What's Left in the Toolbox?

By John Waldman and Thomas Quinn

Restoring Salmon and Other Migratory Freshwater-Marine Fishes

As two aquatic conservation biologists who have long studied riverine fishes, we know of few rivers in the Lower 48 states with migratory fish populations equal to what European colonists first encountered. Indeed, the losses have often been profound, e.g., six orders of magnitude for American shad in the Susquehanna River.

We examined the challenge of restoring salmon, steelhead, shads, sturgeon, eels, lamprey and other diadromous fishes (those migrating between fresh and salt waters) through the prism of “tractability.” That is, regardless of historical causes, what meaningful restoration actions can be taken today? The many “drivers” that reduced these populations differ in how important they are and how tractable they are to resolution.

Drivers Largely Remediated

Overfishing caused many declines but has been reduced by multi-state fisheries commissions on the Atlantic, Pacific and Gulf coasts, and international agreements. Fishery management is imprecise but has benefitted from tribal, state and federal involvement, reducing overfishing in rivers and at sea. Open-cycle electric generating plants using river water for cooling also harm migratory fishes by impinging and entraining them in their early life stages. However, new plants are being built with closed-cycle cooling, reusing water and reducing fish losses.

Continuing to Be Remediated

Water pollution, including sometimes egregious levels of sewage and contaminant discharges to rivers, has been improved by the regulations and funding from the Clean Water Act of 1972. Ohio’s Cuyahoga River no longer catches fire, nor are migrating fishes blocked by the “chemical dam” that formed in the lower Delaware River, though there are effects of “legacy chemicals” and new products being developed and released that may be hazardous.

Irreversible Drivers

Decades of experience has shown that it is nearly impossible to eradicate deleterious non-native species once they are established. Eurasian zebra mussels—an ecological game changer given their filtration abilities— first appeared in the U.S. in the Great Lakes in 1988 and have spread to at least 25 states. Whirling disease, New Zealand mud snails, round goby and many other non-natives have undermined our aquatic ecosystems but likely cannot be eliminated.

Drivers Reversible in Theory but Not in Practice

Climactic warming and associated changes to river flow regimes is shifting fish migration timing, e.g., in Atlantic Coast river herring. And at sea, these fishes may be influenced by “marine heat waves.” Climate change is reversible—in theory. The effects of climate change can be ameliorated by major reductions in greenhouse gases which would translate to cooler water temperatures. However, a modest reduction would not stem the momentum of rising temperatures entrenched in current planetary processes.

Other Drivers

There are several other drivers of decline of these fishes. Hatcheries increase abundance but may introduce less-fit individuals that compete and interbreed with wild fish, and can spur unsustainably high fishing rates. Similarly, In the Pacific Northwest, diversions of rivers for agriculture and drinking water may alter flows, hindering reproduction and survival. Other modifications of rivers such as habitat destruction can spur declines. Many rivers have been channelized, straightened, landfilled and culverted, reducing the ecological value of the altered habitats.

What’s Left?

What is left? Dams, culverts and other blockages have been responsible for losses in many large rivers and countless small streams. In assessing the tractability of options for restoration of American migratory fishes, removal or modification of these barriers to migration offer the greatest potential for restoration. Many dams are relics of an earlier age when the developing nation exploited its rivers to power mills at a time when other sources of energy were so limited. Now, obsolete and derelict dams litter the landscape, preventing or hindering migratory fish from reaching their spawning grounds. In an era of alternative energy sources even dams that generate electricity should be weighed for their value vs. the benefits of removal for safety and the environment. Moreover, culverts are smaller than dams but many block upstream migrations, and important benefits can be obtained by modifying them.

The traditional answer to sustaining populations of migratory fishes in the presence of dams (other than producing them in hatcheries) is the use of ladders, elevators or “trap and haul” operations to drive them past dams. These work better for some species than others but the track record for systems with many dams, for instance, such as the Kennebec and Susquehanna rivers, is abysmal.

Instead, dam removals have restored runs of migratory fishes quickly and to large abundances. For instance, removal of Edwards Dam on the Kennebec allowed river herring to reach its Sebasticook tributary, which the dam had made inaccessible for 162 years. River herring counts soon rose from zero to 6 million, and other success stories are reported around the nation. On the West Coast, removal of the two large dams on the Elwha River to give fishes access to the upper river has justifiably received attention, but many other dams have been or will be removed or modified for fish passage.

The management of its diadromous fishes has been one of America’s worst conservation failures. When so many drivers of decline offer little hope to restore them, there is one broadly tractable tool left in the box—easing their upstream and downstream passage.

John Waldman is a professor of biology at Queens College, New York, and author of Running Silver: Restoring Atlantic Rivers and their Great Fish Migrations.

Thomas Quinn is a professor of Aquatic and Fishery Sciences at the University of Washington, Seattle, and author of The Behavior and Ecology of Pacific Salmon and Trout.

This piece is based on “North American diadromous fishes: Drivers of decline and potential for recovery in the Anthropocene” published in Science Advances, January 2022. Available at https://www.science.org/doi/ full/10.1126/sciadv.abl5486