Western Aleutian Steller Sea Lions Potentially Falling Prey To Sleeper Sharks

(Photo courtesy Bruce Wright, Aleutian Pribilof Islands Association)
(Photo courtesy Bruce Wright, Aleutian Pribilof Islands Association)

There has been plenty of money spent trying to figure out why the sea lion population in the Western Aleutians is not recovering. But nobody has put much money into studying sharks. The latest data from a study that implanted high-tech tags in the animals suggests that maybe they should.

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These are not your ordinary wildlife tags. They are sophisticated pieces of equipment that record temperature, light and other factors throughout an animal’s entire life cycle. They float to the surface after the animal dies and transmit that recording by satellite to Oregon State University ecologist Markus Horning.

In 2005, Horning began tagging young Steller Sea Lions in Prince William Sound and as those animals die, he is putting together a pattern that points to sharks – sleeper sharks, sometimes known as mud sharks – as the cause.

“We don’t have proof of sleeper shark being a major driver of the sea lion population, but we have indirect evidence that suggests we need to consider that,” Horning said.

(Photo courtesy Bruce Wright, Aleutian Pribilof Islands Association)
(Photo courtesy Bruce Wright, Aleutian Pribilof Islands Association)

Prince William Sound is a long way from the Western Aleutians, and Horning only has a small sample so far. His team tagged 45 sea lions and by now 17 of his tags have popped up, indicating an animal died. Two didn’t hold their data set – the rest did.

“In 15 out of the 17 instances we actually got the full data set, so we can tell what happened to those 15 animals, and, lo and behold, all 15 of those young sea lions died by predation,” Horning said.

All of them. They can tell that by the temperature and light recordings. They are warm and dark when they are implanted, but show changes when the sea lion dies or is consumed. And in four of those recordings there is evidence of who the predator was.

“I think, and that’s a bit of an interpretation, but I think that most likely Pacific sleeper sharks ate those Steller sea lions,” he said.

As Horning says, that’s interpretation. But the tags do provide enough information to use a process of elimination. If the sea lion dies and the tag is freed, it should sense light. But if the tag shows continuing darkness, then the tag must still be inside somebody.

“We think what happened was that those tags that remained dark were actually swallowed by the predator that killed the sea lion,” Horning said.

So that could have been any predator. But the four tags in question also show cold temperatures after the sea lion died.  That eliminates warm blooded predators like killer whales, or great white sharks, but leaves cold blooded, deep dwelling sleeper sharks – nocturnal predators. If there is any scientist who knows about the mysterious sleeper shark, it’s Bruce Wright of the Aleutian Pribilof Island Association, who’s been following Horning’s work. He’s not surprised by the findings.

(Photo courtesy Bruce Wright, Aleutian Pribilof Islands Association)
(Photo courtesy Bruce Wright, Aleutian Pribilof Islands Association)

“I’m not surprised at all, no.  I predicted a couple of decades ago that sleeper sharks are not mud sharks and don’t eat mud, despite what a lot of fishermen will tell you, that they’re top predators,” Wright said. “And when I’ve looked inside of sleeper shark stomachs I’ve found chunks of seals, big chunks of great whales, salmon, whole salmon that they’ve sucked down, adult salmon.”

There is not much money to study the sleeper shark, but maybe there should be. Horning says all he’s able to provide is a clue that they might be contributing to the sea lions’ recovery failure – just a clue that leads to a great many un-answered questions, like whether the predators have always been here or have expanded their range to include Alaska.

“We also really don’t know how many sleeper sharks are out there. Are their numbers increasing or are their number decreasing? Which of the sleeper sharks are eating sea lions, if they do?  Is it the big ones, is it the medium size ones, small ones? Probably the big ones. Are there big ones in Prince William Sound and the Gulf of Alaska? We don’t know,” Horning said.

Horning says a study that showed no evidence of sharks consuming juvenile sea lions in the Western Aleutians does not rule out his theory.  Sea lion would be a rare item in the shark diet, because there are so few sea lions there, and nobody knows how many sharks there are.