ICES Journal of Marine Science: Journal du Conseil

Murta, Alberto G.; Vendrell, Catarina

doi: 10.1093/icesjms/fsp027pmid: N/A

Murta, A. G., and Vendrell, C. 2009. Using the EM algorithm to age fish eggs. -- ICES Journal of Marine Science, 66: 607616.We describe an application of the expectation-maximization (EM) algorithm to obtain maximum likelihood estimates of the distribution of ages in fish egg samples. To compare this method with another one based on a non-linear regression of age as a function of development stage and water temperature, which is also suitable for asynchronous spawners, we applied both to simulated and real datasets. The estimates of egg abundance-at-age obtained with the traditional method were biased upwards, compared with estimates obtained with the proposed EM algorithm. The EM method also provided the most accurate and precise estimates, in terms of fitting an exponential-decay, egg-mortality model and the back-calculation of the time of spawning for two datasets used as examples. Besides the ageing of fish eggs, the method described here can be used for any analysis involving the fitting of multinomial models to age and stage classified data, such as the ageing of post-ovulatory follicles.

Gonalves, Patrcia; Costa, Ana Maria; Murta, Alberto G.

doi: 10.1093/icesjms/fsp066pmid: N/A

Gonalves, P., Costa, A. M., and Murta, A. G. 2009. Estimates of batch fecundity, and spawning fraction for the southern stock of horse mackerel (Trachurus trachurus) in ICES Division IXa. ICES Journal of Marine Science, 66: 617622.Since 1995 the annual egg production method has been applied triennially to the southern stock of horse mackerel (Trachurus trachurus) in the Northeast Atlantic (ICES Division IXa). This method assumes that fecundity is determinate, but increasing evidence indicates that horse mackerel are indeterminate spawners. The daily egg production method (DEPM) does not rely on the assumption of determinate fecundity, making it the appropriate method for this species. Therefore, we reanalysed samples collected from previous surveys (2002, 2004, 2005, and 2007) to obtain estimates for batch fecundity and spawning fraction, which are important DEPM parameters. The estimates of batch fecundity are around 200 oocytes g1 of female (total ovary-free weight). Several criteria were used to estimate spawning fraction (migratory nucleus stage, hydrated oocytes, and post-ovulatory follicles) and all showed the same trend among years, varying between 0.10 and 0.30 d1. The estimates were significantly different among methods, but those differences were similar across surveys, indicating that a consistent bias would be reflected in the final spawning-stock biomass (SSB) estimates obtained from the DEPM. Until further information is available regarding the accuracy of the criteria used to estimate spawning fraction, the southern horse mackerel SSB estimates from the DEPM should only be taken as indicative of trends rather than measures of absolute abundance.

Ndjaula, Hilkka O. N.; Hansen, Tom; Krger-Johnsen, Maria; Kjesbu, Olav Sigurd

doi: 10.1093/icesjms/fsp032pmid: N/A

Ndjaula, H. O. N., Hansen, T., Krger-Johnsen, M., and Kjesbu, O. S. 2009. Oocyte development in captive Atlantic horse mackerel Trachurus trachurus. ICES Journal of Marine Science, 66: 623630.This 13-month experimental study focused on developing oocyte production in 101 adult female Atlantic horse mackerel. In addition, proxies of energy patterns in relation to oocyte development were investigated. The fish were captured off western Norway and fed to satiation from October 2005 to November 2006 in two replicate adjacent circular tanks. Monthly histological examination of the ovaries indicated that vitellogenic oocytes were present at all times throughout the study period, but that oocyte development did not progress past the incipient migratory nucleus stage. The oocyte diameter (OD) threshold between pre-vitellogenic and vitellogenic oocytes and the mean OD of the leading cohort were investigated, and no hiatus was observed within the OD size distribution around the 185-m pre-vitellogenic and vitellogenic threshold. Variation in gonadosomatic index, hepatosomatic index, condition factor, and fat content (as measured by the use of a Distell Fish Fatmeter) increased with oocyte development. The observed development of oocyte recruitment and the absence of a hiatus in the oocyte distribution are characteristics of an asynchronous spawner. The species seems to have a prolonged spawning season, judging from the advanced and variety of maturity stages throughout the study period.

Venables, William N.; Ellis, Nick; Punt, Andr E.; Dichmont, Catherine M.; Deng, Roy A.

doi: 10.1093/icesjms/fsp063pmid: N/A

Venables, W. N., Ellis, N., Punt, A. E., Dichmont, C. M., and Deng, R. A. 2009. A simulation strategy for fleet dynamics in Australias northern prawn fishery: effort allocation at two scales. ICES Journal of Marine Science, 66: 631645.We describe simple and effective methods for simulating the movement of a fishing fleet, the spatio-temporal allocation of effort, in the context of harvest policy evaluation. Vessel movements are simulated in a manner that can balance established seasonal fishing patterns with the influence of perceived local variations in abundance. In the longer term, the model adapts smoothly to changes in fleet size and price fluctuations in critical resources, such as fuel. Given an initial allocation of effort at a fine temporal (but coarse spatial) scale, as needed for projecting stock dynamics, a second phase assigns the effort at a finer spatial (but coarser temporal) scale for environmental impact assessment. The first phase is based on a discrete-state, time-inhomogeneous Markov chain, with transition probabilities following a multiple logistic model. Although the ideas are generic, the method has been developed for the specific case of Australias northern prawn fishery, which is used to motivate and describe the method. The approach, in a harvest policy evaluation context, is novel in that it uses economic projections (not surrogates) to reflect realistically the recent pressures many fishing industries are under to change their spatial fishing pattern in response to global fuel price fluctuations.

Campbell, N.; Dobby, H.; Bailey, N.

doi: 10.1093/icesjms/fsp046pmid: N/A

Campbell, N., Dobby, H., and Bailey, N. 2009. Investigating and mitigating uncertainties in the assessment of Scottish Nephrops norvegicus populations using simulated underwater television data. ICES Journal of Marine Science, 66: 646655.Nephrops norvegicus is a commercially significant decapod crustacean that excavates and inhabits burrows in muddy sediments throughout the Northeast Atlantic and Mediterranean Sea. Because of its variable emergence behaviour, traditional surveys are considered a poor indicator of the status of its populations, and underwater television (TV) survey methods have been developed. The approach involves towing a TV camera over patches of muddy sediment and counting the number of Nephrops burrow complexes within a known area. Assuming a 1:1 rate of occupancy, the average population density can be estimated. This is raised to the known area of suitable sediment to give a measure of population size. Recently, underwater TV surveys have been used to provide absolute measures of abundance for a number of Nephrops stocks in the North Sea and Northeast Atlantic. It is therefore imperative to identify, quantify, and control for uncertainties in this process. Previous workers have suggested that visual recognition of Nephrops burrows and edge effects, where single openings of burrow complexes at the sides of the viewed transect are counted, are important sources of uncertainty. This study shows edge effects to be responsible for an overestimation of population size of between 4 and 55%, depending on the width of the field of view and the mean size of the burrow complex. This overestimation is countered to some extent by variability in burrow entrance structure, which leads to Nephrops burrows going unrecognized. The scales of these errors are discussed in relation to observed sediment softness, sediment distribution, and burrow dimensions from sites around Scotland.

Green, Bridget S.; Gardner, Caleb

doi: 10.1093/icesjms/fsp030pmid: N/A

Green, B. S., and Gardner, C. 2009. Surviving a sea-change: survival of southern rock lobster (Jasus edwardsii) translocated to a site of fast growth. ICES Journal of Marine Science, 66: 656664.In an experiment aimed at increasing the yield and value of the fishery for southern rock lobster (Jasus edwardsii), 1998 pale and slow-growing lobsters were translocated to sites where lobsters grow faster naturally and develop higher value market traits. Survival of lobsters in their new habitat was critical to the viability of this experiment. To estimate survival, the release site was surveyed every 13 months for 2 years. Apparent survival of translocated lobsters was compared with survival of resident lobsters tagged in a similar period using CormackJollySeber modelling on markrecapture data. Lobster survival was not influenced by size, gender, or origin (translocated or resident) alone. The four most parsimonious models suggested slightly lower apparent survival in translocated lobsters immediately after release compared with all other lobsters, but support for these models was weak (QAICc weights 62%). The differences in apparent survival were not likely to be significant because of large variance when averaged over all the models. Apparent survival of newly released translocated lobsters was 92% (7298, 95% CI) compared with 97% (9598, 95% CI) for all other lobsters. Potential sources of differences in survival are movement from the release site or greater predation on pale lobsters. Losses of lobster through release mortality were low and unlikely to influence the feasibility of translocation as a tool to enhance the value of the fishery, although it would be prudent to include 5% mortality of translocated lobsters in future models of translocation feasibility.

Bogaards, Johannes A.; Kraak, Sarah B. M.; Rijnsdorp, Adriaan D.

doi: 10.1093/icesjms/fsp038pmid: N/A

Bogaards, J. A., Kraak, S. B. M., and Rijnsdorp, A. D. 2009. Bayesian survey-based assessment of North Sea plaice (Pleuronectes platessa): extracting integrated signals from multiple surveys. ICES Journal of Marine Science, 66: 665679.Dependence on a relatively small sample size is generally viewed as a big disadvantage for survey-based assessments. We propose an integrated catch-at-age model for research vessel data derived from multiple surveys, and illustrate its utility in estimating trends in North Sea plaice abundance and fishing mortality. Parameter estimates were obtained by Bayesian analysis, which allows for estimation of uncertainty in model parameters attributable to measurement error. Model results indicated constant fishing selectivity over the distribution area of the North Sea plaice stock, with decreased selectivity at older age. Whereas separate analyses of survey datasets suggested different biomass trends in the southeast than in the western and central North Sea, a combined analysis demonstrated that the observations in both subareas were compatible and that spawning-stock biomass has been increasing over the period 19962005. The annual proportion of fish that dispersed in a northwesterly direction was estimated to increase from about 10% at age 2 to 33% at age 5 and older. We also found higher fishing mortality rates than reported in ICES assessments, which could be the consequence of inadequate specification of catchability-at-age in this study or underestimated fishing mortality by the conventional ICES assessment, which relies on official landings figures.

Stefnsson, Magns rn; Reinert, Jkup; Sigursson, orsteinn; Kristinsson, Kristjn; Nedreaas, Kjell; Pampoulie, Christophe

doi: 10.1093/icesjms/fsp059pmid: N/A

Stefnsson, M. ., Reinert, J., Sigursson, ., Kristinsson, K., Nedreaas, K., and Pampoulie, C. 2009. Depth as a potential driver of genetic structure of Sebastes mentella across the North Atlantic Ocean. ICES Journal of Marine Science, 66: 680690.A primary question that remains to be answered about the fishery for Sebastes mentella is whether it exploits several stocks within the North Atlantic Ocean. To address this issue, 1240 redfish were collected from several fishing grounds in late 2006 and early 2007 and genotyped at 12 microsatellite loci. Contemporary allele frequencies were compared with archived data to examine the temporal stability of the genetic structure. The analyses all revealed the presence of three genetically distinguishable clusters, which persisted throughout the study period, suggesting that the genetic structure detected is genuine. Cluster D included fish from the deep Irminger Sea and west Faroe Islands, and Cluster I included fish only from the Icelandic shelf. All other fish grouped in a third cluster (S). Further analyses revealed that the genetic pattern observed was not primarily attributable to isolation by distance, but rather to depth distribution.

Hunter, W. R.; Sayer, M. D. J.

doi: 10.1093/icesjms/fsp058pmid: N/A

Hunter, W. R., and Sayer, M. D. J. 2009. The comparative effects of habitat complexity on faunal assemblages of northern temperate artificial and natural reefs. ICES Journal of Marine Science, 66: 691698.Several north temperate marine species were recorded on subtidal hard-substratum reef sites selected to produce a gradient of structural complexity. The study employed an established scuba-based census method, the belt transect. The three types of reef examined, with a measured gradient of increasing structural complexity, were natural rocky reef, artificial reef constructed of solid concrete blocks, and artificial reef made of concrete blocks with voids. Surveys were undertaken monthly over a calendar year using randomly placed fixed rope transects. For a number of conspicuous species of fish and invertebrates, significant differences were found between the levels of habitat complexity and abundance. Overall abundance for many of the species examined was 23 times higher on the complex artificial habitats than on simple artificial or natural reef habitats. The enhanced habitat availability produced by the increased structural complexity delivered through specifically designed artificial reefs may have the potential to augment faunal abundance while promoting species diversity.

Rocklin, Delphine; Santoni, Marie-Catherine; Culioli, Jean-Michel; Tomasini, Jean-Antoine; Pelletier, Dominique; Mouillot, David

doi: 10.1093/icesjms/fsp036pmid: N/A

Rocklin, D., Santoni, M-C., Culioli, J-M., Tomasini, J-A., Pelletier, D., and Mouillot, D. 2009. Changes in the catch composition of artisanal fisheries attributable to dolphin depredation in a Mediterranean marine reserve. ICES Journal of Marine Science, 66: 699707.There is increasing evidence from previous studies, and from fishers observations, that coastal dolphins use fishing nets as an easily accessible feeding source, damaging or depredating fish caught in the nets. This study investigates the impact of dolphin depredation on artisanal trammelnets by analysing the catch composition of 614 artisanal fishing operations in the Bonifacio Strait Natural Reserve (France). Common bottlenose dolphins (Tursiops truncatus) attacked, on average, 12.4 of the nets and damaged 8.3 of the catch. However, attacked nets were characterized by statistically significantly higher catch per unit effort than unattacked ones. Catch composition also differed significantly after dolphin attacks; bentho-pelagic fish were more represented and reef-associated fish less represented. Our results suggest that (i) dolphins are attracted by high fish densities in the fishing area and/or nets, and (ii) their attacks induce specific fish-avoidance behaviour, according to the fish position in the water column. Although dolphins depredate a small part of the catch, damage to nets, not yet assessed in this area, could weaken the benefits that reserves can provide to artisanal fisheries.