ICES Journal of Marine Science: Journal du Conseil

Zheng, X.; Pierce, G. J.; Reid, D. G.; Jolliffe, I. T.

doi: 10.1006/jmsc.2001.1131pmid: N/A

This paper describes spatial relationships between whiting, Merlangius merlangus (Linnaeus, 1758), abundance in the northern North Sea and contemporaneous measures of environmental conditions: sea surface temperature (SST), sea bottom temperature (SBT), and depth, with particular reference to the processes underlying patterns in SST. Generalised additive models (GAMs) were used to provide quantitative descriptions of the relationships between local abundance and environmental conditions. GIS (geographic information system) techniques were used to provide qualitative description of spatial patterns and to confirm the results revealed from GAMs.GAMs fitted to both long-term averaged and individual years' data revealed marked seasonal changes in the spatial relationships between whiting abundance and environmental variables. The GAM results were supported by GIS analysis. In winter and spring (December–April) in the northern North Sea, the spatial pattern of SST apparently has an important influence on the spatial distribution of whiting at the same time. Where the water is relatively warm whiting abundance is relatively high, probably reflecting the indirect influence of North Atlantic waters entering the northern North Sea. However, there are no consistent optimum SST bands for whiting. These positive relationships between abundance and SST disappear in summer.

Robichaud, David; Rose, George A.

doi: 10.1006/jmsc.2001.1160pmid: N/A

Assessing the spawning abundance of marine fishes is difficult if spawning periods exceed the residency of individual fish on the spawning grounds. For Atlantic cod (Gadus morhua), which has a protracted spawning period, we use biotelemetric surveys to estimate the rate at which individual fish vacate the spawning ground and develop a method to adjust multiple acoustic-survey results to account for spawner turnover. Two acoustic surveys conducted one month apart (May and June 1998) on a cod-spawning ground in Placentia Bay, Newfoundland, yielded abundance estimates of 220 000 and 210 000 fish of mean length 63 cm. Rates of evacuation from the spawning ground, observed over two separate spawning seasons, were modelled as logistic decay functions with good fit (r2=0.96 in 1998; r2=0.88 in 2000). Our method estimated that only 8.8% of the fish counted during the second survey were present during the first, and that between 400 976 and 420 842 fish were actually present over the full spawning season. Coupled telemetric and acoustic surveys could be used to estimate spawning abundance in many marine fishes.

Furness, Robert W.

doi: 10.1006/jmsc.2001.1155pmid: N/A

The lesser sandeel, Ammodytes marinus, is a key food for many seabirds and seals, and is also the target of the largest single-species fishery in the North Sea. Despite claims that sandeel fishing has harmed dependent predator populations, census data show that most seabirds and grey seals increased in numbers as the fishery grew and reached peak harvest. Generally high breeding success of black-legged kittiwakes at North Sea colonies also suggests that sandeel abundance has remained good for breeding seabirds at the broad scale, though local and small-scale effects of sandeel fishing should not be overlooked. VPA and CPUE data suggest that abundance increased as the fishery grew. A negative correlation between sandeel recruitment and total stock size preceding spawning suggests that there is now resource competition (bottom-up control). Bioenergetics modelling indicates that predatory fish take far more sandeel than taken by the industrial fishery or wildlife. Effects of decreases in predatory fish stocks have been greater than increases in the take by seabirds and seals and by the fishery. Thus, overall, there appears to have been a reduction in mortality during the last 30 years. Changes in predatory fish abundances, especially mackerel and whiting, may influence sandeel stocks more than changes in industrial fishery, at least at the scale of the North Sea as a whole. These interactions imply that seabird and seal food supply in terms of sandeel may be strongly dependent on decisions regarding management of stocks of mackerel and gadoids. The overwhelming influence of predation on “food-fish” by predatory fish may be a feature of many marine food webs worldwide, where “fishing down the food web” has occurred, and this has clear management implications if wildlife and fisheries are to coexist.

Johansen, Geir Odd

doi: 10.1006/jmsc.2001.1157pmid: N/A

Previous studies indicate that predation by Northeast Arctic cod is an important cause of natural mortality of juvenile Norwegian spring-spawning herring in the Barents Sea. In this paper broad scale temporal and spatial variation in the predator–prey interaction between these two species in the Barents Sea was analysed. The analysis was based on cod stomach data from this area in 1984–1997. The predator–prey interaction between cod and juvenile herring in the Barents Sea was highly variable in time and space. On a yearly basis the most intense predation occurred in years with strong year classes of herring in the Barents Sea. Intensity of predation increased with decreasing abundance of capelin. Seasonal variation in intensity of predation on juvenile herring was low. Maps of the spatial distribution of cod feeding on herring illustrated a difference between the first and second halves of the year. In late winter and spring herring was consumed by cod in a restricted area in the southern part of the Barents Sea. In late summer, autumn and early winter herring was consumed by cod in a wide area south of the Polar Front. Climatic variation influenced the spatial distribution in the second half of the year. The results were discussed in the context of broad scale ecosystem dynamics in the Barents Sea.

Korneliussen, Rolf J.; Ona, Egil

doi: 10.1006/jmsc.2001.1168pmid: N/A

Calibrated and digitized data from two or more discrete echosounder frequencies can be combined for the purpose of separating and extracting the acoustic scattering from zooplankton and fish in mixed recordings. This method is also useful for quantifying the relative contribution of each frequency to the total acoustic-backscattering when scrutinizing records in large-scale, acoustic surveys. Echosounder hardware requirements are defined which would permit the ideal extraction of such information. These include calibration, transducer specification, pulse resolution and digital representation of the signals. During this initial study a special version of the Simrad EK500 multi-frequency, split-beam echosounder and the Bergen Echo Integrator (BEI) post-processing system were used. The echosounder transmitted pulses simultaneously at four frequencies, 18, 38, 120 and 200 kHz and transferred the received signals to the post-processing system in calibrated, raw, digitized format. Methods are described for echogram manipulation and for the construction of new, synthetic, combined-frequency [c(f)] echograms. Examples of extracted scattering information from mixed layers of fish and small scattering-organisms, such as copepods and euphausiids, are shown, and the potential of the method is discussed.

Mistri, Michele

doi: 10.1006/jmsc.2001.1169pmid: N/A

Data collected from a three-year continuing sampling program were used to study the persistence of the macrofaunal communities living in the Valli di Comacchio (44°41′N, 12°10′E), a complex of shallow-water brackish lagoons located in the southernmost part of the Po River deltaic area (Italy). Univariate and multivariate analyses were used to assess the variability and persistence of the community at each of the four sites investigated. A total of 46 benthic taxa were identified from bottom grab samples: Annelida was the most abundant group, followed by Crustacea and Mollusca. The abundance of some species exhibited marked temporal fluctuations. At three out of four sites, abundance fluctuation were unpredictable in terms of both timing and magnitude. There was little evidence that factors operating at lagoon-wide scale contributed to variability in community structure. The balance of taxa within each community was probably most affected by forcing factors acting on a local scale. The perception of macrofaunal communities as being persistent through time decreased as the analysis moved from coarser (presence/absence) to finer (absolute abundance) levels of numerical resolution. Analyses showed that macrobenthic assemblages at some sites were both persistent and variable: this paradox reflected viewpoints of different analytical scales. Because of this, a multi-level approach is probably more useful in gaining insight into community persistence and variability.

Peltonen, Heikki; Raitaniemi, Jari; Parmanne, Raimo; Eklund, Jan; Nyberg, Kari; Halling, Folke

doi: 10.1006/jmsc.2001.1156pmid: N/A

Two methods of determining the age of herring (Clupea harengus L.) from the Bothnian Sea, in the northern Baltic Sea, were compared. Ages were estimated from whole otoliths, which is a routine method for herring age determination, and from neutral red stained otolith cross sections. There was disagreement between the methods, especially in specimens with more than five annual rings. The agreement between the determinations of different age readers was better with the cross section method and, besides, it is clear that with the whole otolith method there is considerable negative bias in old fish. Thus, more herring were classified as more than 15 years old with the cross section method. The study indicates that conventional methodology may generate considerable errors in age distributions, especially in samples which mainly consist of old fish. However, the differences in age reading produced relatively small changes in growth rate estimates, because of the slow growth of the old fish, that are most prone to age determination errors.

Comeau, L. A.; Campana, S. E.; Chouinard, G. A.

doi: 10.1006/jmsc.2001.1153pmid: N/A

The objective of this study was to identify likely proximate cues to the seasonal migrations of cod, Gadus morhua L., in the southern Gulf of St Lawrence. We computed a migration timing index across several years using commercial fishery (1970–1992) and sentinel survey (1995–1999) catch rate data, and closely monitored the seasonal changes in temperature, dissolved oxygen, food intake, and energy stores between 1995 and 1998. The inter-year variability in the timing of the seasonal migrations was relatively low (s.d. = 8 d for spring migration, 10 d for autumn migration), suggesting that photoperiod was involved in the control mechanism of migration. However, other factors were also implicated in the control mechanism, given that cod initiated the autumn migration progressively earlier in the late 1980s and 1990s. At the onset of the autumn migration, dissolved oxygen concentrations were above levels known to induce avoidance behaviour in this species, and food resources were considered to be relatively abundant. Similarly, cod did not initiate the autumn migration at a specific temperature or following a particular temperature regime experienced during summer. However, cross correlation function analysis indicated that a widespread cooling of near bottom waters preceded the mid-1980s shift in migration timing by one to two years, suggesting that the cooling event was linked in some way to the change in migration dates. Other correlations showed that the earlier migrations coincided with older cod becoming more abundant and smaller in size, and also with their principal fish prey (herring and capelin) increasing in number. These results suggest that older cod played a key role in launching the early autumn migrations. One plausible interpretation is that their decline in body size and lipid-rich diets led to a rapid build up of energy reserves over summer grounds, and thus to an earlier readiness to engage in the autumn migration. We discuss the potential impact of earlier departures on stock productivity.

Morales-Nin, B.; Canha, Â; Casas, M.; Figuereido, I.; Gordo, L. S.; Gordon, J. D. M.; Gouveia, E.; Piñeiro, C. G.; Reis, S.; Reis, A.; Swan, S. C.

doi: 10.1006/jmsc.2001.1154pmid:

Maclennan, David N.; Fernandes, Paul G.; Dalen, John

doi: 10.1006/jmsc.2001.1158pmid: N/A

Long-standing problems with acoustical terminology in fisheries applications such as echo-integration indicate the need for a more consistent approach. Based where possible on existing terms, a scheme of explicitly named quantities is proposed, backed by clearly stated definitions and preferred symbols. The emphasis is on scattering phenomena because the terminology in this area presents the main source of difficulty. Starting with the scattering equations for a small target, the volume, area, and line coefficients relevant to multiple, distributed targets are defined, leading to practical formulas for the important application of remote biomass estimation from echo-integration. The aim is to incorporate, as far as possible, common practice in fisheries-acoustics terminology and related fields. The developed scheme has been commended by the ICES Fisheries Acoustics Science and Technology Working Group as a constructive approach to better communication standards in fisheries-acoustics publications.