Sea Lions Beware, Sleeper Sharks May Have Developed Bigger Appetite

Sea lions beware, Pacific sleeper sharks may have developed a bigger appetite, new research shows.

These sleeping giants, a large, slow-moving species thought of as primarily a scavenger or predator of fish, seem to have set their sights higher on protected Steller sea lions in the Gulf of Alaska.

The new study found the first indirect evidence that these cold-blooded killers, measuring more than 20 feet long, may be opportunistic predators. And given that they're bigger than even the feared great white, sea lions should be wary - especially since their numbers have already dropped 20 percent since 1975.

For the past decade, Markus Horning of the Marine Mammal Institute at Oregon State University has led a team in tagging juvenile Steller sea lions. The "life history transmitters," as they call them, rely back information on temperature, light and other properties during the sea lions' lives, and even after the animals die.

Of the tagged sea lions, 17 of them died, 15 of them presumably from predation.

"At least three of the deaths were different," Horning said in a statement. "They recorded abrupt temperature drops, but the tags were still dark and still surrounded by tissue. We surmise that the sea lions were consumed by a cold-blooded predator because the recorded temperatures aligned with the deep waters of the Gulf of Alaska and not the surface waters."

Ruling out the warm-blooded killer whale, as well as the sea lion's common enemies, the great white shark and salmon shark, researchers were left with few other suspects.

Pacific sleeper sharks, meanwhile, are abundant in the Alaska area, as 3,000 to 15,000 of these sharks have been killed annually.

If sleeper sharks are indeed the culprit, however, "it creates something of a dilemma," Horning says. In recent years, fisherman have been limited in how much groundfish they can catch in the Gulf of Alaska to try to reduce competition with sleeper sharks for food. In turn, this creates a problem for sea lions, who might be in danger with so many more of these predators swimming around.

Sleeper sharks caught are usually pretty small, but larger ones are capable of tearing into fishing nets, and possibly sea lions, too.

The findings were published in the journal Fishery Bulletin.