An age-structured population dynamics model, incorporating interactions between Atlantic cod (Gadus morhua), the fishery, and the grey seal (Halichoerus grypus) population, was applied to the cod stock off eastern Nova Scotia (Northwest Atlantic Fisheries Organization Divisions 4Vs and 4W, commonly abbreviated to 4VsW), a stock that has dramatically declined since the late 1980s. Mortality was modeled as having three components: fishing mortality (F), seal predation (Mp), and all other sources of natural mortality (M). Specifically, M was assumed to be distinct for immature cod (ages 1–4; Mi) and mature cod (age 5 and older; Mm), and respective annual variations were estimated. Parameters estimated also included recruitment (cod abundance at age 1; R), F, and Mp. Based on our estimates of F, Mp, and M, it is unlikely that the collapse of the 4VsW cod stock can be attributed to a sudden increase in M; fishing appears to have been the primary cause for the stock's decline. However, after the moratorium on commercial fishing in 1993, increasing Mp and Mm and low R may have contributed to the failure of the 4VsW cod stock to recover.

The Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf collapsed in 1993. Over a decade later, in spite of a fisheries moratorium on cod fishing, this stock is at an all-time low. In parallel with the collapse of the cod stock, the abundance of large cod prey, including forage fish, shrimp, and snow crab, has greatly increased. The key question, which we explore using trophic mass-balance models, is what processes are preventing cod from recovering on the eastern Scotian Shelf? Cod were split into large and small cod. Modelling results indicate high predation pressure on small cod. In addition, small cod compete with the abundant forage fish for decreasing prey, and are in below-average condition. Large cod incur high, but unidentified, mortality that we suggest is derived from the poor condition of small cod carried through to adulthood. As a consequence of the removal of cod by fishing and an ensusing trophic cascade, eastern Scotian Shelf cod are trapped in a vicious circle: their abundance is being kept low by predation, causing an abundance so low that cod cannot compete for prey with their exceptionally abundant competitors. Furthermore, these competitors may also prey on younger stages of cod.

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A spatial, age‐structured population dynamics model was developed for a stock of Atlantic cod Gadus morhua off eastern Nova Scotia to examine alternative management options. The model incorporates stock structure, seasonal migration, predation, Allee effects, and variation in natural mortality. The simulated dynamics of two substocks were compared under three management options: (1) historical catch levels; (2) combined management (substocks are managed as one unit) with a constant fishing mortality rate (F) of 0.4; and (3) separated management (substocks are managed individually) with the same F level. Three major conclusions emerged from the simulations. First, the population dynamics under historical catch levels tended to vary widely from complete extinction to a fourfold increase. Second, constant‐F management produced stable population dynamics, reduced the probability of stock decline, and yielded a higher average catch. Third, the combined management option resulted in overfishing the more vulnerable substock; separated management helped to prevent the more vulnerable substock from collapse, but its performance was compromised when there was net immigration to the more vulnerable substock.

Conservation and management strategies for endangered and threatened species require accurate estimates of demographic parameters such as age and growth. The whale shark, Rhincodon typus, is the largest fish in the world and is highly valued in the eco-tourism sector. Despite conservation concerns and advances in our understanding of their life history, basic demographic parameters for growth, longevity and mortality are of questionable accuracy; previous growth studies could not agree whether the vertebral growth bands were formed annually or biannually. Here, we provide the first validation of the annual formation of growth bands within the vertebrae of the whale shark using bomb radiocarbon assays. Ages of up to 50 years were estimated from sectioned vertebrae of sharks collected in Taiwan and Pakistan. There was no cessation of the formation of growth bands in the vertebrae of older sharks and our study provides the oldest observed longevity for this species. Initial estimates of growth (k = 0.01-0.12) and natural mortality rates (M = 0.09-0.14) are consistent with those expected of long-lived sharks, which highlights their sensitivity to fishing pressure and conservation concerns.

Roff, I. C., L. P. Fanning, and A. €3. Stasko. 1986. Distribution and association d larval crabs (Decapoda: Brachyura) on the Scstian Shelf. Can. 1. Fish. Aquat. Ssi. 43: 587-599. Thirty-five taxa of weustonic larval Brachyura were distinguished and identified to stage of development in 3055 samples from the Scotian Shelf taken from north of Cape Breton to Georges Bank. A maximum of 17 species of larvae was taken in a single tow, with a norm of about 4 per tow. The most abundant species, collected in all stages sf development, were the indigenous Chionoecetes opilio, Hyds araneus and Hyas coarctatus, Cancea borealis and Cancea irroaatus, and Carcinus maenas. Biomass was dominated by the Hyas and Cancer species.Distributions sf the three Majidae species indicated that larvae drift southwest along the shelf, and that they are indigenously derived. Twenty-six expatriate species occurred as megalspas only, predominantly offshore. Reciprocal averaging and factor analysis showed that there was considerable separation of the indigenous and expatriate larval species, but did not reveal strong relationships to environmental variables or ecslogical gradients. Recurrent group analysis indicated that community structure was weak and variable, but confirmed the virtual separation into indigenous and expatriate associations. An examination of co-occurrences of indigenous and expatriate species by stage sf development showed strong patterns. We suggest that concurrent analysis of larval lobster and larval crab distribution patterns on the Scotian Shelf could indicate recruitment origin of lobsters (Homaaus americanus). Trente-cinq taxa de Brachykara neustonique larvaire ont ete distingues et identifies jusqu'au stade de developpernent dans 3 055 6chantillsns preleves sker la plate-forme Scstian entre la partie nord du Cap-Breton et le banc de Georges. Au maximum 17 especes de larves snt ete prises en un seul coup de filet, avec une moyenne de 4 par ramassage. Les especes les plus abondantes, recueillies 3 tous les stades de d6veloppemewt, etaient: Chionoecetes opiiio, Hyas araneus et Hyas coarctatus, Cancer borealis et Cancer irroaatus, et Carcinus maenas. Hyas et Cancer etaient les especes dominantes dans la biornasse. L'analyse des distributions des trois esgeces de Majidae a revele que les larves se dkplacent vers le sud-suest le long de la plate-forme continentale et qu'elles sont d'origine indigene. Vingt-six especes expatriees ont kt6 trouvkes sous forme de mkgalopes seulement, surtout au Barge. Le calcul de la moyenne des inverses et I'analyse factsrielle snt montre que la separation etait considerable entre les especes larvaires expatriees et indigenes mais qu'il n'existait pas d'irnportantes relations avec des variables ewvironwernentales et des gradients ecologiques. L'analyse des groupes recurrents a indiqu6 que la structure de la cornmunaute etait faible et variable mais qu'il y avait bien separation virtkaelle en associations indigene et expatriee. Un examen de la co-occurrence des especes indighes et expatriees pa...

The Canadian Department of Fisheries and Oceans conducts annual bottom trawl surveys to monitor changes in the abundance of the major commercially important groundfish populations. Some of these surveys have been in operation for almost 20 yr. The estimates from these surveys often indicate rapid changes in abundance over time beyond that expected from the population dynamics of the fish. Much of this interannual change has been interpreted as variation, the magnitude of which has often made it difficult to measure anything but the most severe effects of fishing, pollution or any other intervention on the population. Recent studies have shown that some of this variation may be attributed to changes in catchability of fish due to the effects of environmental variables on fish distribution. Annual changes in abundance as estimated from such field surveys may be confounded by changes in catchability due to annual changes in environmental conditions. In this study, trawl catches of age 4 Atlantic cod (Gadus morhua) from surveys conducted during March 1979-1988 were compared with concurrent measurements of bottom salinity, temperature and depth. Large catches of age 4 cod are more likely to occur in water characterized as the intermediate cold layer defined by salinities of 32-33.5 and temperatures<5°C. This relationship also appears to be modified by depth. We further show that internnual changes in the estimated abundance from the surveys were, in a number of cases, coincident with changes in the proportion of the bottom water composed of the intermediate cold water layer. The implications that these patterns may have on interpreting trends in the estimates of abundance from trawl surveys are discussed.