Experts say a dam at Phillips Millpond in Smithtown prevents...

Experts say a dam at Phillips Millpond in Smithtown prevents alewife fish from returning to their upstream spawning areas. Credit: NYSDEC

Conservationists will begin engineering studies next year for a fish passage on the Nissequogue River intended to help tens of thousands of ocean-run alewife bypass a dam built in Colonial times, returning to their inland spawning grounds for the first time in three centuries.

The speedy, iridescent alewife, a type of herring, feeds predators ranging from ocean mammals and birds to the trout that were once abundant in Smithtown’s waters. But researchers began observing drastic declines in river herring populations starting in the 1990s nationwide due to overfishing, habitat degradation and dams like the ones that block almost every river on Long Island. The National Marine Fisheries Services listed the fish as a “species of concern” in 2006. Its experts are now deciding if alewife and another type of river herring should be listed as endangered or threatened.

Work on the Nissequogue River, which is one of the North Shore’s largest and extends to Hauppauge, would take place near the north end of Phillips Millpond. It would restore access to spawning grounds as far inland as Stump Pond in Blydenburgh Park, roughly 6 miles from the Long Island Sound.

The monthslong $209,000 planning phase of the project will be funded by grants from New York State Department of Environmental Conservation and the Long Island Sound Futures Fund. The Connecticut Fund for the Environment’s Save the Sound project will oversee the work.

“This is a unique opportunity on the Nissequogue,” said Gwen Macdonald, the Connecticut Fund’s director of habitat restoration. “You’re making this permanent change in one specific location that impacts species of fish that travel all the way out to the Atlantic Ocean.”

The passage will help fish overcome rushing water and a roughly 6-foot change in elevation at the dam’s spillway. Design of the passage hasn’t been determined yet, Macdonald said, but similar projects have involved building a series of pools allowing the fish to bypass the dam, or installation of a prefabricated metal chute like the one in place at Argyle Lake in Babylon Village.

The total alewife run on Long Island probably averages 150,000 fish, according to the DEC. The runs at the Peconic River and Alewife Creek in Southampton account for about 50,000 fish each, with runs in streams on the North and South shores and Peconic Bay accounting for the remainder. The count on the Nissequogue is unknown, though researchers know from video footage that the alewife is present.

Long Island waterways, like those in most of the settled Northeast, are dotted with dams or remnants of dams used for milling lumber and grain — Phillips was used for both — or flood control. A structure as short as a foot can block the alewife. “They don’t really jump like salmon do,” said Victoria O’Neill, the DEC’s Long Island Sound habitat restoration coordinator.

What the alewife lacks in athleticism it makes up for in determination, O’Neill said. It matters little that no alewife has made the passage in centuries. “If you give them a way to do it, they will get over the dam.”

Since 2008, according to the DEC, 13 fish passages have been built on Long Island.

Brad Harris, Smithtown’s historian, said that he was eager to see a return of the alewife, which supported a food web rich into the 20th century with trout, bass, perch and the like. He described a fisherman’s heaven that fed locals and drew wealthy Manhattanites.

“I hope it works,” he said of the fish passage. “I’d like to see the waters in Smithtown returned to a sportsman’s paradise.”

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