Excessive and unsustainable fishing mortality was the predominant factor in the depletion of Northwest Atlantic cod (Gadus morhua) stocks. However, despite imposition of severe catch restrictions for over a decade, stocks have mostly failed to recover at predicted rates. A number of explanations have been considered. Our analysis of demographic characteristics of 12 of these stocks indicates that recent productivity over the northern portion of the range is much lower than 20 years previous when several stocks recovered from less severe declines. Main contributing factors are, in rank order, increased natural mortality, decreased body growth, and in a few cases, reduced recruitment rates. Continued fishing in directed and bycatch fisheries is also an important factor. Under current conditions, we estimate negative or very low (<2% per year) average growth rates in eight stocks. If fishing ceases, growth rates of >5% would be expected in six stocks, with >10% in four of these. Although productivity is low, we conclude that fishing mortality is further delaying recovery.

Grey seal pup production on Sable Island, Nova Scotia, has been monitored since the early 1960s. We estimated pup production on Sable Island in 1997 using aerial photography with a correction for detection of pups on the imagery and a statistical model to account for the proportion of pups not born at the time of the survey. A total of 20 863 pups were counted on the colour positives. When corrected for the proportion of pups seen on the imagery (0.96) and the proportion of pups born before the survey (east colony 0.94, west colony 0.66), estimated total pup production was 25 400 with approximate 95% confidence limits of 23 500 and 26 900. The 1997 estimate indicates that pup production in this population, now the largest grey seal colony in the world, has been increasing exponentially at an annual rate of 12.8% for four decades in the face of considerable environmental variability.Crown

We modelled trends in the predation mortality of the Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf, Canada, by grey seals (Halichoerus grypus) and the potential effects of this mortality on cod recruitment over the period 1970-1994. Two predation models were explored: one in which the fraction of cod in the diet was constant and another in which the fraction of cod was proportional to cod abundance. The number of grey seals in the study area, from the Gulf of St. Lawrence and Sable Island populations, increased from about 5000 in 1970 to 64 000 in 1994, while total food consumption increased from 10 000 t⋅year -1 in 1970 to 121 000 t⋅year -1 . Under the constant ration model, grey seals consumed about 15 400 t of cod in 1994 compared with about 6300 t under the proportional ration model. Grey seal predation mortality was only 10-20% of estimated fishing until the fishery was closed in 1993, indicating that grey seals were not a major factor in the recent collapse of this stock.Résumé : Nous avons modélisé les tendances de la mortalité du stock de morue franche (Gadus morhua) de l'est de la plate-forme néo-écossaise (Canada) due à la prédation par le phoque gris (Halichoerus grypus), et les effets potentiels de cette mortalité sur le recrutement de la morue pendant la période 1970-1994. Deux modèles de la prédation ont été explorés : dans l'un, la proportion de morue dans l'alimentation des phoques était constante, tandis qu'elle variait en fonction de l'abondance des morues dans l'autre. Le nombre des phoques gris présents dans la zone d'étude, qui appartenaient aux populations du golfe du Saint-Laurent et de l'île de Sable, a grimpé, passant d'environ 5000 têtes en 1970 à 64 000 en 1994, tandis que la consommation totale de nourriture passait dans la même période de 10 000 à 121 000 t⋅an -1 . Dans le modèle à ration constante, les phoques gris consommaient environ 15 400 t de morue en 1994 contre environ 6300 t dans le modèle à ration proportionnelle. La mortalité due à la prédation par le phoque gris correspondait à seulement 10-20% des prises commerciales estimées jusqu'à la fermeture de la pêche en 1993, ce qui permet de penser que ce pinnipède ne représentait pas l'un des facteurs principaux du récent effondrement du stock de morue. [Traduit par la Rédaction]

We examine the effect of introducing an environmental factor — the North Atlantic Oscillation (NAO) index — into a stock–recruit relationship fitted to 13 North Atlantic cod (Gadus morhua) stocks and discuss the implications for management. The NAO has a significant effect on recruitment of four of the stocks (positive effect on recruitment in the North Sea, Baltic Sea, and Irish Sea; negative effect on recruitment at Iceland), and the pattern of positive and negative effects on all stocks is consistent with the geographic influence of the NAO on environmental variables. Observed variability in the NAO should be taken into account in interpreting the causes of past changes in cod stocks. The NAO index for the previous winter is available by April and thus may provide an early indication of the likely range of cod recruitment in the current year. In areas, such as the North Sea, where the effect is strong, medium- and long-term assessments of recruitment and yield of the cod stocks should consider likely future states of the NAO. The NAO can be used to represent environmental variability in stock projections and thereby provide a better basis for the estimation of risk.

The continental shelf ecosystem on the Eastern Scotian Shelf (ESS) has experienced drastic changes. Once common top predators are a small fraction of their historical abundance, and much of the current community structure is now dominated by pelagic fishes and invertebrates. Embedded within this food web, Atlantic cod and gray seal populations have recently exhibited nearly opposite trends. Since 1984, cod populations have decreased exponentially at a rate averaging 17% per year, whereas gray seals have continued to increase exponentially at a rate of 12%. We reexamined the impact of gray seals on Atlantic cod dynamics using more than 30 years of data on the population trends of cod and gray seals while incorporating new information on seal diet and seasonal distribution. The closure of the cod fishery over 10 years ago allowed for a better estimation of natural mortality rates. We quantified the impact of seals on ESS cod by (1) estimating trends in seal and cod abundance, (2) estimating the total energy needed for seal growth and maintenance from an energetics model, (3) using estimates of the percentage of cod in the total diet derived from quantitative fatty acid signature analysis (QFASA) and of the size‐specific selectivity of cod consumed (derived from otoliths collected from fecal samples), and (4) assuming a gray seal functional response. Uncertainties of the model estimates were calculated using the Hessian approximation of the variance–covariance matrix. Between 1993 and 2000, cod comprised, on average, <5% of a gray seal's diet. Our model shows that, since the closure of the fishery, gray seals have imposed a significant level of instantaneous mortality (0.21), and along with other unknown sources of natural mortality (0.62), are contributing to the failure of this cod stock to recover.

No abstract

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.

No abstract