Beaver Ponds... Does a River Really Flow Through It?

By John Zablocki

The question of whether or not trout can successfully negotiate beaver dams in streams has for years been the subject of intense debate by anglers, fisheries managers and scientists. Rightfully so: whether or not beaver dams present barriers to trout has important implications. 

Barriers in streams (e.g., culverts, dams, irrigation structures) that don’t allow fish passage have the effect of fragmenting stream habitats and isolating populations of fish. These isolated populations have lower genetic diversity, have less available habitat, lack a migratory life history, and in general suffer decreased resilience to disturbance. In short, they’re more likely to be lost. Reconnecting streams remedies these problems and is therefore a central part of TU’s approach to aquatic conservation. Our volunteers--anglers, most of them--understand the importanct of reconnecting streams and opening up habitat to fish. It simply makes fishing better.

Above: TU and partners replace culverts with a fish passage structure in a Nevada watershed. 

But humans aren’t the only ones whose construction activities affect streams. Beavers, once pushed to near extinction by fur-trappers in much of North America, are making a comeback. And they are leaving a trail of dams wherever they go. Should these dams be considered barriers and removed in order to reconnect streams? Strong opinions can be found on either side, but the question remains open.

 

Above: In many places, beaver dams are perceived as fish barriers and intentionally removed, mechanically or with explosives by fisheries managers.

Nevertheless, there is a growing awareness among fisheries scientists and restoration ecologists that beavers and their dams may be allies in the effort to restore North America’s streams and armor them against climate change. Beavers can help regenerate floodplains, enhance water storage, and create refugia for trout threatened by wildfire. On the other hand, they accomplish this by creating dams, which have the potential to act as barriers.But do beaver dams really function as barriers to trout?

A group of researchers from the U.S. Forest Service and Utah State University decided to bring science to bear on this question. They studied two Northern Utah streams containing native Bonneville Cutthroat Trout populations along with non-native Brook Trout and Brown Trout. The researchers captured 1,357 trout over several years and implanted them with Passive Integrated Transponder tags. They then tracked the movements of the fish at different times of the year to determine the extent that fish were migrating past beaver dams. Their findings, recently published in the Transactions of the American Fisheries Society show that trout are quite capable of passing beaver dams, at least in the systems they studied. In fact, they observed 481 passes through the 21 beaver dams they monitored.

The ability of fish to pass beaver dams probably depends on a host of factors (stream flow, channel morphology, dam size and geometry, fish species and behavior, and more), but it should hardly be surprising that, in many cases, fish can negotiate beaver dams given that the trout and beavers evolved together in North America. 
 

John Zablocki is a restoration biologist for TU working on Lahontan cutthroat trout recovery in the Great Basin.

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