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Off the coast of Iceland, ecologist Anaïs Remili and her colleagues carefully aim a biopsy dart at a killer whale surfacing nearby. The dart nicks the whale’s back before falling into the water, and the scientists retrieve their precious projectile with a net. The dart contains a thimble-sized sample of skin and blubber—enough flesh for the scientists to use in multiple experiments.
The darts, used under strict research permits, don’t appear to bother the whales, Remili says, “and from this one interaction, we can learn so much.” Remili has developed a method that can estimate what the killer whales have been eating based on the fatty acids in their blubber, and in a new study, she’s mapped the prey of different populations of killer whales across the North Atlantic.
Unlike their well-studied cousins off the west coast of North America, Arctic and North Atlantic killer whales have kept their diet preferences to themselves, until now. “What they’re eating is really important because as top predators, they have effects on the entire food chain,” explains Remili, adding that learning about killer whale diets provides important baseline information for future research and conservation.
Through her new research methodology, Remili teases apart the composition of fatty acids found in whale blubber and estimates the whales’ diet based on the known fatty acid compositions of their prey. For her study, Remili gathered 900 prey samples and 191 whale blubber samples from across the North Atlantic.
Remili found that killer whales off the Arctic coast of Canada largely feasted on belugas, narwhals, and ringed seals, while to the east, near Greenland, killer whales ate hooded, harp, and ringed seals. Whales off the coast of Iceland preferred mostly herring with a side of harbor porpoise, while whales in Norway ate mostly herring and some harbor seal.
Remili was most surprised by how different individual diets were within the same population. For example, half the whales in the Canadian Arctic preferred belugas and narwhals, while the other half preferred ringed seals. Individuals from Greenland ate a mix of seals, whales, and fish. Most of the whales from Iceland subsisted on herring, but a few individuals ate only harbor porpoise and mackerel.
Remili says one of the limitations of the model is that they can’t precisely estimate timing. Killer whale diets may vary seasonally, for instance, with perhaps some whales chasing a herring spawn one season and switching to marine mammals another season. More physiological tests would help scientists develop a clearer picture of what the whales ate and when. Still, Remili says the fatty acid analysis will be an immensely helpful tool for studying whale diets in the future, with the potential for applying the same methods to other whale species.