Wildlife conservation officials trapped river herring in a spillway Wednesday and released them into the Mill Pond in Rockville Centre to spawn.

Hundreds of river herring called alewife were netted by volunteers from Hofstra University and the Islip-based Seatuck Environmental Association wearing galoshes and hip waders. The fish, about a foot long, were tagged and studied before being released into the pond.

“We’re trying to advance fish restoration here and address the health of Mill River,” which is connected to Smith Pond, Seatuck director Enrico Nardone said.

The alewife fish cannot be caught by recreational anglers, officials said.

Laura Munafo, program manager for the state’s Living with the Bay initiative, said Wednesday’s event included a study of water quality and aquatic life in Smith Pond. The event is part of a $125 million program to address water quality and storm mitigation in the Mill River, Munafo said

A dam in Smith Pond prevents the alewife from reproducing there, so they get stuck in the spillway and fewer of the young survive.

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“They’re not jumpers like salmon,” Nardone said. “That’s an absolute barrier for them.”

Officials eventually want to put a “fish ladder” along the dam so they can get back into the pond without human assistance, he said. The fish will spend most of their lives in the ocean, but return to their birthplace to mate.