Sleeper Sharks May Hunt Steller Sea Lions in the Gulf of Alaska

Sleeper sharks could be behind attacks on protected Steller sea lions. While Pacific sleeper sharks are usually seen as a slow-moving, scavenger species, scientists believe that they may be preying on sea lions in the Gulf of Alaska.

For the past decade, researchers have deploying specially designed "life history transmitters" into the abdomens of juvenile Steller sea lions. These tags record data on temperature, light and other properties during the sea lions' lives, and can aid conservation efforts. From 2005 to 2011, the scientists implanted these recorders into 36 juvenile Steller sea lions. A total of 17 of the sea lions died, and fifteen transmitters indicated that the sea lions had been killed by predation.

"The tags sense light and air to which they are suddenly exposed, and record rapid temperature change," said Markus Horning, one of the researchers, in a news release. "That is an indication that the tag has been ripped out of the body, though we don't know what the predator is that did this."

While the researchers can't confirm what the predator was, they do have theories. By process of elimination, they think that sleeper sharks could be to blame.

"We know the predator was not a killer whale, for example, because the temperatures would be much higher since they are warm-blooded animals," said Horning.

So what makes the researchers believe that sleeper sharks are to blame? The number of sleeper sharks killed in bycatch yearly reveals that there are large numbers of these sharks located in Alaska. In addition, larger sleeper sharks are rarely caught, which means that there could be some massive ones preying on juvenile sea lions.

"If sleeper sharks are involved in predation, it creates something of a dilemma," said Horning. "In recent years, groundfish harvests in the Gulf of Alaska have been limited in some regions to reduce the potential competition for fish that would be preferred food for Steller sea lions. By limiting fishing, however, you may be reducing the bycatch that helps keep a possible limit on a potential predator of the sea lions. The implication could be profound, and the net effect of such management actions could be the opposite of what was intended."

The findings are published in the journal Fishery Bulletin.