The other disturbing signal the researchers found in the tusks referred to the changing food sources of the whales. They looked for stable isotopes of carbon and nitrogen, residues from the diet of narwhals that persist in their defenses. Carbon reveals information about the habitat of the prey, such as whether it lived in the open sea or closer to land. Nitrogen tells you its trophic level or its location in the food chain. “Together, they give you an idea of the overall feeding ecology of the species,” Desforges explains.
As with mercury, Desforges could map how this regime has changed over time. Prior to 1990, whales preyed on “sympagic” prey associated with icy habitat – arctic cod and halibut. Then their diet began to shift towards more “pelagic” or deep-sea prey, such as capelin, a member of the smelt family. “We’re not looking at the stomach contents of prey or anything,” Desforges says. “But we’re essentially arguing that this temporal model fits extremely well with what we know about the extent of sea ice in the Arctic, which after 1990 begins to drop quite dramatically.”
Two or three things could happen. As sea ice recedes in the Arctic, ecosystems below may be reshaping, causing populations of arctic cod and halibut to decline. In this case, narwhals should turn to hunting deep-sea species to fill their food deficit. On the other hand, these cod and halibut populations are not necessarily declining, but simply moving north. Or it could be that, as the arctic waters warm, there are more capelin around and the narwhals aren’t about to miss out on a heavy meal.
But if the fish is a fish, why would it matter what narwhals eat, as long as they have enough food? It turns out that not all fish are created equal. “Arctic species are more nutritious and more energy intensive,” says Desforges. To survive the cold, fish must accumulate fat, which means more calories for predators that feed on them, like narwhals. “If they transfer their prey to less arctic species, this could have an effect on their energy intake,” adds Desforges. “Whether this is true remains to be seen, but it is certainly the big question we have to start asking ourselves.”
This dietary shift – which may or may not be a problem for the narwhal – could collide with increased levels of mercury, which are a problem for any animal. These two threats could prove to be more problematic combined than they are alone. “This is the trickiest part,” Desforges says. “We basically have data that suggests things are changing, but we really have no idea how it affects the whales here.”