Killer whales die when abundance of their favorite prey, Chinook salmon, declines.
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.
The killer whale population's ups and downs mirror those of king salmon for the same periods. The researchers suspect El Nino-like conditions in the early 1990's resulted in the deaths of juvenile salmon, which then brought down the entire king salmon population for the region.
It's doubtful that the weather events directly impacted the whales.
"Killer whales are very adaptable to a remarkable range of water temperatures, being found in the tropics as well as ice-covered waters," Ford said. "The range of temperatures associated with El Nino events would be unlikely to have any direct effect."
Ford and his team instead believe some killer whale populations are dependent upon Chinook salmon as their primary year-round food resource, even though these enormous apex predators physically have the ability to eat different types of fish, walruses, other large whales and additional prey.
"Animals born to a particular population learn foraging tactics to efficiently exploit particular prey types, but this specialization seems to constrain their ability to effectively hunt alternative prey," Ford said. "Specialists can often be more successful than generalists because they are more efficient at their specialization, as in the old adage, 'A jack of all trades is a master of none.'"
In a separate study, bioacoustician Whitlow Au of the Hawaii Institute of Marine Biology, and his colleagues found that killer whales prefer king salmon to such a degree that they can identify these fish when the Chinooks are swimming alongside Coho and Sockeye salmon.
Au's team determined that even in winter months when king salmon make up just 10 to 15 percent of the salmon swimming in water, killer whales will use echolocation to reveal differences in the fishes' "swimbladder shape and volume" to pick out king salmon.
Ford's group, which is studying killer whale hunting tactics now, has additionally found that killer whales then "target individual fish and often corral them against steep shorelines."
Given that the fate of some killer whales seems tied to that of king salmon, Fisheries and Oceans Canada calls for strengthening killer whale populations by ensuring adequate availability of prey.
Ford said the Canadian agency also urges "minimizing the potential effects of anthropogenic impacts, such as disturbance and noise, and identifying and protecting critical habitat."