School of Marine Sciences

BANGOR, Me. — Joseph Zydlewski, a research biologist with the Maine Cooperative Fish and Wildlife Research Unit of the United States Geological Survey, drifted in a boat on the Penobscot River, listening to a crackling radio receiver. The staccato clicks told him that one of the shad that his team had outfitted with a transmitter was swimming somewhere below.

Shad, alewives, blueback herring and other migratory fish once were plentiful on the Penobscot. “Seven thousand shad and one hundred barrels of alewives were taken at one haul of the seine,” in May 1827, according to one historian.

Three enormous dams erected in the Penobscot, starting in the 1830s, changed all that, preventing migratory fish from reaching their breeding grounds. The populations all but collapsed.

But two of the dams were razed in 2012 and 2013, and since then, fish have been rushing back into the Penobscot, Maine’s largest river.

“Now all of a sudden you are pulling the cork plug and giving shad access to a truckload of good habitat,” Dr. Zydlewski said. Nearly 8,000 shad have swum upstream this year — and it’s not just shad.

More than 500 Atlantic salmon have made the trip, along with nearly two million alewives, countless baby eels, thousands of mature sea lamprey and dozens of white perch and brook trout. Striped bass are feeding a dozen miles above Bangor in waters closed to them for more than a century.

Nationwide, dam removals are gaining traction. Four dams are slated for removal from the Klamath River alone in California and Oregon by 2020.

Just a few of these removals have occurred on such large rivers, which play an outsize role in coastal ecosystems. But the lessons are the same everywhere: Unplug the rivers, and the fish will return.

In the Elwha River, on Washington’s Olympic Peninsula, engineers removed two dams, one more than 200 feet tall, from 2011 to 2014. Afterward, chinook, chum and sockeye salmon, along with steelhead trout, quickly moved upriver, said Jeffrey Duda, a research ecologist at the Western Fisheries Research Center of the U.S.G.S.

“Once you remove these dams, migratory fish will probe into the watersheds,” Mr. Duda said.

And there are more subtle changes. The migratory salmon quickly began enriching the food web of the Elwha River with oceanic nutrients. A year after the Elwha Dam came down, Mr. Duda and his colleagues found chemical signs of marine-derived nutrients in the blood of American dippers, small aquatic songbirds that forage in rivers.

But the turnout has rarely been as vast as it has on the Penobscot.

Like other large coastal rivers, the Penobscot once funneled millions of pounds of fish inland from the ocean each spring. But fish populations suffered in the 1800s as fishing pressure increased, water quality diminished and, most consequential, dams blocked the fish from their spawning grounds.

Until 2013, fish ran a gantlet of three large dams in the first 10 miles of the Penobscot above head of tide, near Bangor. The Penobscot River Restoration Project, a consortium of government and tribal agencies, conservation groups and hydropower companies, spent $60 million to remove the first two dams and to install a fish lift at the next dam upstream.

In June, the group dedicated the last piece of the project, a bypass channel around a dam on an upriver tributary.

Before the dams came out, biologists began studying the river’s fish to better understand the baseline conditions. “We asked the question, ‘Who’s knocking at the door?’” Dr. Zydlewski said.

Shad were so diminished that fewer than 20 had passed the fishway of the former Veazie Dam over several decades. But Dr. Zydlewski and his colleagues, using sonar, documented a small population that persisted below the dam.

Everything changed with the removal of the Veazie and Great Works dams, Dr. Zydlewski said.

This year, precisely 7,846 shad ventured upriver, past the two demolished dams and through the fish lift at Milford Dam, which is now the first obstacle fish reach. Other shad, like those Dr. Zydlewski was tracking beneath the boat, stayed downstream; he and his colleagues say they are not sure why.

Another research team, led by the University of Maine’s Michael Kinnison and Gayle Zydlewski (who is married to Joe Zydlewski), discovered a previously unknown population of the endangered shortnose sturgeon in the Penobscot, near Bangor. Since the dams have come down, some of the sturgeon have nosed upstream into the newly free-flowing river.

The Penobscot also hosts the nation’s largest run of Atlantic salmon, another endangered species. Historically, salmon runs may have numbered 60,000, but recent returns fell to less than 1,000, and as low as 250 in 2014. Among the salmon’s challenges is changing climate, bringing warmer waters and unfavorable conditions at sea.

In predam days, salmon were far outnumbered by shad and their smaller cousins, alewives and blueback herring, also known as river herring.

In anticipation of the dam removals, state biologists in 2010 began stocking lakes in the Penobscot watershed with the herring; fish that swam up the Penobscot this year are their progeny.

This strategy proved effective on the neighboring Kennebec River, where the Edwards Dam was removed in 1999. There, river herring now return by the millions and support a commercial fishery.

John Banks, the director of the Penobscot Indian Nation Department of Natural Resources, said his tribe long relied on migrating fish like salmon and shad for sustenance, and used river herring to fertilize their gardens.

“It’s just fantastic to see the river coming back to life so quickly after the dams have been removed,” Mr. Banks said. “And the alewives are so key to this. They are the keystone species that helps drive the whole river ecosystem.”

River herring are prey for everything from ground fish to seals. And because they are so numerous, they serve an ecological role as prey buffers.

Juvenile Atlantic salmon, for example, are more likely to avoid predators when migrating seaward through schools of river herring, which allow them to sneak out through the crowd.

Much of the Penobscot’s recovery has been subtle, but some indicators of the river’s link to the ocean are quite conspicuous. Recently, seals showed up in the river, miles above the old Veazie Dam.

Dr. Zydlewski sees the annual migratory cycle as a grand spectacle of predators and prey.

“You don’t see the fish, but it’s hard to miss the eagles and osprey. Just like striped bass, they follow the food,” he said. “It’s a shadow of what it once was, but it’s exciting to see how it might come back.”