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Sea Lions may be decimating seal population in Glacier Bay

SEAFOOD.COM NEWS [Anchorage Daily News] Anchorage - April 23, 2007- By ALEX deMARBAN--Steller sea lions that thrash harbor seals to death in their powerful jaws have become surprise suspects in the mysterious harbor seal decline in Southeast Alaska's Glacier Bay National Park.

The massive, sharp-toothed sea lions are known for eating fish, but evidence of attacks on harbor seals, until recent years, has been very rare, said Beth Mathews, a University of Alaska Southeast professor.

In Glacier Bay, there's no record of any reports until 1995. Since then, the reports have become frequent enough to suggest that the attacks are putting a large dent in harbor seal numbers, she said.

That's a new twist in the decline that's had scientists scratching their heads for years. Some people have blamed cruise ships for stressing the seals, forcing them off ice floes and into the water. That might drain energy reserves, resulting in lower reproduction or reduced survival, the theory runs.

Raymond Sensmeier, a Native seal hunter who does harbor seal surveys near Yakutat for the Alaska Native Harbor Seal Commission, said he's never seen a sea lion killing a harbor seal. He thinks cruise ships are a bigger factor in the decline than sea lions.

'It's cruise ships along with sightseeing charter boats and catamarans,' he said.

Seal counts have fallen from about 6,200 in 1992 to 2,500 in 2002, said Mathews. State surveys conducted since then show continued decline.

The ships, tightly regulated in the park to keep them from coming too close to seals, shouldn't have caused such a large drop, she said. As far as she knows, no one has ever documented a seal death directly from a cruise ship.

Same thing for commercial fishing, she said. The National Park Service heavily curtailed fishing in the park beginning in 1999.

On the other hand, there have been 13 reports of sea lions attacking seals between 1995 and 2006, some in open water, some on sandbars near Spider Island, said Mathews.

LAND, SEA ATTACKS

If the seals are on land, a sea lion attack stirs a 'mass exodus' of seals plunging into the water. The sea lions -- adult males average about 1,250 pounds -- snatch much smaller seal pups or juveniles with their sharp teeth. Seals weigh about 25 pounds at birth and average 180 pounds as adults.

The sea lions shake the animals from side to side like they do with salmon, though probably for longer periods, Mathews said. Some of the action occurs underwater with lots of splashing on the surface.

'It's obviously bloodier,' she said. 'What I've seen is 20 minutes and up to an hour or so of thrashing.'

Mathews is studying the attacks with Milo Adkison of the University of Alaska Fairbanks and plans to complete the analysis this summer.

She has puzzled over the seal decline for years, she said, wondering why few carcasses were found, and why the animals showed no obvious signs of disease or starvation. Aerial photos of surrounding areas, plus DNA studies indicating seals return to the same breeding area each year, suggest the decline wasn't caused by seals leaving the bay.

A conservative calculation, based on reported attacks, shows that sea lions often remove more than 50 percent of pups from the sandbars near Spider Island, the bay's largest terrestrial breeding area. 'It's hard to imagine it didn't have an effect on the population,' she said.

Bruce Paige worked for the park between 1968 and 1994, when he oversaw up to 20 seasonal naturalists who recorded daily observations and talked frequently with commercial fishermen. He never received a single report of a sea lion attacking a seal.

'It's pretty unusual that that's become a potential problem,' Paige said.

Park rangers and other park employees have reported sea lion attacks nine times since then, Mathews said.

ATTACKS A MYSTERY

Seal hunters in the area say they've never seen such attacks, she said, but some elders are aware of it. Also, a 1980s study in the Gulf of Alaska showed that two of 220 sea lion stomachs contained seal remains.

If the attacks have actually increased, she doesn't know why, she said.

There are more sea lions to do the attacking, she said. As seal numbers in the bay have fallen, sea lion numbers have increased, going from 50 to 100 in the early 1990s to more than 500 in 2004, Mathews said.

Older sea lions may be teaching younger ones to snatch and kill seals. She once saw two young sea lions watching an older one kill a seal.

'I wouldn't be surprised if those two later in life tried it out,' she said.

It's worth considering whether Glacier Bay sea lions learned the behavior from watching their peers and are hurting seal numbers, said Peter Boveng, leader of the polar ecosystems program at the Alaska Fisheries Science Center in Seattle.

'It's now something we also need to include among various hypotheses,' he said.

He's watched leopard seals in Antarctica change their feeding habits after a few of the animals discovered they could get into a breeding lagoon for Antarctic fur seals. The leopard seal attacks increased during the multiyear study, causing fur seals to decline.

John Sackton, Editor And Publisher
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