Sex differences and inter-annual variation in diets of adult and juvenile grey seals: evidence from stable isotopes of carbon and nitrogen.
Tucker, Strahan *,1, Bowen, W. Don2, Iverson, Sara1, 1 Department of Biology, Halifax, Nova Scotia, Canada2 Marine Fish Division, Dartmouth, Nova Scotia, Canada
ABSTRACT- While 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 correlated with mass of prey and it is commonly found that predator size determines dietary overlap. Grey seals are a size dimorphic phocid inhabiting the Northwest Atlantic and found in abundant numbers on the Scotian Shelf. Males are 1.5 times larger than females and previous studies have demonstrated significant sex differences in diving and foraging behavior. Males tend to have less overall efforts and dive deeper in square or flat bottom shaped dives, potentially indicative of benthic foraging. In contrast, females tend to display strong diurnal patterns in diving with more left skewed dives suggestive of feeding on pelagic prey. Stable isotopes of nitrogen (delta N15) and carbon (delta C13) provide useful tools for estimating trophic positions and carbon flow to consumers in food webs. N15 and C13 were analyzed for known-aged individual adult grey seals from Sable Island, NS between 1996-2001. Samples of skin were taken in June or September and the same animals were resampled prior to the breeding season in January. In addition, samples were taken from juveniles in January 2004. N15 and C13 were also analyzed for fish (n=12) and invertebrate (n=3) species from the Scotian Shelf. Adult males (n=37) and females (n=35) had significantly different N15 and C13 values (GLMM: F(1,70) = 22.40; p < 0.001). On average males were more enriched in both N15 and C13 than females. Sex differences in isotope values remained significantly different when examined in each individual year of study, however there were significant differences in N15 values between years for males (GLMM, F(5,12) = 4.73; p = 0.01). There were no differences among male and female juveniles though signatures were significantly different from adults. Comparison of isotope values from potential prey suggests that adult male seals feed more heavily on benthic prey, whereas adult females appear to feed more on pelagic prey. Juveniles appear to feed more pelagically. These results are consistent with differences in dive patterns and foraging behaviour. Temporal shifts in signatures coincided with documented shifts in oceanographic conditions and prey distribution.
Key words: stable isotopes, size dimorphism, diet, grey seals
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