Slow and stealthy? Evidence for individual differences in marine mammal consumption by Greenland sharks (Somniosus microcephalus) based on stable isotopes and fatty acids

Background/Question/Methods

Arctic marine consumers are often assumed to feed as generalists. However, recent work has revealed that populations of apparent generalists can be composed of individuals that specialize on a subset of available resources. In Arctic ecosystems, all individuals of a species would be expected to exploit abundant resources, but some individuals may more effectively exploit certain limited resources, therefore leading to individual diet differences. The Greenland shark is a large Arctic consumer whose diet includes a variety of fast-moving fishes (e.g. Gadoids) and marine mammals (e.g. ringed seals, Pusa hispida), which is perplexing due to the shark's slow movement. Greenland sharks are known to scavenge, but they might also actively capture prey through surprise tactics. If this is the case, individual differences in foraging success could have given rise to dietary differences among individuals. The goal of the present study was to test the hypothesis that individual Greenland sharks specialize on marine mammals. Stable nitrogen isotopes (δ¹⁵N) and fatty acids of Greenland shark muscle were used to infer past resource use, and blood plasma (i.e. BP) values were used to infer recent diet. Known Greenland shark prey items (ringed and harp seal (Phoca groenlandica) blubber, Greenland halibut (Reinharditus hippoglossoides) muscle) were also analyzed.

Results/Conclusions

Values of δ¹⁵N were similar between Greenland shark muscle (n=46 mean±SD: 17.16±0.59‰) and BP (17.01±0.67‰) (paired t test, P>0.05). However, individual differences between muscle and BP δ¹⁵N ranged from 0.00 to -1.51‰. Several fatty acids exhibited large variance between seal blubber and halibut muscle. For example, the ratio of 20:5n-3 to 22:5n-3 (i.e. EPA:DPA) was significantly higher in halibut muscle (5.92±1.06) than in harp seal (1.48±0.32) or ringed seal blubber (1.48±0.24) (F_{2, 45} = 211.59, P<0.001). Five individual Greenland sharks had similar EPA:DHA to that of seal blubber in both muscle and BP (ANOVA, P>0.05), which could indicate frequent and consistent consumption of seal relative to the other individuals sampled. These Greenland sharks also had similar δ¹⁵N between muscle and BP, and relative trophic positions consistent with seal consumption (i.e. 4.0-4.5). Results from the present study provide evidence that some Greenland sharks consume more marine mammal than other individuals. These results do not discount the consideration of Greenland sharks as generalists, but do suggest that individual differences in resource use should be considered when attempting to understand the role of these large sharks in Arctic communities.