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ICES WGFE Report 2006. Report of the Working Group on Fish Ecology (WGFE), 13-17 March 2006, ICES Headquarters, Copenhagen
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We used life history traits to categorize vulnerability of elasmobranchs to exploitation. However, the utility of this approach required that the links between life histories and population dynamics be explored. We constructed standardized three-stage matrix models for 55 species of sharks and rays. Using these models we (1) conducted elasticity analyses to determine how the vital rates of mortality (M) and fertility (f) influence elasmobranch population growth rate r, (2) determined the response of elasticity to changes in the levels of exploitation, (3) estimated sensitivity of elasticity to perturbation in vital rates, and (4) examined the taxonomic distribution of model inputs and species vital rates, such as size at maturity (Lmat), and total length (Lmax). We found positive relationships between the elasticity of λ (population growth rate) to changes in juvenile and adult stages to longevity and age of maturity; however, the age of maturity and the elasticity of λ to changes in the adult stage rela- tionship appeared to be invariant. There was a negative relationship between both longevity and age of maturity and the elasticity of λ to changes in inter-stage transitions of the models. Under varying fishing levels, estimates of elasticity were robust to changes in survival. Elasticity and perturbation analyses suggested that compensatory responses to exploitation in elasmobranchs were less likely to be expressed as changes in fertility than as changes in juvenile and adult mortality and stage durations (i.e. changes in age of maturity). Combining vital rates and elasticities, we found similar suites of life histories and demographics within groups at various taxonomic levels.
Density-dependent habitat selection has implications for fisheries management and for the recovery of depleted fish populations. According to ideal free distribution theory, populations contract into areas of highest habitat suitability as their abundance decreases. This can increase their vulnerability to fishing and predation. We detected density-dependent habitat selection by juvenile Atlantic cod (Gadus morhua) (ages 1 and 2) in the North Sea and compared the observed distribution-abundance relationships with those predicted from a model based on ideal free distribution theory and knowledge of optimal temperatures for growth, where temperature was used as a measure of suitability. As predicted by the model, in years when stock size was low, the catches were largely confined to regions with near-optimal bottom temperatures. Conversely, when population size was high, catches were spread across a larger area including regions with suboptimal temperatures. The spatial extent of optimal habitat appears to have decreased from 1977 to 2002, reflecting a gradual warming of the North Sea. The combined negative effects of increased temperature on recruitment rates and the reduced availability of optimal habitat may have increased the vulnerability of the cod population to fishing mortality.
Misinterpretations of elevated catch-per-unit-effort (CPUE) in the northern cod (Gadus morhua) fishery contributed to overestimations of stock size, inflated quotas, and unsustainable fishing mortality in the 1980s and early 1990s. We hypothesize that concentration of the fish and fishery led to extreme hyperstability in the CPUE–abundance relationship. In the late 1980s, migrant cod began to concentrate within the Bonavista corridor, their most southerly cross-shelf migration route. By the spring of 1990, approximately 450 000 t was concentrated within 7000 km2at densities quadruple those of the 1980s. Densities remained high through 1992, while abundance declined fivefold. During this period, cod hyperaggregated (local densities increased with decreasing biomass) in the Bonavista corridor and CPUE increased. To the north, no hyperaggregation occurred, and densities and CPUE declined with biomass. In the Bonavista corridor from 1990 to 1993, CPUE was hyperstable with local cod density. Areas of high cod densities (>0.1 fish·m–2) shrunk as regional estimates of cod biomass declined. The spatial extent of the fishery contracted proportional to the shrinkage in area occupied by the fish. Hence, CPUE was related to abundance at the local scales of a fishing set (local acoustic density) but not to abundance at regional or stock scales.
A dilemma fishery managers sometimes face is how to state with reasonable confidence that a rare fish species has been extirpated from a given system. Data on abundance and distribution are typically scarce for rare species and are mostly in the form of incidental observations, such as fisheries bycatch or museum collections. An objective and quantitative method of evaluating the status of a rare fish species in selected systems—one that is based on incidental observations and that could be used to provide a first warning to managers that a species may be extirpated—would be highly desirable. In this paper, we review two methods that use incidental observations for estimating the probability that a species is extirpated, and we demonstrate the methods with available data sets for Atlantic sturgeon Acipenser oxyrinchus and Alabama sturgeon Scaphirhynchus suttkusi. Special considerations should be made when applying the methods to data sets for fish species. We recommend against using or combining data sets that contain both early life stage and adult observations, and we recommend that priority should be given to data sets obtained in preferred habitat areas. Ancillary information that could provide additional insight regarding the status of the species in the system should be used whenever possible. Finally, we offer a protocol for assembling information and estimating the probability that a fish species is likely to have been extirpated from a system.
Schaefer-model stock assessments can be imprecise when they are fitted to catch rate data (CPUE) because a large, unproductive stock can often explain CPUE trends as well as a small, productive one. However, consideration of life-history characteristics can improve parameter estimates by constraining maximum productivity. Therefore, we applied the methods of McAllister et al. (2001) to Northeast Atlantic spurdog (Squalus acanthias) by using demographic techniques to convert prior distributions for age-specific fecundity and natural mortality (the latter based on published estimates from tagging studies) to prior distributions for the intrinsic rate of population growth (r). The priors for r generated in this manner were then used in a Bayesian, Schaefer-model assessment of spurdog, fitted to bottom trawl survey CPUE data. Results suggest the stock is depleted to about 5% of virgin biomass.
The effects of human exploitation on macroecological patterns have received little attention, although such investigations may highlight unique spatial and temporal changes characteristic of species and assemblages subject to persistent disturbance. In unexploited systems (mainly among temperate avifauna) positive relationships between local abundance and geographic distribution are prevalent for individual species through time (intraspecific pattern) and among species during fixed time periods (interspecific pattern). We investigated intraspecific and interspecific relationships for 24 common marine fishes on the Scotian Shelf and Bay of Fundy, Canada, some of which have been commercially exploited for several decades. Based on extensive fisheries-independent trawl survey data from 1970 to 2001, 16 of the 34 stocks, comprising 13 species, exhibited significant positive intraspecific relationships. Significant relationships were associated mainly with those stocks that demonstrated significant temporal trends in both abundance and geographic distribution. The time-averaged (32 yr) interspecific relationship was positive and significant at the largest scale examined. Significant annual interspecific relationships were also detected over 26 yr. Surprisingly, the slopes of the annual relationships increased systematically and doubled through time, probably due to size-selective exploitation, shifting target species, and associated species interactions. In contrast to previous studies, our results indicate that the contributions of individual species to the interspecific relationship can change through time, and these changes dramatically alter the interspecific abundance-distribution relationship. Temporal trends in the interspecific relationship have not previously been reported, and appear to be due to the large spatial- and temporal-scale effects of exploitation.
Grey gurnard (Eutrigla gurnadus) is a widely distributed demersal species in the North Sea that has been ranked frequently among the 10 dominant species. Since the late 1980s, grey gurnard catch rates in the international bottom trawl surveys showed a pronounced increase and it was included as an "other predator" in the North Sea multispecies virtual population analysis (MSVPA) in 1997. The MSVPA results estimated grey gurnard to be responsible for approximately 60% of the total predation mortality on age-0 Atlantic cod (Gadus morhua). Long-term MSVPA predictions led to the extinction of North Sea cod. As a possible technical reason, the Holling type II functional response implemented in the model was discussed. In the current analysis, it was demonstrated that the Holling type II functional response was not responsible for the extinction of cod in the model, which was rather a true effect of high grey gurnard predation. Further, it was shown that grey gurnard predation had a significant top-down effect on whiting (Merlangius merlangus) and potentially also on cod recruitment, which was linked to the spatial distribution of the three species. Eventually, the implications of the results for North Sea cod stock recovery plans were discussed.
Catchability to commercial fisheries has been predicted to be density dependent due to density-dependent variation in stock area. Previous studies have used indices of stock area based on thresholds of absolute density. These indices will increase with abundance even if density increases uniformly over all areas. We show that spatially uniform changes in abundance can affect catchability given certain models for the distribution of fishing effort, but that this effect is slight compared with the effect of changes in the spatial spread of fish distribution. We describe an index of distribution that depends only on spatial spread: the minimum area over which a specified percentage of the population is spread. We tested the density dependence of this index using data on Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence. Results depended on the percentage of the population for which the index was evaluated. The area containing most (90 or 95%) of the population was density dependent, expanding as population size increased. The area of highest cod concentration (i.e., the area containing 50% of the population) did not expand significantly as population size increased.