Fishes, young and old, are shrinking in Michigan's inland lakes

(Press-News.org) A new study led by the University of Michigan shows that changes in climate are also changing the size of fishes in Michigan's inland lakes. Using data that covered 75 years and nearly 1,500 lakes, researchers have shown that, for several species, old and young fish in 2020 were significantly smaller than their typical size in 1945.

"Climate change is altering the size of different organisms around the world, including fishes in lakes here in Michigan," said Peter Flood, a postdoctoral research fellow at the U-M School for Environment and Sustainability, or SEAS. "And most of those changes we're seeing in Michigan fishes are declines in size through time."

Flood is also the lead author of a new study published in the journal Global Change Biology and was supported by the U.S. National Science Foundation and Michigan Institute for Data and AI in Society, or MIDAS. The report is the latest to make use of data amassed by a community science project that digitized decades worth of observation cards that characterize fishes over time in 1,497 inland lakes in Michigan. 

This allowed Flood and his colleagues not only to track the sizes of 13 different species, but also of different age groups within those species. The team found that, out of 125 species-age class combinations, 58 had changed size. Of those 58 that changed, 46 were smaller.

"The largest decreases in length over time were found in the youngest and oldest fishes," Flood said. "Both of those groups have outsized roles in maintaining healthy fish populations and ecosystem functions and services."

The predators of these fishes are largely gape limited, Flood said, which means they can only eat what fits in their mouths. When younger fishes are smaller, then, they're easier prey, which can limit the population of not only the current generation, but future ones as well. And while older fishes don't have the same impact on population dynamics, they still have important roles in their communities, he said.

"We don't often think about culture with fishes, but they're more social than we realize. They are learning from each other to some extent," Flood said. "And these old individuals, they have this outsized influence on keeping the ecosystems healthy, for a variety of reasons."

Beyond the ecological implications, changes in the size of individual fishes and their populations are important considerations for those who fish and manage Michigan's fisheries. The DNR sets limits on the size and number of fish that anglers can keep to maintain healthy populations. With climate change putting key characteristics in flux, understanding the nature of the changes could help managers stay ahead of the curve.

"Our study is showing that there are differential responses of certain age ranges, so this is a tool in the toolbox managers can use to try to mitigate some of these climate change effects," Flood said.

For those who might be wondering how you determine the age of a fish, the answer is in its scales. As the scales grow, they form ring-like patterns, almost like a tree, that can be similarly analyzed to deduce an age.

The data behind the study were collected and maintained by a collaboration between the university and what would become the Michigan Department of Natural Resources, or DNR. Today that collaboration is known as the Institute for Fisheries Research. But an online platform called Zooniverse invited contributors from around the world to help digitize these records so researchers could more easily reveal trends and insights hidden in the ocean of data.

Unearthing such findings in historical and contemporary data is one of the broader goals of work led by the research team of Karen Alofs, a senior author of the study and an associate professor at SEAS. For example, Alofs and her team have also shown that largemouth bass, which are adapted for warmer temperatures, have become more abundant as Michigan's lakes have warmed. They've also found that mass mortality events for fish in these lakes, which typically happened as ice thawed in the spring, are happening later in the year as lakes experience less ice.

"Each of these changes, whether size, abundance or mortality, has important implications for these ecosystems," Alofs said.

The DNR is still collecting data on the fish, which means researchers like Alofs, Flood and their colleagues can keep an eye on these trends moving forward. But the data also has more dimensions to probe, enabling researchers to ask more questions that could help address new questions, such as how climate change is driving the size changes for different species. 

"There's still so much more that can still be done with this data set," Flood said. "There aren't many out there in the world like this one because of the crowdsourced part of it."

The team is also starting to incorporate fish specimens from the U-M Museum of Zoology into their study. The museum's Division of Fishes has about 3.5 million specimens from around the world, letting the scientists look even further back in time and examine more species. That includes more fish native to Michigan that haven't been studied as much because they aren't fisheries targets, Alofs said.

Katelyn King, a fisheries research biologist, and Kevin Wehrly, research station manager, with the DNR also contributed to the study. U-M team members also included undergraduate researcher Katilin Schiller and Andrew Runyon, who worked on the project as an aquatic biology lab manager. 

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