The alewife spawning season is in full swing on the Peconic River, and conservationist Byron Young is back to mark it with a cast net, a measuring table and a fish lover's enthusiasm.

At a hidden Peconic River tributary a stone's throw from the Suffolk County Center in Riverhead, the former state marine fisheries chief tosses the hand net into a rushing tidal pool; moments later he hauls in the day's first dozen alewives.

The health and the abundance of these river herring, cousins to the American shad, is testament to effort begun in earnest 20 years ago to restore access to centuries-old alewife spawning grounds. Access had been blocked primarily by man-made flood dams and gristmills for nearly 200 years.

The work, conducted under the Peconic Estuaries Program, is a joint project of the U.S. Environmental Protection Agency, Suffolk County, the state Department of Environmental Conservation, Cornell Cooperative Extension and the local community. The aim of the work sounds relatively simple: provide the alewife access to their traditional spawning grounds in the freshwater Peconic River.

At one point the work was done by bucket brigade, with Riverhead students and their teacher, Bob Conklin, shuttling buckets of alewives over the dams into spawning areas. Today, a man-made fishway in Riverhead aids the effort and Conklin, who died before the fishway was completed, is memorialized by a plaque along the waterway.

By one critical measure, as Young's work demonstrates, it has been a big success: The number of alewives moving to the upriver estuaries has more than tripled from 25,000 in 2010 to upward of 80,000 last year, according to estimates.

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The most important step in the Peconic project took place in 2010, when the estuary group completed the fishway at Grangebel Park. Today, as water rushes down the rocky, man-made rapids, tens of thousands of alewives swim up.

"This is one of the biggest runs on Long Island," says Young, over the roar of water. There are smaller ones on Alewife Creek at North Sea Harbor and the Carmans River in Southaven County Park.

Elsewhere on Long Island, similar projects have provided access for alewife spawning at Argyle Creek in Babylon, Massapequa Creek and Beaver Lake at Cold Spring Harbor, Young said.

The spawning season starts around mid-March and continues through April, and sometimes into May, depending on water temperatures. Each female lays around 250,000 eggs during her two-week stay in fresh water, before returning to the ocean through creeks and the river via Peconic Bay.

Unlike spawning salmon, adult alewives survive for years beyond their time in the river. It takes three to five years for new alewives to reach maturity. Young, a marine biologist, says some that have hatched here "are already returning as adults."

Alewives are not just an important part of the marine food chain -- vital to predators such as bluefish and striped bass, and shore birds such as osprey, which perch in trees high over the freshwater pools. Alewives are also filter feeders, helping clean local waters of plankton and algae.

At the Riverhead fishway, the alewives' movement is visible by an underwater camera mounted near a steel gate that directs the fish to open waters.

Alewives' numbers declined so sharply on the East Coast over the last decade that federal regulators last year recommended banning their harvest in regions that could not demonstrate their alewife harvests were sustainable. State regulators could not, and signs posted along the Riverhead run warn residents it's against the law to take them.

Dams and overfishing, including unintended harvesting by big offshore fishing boats, are blamed for depleting the stock to the point of requiring the ban, Young says.

The fishway was the first of a series of projects on Peconic River tributaries that will provide the fish with access to bigger water bodies upstream.

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"They want to go to Wildwood Lake," says Young of a freshwater lake about a mile upstream. A flood dam blocks their access, but in coming months and years, the Peconic estuary program will install a series of fishways to help them bypass it.

"It's important to reconnect these fish to their former spawning grounds," Young says. "Each one of these steps is adding to the overall goal."