FAIRBANKS (AP) -- Yukon River salmon stocks have been dwindling to dangerous levels in the past decade. But ask the biologists where all the fish have gone, and you get only educated guesses.
''Here's the basic problem,'' said National Marine Fisheries Service scientist Jack Helle in Juneau. ''There's basically nothing known about Yukon chum and kings and where they go once they're in the ocean.''
Like motorists passing through a bad neighborhood, kings and chums that originate in fresh waters apparently are being picked off once they reach the Bering Sea.
Just how that happens remains murky. And there's no research being done on the mystery.
Helle, program manager for ocean carrying capacity research at NMFS's Auke Bay Lab, has spent three years studying Bristol Bay salmon once they leave fresh water.
He would like to do the same for Yukon stocks.
Drawing from several disciplines, researchers believe that there are three major factors playing into the decline:
n The environment: The rich and diverse waters of the Bering Sea are undergoing a series of changes on several levels. Temperatures are rising, which has led not only the shrinking of the ice pack, a vital link in the food chain, but to a giant algae bloom. ''It looks like cream of broccoli soup,'' Helle said. ''It's very sterile out there. The fish that are in that bloom have very little in their stomachs. It could be that the Yukon-Kuskokwim juveniles have to go through that.''
n Hatcheries: These fish producers around the Pacific Rim are increasingly becoming suspects. Hatchery-reared chums may well be competing with the wild chums in the Gulf of Alaska.
n Bycatch and piracy: A popular conspiracy theory says pirate factory trawlers from Russia and China swoop in regularly and steal Alaska fish. But while the Coast Guard does occasionally turn up rogue ships on the high seas, the real danger to salmon comes from legitimate fishing.
''There's been some incredible numbers as far as bycatch with some of these (pollock) draggers,'' Tanana fisher Charlie Campbell said.
''They've got no interest in kings. In fact, they have to get rid of kings. They can't possess them. They just dump them, so who knows what's going on there.''
Every little bit of research adds to the picture. With so little known about Yukon fish, a single study can completely change the way scientists think of salmon.
Technology is helping. In a recent study, scientists implanted depth-sensing tags in salmon. Before that, scientists believed that salmon spent their entire marine lives near the surface.
''We've gotten a gold mine of information,'' Helle said. ''It's really changing the way we think of their behavior.
''They are at the surface at night. During the day, they go way down deep, sometimes 200 meters.''
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