WALTHAM — Three hundred thousand baby shad, thin as a hair and the length of a fingernail, zoomed out of a hose into the Charles River in Waltham on Wednesday, sent to repopulate waters where they once abounded.
The release is the seventh of eight scheduled this summer as part of a multiyear effort to restore native American shad, a fish in the herring family, to the Charles River watershed. More than 3 million shad larvae will be released in 2013, bringing the total to 23 million released since 2006.
Scientists say restoring the shad, which roam the oceans before returning to fresh water to mate, is part of a larger effort to revive the river.
“This project is special because the Charles is such an important river to the people in Boston,” said Wendi Weber, regional director of the US Fish and Wildlife Service, on hand with partners from the Massachusetts Department of Fish and Game and the Charles River Watershed Association.
The shad released Wednesday traveled to the riverbank by truck from North Attleborough National Fish Hatchery, where they were born. They are the 10-day-old spawn of shad from the nearby Merrimack River, where they are plentiful following earlier restoration efforts.
Before the release, fishery staff mixed river water into the 5-foot-high tank where the larvae waited, acclimating to the steamy summer temperatures. Then pumps were readied, the hose aimed into the water, and the fish were off.
“Go forth and propagate!” Mary Griffin, commissioner of the Department of Fish and Game, shouted, as she clutched the hose.
Biologists say the young shad will have to escape the clutches of hungry birds, otter, mink, and bigger fish to survive the summer, which is why the returns of restoration efforts are modest by nature. They hope one in 10,000 larvae released will survive. “As they say, strength in numbers” said Bill Archambault of the Fish and Wildlife Service.
Scientists hope to restore a shad population of 30,000 to the Charles. Now there are less than 1,000, according to fish biologist Ben Gahagan of the state Division of Marine Fisheries. Biologists estimate that two-thirds of those fish came from previous releases.
The cost of shad releases is roughly $100,000 each year, Archambault said.
Colonial New Englanders highly valued shad for subsistence and sport.
“Shad were used for springtime planking, an old pastime of smoking fish on a wooden board,” he said, adding that the species remains popular among fishermen.
“They’re a good fighting fish, fun to catch,” said Frank Peace, 68, of Newton, who came to watch the release. “Tasty, too.”
Shad, which teemed in the Charles as recently as the early 1900s, dwindled as the river became polluted from industrial and residential development. But the main cause for the shad’s decline, Archambault said, was construction of dams at the beginning of the century.
“That was the beginning of the end, because the shad couldn’t make it over the dams to get to the ocean for winter,” he said. “It was the death knell.”
But with recent efforts to clean up the water and construct pathways for fish to swim over the dams, the Charles is now home to more than 80,000 fish, Weber said.
In the fall, biologists will go out on the river to check on the shad, which will have grown to about 4 inches. That will be the last sight of them before they swim to the Atlantic.
There they will mature for three to four years before they return to the river as adults to spawn.
It is because they make this trip, Gahagan said, that the shad is a staple fish for Massachusetts’ waterways.
He added that each of the shad released Wednesday was tagged with a unique pattern of dye applied to their inner ear bone.
That is so that when they return as adults, biologists will be able to identify the year they were released.
The shad in this year’s “class,” Archambault said, will begin returning to the Charles in 2016.
“We’ll be waiting for them,” he said.
Alyssa A. Botelho can be reached at alyssa.botelho@
globe.com. Follow her on Twitter at @AlyssaABotelho.