In one of the toughest judicial actions taken under the Endangered Species Act in the last decade, a federal judge yesterday banned trawl-net fishing for pollock, cod, and other bottom fish in large ocean areas off Alaska, citing a need to protect endangered Steller sea lions. The sea lions forage for the same fish chased by factory trawlers. As factory-style fishing has grown, the Steller sea lion population has dropped from 230,000 in 1965 to fewer than 40,000 today. Enviros, who sued the federal government two years ago seeking greater sea lion protections, were thrilled by the ruling. The commercial fishing industry is predicting that the ban, which will stay in place until the government formulates adequate sea-lion protection plans, could cost $100 million.