levels do not appear to be impacting the populations of the more common species, but the status of species rarely found in the survey catches is unclear.

Fiorentino, F.; Badalamenti, F.; DAnna, G.; Garofalo, G.; Gianguzza, P.; Gristina, M.; Pipitone, C.; Rizzo, P.; Fortibuoni, T.

doi: 10.1093/icesjms/fsn104pmid: N/A

Fiorentino, F., Badalamenti, F., DAnna, G., Garofalo, G., Gianguzza, P., Gristina, M., Pipitone, C., Rizzo, P., and Fortibuoni, T. 2008. Changes in spawning-stock structure and recruitment pattern of red mullet, Mullus barbatus, after a trawl ban in the Gulf of Castellammare (central Mediterranean Sea). ICES Journal of Marine Science, 65: 11751183.The increase in biomass of red mullet, Mullus barbatus, in the Gulf of Castellammare (northwestern Sicily, central Mediterranean) after a 14-year trawl ban, prompted us to compare the spawning-stock structure and the recruitment pattern before and after the closure. Datasets obtained from three experimental trawl surveys were available before the ban (April and September 1985; AprilMay 1986) and four post-ban (September and November 2004; March and May 2005). Spawning-stock biomass increased significantly after the ban. Moreover, females at depths >50 m in the post-ban period were larger than those collected before the ban at the same depth. The recruitment pattern of the population also changed. Notably, recruit numbers increased and recruitment occurs over a broader period. The increase in biomass after the trawl ban seems to be the result of a combination of different processes, mainly associated with the lowering of fishing mortality. A positive trend in sea surface temperature in the area may have played a role too.

Mauna, A. Cecilia; Franco, Bárbara C.; Baldoni, Ana; Acha, E. Marcelo; Lasta, Mario L.; Iribarne, Oscar O.

doi: 10.1093/icesjms/fsn098pmid: N/A

Abstract Mauna, A. C., Franco, B. C., Baldoni, A., Acha, E. M., Lasta, M. L., and Iribarne, O. O. 2008. Cross-front variations in adult abundance and recruitment of Patagonian scallop (Zygochlamys patagonica) at the SW Atlantic Shelf Break Front. – ICES Journal of Marine Science, 65: 1184–1190. We studied cross-front variations in adult abundance and recruitment of Patagonian scallop (Zygochlamys patagonica) and their relationship with the Shelf Break Front and satellite-derived chlorophyll a in the Southwest Atlantic between 38°S and 39°30′S. Integrated data from commercial fleets (CF, 1996–2005), research cruises (RC, 1998–2005), sea surface temperature (SST; 1985–1997), and satellite-derived chlorophyll a (CSAT; 1998–2004) were included in the analysis. One-way ANOVA was used to assess differences in recruitment and scallop abundance in a cross-front direction. The abundance of scallops was greatest (RC > 200 kg h−1) closer to the onshore side of the front or beneath it, and the East–West extension of scallop beds (∼40 km) matched the seasonal zonal displacement of the front (∼37 km). The highest CSAT gradients were west of or matched the position of the front. The annual mean position of the front correlated with the spatial variability in recruitment within areas where the species recruited successfully (RC > 2000 h−1). The spatial variability in adult abundance and recruitment of scallops is strongly related to the spatial variability in the position of the front and with the high CSAT concentrations associated with it. Introduction Marine fronts are usually associated with high biological primary production promoted by physical mechanisms that affect planktonic organisms, such as vertical fluxes, retention, and convergence (Mann and Lazier, 1991). At large scales, spawning of fish (many economically important) and aggregations of their larvae occur associated spatially with fronts (Acha et al., 2004). Moreover, species abundance and community diversity are usually greater in these areas than in the surrounding waters (Dauwe et al., 1998). Therefore, marine fronts play a key role in marine systems, because of the transfer of primary production to upper trophic levels, and because the physical mechanisms favour the aggregation of organisms (Olson, 2002; Acha et al., 2004). The convergence of water masses at a frontal interface may aggregate and retain benthic invertebrate larvae, which then settle, supplying recruits to the seabed (Sinclair, 1987; Acha et al., 2004). Moreover, many benthic species depend on bentho-pelagic coupling mechanisms by which food particles, generated in the euphotic layer, sink to the bottom (Billet et al., 1983; Bett et al., 2001). Therefore, frontal variability (i.e. position and extension, which can change within and among years) may have consequences on (i) recruitment (Shanks, 1995; Alexander and Roughgarden, 1996), (ii) the distribution and abundance of benthic species (Nodder et al., 2003), or (iii) all the processes that contribute to a dynamic community (Dewicke et al., 2002). Although these factors are important in understanding patterns in species abundances and community functions, few studies have aimed to understand those processes better (Josefson and Conley, 1997). The Patagonian scallop (Zygochlamys patagonica) is a suspension-feeder that inhabits soft substrata from Chiloé Island in the SE Pacific (42°S) to Cape Horn (56°S), and in the Southwest Atlantic north to 35°S, the latitude of the mouth of the Río de la Plata (Bogazzi et al., 2005). At large scales, important aggregations of this species match the location of three major frontal systems: the Shelf Break Front (SBF), the Northern Patagonian Front, and the Southern Patagonian Front (Bogazzi et al., 2005). Under the influence of the SBF and along the 100-m isobath, the most profitable scallop beds are located, and they produce ∼50 000 t of commercial scallops annually (Lasta and Bremec, 1998; Orensanz et al., 2006). The fishery, certified as sustainable by the Marine Stewardship Council in 2006, has been prosecuted since 1996, and it is an important one for Argentina, which deploys a factory fleet to harvest the resource (Ciocco et al., 2006). In addition to the large-scale association of scallop aggregations with frontal systems, observations at higher resolution (∼40 km) suggest that their spatial distribution varies in a cross-front direction (Lasta et al., 2001), probably as a result of variability in the SBF. Given these antecedents, we evaluate the hypothesis that scallop distribution varies in a consistent pattern across the axis of the front. With this purpose, we integrated high-resolution data from fishery research cruises (RC, dredge benthic samples), fishing fleets (logbooks and observer data), satellite-derived sea surface temperature (SST), satellite-derived chlorophyll a (Chl a), and in situ CTD data to determine the relationship between recruitment and adult abundance of Patagonian scallop and both spatial variability of the SBF and surface Chl a concentration. Material and methods Study area The study was performed in an area under the influence of the SBF, located between 38°00′ and 39°30′S (Figure 1) in the Argentine Sea (SW Atlantic). The SBF is a thermohaline front characterizing the offshore border of the continental shelf. Its inner boundary is located between the 90- and 100-m isobaths (Martos and Piccolo, 1988). The front has a West–East extension of ∼80 km at the surface and 40 km at the seabed (Bogazzi et al., 2005). It is the result of the meeting of warm (6.5–21°C), fresher (33.2–33.8 psu), nutrient-poorer (1–8 µg at l−1 nitrate; <1 µg at l−1 phosphate) shelf waters of Subantarctic origin with colder (4.5–6°C), more saline (33.8–34.2 psu), nutrient-rich (10–27 µg at l−1 nitrate; >1 µg at l−1 phosphate) waters of the Malvinas Current (Martos and Piccolo, 1988; Lutz and Carreto, 1991; Carreto et al., 1995; Brandini et al., 2000). The front shows strong gradients in the thermal field (>0.08°C km−1; Martos and Piccolo, 1988), mainly during summer, but salinity gradients are weak (0.002 psu km−1; Romero et al., 2006). The typical density structure of the SBF defines a retrograde front in which the slope of frontal isopleths is opposite to that of the cross-shelf topography. Mesoscale oceanographic processes such as a subsurface anticyclonic eddy and cross-front mid-level intrusions (thickness 5–40 m) have been identified in the area (Bogazzi et al., 2005). The location of the SBF varies seasonally at 38–39°S, moving offshore during summer and onshore during spring and autumn (Carreto et al., 1995). It has been suggested that upwelling at the shelf break could lead to the development of the strong band of Chl a shown in satellite images (Romero et al., 2006). This is a quasi-continuous band, between 38 and 50°S, characterized by a strong annual cycle with maximum values during spring and summer (Saraceno et al., 2004). Figure 1. Open in new tabDownload slide The study area in the Southwest Atlantic. Boxes divide the scallop grounds in a cross-front direction. The schematic cells within each box are shown in the lower right corner. The grey area represents the location of the SBF, and arrows indicate the direction of flow of Shelf Waters (SW). The contours correspond to the 100, 200, and 1000-m isobaths. Inset: schematic circulation, showing the Brazil Current (BC), Malvinas Current (MC), Brazil Malvinas Confluence (BMC), and Antarctic Circumpolar Currents (ACC). Adapted from Piola and Rivas (1997). Figure 1. Open in new tabDownload slide The study area in the Southwest Atlantic. Boxes divide the scallop grounds in a cross-front direction. The schematic cells within each box are shown in the lower right corner. The grey area represents the location of the SBF, and arrows indicate the direction of flow of Shelf Waters (SW). The contours correspond to the 100, 200, and 1000-m isobaths. Inset: schematic circulation, showing the Brazil Current (BC), Malvinas Current (MC), Brazil Malvinas Confluence (BMC), and Antarctic Circumpolar Currents (ACC). Adapted from Piola and Rivas (1997). Variability of the SBF system Annual and seasonal mean climatologies in SST gradients were used to locate the SBF mean position and to determine its variability. Although the front is haline, we employed SST to study the position of the front and its dynamics, because the spatial and temporal resolution of satellite data is better than the in situ data available for both salinity and temperature. However, density sections were drawn to describe the general front structure and the connection between surface and bottom conditions. The oceanographic data used in characterizing the vertical structure of the front correspond to density (σt, kg m−3) synoptic sections computed from depth profiles of temperature and salinity measured with a Sea-Bird 19 and 911 CTD. CTD stations were regularly spaced at intervals varying from 10 to 43 km along-transects run to encompass the condition of the front in summer (February 2004), autumn (May 2000), and spring (November 1993). Samples to calibrate salinity data were taken at bottom depths, and the temperature sensor maintained the factory calibration. The SST gradients were computed using a centred difference scheme based on 13 years (1985–1997) of satellite data of the Pathfinder + Erosion monthly climatology at 9.28 × 9.28 km resolution (Casey and Cornillon, 1999, 2001). Years covered by the satellite information do not match those with biological data and cannot be used to explain the occurrence of years with high or low values of catch per unit effort (cpue). Climatological analyses of satellite-derived chlorophyll a concentration were employed based on 7 years (1998–2004) of sea surface colour images from Standard Mapped Images (SMIs), supplied by the Sea–viewing Wide Field-of-view Sensor (SeaWiFS). The SMIs are derived from level-3 monthly binned data and mapped with a resolution of 0.09 × 0.09° (corresponding to ∼7.8 × 10 km in the study area), in a two-dimensional array of equidistant cylindrical projection. Following Yoder (2000) and Romero et al. (2006), the satellite-derived sea surface chlorophyll a concentration is referred to as CSAT (mg m−3). Cross-front variability in adult scallop abundance and recruitment from two sources of data Recruitment and adult scallop abundance: research cruise data To evaluate the relationship between the position of the front and scallop abundance, a scallop ground which include three beds was divided into six boxes, located at different latitudes. Inside each, 20 cross-front cells (∼3 km longitude by 30 km latitude) were created (Figure 1). Data were assigned to the cells according to the initial position of the tows obtained by GPS (±20 m). Data from RC (1998–2005) on the cpue of recruits (cpue R, number h−1), corresponding to scallops <16 mm total height (i.e. up to 1 year old; Lomovasky et al., 2008), and cpue of adults (cpue A, kg h−1), scallops that reach the commercial size (>55 mm total height, individuals 4–14 years old; Lomovasky et al., 2008) were used. RC were performed annually, and station locations (tows) were regularly spaced at ∼6 km intervals. The vessel operated with a non-selective dredge 2.5 m wide, with a mesh size of 10 mm and an efficiency of 43% (Valero, 2002). The standard towing time and speed were 10 min and 5.5 km h−1, respectively. Data from the unsorted catch (UC) were used to estimate the scallop catch. Total UC was weighed (±1 kg), and subsamples (±0.1 kg) were obtained randomly. The scallops present in the subsamples were counted, weighed, and measured to the nearest millimeter. The null hypotheses of no differences in cpue (recruits or adult scallops) across the front were evaluated with one-way ANOVA, and post hoc differences between averages were evaluated with a Bonferroni test. The cpue data were ln-transformed to satisfy ANOVA assumptions (Zar, 1984). Commercial scallop abundance: commercial fleet data The relationship between the position of the front and adult scallop abundance was evaluated with data from commercial fleets (CF, 1996–2005; cpue F), using the scheme described above. CF completed 331 fishing trips (1 trip ∼30 d) during the study period, making an average of 50 tows d−1 (range 30–80 tows d−1). The vessels operated non-selective bottom otter trawls 13 m long, with a mesh size of 100 mm, headrope and footrope 17 and 22 m long, respectively (Lasta and Bremec, 1998), and with an efficiency of 48% (Valero, 2002). UC per tow was assessed visually (extent of codend filling), based on categories of 10% before it was opened on deck. The UC weight for different proportions of codend fullness (20, 50, 80, and 100%) was determined by weighing a series of tows. A full net was estimated to contain 2298 kg of UC (Lasta and Bremec, 1998). Scallop catch was estimated from the scallop yield: subsamples (10 kg) were taken randomly from the UC, and scallop and bycatch portions were weighed (±0.1 kg). The cpue of commercial scallops (cpue F) was calculated based on their proportion in the yield. Based on an underlying error of 15% in biomass estimates from a collection of 30 samples per boat, the use of more than one subsample per tow would only reduce the uncertainty by 2% (Lasta et al., 1998). Results Variability of the SBF system There was weak evidence of the SBF in the northern part of the study area around 38°30′S (boxes 1 and 2, Figure 2). Maximum SST gradients reached 0.02°C km−1 during austral autumn and winter at the western side of box 1, where the mean annual position of the front is located. Maximum gradients of ∼0.02°C km−1 were in autumn at the eastern side of box 2. The SBF front becomes clearer south of box 2 (boxes 3–6), SST gradients increasing gradually southwards and towards the shelf break, reaching values of 0.05°C km−1. The eastern side of most boxes was influenced by the biggest gradients in SST, mainly during spring and summer. The mean seasonal position of the front had greater spatial variability than the mean annual position, with an onshore displacement during austral autumn (April–June) and winter (July–September), and more offshore positions during summer (January–March) and spring (October–December). Zonal displacement of the SBF mean position may reach ∼37 km (boxes 5 and 6; Figure 2), with extreme locations (on- and offshore) in autumn and summer. Figure 2. Open in new tabDownload slide Annual and seasonal mean climatologies of SST gradients (°C km−1) and derived surface chlorophyll a concentrations (CSAT, mg m−3) crossing the shelf break in each box. Figure 2. Open in new tabDownload slide Annual and seasonal mean climatologies of SST gradients (°C km−1) and derived surface chlorophyll a concentrations (CSAT, mg m−3) crossing the shelf break in each box. Vertical sections of density show at the surface a similar spatial pattern of SST gradient fields (Figure 3). The maximum surface gradient of the isopycnals (SBF) during summer and spring is offshore (Figure 3a and c), whereas in autumn the accentuated gradient of density was onshore (Figure 3b). Slope waters appear to be more influenced by the Malvinas Current in spring, shown by the displacement up to 150 m deep of the 27.0 isopycnal (Piola and Gordon, 1989), whereas in summer the same isopycnal was deeper than 200 m (not shown). During autumn, this feature is not observed, probably because the section extension is limited offshore. The strongest vertical stratification was during summer, at the western side of the box, reaching up to 0.12 kg m−3 m−1, diminishing during autumn and spring to a value around 0.04 kg m−3 m−1. The highest CSAT concentrations were at the western location of the SBF in most climatological seasons (Figure 2; note the logarithmic scale for CSAT). However, in the boxes under strong influence of the front (boxes 3 and 5), the highest values of CSAT during spring were related to a central–western position crossing the front. Maximum CSAT values increased up gradually to the south, attaining values of 5 mg m−3. Figure 3. Open in new tabDownload slide Vertical fields of density (σt, kg m−3) along sections sampled during the austral (a) summer, (b) autumn, and (c) spring, crossing the shelf break (see insets). Figure 3. Open in new tabDownload slide Vertical fields of density (σt, kg m−3) along sections sampled during the austral (a) summer, (b) autumn, and (c) spring, crossing the shelf break (see insets). Cross-front variability in adult scallop abundance and recruitment from two sources of data Recruitment and adult scallop abundance: research cruise data The cpue of recruits (cpue R) varies in a cross-front direction for almost all boxes with successful recruitment (Table 1). The spatial correlation between the patterns of distribution of higher values of cpue R (Figure 4) and of annual SBF position (Figure 2) is easily seen. A Bonferroni test revealed that the highest averages (e.g. 13 935 h−1, n = 7, s.d. = 23 643; box 2), located in central positions within the scallop bed (Figure 4), were significantly different from the rest. In box 3, probably because of low statistical power, the Bonferroni test was unable to identify the source of the differences detected by the ANOVA. The largest values of mean cpue R were in central–eastern positions of the bed, although there was an exception in box 4 where the highest values were in a central–western position. In 2000 and 2001, when there was successful recruitment, the cpue was, on average, >2000 h−1 in almost all tows (i.e. throughout the scallop bed). In other years, however, when recruitment was less (<500 h−1 in almost all tows), the spatial pattern was heterogeneous, only a few patches having above-average recruitment (Figure 4). Figure 4. Open in new tabDownload slide Cross-front distribution of cpue of scallop recruits (cpue R: numbers h−1). The years 1998 and 2003 represent poor recruitment years, and 2000 and 2001 represent successful recruitment years (note the change of scale in the symbol). The contours correspond to the 100 and 200-m isobaths. Figure 4. Open in new tabDownload slide Cross-front distribution of cpue of scallop recruits (cpue R: numbers h−1). The years 1998 and 2003 represent poor recruitment years, and 2000 and 2001 represent successful recruitment years (note the change of scale in the symbol). The contours correspond to the 100 and 200-m isobaths. Table 1. Summary of ANOVA results by box. Box and parameter . d.f. effect . MS effect . d.f. error . MS error . F . p-level . Box 1 Cpue A 10 32.186 173 11.802 2.727 0.004 Cpue R 9 16.181 172 2.357 6.685 <0.001 Box 2 Cpue A 10 43.897 142 10.734 4.089 <0.001 Cpue R 10 13.241 142 1.414 9.366 <0.001 Box 3 Cpue A 12 35.274 146 13.591 2.595 0.004 Cpue R 12 26.186 148 2.267 11.551 <0.001 Box 4 Cpue A 8 18.307 88 14.215 1.288 0.260 Cpue R 8 0.795 89 0.957 0.830 0.578 Box 5 Cpue A 10 20.435 194 13.401 1.525 0.133 Cpue R 10 12.261 258 0.837 14.645 <0.001 Box 6 Cpue A 12 11.822 76 10.425 1.134 0.346 Cpue R 12 5.104 77 1.388 3.678 <0.001 Box and parameter . d.f. effect . MS effect . d.f. error . MS error . F . p-level . Box 1 Cpue A 10 32.186 173 11.802 2.727 0.004 Cpue R 9 16.181 172 2.357 6.685 <0.001 Box 2 Cpue A 10 43.897 142 10.734 4.089 <0.001 Cpue R 10 13.241 142 1.414 9.366 <0.001 Box 3 Cpue A 12 35.274 146 13.591 2.595 0.004 Cpue R 12 26.186 148 2.267 11.551 <0.001 Box 4 Cpue A 8 18.307 88 14.215 1.288 0.260 Cpue R 8 0.795 89 0.957 0.830 0.578 Box 5 Cpue A 10 20.435 194 13.401 1.525 0.133 Cpue R 10 12.261 258 0.837 14.645 <0.001 Box 6 Cpue A 12 11.822 76 10.425 1.134 0.346 Cpue R 12 5.104 77 1.388 3.678 <0.001 The analysis contrasts the cpue of scallop recruits (cpue R, numbers h−1) and the cpue of adult scallops (cpue A, kg h−1) at the SBF with those in surrounding areas. The data were obtained from RC. d.f., degrees of freedom; MS, mean square. Open in new tab Table 1. Summary of ANOVA results by box. Box and parameter . d.f. effect . MS effect . d.f. error . MS error . F . p-level . Box 1 Cpue A 10 32.186 173 11.802 2.727 0.004 Cpue R 9 16.181 172 2.357 6.685 <0.001 Box 2 Cpue A 10 43.897 142 10.734 4.089 <0.001 Cpue R 10 13.241 142 1.414 9.366 <0.001 Box 3 Cpue A 12 35.274 146 13.591 2.595 0.004 Cpue R 12 26.186 148 2.267 11.551 <0.001 Box 4 Cpue A 8 18.307 88 14.215 1.288 0.260 Cpue R 8 0.795 89 0.957 0.830 0.578 Box 5 Cpue A 10 20.435 194 13.401 1.525 0.133 Cpue R 10 12.261 258 0.837 14.645 <0.001 Box 6 Cpue A 12 11.822 76 10.425 1.134 0.346 Cpue R 12 5.104 77 1.388 3.678 <0.001 Box and parameter . d.f. effect . MS effect . d.f. error . MS error . F . p-level . Box 1 Cpue A 10 32.186 173 11.802 2.727 0.004 Cpue R 9 16.181 172 2.357 6.685 <0.001 Box 2 Cpue A 10 43.897 142 10.734 4.089 <0.001 Cpue R 10 13.241 142 1.414 9.366 <0.001 Box 3 Cpue A 12 35.274 146 13.591 2.595 0.004 Cpue R 12 26.186 148 2.267 11.551 <0.001 Box 4 Cpue A 8 18.307 88 14.215 1.288 0.260 Cpue R 8 0.795 89 0.957 0.830 0.578 Box 5 Cpue A 10 20.435 194 13.401 1.525 0.133 Cpue R 10 12.261 258 0.837 14.645 <0.001 Box 6 Cpue A 12 11.822 76 10.425 1.134 0.346 Cpue R 12 5.104 77 1.388 3.678 <0.001 The analysis contrasts the cpue of scallop recruits (cpue R, numbers h−1) and the cpue of adult scallops (cpue A, kg h−1) at the SBF with those in surrounding areas. The data were obtained from RC. d.f., degrees of freedom; MS, mean square. Open in new tab  The cpue A varies in a cross-front direction for almost all boxes, except for box 4 (Table 1). In the northern boxes, the differences were caused by lower averages, ∼60 kg h−1, on the western side of the scallop bed in contrast to the higher values in central or eastern areas, with values, on average, ∼220 kg h−1. The highest mean cpue A was 371 kg h−1 (n = 7, s.d. = 342) in a central position of the bed in box 2 (Figure 5). Although there were differences across the front, the southern boxes showed no clear spatial pattern. Figure 5. Open in new tabDownload slide Box-plot of the cross-front distribution of the cpue of adult scallops (cpue A: kg h−1) according to their position from the western side of each box. Letters indicate differences in the average cpue A from the Bonferroni post hoc test. Figure 5. Open in new tabDownload slide Box-plot of the cross-front distribution of the cpue of adult scallops (cpue A: kg h−1) according to their position from the western side of each box. Letters indicate differences in the average cpue A from the Bonferroni post hoc test. Commercial scallop abundance: CF data Commercial scallop cpue (cpue F) varied in a cross-front direction in all boxes (Table 2). There were two clear cross-front patterns and an intermediate version (Figure 6). One pattern presents the lowest mean cpue F, located at the western side, and the larger mean cpue F in the central part of the bed (box 3). The opposite spatial pattern shows the largest mean values of cpue F at the western side of the scallop bed (boxes 1 and 2). The intermediate pattern in the rest of the boxes shows several peaks in cpue F. The Bonferroni analysis showed that almost all values of cpue F were different from each other. The highest was 12 t h−1 (n = 7, s.d. = 3.16) in box 2, although it was well away from the front. Although differences in a cross-front direction were detected for all boxes, in box 3 only was there a spatial distribution similar to that of cpue A in the first three boxes. Box 3 had considerable variability in cpue F, ranging between 1.50 t h−1 (n = 30, s.d. = 0.43) and 11.69 t h−1 (n = 38, s.d. = 8.5). Figure 6. Open in new tabDownload slide Box-plot of the cross-front distribution of the cpue of the commercial scallop fishery (cpue F, t h−1) at the SBF according to the position of the tows. Letters indicate differences in the average cpue F from the Bonferroni post hoc test. Figure 6. Open in new tabDownload slide Box-plot of the cross-front distribution of the cpue of the commercial scallop fishery (cpue F, t h−1) at the SBF according to the position of the tows. Letters indicate differences in the average cpue F from the Bonferroni post hoc test. Table 2. Summary of ANOVA results by box. Box . d.f.effect . MSeffect . d.f.error . MSerror . F . p-level . 1 10 1.703 1 280 0.194 8.790 <0.001 2 10 2.318 512 0.182 12.727 <0.001 3 12 7.043 791 0.379 18.576 <0.001 4 10 4.187 636 0.211 19.886 <0.001 5 13 2.759 510 0.177 15.593 <0.001 6 9 2.046 431 0.133 15.410 <0.001 Box . d.f.effect . MSeffect . d.f.error . MSerror . F . p-level . 1 10 1.703 1 280 0.194 8.790 <0.001 2 10 2.318 512 0.182 12.727 <0.001 3 12 7.043 791 0.379 18.576 <0.001 4 10 4.187 636 0.211 19.886 <0.001 5 13 2.759 510 0.177 15.593 <0.001 6 9 2.046 431 0.133 15.410 <0.001 The analysis contrasts the cpue of commercial scallops (cpue F, t h−1) at the SBF with that in the surrounding areas. The data were obtained from the CF. d.f., degrees of freedom; MS, mean square. Open in new tab Table 2. Summary of ANOVA results by box. Box . d.f.effect . MSeffect . d.f.error . MSerror . F . p-level . 1 10 1.703 1 280 0.194 8.790 <0.001 2 10 2.318 512 0.182 12.727 <0.001 3 12 7.043 791 0.379 18.576 <0.001 4 10 4.187 636 0.211 19.886 <0.001 5 13 2.759 510 0.177 15.593 <0.001 6 9 2.046 431 0.133 15.410 <0.001 Box . d.f.effect . MSeffect . d.f.error . MSerror . F . p-level . 1 10 1.703 1 280 0.194 8.790 <0.001 2 10 2.318 512 0.182 12.727 <0.001 3 12 7.043 791 0.379 18.576 <0.001 4 10 4.187 636 0.211 19.886 <0.001 5 13 2.759 510 0.177 15.593 <0.001 6 9 2.046 431 0.133 15.410 <0.001 The analysis contrasts the cpue of commercial scallops (cpue F, t h−1) at the SBF with that in the surrounding areas. The data were obtained from the CF. d.f., degrees of freedom; MS, mean square. Open in new tab Discussion Our results have revealed that seasonal variability in the position of the front and the location of the surface CSAT maximum correlates spatially with a zone of high recruitment probability and adult scallop abundance. The highest values of scallop abundance were in central–eastern areas within the scallop bed, matching the highest gradients in SST and CSAT concentration at the surface. Therefore, scallop abundance seems to be closely related to oceanographic processes. The zonal displacement of the SBF is close to the cross-shelf break extension of Patagonian scallop beds (∼37 km). This pattern suggests that the autumn (spring) mean climatological position of the SBF may determine the westernmost (easternmost) limit of the scallop bed, because spawning of scallops takes place mainly between spring and early autumn (Walossek and Waloszek, 1986; Campodónico et al., 2008). This seasonal spatial variability in the position of the SBF is seemingly associated with the dynamics of the Malvinas Current (Carreto et al., 1995), and seasonal variations in front intensity may be related to sea–air heat fluxes, horizontal advection (Saraceno et al., 2004; Romero et al., 2006), and/or coastal-trapped waves (Saraceno et al., 2005). Moreover, density sections (Figure 3) show shelf waters with moderate to strong pycnoclines during warmer months, and the water column at the shelf break (near the location of the scallop beds) shows weaker stratification. This situation could promote bentho-pelagic coupling (larval settlement, food supply to adults), along with dynamic processes such as inertial tide mixing and/or internal waves (Mann and Lazier, 1991). Blooms of CSAT occur at the shelf break mainly during spring and early summer, and maxima are located onshore of the SBF (Romero et al., 2006). Although we found some differences between the spatial patterns in SST and CSAT concentration, possibly because of the different years in the climate dataset, the CSAT maximum during spring and summer were located both onshore and above the SBF. Therefore, despite the different years being analysed, CSAT maximum concentrations showed a clear relationship with front position. The strong association between larval retention systems and recruitment is widely recognized (Sinclair et al., 1985), but their effect on community structure (“supply side ecology”; Lewin, 1986) in offshore communities is poorly understood. For the Patagonian scallop, the cross-front pattern of the highest cpue of recruits in the central–eastern portion of the bed (the western position of the SBF) could be a complex phenomenon which implies that the front constrains zonal displacement of larvae, but allows their drift along the front and their subsequent return to the seabed (Shanks, 1995). Mechanisms to explain this process, once larvae are near or in the frontal zone, may involve accumulation caused by the convergence of currents (Shanks, 1995), vertical mixing of the water column, larval transport to the seabed by fluid injection along the isopycnal surfaces (Richards, 1990), or larval transport by internal waves or tides (Pineda, 1991, 1994). There was no distinct pattern in the spatial distribution of scallop recruits in the southern boxes, probably because of the small number of recruits there, and because of their patchiness during the study period. The highest CSAT concentrations during spring and summer were west of the front, suggesting that food supply could be a major factor in determining the persistence and greatest abundances of scallops in the area, reinforcing recruitment patterns. This pattern was confirmed by RC, considered more representative of scallop distribution, because factors that control the allocation of fishing effort could promote the patterns obtained from commercial data (Orensanz et al., 2006). The spatial pattern of scallops across the front obtained from RC suggests that food availability on the western side of the scallop beds is not as good as on the eastern side. A similar pattern was found at other fronts, where chlorophyll levels in sediments (Josefson and Conley, 1997) and more labile organic material (Nodder et al., 2003) are closely associated with the pattern of variation in benthic biomass. Interannual variability in CSAT could also be responsible for the several peaks in commercial cpue in the southern boxes. It is important to note, however, that although the substratum is generally assumed to be a determinant of the spatial distribution of scallops (Katsanevasky, 2005), the spatial distribution of Patagonian scallops at a macroscale does not appear to be related to substratum characteristics over the fairly uniform substrata of the SW Atlantic (Bogazzi et al., 2005). Moreover, high densities of diatoms in the area (Gayoso and Podestá, 1996), a high rate of diatom sedimentation even during stratified condition (Hansen and Josefson, 2001) and the confirmation of greater diatom abundance in the stomach contents of scallops during summer (Schejter et al., 2002) support our interpretation that food availability plays an important role in the mesoscale distribution patterns of the Patagonian scallop. To conclude, front activity in the SW Atlantic appears to generate strong structure in the recruitment of the Patagonian scallop, and that pattern remains stable in terms of the distribution of adults. The spatial variability we observed in scallop abundance seems to be a result of (i) the effect of frontal variability on recruitment and/or (ii) spatial variability in the food supply to the scallop beds. This may be an example of how surface oceanographic patterns are closely related to recruitment localization and the abundance of a benthic species (a commercial fishery resource in this case). Acknowledgements We thank S. Campodónico for her help and advice, and S. Romero for the SeaWiFS data processing. The work was supported by INIDEP, the Universidad Nacional de Mar del Plata (EXA355/06), the Fundacion Antorchas (Argentina; 13900-13), CONICET (Argentina PIP2851), and Glaciar Pesquera. ACM was supported by a Doctoral scholarship from CONICET–Glaciar Pesquera (Argentina). 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