Largemouth bass swimming in a Michigan inland lake. (Credit: Focused Adventures on Shutterstock)

In A Nutshell

Across 75 years and 1,497 lakes, many Michigan fish got smaller, especially the youngest and oldest.

How you measure warming matters: degree days often link to growth, while higher surface temps often link to shrinkage.

Warm water speeds metabolism; if food doesn’t keep up, growth lags, especially in dense juvenile cohorts.

Managers and anglers should track both metrics to spot risk early and protect big spawners.

Fish across Michigan are getting smaller, and attempts to figure out why have left scientists scratching their heads. A study spanning 75 years and nearly 1,500 lakes was intended to provide clarity, but instead revealed something unexpected. The way researchers measure temperature can completely flip their conclusions about whether fish are growing or shrinking.

University of Michigan researchers analyzed more than 84,000 fish measurements from 1945 to 2020, tracking 13 species across the state’s inland lakes. The youngest and oldest fish showed the steepest declines in size, with some year-old fish shrinking by as much as 0.6 millimeters annually. Over 75 years these changes pile up, and they’re happening in ways that could harm fish populations.

Moreover, a puzzling pattern emerged. Whether fish appeared to be growing or shrinking depended on which climate measurement scientists used. Growing degree days (cumulative warmth over time) often showed fish getting bigger, especially cold and cool-water species. But average water surface temperature told the opposite story (shrinking fish). Both measurements track warming, yet they pointed in different directions.

Published in Global Change Biology, the study notes that when coupled with previous conflicting studies, the results “highlight that the variable chosen to represent climate warming can reverse the direction of the observed trend.”

Two Ways to Measure Heat, Two Different Stories

Growing degree days measure the total heat fish experience throughout the year. More degree days means a longer growing season, giving fish more time to eat and bulk up before winter.

Average surface water temperature captures how hot the water gets overall. Higher averages can push fish beyond their comfort zones, especially during summer. For fish, this resembles the difference between enjoying a longer mild season versus living in a sauna.

Cold and cool-water adapted fish typically grew longer with more degree days but shrank as surface temperatures climbed above 17°C (about 63°F). Warm-water species like largemouth bass and bluegill decreased in size with both measures, suggesting they face different pressures—possibly fiercer competition for food as their populations expand in warmer water.

This split response might explain why previous studies disagree about climate’s effects on fish. Lake whitefish appeared larger in one study using surface temperatures but smaller in another using degree days. Same species, different temperature lens, opposite conclusions.

Researchers will also include museum samples, like this lake trout, in their future work studying the impacts of climate change on fishes in Michigan’s inland lakes. (Image credit: Peter Flood.) Babies and Seniors Suffering Most

One of the study’s most troubling findings was which fish are shrinking most. Age-0 and age-1 fish showed some of the steepest decreases in body length. This contradicts the Temperature Size Rule, a longstanding theory predicting juvenile fish should grow faster in warmer water even if adults shrink.

The research team controlled for many factors affecting fish growth, including lake size, depth, water clarity, and surrounding land use. They used boosted regression trees, a machine learning approach, to disentangle climate effects from these other variables. The models explained about 43% of variation in fish length, with timing of when fish were caught emerging as the most influential predictor for most species.

Older fish weren’t faring much better. Age-10 and age-12 fish also ranked among those with the biggest size declines. This creates a problem at both ends of the age spectrum. Smaller young fish face higher predation risk and may not survive to adulthood. Larger, older fish are reproduction superstars. Their egg production increases exponentially with body size. When they shrink, populations can crash.

Lead author Peter Flood and colleagues used Bayesian statistical models and machine learning to separate climate effects from other growth factors like lake depth, water clarity, and surrounding land use. Analysis showed 37% of fish age groups had statistically significant length decreases over time.

Northern pike stood out as particularly affected. All age classes studied (ages 0 through 10) showed declining lengths. Largemouth bass told a similar story, with nearly all age groups shrinking over 75 years. Even species holding steady or growing in some age classes, like bluegill, showed shrinkage among their youngest members.

When Warmer Doesn’t Mean Bigger

Why aren’t warmer temperatures simply making fish grow larger across the board? The answer likely involves food availability.

Fish body temperatures match their surroundings. Warmer water speeds up metabolism, forcing fish to eat more just to maintain current size, much less grow bigger. When food sources don’t increase with metabolic demands, fish can lose weight or grow more slowly despite their revved-up metabolism.

Competition might be intensifying for some species. Warmer temperatures allow fish to reproduce more successfully and at smaller body sizes, flooding lakes with juveniles competing for the same food. This density-dependent growth suppression could explain why young bluegill and largemouth bass are shrinking even as adult sizes hold steady or increase.

Cold-water species like rainbow trout bucked the overall trend, with some age classes growing larger over time. These fish may benefit from longer growing seasons linked to increased degree days. Yet half of cold-water fish age classes still decreased in size when surface temperatures exceeded 17°C, suggesting even cold-water specialists hit a breaking point when lakes get too warm.

Largemouth bass today aged 0 through 11 years are statistically smaller today than they were 45 years ago, according to new research from the University of MIchigan. (Credit: Andrew Runyon) Beyond the Fish Themselves

Shrinking fish ripple through ecosystems. Smaller fish produce fewer eggs, disperse less effectively, and face greater predation pressure. These changes can destabilize populations and alter food webs, affecting everything from algae to eagles.

For anglers, smaller fish mean fewer trophy catches and less meat on the table. Recreational fishing contributes billions to local economies, particularly in lake-heavy states like Michigan. If fish continue shrinking, particularly sought-after species like walleye and bass, the economic consequences could mount.

Researchers examined fish from 1,497 lakes across Michigan, capturing a wide range of lake types, sizes, and environmental conditions. This geographic and temporal breadth strengthens the findings. The dataset combined historical records that community scientists recently transcribed through the platform Zooniverse with contemporary surveys conducted by the Michigan Department of Natural Resources.

Why Past Studies Disagreed

Scientific literature on fish body size and climate change has been sending mixed signals for years. Some studies show fish shrinking as predicted; others show them growing. This research suggests scientists might have been measuring different dimensions of the same problem.

Previous work on North American salmonids found several species growing larger despite warming temperatures, contradicting predictions. The Michigan study’s finding that degree days often correlate with increased growth in cold-water fish supports this pattern. But when those same species experienced higher average surface temperatures, growth declined.

Scientists and policymakers need accurate predictions about how fish will respond to continued warming. Using only one temperature metric might miss key aspects of how climate change affects fish growth. The study authors recommend using multiple climate variables—capturing both season length changes and peak temperatures—to get the complete picture.

What Happens Next

The researchers acknowledge limitations. No individual lake had data spanning the full 75 years, preventing them from tracking specific lake populations over time. They also couldn’t account for all growth factors, like nutrient levels, fishing pressure, or changes in available prey. Different sampling methods used over the decades could introduce biases, though the age-specific analysis helps minimize this concern.

Climate change shows no signs of slowing. Michigan’s lakes continue warming, with growing seasons lengthening and summer temperatures climbing. If current trends continue, the researchers predict further declines in body size for the youngest and oldest fish, leading to population crashes, altered food webs, and diminished ecosystem services.

It’s clear that ecological responses to climate change rarely follow simple rules. Temperature matters, but which temperature metric scientists examine can lead to vastly different conclusions. For Michigan’s fish, the picture is complicated, and understanding those complications might be key to predicting what comes next.

Paper Summary Methodology

Researchers analyzed 84,452 fish length measurements from 13 freshwater species collected between 1945 and 2020 across 1,497 inland Michigan lakes. The dataset combined historical records digitized through community science efforts with contemporary surveys from the Michigan Department of Natural Resources Status and Trends Program. Scientists used Bayesian hierarchical linear regression models to test for changes in length-at-age over time while controlling for lake characteristics (area, depth, water clarity) and sampling timing. They then employed boosted regression trees, a machine learning technique, to separate climate effects from other environmental factors including land use, lake attributes, and sampling biases. Climate was measured using two variables: growing degree days (cumulative warmth over a year) and mean annual surface water temperature during the ice-free period. All analyses were conducted separately for each age class within each species to account for age-specific growth responses.

Results

The study found that 37% of species age classes showed statistically significant decreases in length over the 75-year period, with 69% showing qualitative decreases. The youngest fish (ages 0-1) and oldest fish showed the largest declines, with age-1 fish decreasing by an average of 0.59 millimeters per year across species. Northern Pike showed declines across all age classes (0-10), while Largemouth Bass decreased for nearly all ages except age-12. Rainbow Trout ages 2-4 increased in length over time. Boosted regression trees revealed divergent responses to climate variables: cold and cool-water adapted fish typically increased in length with higher degree days but decreased with elevated surface temperatures above 17°C. Warm-water species decreased in response to both climate measures. The direction of length change over time corresponded with surface temperature responses for 62% of species. Day of year when fish were caught was the most important predictor overall, followed by lake area, maximum depth, and climate variables.

Limitations

The study faced several constraints. No individual lake contained data spanning the full 75 years, preventing researchers from tracking specific lake populations over time or using lake identity as a random effect. The dataset lacked sufficient measurements of factors like nutrient status, primary productivity, and detailed trophic interactions that influence fish growth. Adult age classes for cold-water species were poorly represented, limiting comparisons across life stages and thermal guilds. Different sampling methods were used historically versus contemporarily, though age-specific analyses minimized potential biases. Some species changed aging structures in 2015 (from scales to fin rays/spines), though no obvious trends appeared post-transition. The models explained on average 43% of variation in length, indicating unmeasured factors still influence fish growth. Finally, effects of stocking, harvest, and introduced species could not be fully disentangled from climate impacts.

Funding and Disclosures

This study was supported by the National Science Foundation Directorate for Biological Sciences (Grant No. 2228256). Funding for data curation was provided by a Michigan Institute for Data Science Propelling Original Data Science Grant. The authors declared no conflicts of interest. The research involved collaboration between the University of Michigan School for Environment and Sustainability and the Michigan Department of Natural Resources Institute for Fisheries Research.

Publication Information

Flood, P.J., Schiller, K.E., King, K.B.S., Runyon, A.D., Wehrly, K.E., & Alofs, K.M. (2025). “Long-term and regional-scale data reveal divergent trends of different climate variables on fish body size over 75 years,” published in Global Change Biology, 31, e70584. doi:10.1111/gcb.70584