Proceedings 2009/020
Proceedings of the National Workshop on the Impacts of Seals on Fish Populations in Eastern Canada (Part 2); 24-28 November, 2008
Chairperson: W.D. Bowen
Summary
A 5-day day meeting was held in November 2008, in Halifax, Nova Scotia, during which some 24 presentations were given on the potential impacts of seals on fish stocks in Eastern Canada. These presentations, for the most part, were of new analyses and model results of research identified at the first Workshop that participants felt could be done with existing data. The meeting was attended by 33 invited ecologists and modelers from Canada, Norway, and France, and members of the fishing industry in eastern Canada and northeastern United States of America.
Representatives of the fishing industry stated that grey seals pose a direct threat to depleted cod stocks and indirectly affect the Industry through the transmission of the sealworm parasite to commercial species across an expanding geographic range. Fishers also suspect that spawning success of cod is being negatively affected as grey seals find it profitable to prey on spawning aggregations on shallow banks.
A recent study on cod from the southern Gulf of St. Lawrence (sGSL) found that contrary to expectation, there was a weak but statistically significant positive relationship between parasite abundance and condition, likely reflecting an effect of variation in foraging success on both condition and parasite abundance: cod with higher foraging success achieve better condition, but also ingest more parasites. This would suggest that any negative effect of these parasites on cod condition appears to be slight compared to the effect of variation in foraging rate or success. Analyses of infection parameters from 2007 provided no evidence of nematode-induced cod mortality.
Monte Carlo simulations were used to estimate the probability that different diet compositions of seals could explain natural mortality (M) for southern Gulf of St. Lawrence cod, white hake, and winter skate. Results suggested that: 1) to explain elevated M, seals must consume cod and white hake that are larger than indicated by the available stomach contents data; 2) whole fish consumption by seals is likely to explain less than 40% of M in white hake and cod, but can easily explain elevated M in winter skate, and 3) if seals consume some groundfish by belly-feeding, the probability that seal predation explains a substantial component of M becomes much more likely.
Several studies underscored the importance of the proportion of cod in the diet and the size frequency distribution of cod eaten on estimates of seal predation mortality. Restricted sample collection (in both time and space) and the method used to estimate the diet can affect both the estimated proportion and size of cod eaten by seals.
Preliminary results where presented from bioenergetic allometric and statistical catch-at-age models to evaluate the effects of seal predation, food competition, and environmental variability on the dynamics of several cod stocks. These and previous multi-species models underscore the importance of considering seal predation in a broader ecosystem context as the effects of predation may often be conditional on the presence of competitors and quality of the environment for cod.
Most cod stocks in the northwest Atlantic collapsed in the late 1980s and early 1990s, and 15 years later have shown little or no sign of recovery. Reasons for the lack of recovery vary between stocks. However, an important factor in the lack of recovery for many stocks is elevated natural mortality of adult cod.
Natural mortality (M) of adult cod is very high in stocks south of the Laurentian Channel: the southern Gulf of St. Lawrence (Northwest Atlantic Fisheries Organization (NAFO) Division 4T), eastern Scotian Shelf (4VsW), and western Scotian Shelf/Bay of Fundy (4X) stocks. M is estimated to be slightly elevated in some stocks north of the Channel, but levels of M in these stocks appear to be much lower than in the stocks south of the Channel. Until recently, the offshore component of northern cod was an exception, with very high M; however, M in the offshore appears to have declined to normal levels in recent years. Northern cod in the inshore of 3K were also an exception, with a high estimate of M for the early 2000s.
There are a number of hypotheses regarding the factors that could be limiting cod recovery by contributing to high M including: unreported catch (i.e., the mortality is due to fishing, not natural mortality), disease, contaminants, availability and quality of food, life-history changes, impacts of seal predation, predation by other species, parasites, and other impacts. For most of the cod stocks that have not recovered, natural mortality remains above the “normal” value of 0.2. However, natural mortality is especially high in all stocks south of the Laurentian Channel. Industry has a strong view that seals are having a significant impact on cod recovery, but there is considerable uncertainty in the factors affecting cod dynamics and in the magnitude of stock-specific seal predation mortality on cod.
A population modeling study of the northern Gulf stock (nGSL; 3Pn4RS cod) suggests that predation by harp seals on juvenile cod may limit stock rebuilding under certain environmental conditions. Earlier ecosystem models suggested that harp seals were limiting the recovery of northern cod; however, a recent modeling study using a different approach suggests that effects of prey (capelin) availability are more important than effects of harp seal predation. Based on weight of evidence, seals could be limiting recovery in 4T cod, i.e., spatial and temporal correlations between grey seal abundance and estimated cod M support the hypothesis that grey seal predation is an important cause of the elevated M, whereas other hypotheses for the elevated M of 4T cod are not supported by the correlations between M and the hypothesized forcing factor. Grey seals may also play an important role in the recovery of 4VsW cod. The impact of grey seal predation in 4X cod is not known as there are no seal diet estimates specific to this stock, but if the diets are similar to those estimated in neighbouring 4VsW then the impact of grey seals would be negligible.
However, two key uncertainties that can be applied to all areas are the true proportion of cod in the diet of seals and the age distribution of cod eaten by seals. Together these aspects of diet have a large effect on the putative impact of seal predation on the dynamics of cod. Furthermore, despite their importance, estimating the diet of seals is extremely difficult, given the challenges in obtaining representative samples from large, mobile and widely distributed seal populations.
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