State water board regulators have released a plan to stop power companies from using ocean water to cool their machinery.
Environmentalists say the practice destroys too much sea life, but utility advocates argue the impact is minimal.
Screens prevent larger animals from entering the plants, but fish can die while trapped against these barriers. Anything smaller than the openings in the screens, including millions of tiny fish larvae, can enter the power plants and also die.
Federal rules discourage new operations from drawing in seawater for such cooling systems.
The draft plan released Tuesday proposes that existing plants from Eureka to San Diego reduce the use of ocean water by 93 percent. The board is expected to vote on the new policy on May 4.
Diablo Canyon nuclear power plant and the gas-fired power plant in Morro Bay use once-through cooling systems in their operations.
Pacific Gas and Electric Co., which owns Diablo Canyon, is reviewing whether to apply for exemptions to any plan to restrict such cooling.
In one exemption, PG&E could argue that the rules conflict with federal Nuclear Regulatory Commission safety requirements. In another, the utility could argue that the cost of the new rules is "wholly disproportionate to the environmental benefits."
In either case, the utility would have to propose alternate methods of reducing ocean impacts. The Morro Bay plant would have until 2016 to be in compliance; Diablo Canyon's deadline is 2022.
PG&E biologists have argued that the effect of larvae mortality is minimal because most would not reach adulthood under normal circumstances.
Of the state's 19 power plants that use once-through cooling, others have more dramatic impacts on wildlife. Some plants use narrow offshore pipes to draw in their cooling water.
These pipes create enough suction to draw in fish and marine mammals. The worst is the state's other nuclear power plant, San Onofre in Southern California.
State records show that between 1978 and 2000, 64 harbor seals, 153 sea lions and four sea turtles were found dead in San Onofre's intake structure. Power plants in Ormond Beach and Redondo Beach, both in Los Angeles County, have killed lesser numbers of seals.
Diablo Canyon avoids this problem because its intake pipes are large enough that the suction they create is weak enough that fish and animals can swim out of it. No marine mammals have died in Diablo Canyon's intake structure although divers removed seven live sea turtles from the structure from 1985 to 2001.