ANCHORAGE - They're brainless predators that can grow to the size of basketballs, pulsing on the currents of the eastern Bering Sea.
At the height of spring growth, they can become a net-clogging nightmare for fishermen, an armada of translucent globes with 19-foot tentacles and an appetite for fish eggs, plankton and tiny pollock.
And for reasons no one can explain, the numbers of these jellyfish off southwest Alaska soared tenfold during the 1990s to unprecedented levels in 2000, only to crash during the past three years.
Federal biologist Ric Brodeur, who investigated whether the jellyfish population increased because of shifts in ice cover, currents, climate or declines in forage fish, said he is unsure why the region's population grew so fast and then suddenly shrank.
"There were so many jellies, they were pretty much closing down fisheries" in some regions of the Bering Sea, said Brodeur, now working at the Northwest Fisheries Science Center.
Figuring out what happened won't be simple. Scientists suspect multiple causes, said federal biologist Gary Walters, at the Alaska Fisheries Science Center, in an e-mail message.
"I wish I could tell you I understand why, but that would not be true," Walters wrote.
But their research about jellyfish, combined with a new study by University of Alaska Fairbanks graduate student Steve Whitney, hints at a durable creature that might play a far more complex role in the Bering Sea's marine ecosystem than previously thought.
"It's kind of dangerous to make any assumptions about them," Whitney said. "They were ignored for so long, and we still don't know much about their life cycle or ecology."
One of the most ancient creatures in the sea, jellyfish are gelatinous animals of alien beauty and a mysterious life cycle that includes time spent as a tubelike polyp on the sea floor.
The most common species in the eastern Bering Sea, Chrysoara melanaster, spends its adult season as a medusan bell with 16 brownish rays. It uses stinging tentacles to catch krill, tiny crustaceans, eggs and juvenile fish from the turbulent sea and pull them into its oral cavity. Yet some fish later find refuge under its tentacles and don't get stung.
An area off the Alaska Peninsula known as the "Slime Bank" had always bloomed with jellies during spring, but by the mid-1990s, fishermen were reporting more than ever.
As Brodeur, Walters and other scientists documented the surge in jellyfish numbers, they found that it gradually shifted northwest toward the Pribilof Islands. They believed that native species accounted for the growth. Trawl surveys by the National Marine Fisheries Service found that nearly all of it came from Chrysoara melanaster.
But why did they suddenly thrive?
In scientific papers, the biologists discussed possible links to changes in sea temperature, atmospheric circulation, sea ice cover or release from competition with forage fish like herring and capelin.
In a paper published in 2002, Brodeur and two authors concluded that Chrysoara melanaster ate largely the same diet of tiny crustaceans as juvenile pollock and other forage fish. So they were potential competitors for the region's most important commercial fish species.
Yet the rise and fall of the jellies doesn't track closely with other species, or closely match the most recent climate changes.
"I have talked to other scientists here at the Alaska Fisheries Science Center and no one knows of any organism that has had the same pattern of abundance over this time period," Walters said. "There certainly doesn't seem to be any correlation to fish."
During dives of an unmanned submersible in the late 1990s off the Pribilof Islands, Brodeur observed another surprising detail: juvenile pollock were sometimes hiding within the tentacles of melanaster 30 to 120 feet below the surface, apparently to avoid other predators.
"They're only found (together) during the daytime, and then at night, (the pollock) come up to the surface to feed," Brodeur said. "The jellies eat them when they're young, but they also offer shelter to them when they're a little bit older."
Intrigued by the increases and what they might mean for fish stocks, Whitney decided to investigate what jellyfish were actually digesting as research for his master's degree. With about $212,000 in grants from the National Undersea Research Program and the Pollock Cooperative Conservation Research Center, Whitney chartered a fishing boat and went out from False Pass in April 2000 to collect 100 prime jellyfish.
At first, Whitney tried sending down divers to snatch them. It didn't work. "It was basically like swimming in pea soup," he said. Above the sea floor "was a ground fog of krill." They finally were able to collect and preserve enough jellies by towing very slowly with a special net. About 75 percent were melanaster, which fortunately didn't sting humans.
"It's kind just a glob of Jell-O the size of a basketball," Whitney said. "I've never been stung by a melanaster, and I've been waist deep in them."
Over the next few years, Whitney systematically dissected each creature and catalogued what he found.
"The stomachs are not like a vertebrate's or a fish with a discrete sack," he said. "It's kind of like sorting through spaghetti with the consistency of masking tape."
Whitney, who is finishing his analysis this winter and working as a biologist in Juneau, found a surprising range of food in jellyfish guts. Along with tiny crustaceans, krill, worms and small fish, Whitney also discovered eggs from pollock and a few species of flatfish.
"Even though they seem to be eating a lot of pollock eggs, they don't seem to be affecting the number of pollock," he said. Perhaps the negative impact of eating pollock gets canceled out by giving them shelter.
"When you look at (jellyfish), you really have to think of the ocean differently than when you look at fish," Whitney said. "They don't fit the mold of how things work."
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