UWL study uncovers key brook trout habitat factors

Thill, who now works as a water resource management specialist for the Wisconsin DNR, conducted his UW-La Crosse graduate research on the complexities of trout habitat preferences and the impacts of environmental changes. His work aimed to identify and protect critical habitats for brook trout, a species facing severe challenges in the years ahead. Brook trout have a thermal max temperature of 25 degrees C. Their numbers have been dwindling, in part, due to increasingly warm stream temperatures. According to a 2019 study, suitable brook trout habitats in Wisconsin could diminish by up to 68 percent by 2050 if current trends continue. Meanwhile, brown trout, which prefer slightly warmer waters and have been thriving in Wisconsin streams, are pushing out native brook trout.

The Driftless Area has emerged as one of the few regions in the state where brook trout can still thrive. Thill's research pinpoints specific habitat features in this area that make it more hospitable to brook trout, with the goal of implementing long-term conservation strategies. Under the guidance of UWL Assistant Teaching Professor Jason Freund, Thill has shared his findings with several conservation organizations, including Trout Unlimited and the Wisconsin Chapter of the American Fisheries Society. His research highlights the critical importance of preserving and enhancing these habitats to support brook trout populations.

Snorkeling in streambeds alongside trout was one of Thill's primary methods for data collection. He studied the fish feeding and resting spots, meticulously recording many data points. This helped numerically paint a picture of the stream, accounting for what variables influence the stream temperature in brook trout vs. brown trout dominated streams.

His findings indicate a marked preference among brook trout for sandstone streambeds over dolostone ones. Sandstone streams, characterized by cooler temperatures and slower flow rates, provide more suitable conditions for brook trout compared to the warmer, faster-flowing dolostone streams. During his July-August 2023 study, Thill recorded temperatures in some dolostone streams approaching the critical thermal limit of 25°C for brook trout, while sandstone streams maintained more favorable temperatures.