04 Apr 2013
Blocked Migration: Fish Ladders On U.S. Dams Are Not Effective
Fishways on rivers in the U.S. Northeast are failing, with less than 3 percent of one key species making it upriver to their spawning grounds, according to a new study. The researchers’ findings provide a cautionary tale for other nations now planning big dam projects.
In most major rivers in the U.S., maintaining some semblance of the integrity of migratory fish runs past hydropower dams is dependent upon the fish using ladders and elevators as freely as do two-legged humans. But is this asking too much?
Six colleagues and I undertook a study
of the success — or, rather, failure — of Atlantic salmon, American shad, river herring, and other species in migrating from the sea to their spawning grounds past a gauntlet of dams on three rivers in the northeastern U.S. — the Susquehanna, Connecticut, and Merrimack. What we found was grimmer than we expected. For one species, American shad, less than 3 percent of the fish made it past all the dams in these rivers to their historical spawning reaches.
Results for other anadromous species (those that spawn in fresh water and migrate to the ocean and back again) were nearly as bad. And the sobering aspect of these contemporary studies is that they are based on the insubstantial number of fish today as compared to earlier massive migrations of these species, which numbered in the many millions. While investigating fish passage on the Merrimack River in New Hampshire, our project’s lead researcher, Jed Brown of the U.S. Fish & Wildlife Service, was struck by the long-term lack of recovery of the targeted fish populations — at some fish restoration meetings there were more people in the room than salmon in the river.
What has happened on the U.S. East Coast, as reported in our study published in the journal Conservation Letters
in January, is of more than regional or national interest. There are important global conservation
What’s clear is that providing fish passages at a dam is not a panacea.
lessons, as well. Even as some large dams in the U.S. begin to be removed for environmental reasons, a hydropower boom is occuring worldwide. Thirty large dams have been announced for the Amazon River alone. Eleven major dams are planned for the lower Mekong River. The dam industry in Canada wants to dramatically expand its recent hydropower initiative.
And dam projects are proposed, planned, or in the works for Africa’s upper Nile, the Patuca in Honduras, the Teesta in India, the upper Yangtze in China, the Tigris in Turkey, the Selenge in Mongolia, and many others. Though most of these rivers lack anadromous fishes, many are home to richly diverse freshwater fish communities that make important seasonal migrations within these river systems.
For the international community, the record of fish passage on rivers in the northeastern U.S. is a cautionary tale. Hydropower has often been billed as a clean source of renewable energy, and generating electricity without polluting the air or producing greenhouse gases is commendable. But “clean” is in the eye of the beholder, and any claims to being sustainable ignore its multifarious aquatic effects, including blocking fish passage, fragmenting habitat, and undermining a river’s fundamental ecological services.
What’s clear is that providing fish passage facilities at a dam is not a panacea. Fishways are to be included in some of these large international projects, but not in others. Yet the options are dismal: To not include fish passage on a large dam is to ensure disruption of critical fish migrations; but to include fish passage is to likely diminish and maybe even endanger critical fish migrations.
Brown’s research began when, as a biologist for the U.S. Fish & Wildlife Service, he relocated in 2005 from the free-flowing mainstem-Delaware River to the thoroughly dammed Merrimack. Brown was struck by the small number of fish making it past the dams. Most fish passage research seeks to engineer improvements to existing technologies; Brown instead
These rivers and others have multiple dams blocking access to historical spawning reaches.
decided to launch a survey of the actual long-term results of fish passages on large, heavily dammed rivers.
What Brown and I and our coauthors found was bleak. One metric used was the percentage of fish passing the first dam that also passed just the second dam. For shad, the numbers were 16 percent on the Merrimack, 4 percent on the Connecticut, and 32 percent on the Susquehanna. But on these rivers the second dam is only the beginning of the journey — these rivers and many others have multiple dams blocking access to historical spawning reaches.
It’s important to put these results in perspective because they are merely relative
to the present paltry numbers of fish that even attempt to migrate up these rivers. For an anadromous fish population in North America, there are three absolute
numbers that matter. One is how many ran annually before European colonization. The second is the numbers targeted for restoration in fish passage programs. And the third are the numbers that actually show up each year.
On all three rivers examined, restoration goals were in the hundreds of thousands of fish — at least one, if not two, orders of magnitude less than historic, pristine runs. Yet run sizes obtained across three decades ranged annually from a high of about 10 percent to, more commonly, 2 percent or less of the stated goals. To put it in historical context, despite vast spending on modern technologies, contemporary shad migrations on these rivers are at least three to four orders of magnitude below the original unfettered run sizes, with similar results for salmon and river herring. Dams alone don’t explain these results — overfishing, habitat destruction, and alien species contribute — but there is widespread consensus among fish biologists that dams are a primary cause.
No East Coast river has been as adulterated as the Susquehanna, once a veritable shad factory. Shad ran up the Chesapeake Bay, entered the river’s mouth, and swam throughout its tributaries and mainstem through much
With very low or high waters, fishways don’t work well or shut down altogether.
of Pennsylvania and almost 500 miles to Cooperstown in central New York. Shad schools driving upriver on the Susquehanna were so enormous that they were visible in the distance to commercial fishermen by the waves they pushed ahead of them. One notable haul of mixed shad and river herring made in 1827 was estimated at 15 million fish; it took more than three days to offload the catch into wagons.
Contrast the open river of yesteryear with the occluded present. A shad fresh from the Atlantic entering the Susquehanna according to its natural rhythms encounters the almost 100-foot-tall Conowingo Dam only 10 miles from the river mouth. There it must somehow sense a tongue of water — the “attraction flow” — at the dam’s base in order to allow itself to be lifted in a metal trough to the reservoir above. Next it must orient in the strangely still water and then move upriver past three more dams using fish ladders — lengthy angled chutes with baffles that break up the flow.
With these serial delays it is unlikely that the few shad that make it to the spawning reaches of the Susquehanna arrive at the optimal time in the river’s seasonal ecological cycle. Worse yet, the numbers of adults successfully returning downstream past the dams to the sea are nil, sacrificing their future spawning potential. And with very low or high waters, fishways either don’t work well or shut down altogether, further delaying migrations.
Electric utility companies have nearly de facto sovereignty over migratory fish on these rivers, with the installation of fishways providing legal but largely ineffectual mitigation for their operations. Exploring technological
Rarely does even a single salmon or shad make it as far as the Amoskeag Dam.
improvements is limited by costs and the inflexibility of the utilities. That industry is in control may be atoned for with feel-good shad fishing derbies or informational facilities. The Amoskeag Fishways Learning and Visitors Center on the Merrimack in New Hampshire, for example, features a giant sculpture of a leaping American shad. Sadly, though, during most recent years that is the only
anadromous fish you will see at the center, for rarely does even a single living salmon, shad, river herring, or sea lamprey make it as far as the Amoskeag Dam.
In the U.S., the overall record of fish passage is mixed. Fish ladders often work well for river herring on smaller Atlantic rivers. Fish ladders at dams on the West Coast’s giant Columbia River system allow large numbers of salmon and also non-native shad to pass, but despite this apparent success contemporary runs of salmon are likely an order of magnitude lower than historic abundances. Chum salmon runs once numbered well more than a million; today they are about three percent of that.
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Is it the nature of fishway technology itself or is it less than optimal implementation that is at fault? John Hay, author of The Run
(1959), was a keen observer of river herring on Cape Cod, where fish ladders work relatively well. He wrote nonetheless, “There is no such thing, I have been told by men who were in the business of making them, as a good or even adequate fishway. There is always an imbalance between the purposes they serve and the results.”
My friends in the fish passage world disagree and say the fault is the difficulty in being able to fine-tune and test new ideas at real-world fishways. Fish passage researchers are earnest, hard workers who need to be optimistic; they tend to believe they are just a tweak or an insight away from a breakthrough. Perhaps they are. Clearly, with the existence of hydropower dams a continuing reality, any enhancements they can wring from fishways will be welcome.
One simple and promising idea being tested in Europe is to line the bottom of fish ladders with rubble to make the ladders seen less artificial. And in some suitable locations in the U.S. and elsewhere, “naturalized” fishways are being built that more closely resemble actual river reaches. In Germany, researchers are building fishways of different designs and then testing them, before applying the new knowledge to the next set of fishways. It’s not clear how well these new approaches will work, but it’s imperative to find out.
In the end, the challenges are daunting, and for a simple reason: It’s asking a lot for a finned creature to take an elevator or to climb a ladder.
ABOUT THE AUTHOR
, a professor of biology at Queens College, New York, works on the ecology and evolution of anadromous fishes, historical ecology, and urban waterways. He is the author of the book Running Silver: Restoring Atlantic Rivers and Their Great Fish Migrations
. In previous articles for Yale Environment 360
, he has written about the lessons fisheries can learn from agriculture
on sustainable management and how from generation to generation, certain species cease to matter