[by Alexander Simolaris]
According to a report released in Nature last month, Orcas (Orcinus orca) of the Pacific U.S. have been actively predating on sleeper sharks (Somniosus pacificus). This development is contrary to the popular notion that Killer whales feed almost exclusively on marine mammals and large schools of small fish. However, the catch for the whales is severe damage—and often times complete destruction—of their teeth.
While interesting, the find is reminiscent of the discovery that some whale pods feed on sea-otters—a sign of desperation stemming from a crippling reduction in biomass. If the cost of predating sharks is equally high, then the results of the study suggest that the the whales' situation is more alarming than previously imagined—or is it?
Interestingly enough, seals pose a more direct risk to whales than sharks. Seals, among other prey, are more maneuverable and defensive during an attack; they are more likely to bite an oncoming whale than any other prey item. As counter-intuitive as this may be, sleeper-sharks—during the actual hunt, anyway—are rather innocuous targets. Incurring accelerated tooth decay may simply be a worth-while trade-off that has no significant, net effect on the whales' fitness.
Under this paradigm, there is an argument in the marine community to classify the shark-eating orcas, collectively known as “residents,” as a separate species from the “transients,” which predate almost exclusively marine mammals. These populations do not exchange vocalizations or genetic material despite being fully capable of doing so, indicating the potential development of incipient species. Should the designation be made official, conservation efforts in the region may be, at the very least, administratively affected by the change.
While these possibilities are both interesting and timely, the results of the study are difficult to interpret with any degree of certainty. Populations of pacific whales were discovered in the 70s, specific designations to residents and transients classified in the 80s while dietary habits have only been analyzed in the last decade. Despite years of work, scientists really know very little about the nuances of regional whale behavior. This problem, in addition to the notable absence of a second longitudinal study, forms only one line in a laundry list of variables that makes the data so difficult to interpret. As the pace of research is dictated by ever dwindling numbers of slowly reproducing whale pods, the important implications of the article's results may take decades before coming to fruition.