Research Document - 2010/128

Winter diet of grey seals in Cabot Strait

By G.B. Stenson, M.O. Hammill, and J. Flight

Abstract

The winter diet of grey seals, in the Cabot Strait, was examined to determine if they feed extensively on overwintering southern Gulf of St. Lawrence (NAFO area 4T) Atlantic cod that concentrate in this area. The stomach and intestines of 100 grey seals, collected between Cape Breton and St. Paul’s Islands, were examined. The majority of samples, of which 50% contained food, were from males. Numerical correction factors (NCF) were applied to the intestine contents to account for otolith loss, but no correction was applied to the stomach content data. The diet of males and females differed greatly. Using the stomachs, Atlantic cod (50%), herring (21%), and white hake (13%) accounted for 84% of the male diet. Females fed primarily on herring (72.6%), winter flounder (17.3%), white hake (3.5%), sandlance (3.0%), and capelin (2.5%). Large robust otoliths of cod appear to be retained in the stomach, leading to an overestimate of the contribution of cod to the diet. The average size of cod consumed was 43.2 cm which is larger than that observed in other studies. NCF increased the importance of species with small, fragile otoliths such as herring, capelin, and sandlance while reducing that of fish with robust otoliths such as Atlantic cod and flatfish. Based on numerically adjusted data from the intestines, the importance of the many prey species differed when compared to the results from the stomach contents. Flatfish (Pleuronetidae sp.) and herring were the most important prey species in both males (27.5% and 28.2% respectively) and females (27.8% and 21.2% respectively) while the percentage of cod in the diet of males and females was 16.24% and 2.55%, respectively. The proportion of Atlantic cod and Gadus sp. combined in males was lower in the intestines than in the stomachs (24% vs. 52.9%). Using data from the intestines, females had a lower reliance on herring (21.2% vs 72.6%) and winter flounder (2.3% vs. 17.3%) when compared to stomach data. Only 12 of the 100 stomachs contained Atlantic cod otoliths while only 6 tested positive for cod DNA. No DNA was found in stomachs that did not also contain Atlantic cod otoliths in their stomachs suggesting that if belly biting occurs, it is not common in this area. Although no method of diet analysis is unbiased, the potential retention of large cod otoliths and the lack of correction factors for the stomach data suggest that the diet based upon the intestine contents is likely a more realistic description grey seal feeding during this study.

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