For more than a century, a small dam in Mill Neck has blocked river herring returning from Long Island Sound to reproduce in freshwater wetlands.

A fish passage that opened on Aug. 30 allows the herring to spawn in Beaver Lake and Beaver Brook. It’s the latest in a series of about 10 fish passages constructed across Long Island to increase fish populations.

The Mill Neck fish ladder is designed to increase the number of alewives and blueback herring, two types of migratory river herring that have dramatically declined in population in the area since the dam was built in about 1910, said Sally Harold, director of river restoration and fish passage for the Connecticut chapter of the nonprofit Nature Conservancy, which oversaw the Mill Neck project.

The herring are important prey for larger fish such as striped bass and tuna, as well as osprey, heron, eagles and other birds, and mammals such as otter.

“They are critical to the survival of so many other species,” Harold said of the herring.

Over the years, a small number of herring were able to swim over the dam during high tides and storm tides, but far fewer than the number that naturally migrated to the area when what is now a freshwater lake was an estuary open to Long Island Sound, said Chart Guthrie, regional fisheries manager with the New York State Department of Environmental Conservation.

The herring live most of their lives in the open ocean, but they can only spawn in freshwater or in low-saline saltwater, Guthrie said.

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The fish typically return to the freshwater area where they were born, so this spring, the DEC and scientists from Hoftra University and Cornell Cooperative Extension collected adult alewives from the Little River in Southampton Town and stocked them in Beaver Lake.

“The effort is to jump-start the population and get the fish we transported to breed there, so the young will home in on that location and come up the fish ladder to spawn in the future,” Guthrie said.

The fish passage, which Harold described as “like a watery ramp,” is at a 20-percent slope. The fish swim up from the saltwater into Beaver Lake.

Herring spawn elsewhere on Long Island, but their population has fallen because of dams and other factors such as rising ocean temperatures, Harold said.

Urban runoff — which can carry pollutants and warmer surface water into waterways — mean the fish can have a harder time surviving even in some freshwater spawning areas, she said.

“The more habitat they have available, the better they’re going to do,” she said.

The state and federal governments, Nature Conservancy, Oyster Bay-based Friends of the Bay and private donors contributed the more than $300,000 for the project, Harold said.

Guthrie said all the fish passages on Long Island have been built in the past nine years, including one on the Carmans River at Upper Lake in Brookhaven that opened last month. Plans are underway to build about 10 more, he said.