Taking down dams and letting the fish flow

Joseph Zydlewski, a research biologist with the Maine Cooperative Fish and Wildlife Research Unit of the US 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.

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 (USA) largest river.

“Now all of a sudden you are pulling the cork plug and giving shad access to a truckload of good habitat,” Joseph 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 (Maine) in waters closed to them for more than a century.

In the US, 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.

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?’” Joseph said.

Shad were so diminished that fewer than 20 had passed the fishway of the former Veazie Dam over several decades. But Joseph 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, Joseph 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 Joseph 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, 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 unfavourable conditions at sea.

Predators and prey

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 neighbouring Kennebec river, where the Edwards Dam was removed in 1999. There, the river herring now return by the millions and support a commercial fishery.

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.

Joseph 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.”