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Killer Whales Die Without King Salmon by Jennifer Viegas Discovery Channel, September 16, 2009

Some killer whale populations favor king salmon so much that the whales will actually die when numbers of this largest member of the salmon family drop, according to new research.

The study, published in the latest Royal Society Biology Letters, suggests that although killer whales may consume a variety of fish species and mammals, many are highly specialized hunters dependent on this single salmon species.

Lead author John Ford explained to Discovery News that "nutritional stress" probably leads to killer whale deaths because it can make the whales "susceptible to other factors leading to mortality, such as disease and parasitism," and possibly also more vulnerable to the "immuno-suppressive effects of PCBs" and other ocean pollutants.

Ford, a research scientist at Fisheries and Oceans Canada, and his colleagues used 25 years of demographic data from two populations of fish-eating killer whales in the northeastern Pacific Ocean, along with data on numbers of chum and Chinook salmon, commonly known as king salmon.

The data consisted of annual photo ID censuses for the whales and Pacific Salmon Commission population estimates for the fish.

The scientists determined that from 1974 to the mid 1990's, resident killer whale populations steadily grew in number at an increase of nearly 2.6 percent per year. That pattern abruptly ended in the mid 1990's, when both analyzed populations of killer whales entered a period of prolonged decline, dropping by up to 17 percent as of 2001. By 2004, the populations began to recover and show growth again.