National Grid must incorporate new controls into its Northport power station to reduce the plant’s destruction of aquatic life by 85 percent by June 2021.

The state Department of Environmental Conservation outlined the new conditions this month in response to National Grid’s 2011 request for a state-mandated pollutant discharge permit. When operating, the nearly 50-year-old plant draws more than 939 million gallons of cooling water each day from the Long Island Sound, and with it an estimated 8.4 billion fish eggs and larvae each year. In addition, the practice injures up to 127,118 fish, which can get trapped in intake filters and other plant gear.

The DEC permit requires that National Grid install a new system so that live fish can be returned unharmed, an “impingement barrier net” to prevent aquatic life from being sucked into the cooling system, a new system of screens and barriers, and a variable-speed pump.

National Grid spokeswoman Wendy Ladd said the company “will be installing the new technologies over the next five years, an investment of $60 million to $70 million.” She later sent a statement saying that those cost figures were “inaccurate,” and that the new fixes would be “a fraction of the cost” of more expensive cooling towers but it was “too early to speculate on a number.”

She didn’t comment on how upgrades would be paid for. The plant is operated under a 20-year contract to the Long Island Power Authority, which covers maintenance expenses.

Environmentalist and former Connecticut bayman Terry Backer and his group, Soundkeeper, last year sued National Grid and the DEC to eliminate the cooling system at the plant, which they called a “fish-killing machine.” Backer died in December.

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Opponents of the old system want National Grid to install modern closed-loop cooling towers which would vastly eliminate intake from the Sound and the impact on fish. But the DEC said installation of such a system was not feasible because of space constraints at the plant.

Reed Super, an attorney for Soundkeeper, said the new permit requirements were a tribute to Backer, whom he called the “most hard-working and diligent protector of the Sound.” The lawsuit was withdrawn earlier this year when the DEC issued a draft permit.

National Grid’s permit to operate the plant using the old cooling system expired in 2011, but it was able to continue to operate until the DEC issued a final ruling on the permit.

Newly named DEC Commissioner Basil Seggos said in a statement that the agency “worked hard to strike the right balance for the people, businesses and marine ecosystems of Long Island.”

The Northport plant, Long Island’s largest, has a capacity of 1,452 megawatts, enough to power more than 100,000 homes, but it has been used less often as newer power sources brought access to cheaper electricity.

The DEC has yet to issue a final permit for a second National Grid-owned plant in Island Park. Super said the permit has been delayed even longer than the Northport plant’s, in part because National Grid has proposed repowering the plant with a modern new cooling system. The E.F. Barrett plant in Island Park draws 294 million gallons a day from Western Bays during operation, destroying some 906 million fish eggs and up to 176,044 fish a year.