NEW YORK -- The Environmental Protection Agency must force power plants to protect fish and other aquatic life even if it's expensive, a federal appeals court said in a ruling favoring states and environmental groups.

The decision late Thursday by the 2nd U.S. Circuit Court of Appeals concluded that it was improper for the EPA to let power plants circumvent environmental laws - for instance, restocking polluted water with new fish instead of paying to upgrade their technology.

It said the EPA's decisions must "be driven by technology, not cost," unless two technologies produce essentially the same benefits but have much different costs.

"EPA's goal is to protect fish and the ecosystem while meeting the nation's need for reliable energy sources," said Benjamin H. Grumbles, the agency's assistant administrator for water. The agency was reviewing the decision, he said.

The ruling drew praise from environmental groups and six states that had sued.

"This decision is a strong and stinging rebuke of the Bush administration's underhanded practice of issuing rule changes to undercut environmental laws," Connecticut Attorney General Richard Blumenthal said in a statement Friday.

The other states involved are Rhode Island, Delaware, Massachusetts, New Jersey and New York.

They sued after the EPA published regulations in July 2004 describing how power plants must protect aquatic life when they use water from bays, rivers, lakes, oceans and other waterways for cooling.

Scientists say fish, larvae and eggs are killed in the water-cooling process, which is used heavily in states with many older, mostly fossil-fuel plants.

The appeals court previously rejected arguments that some species are nuisances and require eradication. The court had also dismissed the claim that other species respond to population losses by increasing their reproduction.

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