Major hydropower dams in the northeastern United States, constructed with state-of-the-art features designed to allow migratory fish to pass through them on their way to spawn upstream, have failed in that regard, raising questions that should be addressed as more dams are planned worldwide.
These findings were reported in a study published today in the journal Conservation Letters.
A team of ecologists and economists, led by J. Jed Brown of the Masdar Institute of Science and Technology in Abu Dhabi in the United Arab Emirates, reported that despite the presence of fish-passage facilities that were built into the dams, the actual numbers of fish that passed through them over several decades were only tiny fractions of targeted goals.
Brown completed his doctorate in renewable natural resources at the University of Arizona under the direction of Edward Glenn, a professor in the department of soil, water and environmental science at the UA College of Agriculture and Life Sciences.
“It may be time to admit failure of fish passage and hatchery-based restoration programs and acknowledge that ecologically and economically significant diadromous species restoration is not possible without dam removals,” Brown and his colleagues wrote.
The three large river systems studied – the Merrimack, Connecticut and Susquehanna – are historically important rivers for a suite of fishes that migrate from the sea to rivers; they are called diadromous fishes by scientists and include species such as sturgeon, salmon, shad, river herring and eel.
“Once these rivers supported tens of millions of pounds of biomass of these species and provided valuable protein to a growing nation,” said Karin Limburg, a fisheries ecologist at the SUNY College of Environmental Science and Forestry in Syracuse, N.Y.
Today the river systems contain hundreds of dams. The dams with the largest impacts on the fish populations are those constructed on the main stems of the rivers for hydropower generation. There are four on the Susquehanna, including one less than nine miles from its mouth at the head of the Chesapeake Bay, more than 10 on the Connecticut and five on the Merrimack.
Using publically available data collected by various agencies since the 1960s, the research team shows that these state-of-the-art fish passage facilities have been unsuccessful. Some migratory species, such as sturgeons, do not pass through at all. But even the species that do make it through do so in numbers far less than stated targets.
In recent years, for example, the number of American shad, which was once one of America’s premier food fish, that passed through the dams has hovered around 2 percent of the target in the Merrimack River and close to zero percent of target in the other two rivers. System-wide passage efficiencies, defined as passage from the most downstream dam in a river up past the uppermost main stem dam with fish passage facilities, hover at less than 3 percent. Although some fish spawning does occur downstream of the lowest dams, most of these migratory species require habitat above the farthest upstream dam.
River herring, a term used to describe both blueback herring and alewife, are important migratory forage fish, which have been proposed for listing under the federal Endangered Species Act.
“These dams are contributing to reduced resilience of not only shad, but all diadromous species,” said Adrian Jordaan of the University of Massachusetts, Amherst. “The result is that other factors including climate change will have a greater impact on these populations that are at fractions of their historical levels.” For example, restoration targets for river herring vary from several hundred thousand to millions of fish – however, in recent years, river herring returns on these rivers have averaged less than 1,000 fish.
“The consequences of the use of these failed technologies include declines in diadromous species, but also an odd, self-perpetuation of attempts to mitigate dam impacts by continuing restoration programs despite their inability to show success,” said John Waldman of Queens College. “Not only are these losses felt locally, possibly with major ramifications to fishery and biodiversity resources, but in fact they translate into lost marine production and weakened marine food webs.”
The authors note that – ironically – at one fish passage facility with an educational center, no fish passed in a typical year.
The authors support innovative solutions to this quandary, inviting more scrutiny to finding alternatives to main stem dams. In the state of Maine, for example, a creative solution developed by a broad coalition was to purchase two main stem dams on the state’s largest river, the Penobscot, but compensate the electric company with alternative power generation in tributaries considered less critical for fish reproduction.
The researchers say the case study serves as a cautionary tale not to count on fish passage facilities to mitigate dam projects, even as many developing nations look to their undammed rivers – the Amazon and the Mekong, among others – as valuable potential sources of hydropower.
“Electricity from hydropower dams is considered mature technology that is seen as ‘green energy’ because it does not generate greenhouse gases,” said Brown, the lead author.
“Although hydropower dams are criticized for obstructing the movement of fishes and other creatures, many hydropower dams have fish passage facilities of one sort or another. These passage facilities appear to create a ‘win-win’ situation that allows us to enjoy both hydroelectricity and healthy fish populations. The problem is it doesn’t seem to work.”