A Tale of Two Fishes: Study Maps the Future of Trout Populations in Wisconsin

FOR IMMEDIATE RELEASE: [Madison, WI] – Climate change is being felt across the world in many different ways. Atlantic hurricanes intensify. Western droughts deepen. And, here in Wisconsin, the future of fishing gets more uncertain. 

According to a new study published in the journal Ecosphere on December 19th, the story of Wisconsin’s trout fishery over the last thirty years has been a tale of two species responding differently to changing conditions. Populations of smaller, brightly colored brook trout have declined, while the abundance of the larger brown trout has generally increased.  

This story of “winners versus losers, with brookies declining and brownies taking advantage of current conditions,” has mainly to do with temperature, says Bryan Maitland, a postdoctoral researcher at the UW-Madison Aquatic Sciences Center’s Water Resources Institute and lead author of the study. Brook trout, which are native to Wisconsin, thrive in colder waters while brown trout, a popular non-native species in the state’s trout fishery,  prefer their streams just a few degrees warmer. 

But temperature only tells part of the tale, Maitland says. Other issues also trouble the waters.

Bryan Maitland.

In the Midwest, more frequent intense rain events and earlier winter thaws – both the result of climate change – can dramatically impact populations of trout.  ”Extreme events are equally bad for both species,” Maitland says, because these high streamflow events scour out the gravel beds where eggs are incubating in the winter or, in spring, overstress newly hatched fish. 

But, Maitland says, the main takeaway from his study is that climate impacts are being felt differently around the state. “Some parts of the world are just going to get hit harder because of where they are,” he says. “Southern Wisconsin has been hit the hardest by temperature changes so far, making stream conditions warmer and closer to the temperature threshold for trout.” As that threshold is projected to advance northward,Maitland says, “streams with more cold groundwater inputs like spring-fed streams in Central Wisconsin will have the greatest capacity to continue to support trout.”

Location matters, and studies like Maitland’s can help resource managers pinpoint both “bright spots” where the state’s trout fishery seems likely to withstand current climate impacts, as well as areas of concern that may need extra attention. 

Knowing where impacts are most likely to be felt the strongest , Maitland says, can help fisheries managers tailor their response at a more focused, regional scale.”It helps us be able to say, ‘Okay, in the southern part of the state we need to focus on temperature and keeping streams colder by encouraging more shade cover,” while the more forested streams in northern Wisconsin’s less-developed watersheds would benefit more from things like in-stream habitat restoration and watershed protection.

This is invaluable information as fisheries managers try to mitigate both current and future climate impacts to the state’s trout fishery, says Alex Latzka, a fisheries biologist at the Wisconsin Department of Natural Resources and a co-author on the Ecosphere paper. Future projections have temperatures creeping higher, beyond the brown trout’s optimal range, Latza notes. And that underscores the need to improve conditions for trout right now – even for populations that are currently doing well.

“Anything we can do to reduce flooding will help [and] anything we can do to keep streams cold as air temperatures warm will keep trout thriving longer,” he says. “The Wisconsin DNR has already identified brook trout populations that are most likely to be able to persist in a warmer climate, and we are working on implementing these and other conservation measures that could benefit trout over the long term.” 

The full text of the Ecosphere paper can be found here. 

Funding for this project was provided by the University of Wisconsin Water Resources Institute (USGS, US Department of the Interior federal base grant number G16AP00092) and the Wisconsin Department of Natural Resources. Bryan Maitland was funded by a Wisconsin Water Resources Science-Policy Postdoctoral Fellowship. 

Cover photo by Kirk Olson, Wisconsin DNR