by Adam Hinterthuer – When Holly Embke was a graduate student at the UW-Madison’s Center for Limnology, she began an extremely labor-intensive research project – one that required catching and removing as many warm-water fishes from a lake in northern Wisconsin as possible. By the time she had earned her PhD, Holly and her team had trapped, netted and removed almost 300,000 fish.
The goal of that project was to tip the scales in favor of walleye living in the lake. Walleye prefer cooler waters and, as lakes around the U.S. and Canada have warmed, their populations have declined while fish that prefer warmer water – like bass and bluegill – have flourished.
These warm-water fishes compete with walleye for food and also prey on young walleye, so the researchers hoped that as their populations went down in the lake, walleye might get room to respond and their population numbers would go up.
The idea behind the study, says Steve Carpenter, director emeritus of the Center for Limnology and a co-author on the paper, was to look into potential management tools for keeping healthy walleye fisheries in our lakes. “While we can’t manage climate change here in Wisconsin,” Carpenter says,” “the climate is still interacting with factors we can potentially change and we ought to take stock of those and see what we can do.”
The results of the study were published this summer in the journal Ecosphere and, while they don’t tell a very promising tale for walleye, they do reveal that their cool-water cousins, yellow perch, were more than happy to take advantage of the experiment.
Embke, who is now a a fish biologist with the U.S. Geological Survey’s Midwest Climate Adaptation Science Center, says that while walleye “didn’t really respond” to the reduction in warm water fish populations, yellow perch biomass went up more than 900 percent. What’s more, ”that food web shift trickled down to other levels.” Aquatic insect communities grew as predation from warm-water fish decreased, while zooplankton numbers took a hit, since they are a preferred part of young yellow perch diets.
Their study underscores that a “one-size-fits-all” approach to fisheries management isn’t optimal in climate adaptation strategies, says Embke, who was lead author of the report.
“Thermal characteristics are not the be-all-end-all of how fish will respond to warming lakes,” she says. “You have to look beyond temperature.” And, when you do look beyond temperature, she says, you see one factor emerge that is an especially important predictor of how a species will fare as lake conditions change.
“Yellow perch are way more flexible in their life history and ecological roles,” she says.”They can spawn in different places and at different times. They target new food sources when what they prefer isn’t available. Unlike walleye, their adaptive capacity is way higher.”
That trait is likely shared by other species of fish in other lakes, Embke says, as it isn’t unusual for top predators to be more specialized in their diets and habitat preferences while species lower in the food web can switch between food sources and available habitat.
“If you have that flexibility as a fish, then you’re going to be way better positioned to adapt to change,” she says.
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The full Ecosphere article is available here: https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecs2.70297