ICES Journal of Marine Science: Journal du Conseil

Payne, Andrew I. L.; Kennedy, Cathy

doi: 10.1093/icesjms/fsl008pmid: N/A

Roa-Ureta, Rubn; Arkhipkin, Alexander I.

doi: 10.1093/icesjms/fsl017pmid: N/A

Roa-Ureta, R. and Arkhipkin, A. I. 2007. Short-term stock assessment of Loligo gahi at the Falkland Islands: sequential use of stochastic biomass projection and stock depletion models ICES Journal of Marine Science, 64, 317.Two short-term stock assessment models are combined to examine the pre-season, in-season, and post-season dynamics of the Loligo gahi fishery off the Falkland Islands over four consecutive fishing seasons. A stochastic biomass projection model (SBPM) projects a pre-season survey-based biomass estimate from the date of the survey to the start of the season. A stock depletion model (SDM) assesses in-season biomass from commercial daily catch-and-effort data. The SBPM projects the SDM biomass estimate at the end of the season to a post-season date of spawning. Combining the SBPM and the SDM helps to clarify the spatio-temporal functioning of the stock and to assess the comparability of survey- and fishery-based estimates of biomass. For the first 2005 season, projected length frequencies indicate two pulses of recruitment onto the fishing grounds. Survey-based projections of biomass were lower than equivalent fishery-based estimates. Over two surveys, the sex ratio was balanced, suggesting full recruitment of both sexes onto the fishing grounds, and the ratio of survey-projected to fishing-estimated biomass was constant. This constant is interpreted as a scaling factor between survey biomass and absolute biomass.

Pepin, P.; Curtis, K. A.; Snelgrove, P. V. R.; de Young, B.; Helbig, J. A.

doi: 10.1093/icesjms/fsl016pmid: N/A

Pepin, P., Curtis, K.A., Snelgrove, P.V.R., de Young, B., and Helbig, J.A. 2007. Optimal estimation of catch by the continous underway fish egg sampler based on a model of the vertical distribution of American plaice (Hippoglossoides platessoides) eggs ICES Journal of Marine Science, 64, 1830.We investigate how the vertical stratification of the water column (specifically density) affects predictions of the catch of American plaice eggs (Hipploglossoides platessoides) from a fixed-depth sampler [the continuous underway fish egg sampler (CUFES)] relative to the integrated abundance in the water column measured in bongo tows. A steady-state model of the vertical distribution of fish eggs coupled with a simple model of the vertical profile of eddy diffusivity (i.e. mixing) is applied. Key model parameters are estimated through optimization of a one-to-one relationship between predicted and observed catches fit, using a generalized linear model with a Poisson, negative binomial, or gamma error structure. The incorporation of data on the vertical structure of the water column significantly improved the ability to forecast CUFES catches when using Poisson or negative binomial error structure, but not using a gamma distribution. Optimal maximum likelihood parameter estimates for eddy diffusivity and egg buoyancy fell within the range of expected values. The degree of uncertainty in the parameterization of eddy diffusivity suggests, however, that greater understanding of the forces that determine the vertical profile of mixing is critical to achieving strong predictive capabilities. The inverse problem of predicting integrated abundance from CUFES catches did not benefit from the environmental-driven model because of the high uncertainty in the catches from the CUFES.

Kilada, Raouf W.; Campana, Steven E.; Roddick, Dale

doi: 10.1093/icesjms/fsl001pmid: N/A

Kilada, R. W., Campana S. E., and Roddick, D. 2007. Validated age, growth, and mortality estimates of the ocean quahog (Arctica islandica) in the western Atlantic. ICES Journal of Marine Science, 64: 3138.The age structure of offshore (Sable Bank) and inshore (St Mary's Bay) populations of eastern Canadian ocean quahogs (Arctica islandica), and of a northwestern Iceland population, is investigated. Age estimates for eastern Canadian ocean quahogs were validated through analysis of bomb-produced 14C in quahog shell growth increments deposited before, during, and after the atmospheric atomic bomb testing periods of the 1950s and 1960s. Delta 14C from shells with presumed birthdates between the late 1950s and 1970s clearly reflects the sharp increase in oceanic radiocarbon attributable to nuclear testing. The results validate our age interpretations of Sable Bank quahogs to an age of 45y, and support longevity estimates of more than 200y for the same population. Longevity calculations for the other populations exceeded 60y. Von Bertalanffy growth parameters were estimated for the three populations; the growth rate of all three was relatively rapid for the first 2030y of life, but thereafter was very slow. The instantaneous rate of natural mortality (M), calculated using the agefrequency distribution of the unexploited populations, was estimated to be 0.03 and 0.10 for the Sable Bank and St Mary's Bay populations, respectively.

Everson, Inigo; Tarling, Geraint A.; Bergstrm, Bo

doi: 10.1093/icesjms/fsl010pmid: N/A

Everson, I., Tarling, G. A., and Bergstrm, B. 2007. Improving acoustic estimates of krill: experience from repeat sampling of northern krill (Meganyctiphanes norvegica) in Gullmarsfjord, Sweden ICES Journal of Marine Research, 64, 3948.A series of eight replicated acoustic surveys, four by day and four by night, was undertaken in Gullmarsfjord on the Swedish west coast during two 24-h periods on 8 and 10 September 2003, using a calibrated echosounder operating at 120 and 38kHz. The difference in signal strength (Sv) was used to distinguish northern krill (Meganyctiphanes norvegica) from other acoustic scatterers. The approach is concluded to be very effective, but it can be improved greatly by applying the following series of simple extensions to current protocols: first, set a very low threshold on both frequencies to minimize sampling bias; second, undertake tests to confirm that the data extracted from each acoustic frequency apply to the same scatterers; third, ensure that the range of Sv is not greater than the TSrange at either frequency; and finally, when abundance estimation is the primary aim, arrange for sampling at the time of day and using the acoustic frequency that together provide the least variance.

Brickman, David; Marteinsdottir, Gudrun; Logemann, Kai; Harms, Ingo H.

doi: 10.1093/icesjms/fsl019pmid: N/A

Brickman, D., Marteinsdottir, G., Logemann, K., and Harms, I. H. 2007. Drift probabilities for Icelandic cod larvae ICES Journal of Marine Science, 64, 4959.The climatological distribution of juvenile Icelandic cod is characterized by a negative spatial age gradient, with a fairly abrupt decrease in age near the northwest corner of Iceland, and a spatial abundance gradient with higher concentrations of 0-group fish inshore. Flowfields from a high-resolution circulation model developed for Icelandic waters were used to investigate larval drift from the various spawning grounds in Icelandic coastal waters to understand the distribution of 0-group fish. To present the results clearly, drift probability density functions (pdfs) are derived describing the probability of drifting from a given spawning ground to a given spatial region over a specified time interval. These pdfs are used to determine the spawning grounds most probably contributing to the observed age distribution. The observed spatial gradient in age is likely due to differences in the spawning location of larvae, with older larvae originating in spawning grounds in the southwest and younger larvae from farther north. In general, the contribution from the main spawning grounds in the southwest is predicted to decrease with clockwise distance from the source region. The pdf technique was also used to investigate drift from regions on the south coast of Iceland corresponding to known or possible subpopulation spawning grounds, to see whether these spawning areas are associated with distinct drift patterns. This technique is a useful way to present larval drift results and to facilitate comparison with real data.

Segers, F. H. I. D.; Dickey-Collas, M.; Rijnsdorp, A. D.

doi: 10.1093/icesjms/fsl002pmid: N/A

Segers, F. H. I. D, Dickey-Collas, M., and Rijnsdorp, A. D. 2007. Prey selection by North Sea herring (Clupea harengus), with special reference to fish eggs. ICES Journal of Marine Science, 64: 6068.he herring stock in the North Sea in recent years has recovered to a relatively high biomass, and here we investigate prey selection of individual North Sea herring when population numbers are high. The diet composition, and specifically pelagic fish eggs, was investigated in February 2004. Samples of herring from the International Bottom Trawl Survey were used for stomach analysis, and ichthyoplankton samples from the southern North Sea were used to investigate selection. Crustaceans were the main diet component. The average diameter of the fish eggs recovered from the stomachs was significantly larger than that of the eggs collected in the field. In addition, the frequency at which the latest developmental stages occurred in the herring stomachs was significantly different from the frequency at which these stages were found in the field. This shows selective foraging. There was a relationship between the amount of food and the number of eggs in a herring stomach: the fullest stomachs tended to contain fewer fish eggs. This suggests that herring forage on eggs when other prey are not available. Hence, it is likely that the dynamics of multiple trophic levels influence the ecological impact of a large herring stock on the North Sea ecosystem.

Melvin, Gary D.; Stephenson, Robert L.

doi: 10.1093/icesjms/fsl018pmid: N/A

Melvin, G. D., and Stephenson, R. L. 2007. The dynamics of a recovering fish stock: Georges Bank herring ICES Journal of Marine Science, 64, 6982.The decline and subsequent recovery of Georges Bank Atlantic herring (Clupea harengus) provides a rare opportunity to examine the dynamics of a recovering fish population. Moreover, the near absence of a commercial fishery on Georges Bank between 1978 and 1995 removes the confounding effects of exploitation during the recovery period. Herring abundance on Georges Bank increased and the distribution of adult spawning fish evolved from a few isolated locations to most of the northern fringe during the period 19831995. The distribution of recently hatched larvae also expanded in a manner consistent with progressive occupation of historical spawning grounds. Changes in the size composition and age structure of herring during the spawning season broadened from the dominance of a single age class to multiple year classes as the stock recovered and expanded. Growth, as reflected by length-at-age, decreased significantly and was correlated with the number of fish estimated to be in the stock. This and the observed difference in mean length and length at first spawning during the recovery provide strong evidence of density-dependent growth. In particular, there is a highly correlated (p<0.01) relationship between the number of 4 herring in the stock at the start of the year and the mean length of herring aged 3 recruiting to the spawning stock in autumn of the same year. A mechanism based on an extended period of prespawning interaction is proposed to explain the density-dependence.

Overholtz, W. J.; Link, J. S.

doi: 10.1093/icesjms/fsl009pmid: N/A

Overholtz, W. J. and Link, J. S. 2007. Consumption impacts by marine mammals, fish, and seabirds on the Gulf of MaineGeorges Bank Atlantic herring (Clupea harengus) complex during the years 19772002. ICES Journal of Marine Science, 64: 8396.A comprehensive study of the impact of predation during the years 19772002 on the Gulf of MaineGeorges Bank herring complex is presented. An uncertainty approach was used to model input variables such as predator stock size, daily ration, and diet composition. Statistical distributions were constructed on the basis of available data, producing informative and uninformative inputs for estimating herring consumption within an uncertainty framework. Consumption of herring by predators tracked herring abundance closely during the study period, as this important prey species recovered following an almost complete collapse during the late 1960s and 1970s. Annual consumption of Atlantic herring by four groups of predators, demersal fish, marine mammals, large pelagic fish, and seabirds, averaged just 58000t in the late 1970s, increased to 123000t between 1986 and 1989, 290000t between 1990 and 1994, and 310000t during the years 19982002. Demersal fish consumed the largest proportion of this total, followed by marine mammals, large pelagic fish, and seabirds. Sensitivity analyses suggest that future emphasis should be placed on collecting time-series of diet composition data for marine mammals, large pelagic fish, and seabirds, with additional monitoring focused on the abundance of seabirds and daily rations of all groups.

Miller, Timothy J.; Skalski, John R.; Ianelli, James N.

doi: 10.1093/icesjms/fsl013pmid: N/A

Miller, T. J., Skalski, J. R., and Ianelli, J. N. 2007. Optimizing a stratifield sampling design when faced with multiple objectives ICES Journal of Marine Science, 64, 97109.For many stratified sampling designs, the data collected are used by multiple parties with different estimation objectives. Quantitative methods to determine allocation of sampling effort to different strata to satisfy the often disparate estimation objectives are lacking. Analytical results for the sampling fractions and sample sizes for primary units within each stratum of a stratified (multi-stage) sampling design that are optimal with respect to a weighted sum of relative variances for the estimation objectives are presented. Further, an approach for assessing gains or losses for each estimation objective by changing allocation of sample sizes to each stratum is provided. As an illustration, the analytical results are applied to determine optimal observer sampling fractions (coverage rates) for the North Pacific Groundfish Observer Programme (NPGOP), for which the multiple objectives are assumed to be bycatch (seabird, marine mammal, and non-targeted fish species) and total catch, and catch-at-length and -age of targeted fish species. Simultaneously optimizing a criterion that defines the strata of the NPGOP sampling design is also considered. When observer coverage rates are allowed to be gear-specific for the NPGOP design, the optimized objective function is between 10 and 28 less than the value corresponding to current sampling for annual data (20002003) and 12 less when optimized over all years combined.