A fast-moving pack of 200 to 300 hungry sea lions prowled Berners Bay this spring, hunting together in a manner that has astounded scientists.
"They were chasing something up and down the bay in football lines of hundreds of lions, driving them, working in a big swath," said research ecologist Mary Willson. "They'd come up for a minute, breathe and swim, then all go down for about two minutes, then come up again several hundred yards further on. It's all very synchronized."
Willson, a former Forest Service scientist now working as a consultant, has studied predator behavior in Berners Bay, 40 miles north of downtown Juneau, since 1996. She and other scientists have witnessed a phenomenon never documented before - Steller sea lions hunting together shoulder-to-shoulder in enormous packs. It contradicts the generally held belief that Steller sea lions hunt solitarily or perhaps in pairs.
"It was really a surprise to us," said research ecologist Scott Gende of the federal Forestry Sciences Laboratory in Juneau. "This was the first real observation that was quantified, a massive congregation of sea lions cooperatively foraging together."
On the Rocks: Steller sea lions amass at a haulout on Benjamin Island, 30 miles north of Juneau.
MICHAEL PENN / THE JUNEAU EMPIRE
Steller sea lions are thriving in Southeast Alaska. But in Western Alaska, the animals are in serious decline. Populations in the Gulf of Alaska and the Aleutians have plummeted 80 percent in the past 25 years, as populations in Southeast Alaska have slightly increased.
"It's not clear what makes the difference," Willson said. "That's one of the things we'd like to know."
This decline in Steller sea lions has resulted in lawsuits and federal action, and has profoundly affected the billion-dollar groundfish industry in Western Alaska. Scientists hope the sea lions of Southeast will offer insights to why populations are dwindling in other parts of the state.
Gende described one of the first times he and biologist Jamie Womble saw the lions in Berners Bay hunting cooperatively. They were in a Zodiac raft one morning when they saw what looked like the wake of a large boat. As it came closer they saw it was a half-mile-long line of hundreds of rapidly porpoising sea lions. Swimming on the surface just a few yards apart, the entire group suddenly disappeared beneath the water. Gende said the lions stayed down for nine or 10 minutes before coming up.
"You would not be able to tell that two or three hundred sea lions were foraging when they were down," Gende said. "Then they'd pop up in a different area of the bay."
They watched the animals for several hours as they moved through the bay. He said the lions would swim together on the surface in a long line for 6 to 15 seconds, then dive simultaneously.
"Sometimes individuals would break off and others would join," Gende said. After an hour the number of lions participating decreased, and after two hours no more lions were seen foraging in a group.
The morning hunting sessions were followed by afternoon siestas. Gende said the sea lions formed large rafts of 10 to 80 sleeping or resting individuals in the middle of the bay.
Swimming in packs: Scientists believe sea lions such as these hunt in Berners Bay in the spring before heading to rookeries on the Outer Coast, where they breed and have pups in June and July.
MICHAEL PENN / THE JUNEAU EMPIRE
"In the afternoon they'd raft up in these huge groups, sleeping in the surface of the water," he said. "They'd float by our boat at arm's length. We'd see parts of the hooligan hanging out of their mouths, and they'd spend hours napping in the rafts, in tight little packs."
It wasn't just sea lions that brought Willson, Gende and other scientists to Berners Bay. They've been documenting the numbers and types of predators attracted to the area to feed on the spring run of hooligan, slim, 7-inch long fish also know as eulachon. Hooligan, a type of smelt, spawn in late April or early May in the fresh water rivers of Berners Bay.
"It's absolutely invaluable as a resource around here," Gende said.
Tens of thousands of predators are drawn to the bay to prey on the oily, nutritious fish. Willson said scientists have counted as many as 1,000 eagles, 800 seals and sea lions and 50,000 gulls in the bay during the hooligan run.
The spring phenomenon offers valuable insight to the importance of what's called "pulse prey" for sea lions. Pulse prey isn't a staple of an animal's diet but provides an important food source in the brief time they're available. It could be critical to pups and young lions still learning to hunt and for adults preparing to breed.
"The timing is such that they could be loading up on this resource before heading out to the Outer Coast, for the most energetically taxing time," Gende said, referring to the lion's early summer breeding and pupping season.
"Our idea is these little fish that are dripping with fat are important to females who are lactating and males who fast while guarding their harems," Willson said, adding that nursing young takes more energy than anything else in a female's life.
Southeast lions breed in May and June on rookeries on the Outer Coast. They disperse in late summer, and the inside waters of northern Southeast are home to several thousand lions.
From September to May, 300 to 600 sea lions use a haulout on Benjamin Island, about 10 miles south of Berners Bay. As many as 1,000 have been counted at a haulout at Grand Point near the Katzehin River about 40 miles north of Berners Bay. There are smaller haulouts all up and down Lynn Canal. The lions move throughout the area, and scientists suspect these are the same animals they've seen hunting together in Berners Bay in April.
"We'd have to mark individual animals to be sure," Willson said.
Willson and Gende said it's possible the sea lions may be feeding on herring as well as hooligan. But the cooperative hunting occurred in 1997 and '98 as well, years herring did not spawn in the bay. Evidence indicates the lions were after the oilrich hooligan.
"We would see blood and oil on the water where they'd been feeding, loose body parts, and see hooligans in the lions' mouths," Willson said.
Willson also saw another behavior she said is unusual for sea lions - lions entering the river systems, apparently in pursuit of the schools of hooligan.
"They even chase them up the rivers for several kilometers," she said. "You see these great big animals humping themselves over the tide bars. They look pretty silly, but they're serious predators."
Willson said they're also curious predators. She said one sea lion swam up and gently bit down on the Zodiac, but she doesn't think it was being aggressive.
"He just sampled it," she said. "It was more out of curiosity."
This spring Willson was in the bay with Mike Sigler, a fish biologist with the National Marine Fisheries Service in Juneau. They were over an underwater gully near the mouth of the Berners River when they found a mass of hooligan below.
"We saw it on the depth finder on the boat, then dropped an underwater camera and estimated between 10 and 20 million hooligan in a blanket on the bottom," she said. "They were in about 40 or 50 meters of water."
"That's amazing, that magnitude," Gende said.
Sigler has been to Berners Bay seven times this year. He's working with biologist Womble, estimating the numbers of eulachon in the spawning run, documenting how long they're available and how the sea lion numbers change in relation to the eulachon numbers.
"How long do they get to eat at the eulachon restaurant?" Sigler said with a laugh. "This oily, greasy restaurant."
Scientists suspect that sea lion diets may be the reason why populations are declining in Western Alaska, and there may be several reasons why the western lion's diets are changing. Gende said comparing the diversity of diets between eastern and western populations may provide answers.
Riley Woodford can be reached at email@example.com.
Juneau Empire ©2014. All Rights Reserved.