Whales Eat Sharks

January 31, 2011

Killer whales (Orcinus orca) are common off the northwestern coast of the US and Canada.  Decades ago researchers learned that there were two separate populations of these animals - “resident” whales who remain in the area for long periods of time, and “transient” whales who pass through more briefly.  Resident whales eat primarily fish, such as salmon, and squid, while transient whales eat marine mammals like seals and porpoises.  These two populations are genetically distinct, do not interact and do not interbreed.  This food specialization may allow different groups of whales to live in close proximity without competition.

More recently a third group of whales was identified in the area, called “outsides” because they spend their time in more distant water near the edge of the continental shelf.  The outsides are also a genetically unique group, and do not breed with either residents or transients.  If killer whales are such dietary specialists, what do the outsides eat?

A recently published study by Ford et al describes these researchers’ observations of two groups of outside whales feeding off the coasts of Alaska and Canada.  The intrepid researchers watched the whales feed above and below the water in groups, then slipped in to recover the remains of their meal.  They used genetic analysis to determine that these outsides fed almost entirely on Somniosus pacificus, the Pacific sleeper shark.  Over a period of several hours the whales ate at least 16 of the 12-foot average sharks.  This is the first definitive report of the outsides’ diet, and the number of sharks consumed suggests these whales may specialize in eating sharks.

The researchers also saw something else very unusual.  Whales, being mammals, don’t replace their teeth as sharks do, but since most toothed whales eat fish and mammals, tooth wear is not a problem.  Unlike these soft prey, however, shark skin is covered in hard dermal denticles.  The outside whales showed extreme tooth wear, with most adults having teeth worn completely to the gum line!  (Look at the article, the photos are amazing.)  It’s likely these offshore whales wear down their teeth prematurely by chewing through tough shark skin.  Sleeper sharks have large oily livers, which makes them a rich food for whales.  It seems that tooth loss may be a trade-off the outside whales make to be able to feast on this valuable resource.

The article is: Ford, JKB et al. 2011 Shark predation and tooth wear in a population of northeastern Pacific killer whales. Aquatic Biology, 11:213-224.  It is available here: http://netcommunity.vanaqua.org/Page.aspx?pid=542