Currents

UWL study uncovers key brook trout habitat factors

Brandon Thill grew up fishing with his dad and grandpa. His love for the outdoors has fed his desire to understand ways to improve fish habitat so populations can thrive in Wisconsin waters.  

That’s an ambitious goal for the species he studies: brook trout. These native Wisconsin fish have been declining in the state primarily due to climate change and competition from non-native brown trout.  

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. 

Thill's research highlights several factors contributing to the decline of brook trout

• Climate Change: Increasing stream temperatures reduce suitable habitats. 
• Competition: Brown trout, which thrive in warmer waters, outcompete brook trout for resources. 
• Predation: Brown trout prey on brook trout. 
• Decline of Insects: Reduced insect populations affect brook trout food sources. 
• Parasites: Infestations such as gill lice hinder brook trout health. 
• Hybridization: Crossbreeding with brown trout produces sterile hybrid offspring known as tiger trout. 

To combat these issues and support brook trout survival, Thill recommends

• Clean fishing gear: To prevent the spread of parasites like gill lice, anglers should clean their boots and equipment when moving between streams.
• Target brown trout: Following Wisconsin fishing regulations, anglers can help by fishing for and keeping brown trout to reduce their numbers.
• Enhance stream shade: Property owners can plant native vegetation along streams to provide shade and help cool water temperatures.
• Focus on brook trout habitat restoration: Direct restoration efforts specifically toward brook trout-dominated streams to ensure they benefit from conservation activities and are less likely to be overtaken by brown trout. Thill's work is crucial for developing targeted conservation strategies that will help preserve brook trout populations and maintain the ecological balance of Wisconsin's freshwater systems. For more information, view his presentation to Wisconsin Trout Unlimited.

As Thill emphasizes, "Through initiatives like conservation maps and brook trout reserves, we can identify and protect vital habitats to ensure the survival of this species for future generations." 

Learn more about UWL graduate program in biology.