Marine mammals may play a role in structuring marine ecosystems as top predators (for example, killer whales, belugas), fish-eaters (for example, sea lions, seals), or bottom feeders (for example, sea otters, bowhead whales, gray whales). However, the effects of marine mammals on the functioning of marine ecosystems are poorly understood. Some marine mammals, such as sea otters, are known to be keystone species because their removal results in a significant ecosystem shift. Sea otters feed on sea urchins, which, in the absence of predation by sea otters, overgraze kelp.
Several marine mammal populations are recovering from past overharvesting including grey seals in the Scotian Shelf and Gulf of St. Lawrence,23 harp seals in the Gulf of Maine and Scotian Shelf,24 western Arctic bowhead whales in the Beaufort Sea,25 the B.C./Alaska sea lions,26 sea otters,5 and the Pacific harbour seal.27 Resident killer whale populations off the coasts of Vancouver Island have also recovered from previous commercial overexploitation but have begun to decline in recent years, possibly related to declines in chinook salmon, an important food source.28
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