It's getting hot in here: Thermal refuge use by Brown Trout

Chris Sullivan, PhD Student, University of Connecticut
Dr. Jason C. Vokoun, University of Connecticut

The availability and use of thermal refuges (i.e., an area with preferential temperatures that is not in a fish’s normal habitat) throughout a riverscape is a concern for cold-water fish in temperate regions, particularly during the summer. Cold-water adapted fish are sensitive to warm water temperatures, and individuals must often abandon their territories and move to thermal refuges for summer survival. Thermal refuges in rivers and streams are shallow patches of groundwater seeps, streambed hyporheic flows, or tributary confluences where water temperatures are >3 °C cooler than mainstem river temperatures. Thermal refuge use can increase summer survival of cold-water fish and thus play a pivotal role in their persistence in marginal, warmer rivers.

In Connecticut, trout and salmonids, particularly Brown Trout, were stocked into many streams and rivers beginning in the 1860s. The Connecticut Department of Energy and Environmental Protection (CT DEEP) currently has an active program of stocking Brown Trout in many rivers and streams throughout the state, annually stocking nearly 452,000 9-12” and 48,000 6-8” trout (includes Brook and Rainbow Trout). The Brown Trout fishery, in addition to the other stocked trout species, promoted by these stocking efforts provide economic benefit, resulting in an annual net economic impact of US$4.9-10 million and is one of the state’s most popular fisheries. Roughly 7,800 km of rivers and streams provide habitat for stocked Brown Trout throughout Connecticut; however, warm summer temperatures that exceed their thermal tolerances occur regularly and limited or negligible numbers of stocked Brown Trout can survive >1 yr post-stocking (i.e., holdover). Brown Trout survival and persistence throughout Connecticut rivers and streams relies upon their ability to find and use thermal refuges, which are among the most threatened habitats statewide. As a result of both warm summer temperatures and increasingly limited cold-water habitats, a substantial funding investment by CT DEEP is required to produce, stock, and maintain Brown Trout for this popular fishery.
Currently, only limited sections within two large rivers consistently support holdover Brown Trout. Most notably, the Housatonic River is Connecticut’s second largest river (240 km) and supports some of the state’s premier “blue-ribbon” Brown Trout fisheries. Two Trout Management Areas (TMA), the 15 km Housatonic River TMA and the 7 km Bulls Bridge TMA, offer numerous large and small cold-water tributary confluences that are known to provide thermal refuge for trout. Brown Trout use these thermal refuges for days to months during the summer, yet year-round survival is considered low and populations must be sustained by the annual stocking of roughly 18,000 6-15” Brown Trout (B. Eltz, personal communication). The success of stocking, and therefore the success of the fishery, relies upon the behavior and survival of Brown Trout within and around the TMAs. To better manage the fishery now and into the future, an understanding of how stocked Brown Trout behave and survive once stocked is necessary. Therefore, we are conducting a rigorous assessment of stocked Brown Trout behavior within and around thermal refuges in the Housatonic River TMA, the larger of the two TMAs.
Working with collaborators from CT DEEP, we will begin monitoring stocked Brown Trout movements in the spring of 2021 and continue until late 2023. We will focus on three of the larger refuges in the Housatonic River TMA: Kent Falls, Furnace Brook, and Mill Brook. Using radio telemetry technology, we will be able to monitor how Brown Trout use thermal refuges, and the Housatonic River at large, by both actively and passively tracking tagged trout. We hope that these data will be used to further enhance and protect cold-water fish populations in Connecticut’s rivers and streams.