Alewives have swum from the Carlls River into Argyle Lake in Babylon Village for perhaps the first time since the waterway was dammed more than 120 years ago, conservationists said.

Video equipment at the mouth of a "fish ladder" built to bypass the dam recorded the first alewife -- distinguished by its forked tail and tall dorsal fin -- making its passage shortly after midnight on the morning of April 6.

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Two more alewives followed, said Enrico Nardone, executive director of the Seatuck Environmental Association. "This is what we were hoping for," he said. "There's a lot of great habitat there for them."

Millions of alewives once made the annual journey from Long Island's ocean waters to its rivers and lakes to spawn, but those numbers dropped as many of the routes were blocked by dams or other man-made structures in the 1800s, Nardone said.

Fish ladders like the one in Babylon Village and elsewhere on Long Island -- there are similar projects on the Peconic River, Massapequa Creek and Beaver Lake at Cold Spring Harbor -- could reverse that slide by restoring access to the spawning grounds.

In Babylon, success of the Argyle fish ladder could lead to further work to extend the spawning grounds farther north, making Southards Pond and Belmont Lake accessible, Nardone said.

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In coming weeks, in the relative safety of the upland tributaries, female alewives will discharge about 250,000 eggs each, Nardone said. About 2 percent to 3 percent of those eggs will survive and make the trip back out to sea to join schools of their brethren over the Atlantic's continental shelf.