WHITE PLAINS — The huge numbers of fish sucked to their death by the cooling system at the Indian Point nuclear plant prove that the system harms the Hudson River environment, a state official has ruled.

The finding by the assistant commissioner of the Department of Environmental Conservation, J. Jared Snyder, is a victory for plant critics who claim that up to 1.2 billion fish and eggs are killed each year as the plant continuously draws in river water for use as a coolant.

“For decades, Indian Point has maintained that its cooling systems have no impact on Hudson River fish,” the general counsel to the environmental group Riverkeeper, Robert Goldstein, said. “At long last, the DEC has pout an end to this fiction.”

Mr. Snyder said that even the lowest estimate of fish deaths — 900,000 annually — “represents excessive fish kills” and establishes an adverse environmental impact.

The ruling, issued this month, means the plant’s owner, Entergy Nuclear, may no longer raise the environmental-impact issue as it battles the state’s order to build costly towers that recycle cooling water and make big river intakes unnecessary. Entergy had argued that the river’s adult fish populations have been stable.

The towers, known as closed-cycle cooling, could cost Entergy more than $1.6 billion.

A company spokesman, Jim Steets, said yesterday that Entergy will continue to argue against the towers, using several other issues that Mr. Snyder said can be raised at hearings that will start next year. Those issues include whether cooling towers are the best available technology for the money, the visual impact of the towers, and the effect of outages that might be caused by construction.

“This ruling actually gives us the opportunity to bring in the argument that this could impact electric power reliability in New York, that the actual construction challenges may make it unfeasible,” Mr. Steets said. “We still have to determine if building cooling towers is in the public interest.”