River dams control water flow and help generate electricity, but they're a daunting barrier to fish swimming upstream to spawn. Various structures called fish passages are designed to get fish past dams, and they dot rivers across the Northeast United States. But a new analysis suggests they aren't working like they're supposed to, and fish aren't making it to where they need to go.
To help fish surmount the looming wall of a dam and reach upstream waters, dams are fitted with stairlike structures called ladders (fish leap up a series of pools) and elevatorlike contraptions called lifts (fish are channeled into a hopper that gets raised). Such fish passages are a key component of restoration efforts for migratory fish such as American shad and Atlantic salmon, whose populations are at historic lows—less than 10% of previous generations. State laws have required fish passages for hundreds of years—some date back to the 1700s—and the Federal Energy Regulatory Commission has mandated them for relicensing hydropower projects since the 1960s.
Data on fish passages is collected by power companies and is publicly available, but until now no one had pulled the information together. So Jed Brown, a fish ecologist who was working at the U.S. Fish and Wildlife Service in Nashua, New Hampshire, and his colleagues compiled fish passage data from multiple "mainstem" dams—those closest to the mouth—on three major rivers: the Merrimack, which runs from New Hampshire and empties into the Atlantic Ocean north of Boston; the Connecticut, which runs from New Hampshire south to the Long Island Sound; and the Susquehanna, which runs from upstate New York to the Chesapeake Bay.
Scientists and engineers set targets for the transport capacity of fish passages. And yet, the study lays bare that those targets are being missed by orders of magnitude. For instance, the first Merrimack River dam aims to let 300,000 river herring pass through; the mean number for the years 2008 to 2011 was 706 per year. The goal at the first Connecticut River dam is 300,000 to 500,000 fish. There, the mean for those same years was 86. And for the Susquehanna, the goal is 5 million river herring spawning above the fourth dam, which passed an average of seven herring from 2008 to 2011. This means that very few fish are reaching quality breeding grounds, which has likely contributed to the decimation in river herring populations.
"It's an old problem and it hasn't gotten solved," Brown says of getting fish around dams. (Brown now directs the Integrated Seawater Energy and Agriculture System Project in Abu Dhabi.)
It's not like fish ladders never work. American shad climb ladders in Western U.S. rivers with apparent ease, says co-author Karin Limburg, a shad expert at the State University of New York's College of Environmental Science and Forestry in Syracuse. But for reasons no one completely understands, they're not helping fish at these mainstem dams in the East. Many fish have trouble finding the passages in these large waterways, Limburg says.
So what's the solution? The authors, who publish their work online this month in Conservation Letters, suggest it's time to admit failure that the fish passages they studied aren't working. They make a case for dam removal in these areas and point to Maine's experience removing two dams from the Penobscot River. In that case, the power company was allowed to increase generating power at other, less ecologically important sites. Removing mainstem dams can allow free access to lower tributaries and their spawning habitats, while dams farther upstream can keep producing electricity (while they limit access to upper tributaries and ancestral habitat).
Brown knows that removing dams will be an uphill battle, so to speak. "I hear this a lot: 'These dams will never come out,' " he says. "Maybe our paper will change that."
James McCleave, professor emeritus at the University of Maine, Orono, agrees that it's time to consider different options. Migratory fish, he notes, readily move into newly opened habitat when dams are removed. "So many people are focused on making better fishways," McCleave says. "I think Brown is saying, 'Let's step back and take a different tack.' "