Ann Arbor Times

A study led by the University of Michigan has found that fish in Michigan’s inland lakes have become smaller over the past 75 years, with both young and old fish showing notable declines in size. The research analyzed data from nearly 1,500 lakes, covering observations from 1945 to 2020.

“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 (SEAS). “And most of those changes we’re seeing in Michigan fishes are declines in size through time.”

Flood, who is also lead author of the study published in Global Change Biology, explained that his team used digitized observation cards from a community science project to track changes across 13 species and multiple age groups. Out of 125 combinations of species and age class examined, 58 showed significant size changes; among these, 46 had become 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.”

Flood noted that because many predators can only eat prey small enough to fit into their mouths—a trait known as gape limitation—smaller young fish may be more vulnerable to predation. This can impact not just current but also future fish populations. He added that older fish play important roles within their communities: “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,” he said. “And these old individuals, they have this outsized influence on keeping the ecosystems healthy, for a variety of reasons.”

The findings are relevant for fisheries management as well. The Department of Natural Resources (DNR) sets rules on how many and what size fish anglers can keep to help maintain populations. As climate change alters key characteristics like body size, having detailed information helps managers adapt their strategies.

“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.

Researchers determine a fish’s age by analyzing its scales for growth rings similar to those found on trees.

The data used for this study was collected through collaboration between the university and what is now known as the Michigan Department of Natural Resources (DNR), under an effort called the Institute for Fisheries Research. An online platform called Zooniverse enabled global contributors to help digitize decades-old records so researchers could analyze long-term trends.

Karen Alofs, associate professor at SEAS and senior author on the study, leads work using historical and modern data sets to understand ecological changes. Her team has previously shown that largemouth bass—fish adapted for warmer temperatures—have increased as lake temperatures rise. Additionally, mass die-offs among lake fish due to ice thawing now occur later as lakes experience less ice cover.

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

The DNR continues collecting data on inland lake fishes so researchers can monitor ongoing trends. The dataset also allows further questions about how climate change affects various species’ sizes.

“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.”

Researchers are beginning to use specimens from the University of Michigan Museum of Zoology’s Division of Fishes—which holds about 3.5 million samples worldwide—to expand their analysis even further back in time and include lesser-studied native species.

Katelyn King and Kevin Wehrly from DNR contributed to this research along with U-M undergraduate Katilin Schiller and aquatic biology lab manager Andrew Runyon.