FISH stocks and populations of marine mammals and birds are declining in the Bering Sea region, where the biggest commercial fisheries in the United States overlap with a key migration area for whales, sea lions, walruses, and seals and some 200 bird species from all seven continents.Environmentalists say the ecological decline is a chain reaction to booming growth in the $1 billion-a-year US groundfish industry and unrestricted harvests in international waters. Government officials are warning the Bering Sea, the world's last great commercial fishing grounds, is being overharvested and some wildlife species dependent on the waters dividing Asia and North America could face extinction. Unrestricted harvests in the international zone controlled by neither the US nor the Soviet Union have devastated stocks in US waters off Alaska, state fisheries managers say. Because of stock reductions, commercial pollock fishing will likely be banned next year in the Bering Sea's Bogoslof region, a triangular fishery zone surrounding tiny Bogoslof Island in the Aleutian chain. The region usually accounts for 15 percent of the annual Bering Sea pollock catch. The 20,000-to-30,000-square-mile zone is usually harvested during the early part of the season, when fish are carrying the roe that is sold as an expensive delicacy in Japan. Bogoslof pollock stocks were down by 75 percent in September, according to the North Pacific Fishery Management Council. Alaskan officials put the blame for the Bogoslof declines squarely on the Japanese, Chinese, South Korean, and Polish fleets that for years have fished without restriction in the "Donut Hole the 50,000-square-mile international zone lying between US and Soviet waters. By some estimates, about 80 percent of the pollock in the Donut Hole originate in Alaska's Bogoslof region. Fearing for fish stocks within their own 200-mile exclusive economic zone limits, US and Soviet officials are pushing for a treaty that would sharply curtail what they say is overharvesting by the other four nations' fleets. The first session of negotiations between the six nations was held in February in Seattle; another session is scheduled later this month in Washington, D.C. David Bentson, international fisheries manager for the Alaska Department of Fish and Game, is one of the US negotiators. He says the four nations that fish the Donut Hole have rebuffed US and Soviet attempts to establish a management regime. US fisheries officials fear that stocks will be wiped out before they can put a Donut Hole management plan into place. "By the time the economics drives" the four nations out of the international waters, Mr. Bentson says, "they will have done real damage to our fishing resources." Through the first six months of this year, the Donut Hole pollock catch was 170,150 metric tons, less than half the 388,748 metric tons caught during the same period last year, according to the National Marine Fisheries Service (NMFS). Total Donut Hole pollock catches had jumped from 363,400 metric tons in 1985 to over 1.4 million metric tons in 1989, according to the Alaska Department of Fish and Game. Environmentalists, meanwhile, blame the US-flagged fleet of deep-sea trawlers for sharp declines among Steller sea lions and other marine mammals. Once plentiful along Alaska's rocky shores, Steller sea lions may be headed for extinction, scientists warn. If the huge mammal is placed on the endangered species list, that could shut down the entire groundfish industry, says Dick Merrick, a Seattle-based NMFS zoologist. Statewide, the population totals about 37,000, a decline of more than 70 percent since the mid-1970s. From the western Gulf of Alaska to the Aleutian Islands, the population has dropped about 85 percent since 1960, NMFS figures show. In the critical area stretching from the Kenai Peninsula to the central Aleutian Islands, the Steller sea lion population dropped another 5 percent in the past year, a NMFS survey this summer showed. The sea lions are particularly vulnerable because the remaining population is grouped in isolated clusters rather than spread out evenly over the region, Mr. Merrick says. Scientists this spring postulated that herring and capelin are the proper food sources for the sea lions, but they had to turn to pollock because of overaggressive harvests by the Japanese in the 1950s and 1960s. While NMFS declines to draw links between the groundfish industry and pollock decline, Greenpeace has not hesitated to do so. The environmental group sued NMFS unsuccessfully in US District Court in Seattle to block further pollock fishing in the Gulf of Alaska, where NMFS hiked the 1991 pollock quota by over 40 percent, from 73,400 to 103,400 metric tons. Scientists also are troubled by bird declines in Alaska's Pribilof Islands, a region famous among bird watchers for its huge colonies of rare species. The red-legged kittiwake, one of several species unique to the Bering Sea, is located almost entirely on St. George Island in the Pribilofs. But by 1989, the red-legged kittiwake population had dropped to about half its 1976 total of 250,000, an April US Fish and Wildlife Service study showed. The study, based on 1989 surveys, also discovered abnormally high reproductive failures among the more common black-legged kittiwakes, as well as a complete reproductive failure for both kittiwake species on St. George Island. To Larry Merculieff, an Aleut leader who grew up on neighboring St. Paul, the northern Pribilof Island, the scientists' focus on individual species understates the danger to the region's ecology. A former president of the village native corporation, state commerce commissioner, and now St. Paul's city manager, Mr. Merculieff argues that fish and wildlife declines and reproductive failures throughout the region are linked. He points to NMFS's measured 5 percent annual fur seal and harbor seal declines at St. George Island and throughout the area. This summer, St. Paul hosted a famous Soviet bird specialist who confirmed Merculieff's suspicions. Alexander Golovkin, head of the Soviet Union's endangered species program, concluded after three months on the island that the major bird species there have declined by 80 percent since the mid-1970s, and the reason is lack of fish. Merculieff says Mr. Golovkin also believes the Bering Sea holds four distinct stocks, each requiring different harvest times and limits. So federal fisheries officials are erring by managing Bering Sea pollock as a single fishery. "By managing them as a single unit, we increase the probability of overfishing certain stocks," Merculieff said. Nonetheless, the North Pacific Fishery Management Council, which sets area-wide fishing quota each December, may be pressured by seafood companies to expand the overall harvest. Demand and prices for surimi, the seafood paste made with pollock and other groundfish species, are up sharply. Some biologists worry that economic forces will overwhelm ecological considerations, and that the Bering Sea will go the way of the overharvested Atlantic. "What we're doing in the Bering Sea is analogous to cutting down the entire Costa Rican rain forest in a year," says a biologist here. "Humans are decimating the oceans, and no one cares."