Continued decline of the threatened Eastern Scotian Shelf Atlantic cod population: How important is grey seal predation

Population Ecology Division, Bedford Institute of Oceanography, Department of Fisheries and Oceans, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada.
Ecological Applications (Impact Factor: 4.09). 01/2007; 16(6):2276-92. DOI: 10.1890/1051-0761(2006)016[2276:CDOAAC]2.0.CO;2
Source: PubMed

ABSTRACT 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.

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Available from: William Don Bowen, Jan 16, 2015
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    • "This would theoretically result in a depensatory PR that would accelerate the decline and even inhibit recovery , particu - larly if a 2 - state system is realized ( Fu , Mohn & Fanning 2001 ; Frank et al . 2005 ; Trzcinski , Mohn & Bowen 2006 ) . "
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    • "Estimating the diets of marine mammals is needed to understand how individuals and populations respond to ecological and environmental variability and their functional roles in marine ecosystems (Bowen 1997). An understanding of diet is also of practical importance in marine mammal conservation (Parrish et al. 2002) and to evaluate the contribution of marine mammal predation to sources of natural mortality in prey populations, some of which may be commercially fished or of conservation concern (e.g., Trzcinski et al. 2006). In many terrestrial and avian predators, the species composition of diets can be estimated from direct observation of feeding or provisioning, although indirect methods are also used (Pierce and Boyle 1991). "
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