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Although many studies have documented diets for terrestrial mammals, much less is known about prey selection in marine mammals. Variation in body size of terrestrial carnivores is cor- related with mass of prey consumed, and predator size often determines dietary overlap. Grey seals Halichoerus grypus are a size-dimorphic phocid seal inhabiting the Northwest Atlantic, with males 1.5× larger than females. Stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) provide useful tools for estimating trophic positions and carbon flow to consumers in food webs. δ 13 C and δ 15 N were analyzed for individual adult grey seals from Sable Island, Canada, between 1996 and 2001. Samples of skin were taken in May or September and the same animals were re-sampled when they returned to the breeding colony in January. In addition, samples were taken from juveniles (n = 29) in January 2004. δ 13 C and δ 15 N were also analyzed for 12 fish and 3 invertebrate species from the Scotian Shelf. Adult male (n = 36) and female (n = 36) grey seals exhibited significantly different δ 13 C and δ 15 N values (F1, 70 = 22.40; p < 0.001). On average, males were more enriched in both δ 13 C and δ 15 N than females. While there were no differences between sex in juveniles, signatures were significantly different from adults (F2,101 = 28.05; p < 0.001). Comparison of isotope values from potential prey suggests that adult males feed more heavily on benthic prey, whereas adult females appear to feed more on pelagic prey. Juveniles also appear to feed more on pelagic prey. These results are consistent with known dif- ferences in dive patterns and foraging behaviour in this population. In addition, there was a positive effect of body size on both δ 13 C (t = 2.54, p = 0.014) and δ 15 N (t = 2.48, p = 0.016) and a positive rela- tionship between δ 15 N and the rate of fat accumulation (kg d -1 ). These results suggest an underlying energetic basis to diet variation in grey seals. Finally, examination of the variance in stable isotope ratios suggested that while individual grey seals appear to be relative specialists in their diet, niche breadth was 1.5× greater in males than in females. On average, adult grey seals had a 2 × higher niche breadth than juveniles.

Dimensions of diet segregation in grey seals Halichoerus grypus revealed through stable isotopes of carbon (δ13C) and nitrogen (δ15N)