A new study has revealed that changes in climate are causing fish in Michigan’s inland lakes to shrink.
After analysing fish size records spanning the past 75 years across nearly 1,500 lakes, researchers from the University of Michigan (U-M) found that numerous fish species are significantly smaller today than they were in 1945.
This reduction in animal size, which was observed across both young and old individuals, highlights how climate change is altering the biology of freshwater ecosystems, say the study authors, who published their findings in Global Change Biology.
“Climate change is altering the size of different organisms around the world, including fishes in lakes here in Michigan,” says lead author of the study, Peter Flood, a researcher at the U-M School for Environment and Sustainability. “And most of those changes we’re seeing in Michigan fishes are declines in size through time.”
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The study enlisted the help of citizen scientists to digitise decades of handwritten observation cards describing fish from 1,497 Michigan lakes. This long-term record allowed the team to track 13 species and compare sizes across age groups.
“The largest decreases in length over time were found in the youngest and oldest fishes,” says Flood. “Both of those groups have outsized roles in maintaining healthy fish populations and ecosystem functions and services.”
The shrinking of the youngest fish is particularly concerning, say the researchers. Many predators are gape limited, meaning they can swallow only what fits into their mouths. Smaller young fish become easier prey, reducing the number that survive to adulthood. This can disrupt not just current fish populations but those of future generations.
Older fish may not influence population numbers in the same way, yet their ecological and social roles are significant, says Flood. “We don’t often think about culture with fishes, but they’re more social than we realise. They are learning from each other to some extent,” he explains, adding that as old fish get smaller, their chances of being preyed upon increase, meaning their ability to pass on valuable information to new generations diminishes.
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The study says that temperature changes are a major contributor to reductions in fish size, but other factors, particularly food consumption, are also important “and may interact with climate change to influence fish growth.”
The findings also raise practical questions for fisheries management. Michigan’s anglers must follow size and catch limits set by the Michigan Department of Natural Resources, but those guidelines depend on stable expectations of fish size. As warming conditions change those baselines, managers may need to adapt, suggests the study..
“There's still so much more that can still be done with this data set. There aren't many out there in the world like this one because of the crowdsourced part of it,” says Flood, who believes the study can be used by land managers to mitigate some of the effects of climate change.
Next, the researchers plan to analyse fish from the U-M Museum of Zoology’s Division of Fishes, where around 3.5 million specimens from around the world are stored. This will allow them to look even further back in time and examine more species, including fish native to Michigan.
Top image: Michigan lake. Credit: Getty
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