A groundbreaking new study led by researchers at the University of Michigan reveals that climate change is profoundly impacting the body sizes of fish inhabiting Michigan’s inland lakes. By analyzing an extensive dataset spanning 75 years and encompassing nearly 1,500 individual lakes, the study uncovers striking evidence that, for multiple fish species, both juvenile and adult specimens captured in 2020 were noticeably smaller than their counterparts observed in the mid-20th century, precisely around 1945. This large-scale temporal analysis offers vital insights into how global warming is reshaping freshwater ecosystems on a regional scale.
The research, helmed by Peter Flood, a postdoctoral fellow at the University of Michigan School for Environment and Sustainability (SEAS), draws attention to a pattern of shrinking fish sizes attributable to ongoing climatic shifts. Using historic data digitized through a pioneering community science initiative, Flood and colleagues document a consistent trend of diminished lengths across numerous species and age classes. Their findings, recently published in the journal Global Change Biology, show that out of 125 species-age groups studied, nearly half exhibited changes in size, with 46 displaying statistically significant reductions.
One of the pivotal technical advancements underpinning this study was the digitization of decades-old field data collected by the Michigan Department of Natural Resources (DNR) and its predecessors, made accessible through the collaborative platform Zooniverse. This crowdsourced effort enabled research teams to efficiently quantify fish sizes and ages from community-curated observation records, unlocking a treasure trove of ecological information that would have otherwise remained inaccessible. This novel approach exemplifies how citizen science can directly empower high-resolution, longitudinal ecological research.
The shrinking size trends identified were especially pronounced in the youngest and oldest fish within the surveyed populations. This is ecologically consequential because both age groups serve critical, yet distinct, roles in sustaining population dynamics and ecosystem functions. Juvenile fish size affects their vulnerability to gape-limited predators—predators restricted by the maximum size of prey their oral cavity can accommodate. Smaller juveniles face elevated predation risks, potentially reducing recruitment and future population stability. Meanwhile, older fish, although less pivotal for reproduction, exert substantial influence over social dynamics and ecological resilience within fish communities, acting as reservoirs of behavioral knowledge and ecosystem regulation.
Beyond ecological ramifications, these shifts in fish body size have profound implications for fisheries management and conservation efforts. Agencies like the Michigan DNR rely heavily on size and catch limits to maintain sustainable fish populations. As climate change alters the expected growth and survival patterns of fish, these management frameworks must adapt to preserve both ecological stability and angling opportunities. Flood emphasizes that understanding size trajectories across age classes equips resource managers with refined tools to anticipate and mitigate climate-driven biological perturbations.
The methodology for aging fish employed in the study involves detailed analysis of scale ring patterns, similar to dendrochronology in trees. As fish grow, their scales develop incremental growth rings that serve as annual markers, enabling precise age determination. This scale-based aging technique, combined with extensive sampling efforts across lakes and timeframes, allowed researchers to stratify size data by fish age, revealing nuanced growth trends obscured in bulk population analyses.
Lead author Flood’s team also benefited from data spanning the Institute for Fisheries Research, a long-standing collaboration between the university and Michigan’s DNR. This partnership has amassed unparalleled records on inland lake fishes, now further enhanced by modern digitization efforts. The continual collection and integration of contemporary data permit ongoing monitoring of population responses in real time, a crucial advantage for adapting to rapid climate shifts.
Senior author Karen Alofs, an associate professor at SEAS, has been instrumental in contextualizing these findings within broader ecological change. Her research integrates historical and present-day population metrics to uncover how warming waters facilitate species shifts, such as increased abundance of warm-adapted largemouth bass, and delayed fish mortality events associated with altered ice phenology. These complementary trends underline the cascading effects of climate change across multiple ecological axes—size, abundance, phenology.
Intriguingly, the study team is now expanding their temporal horizon by incorporating fish specimens from the University of Michigan Museum of Zoology’s extensive collections, which house over 3.5 million global fish specimens. This unique archival resource enables retrospective analyses extending much further back in time and across species less commonly studied due to their minimal commercial importance. Such deep-time perspectives promise to illuminate evolutionary and ecological responses to environmental variability on scales rarely documented in freshwater systems.
While this research spotlights Michigan’s inland lakes, the implications resonate more broadly. Freshwater ecosystems worldwide are vulnerable to climate-driven stressors, with size shifts in fish representing a biomechanistic indicator of environmental change that influences trophic interactions, ecosystem services, and human livelihoods. Flood and colleagues’ study illustrates how historic data, coupled with innovative community science, can transform our understanding of these complex biological responses.
“Fish size is more than a biological trait; it’s a vital signal of ecological health and stability,” Flood notes. “Our findings highlight the urgent need to factor body size dynamics into conservation and management strategies as climate change reshapes aquatic ecosystems globally.” Continued interdisciplinary collaboration, innovative data integration, and public engagement remain critical for advancing this frontier of climate biology.
In conclusion, this comprehensive investigation underscores the multifaceted ways climate change influences fish morphology and community structure across temporal and spatial scales. By leveraging historic records, community science platforms, and museum archives, researchers present a nuanced and compelling narrative of ecological transformation. This emerging knowledge frontier sets the stage for targeted interventions aimed at preserving biodiversity, fisheries productivity, and ecosystem function in a warming world.
Subject of Research: Climate change impacts on fish body size in Michigan’s inland lakes over 75 years
Article Title: Long-term and regional-scale data reveal divergent trends of different climate variables on fish body size over 75 years
News Publication Date: 5-Nov-2025
Web References:
- Peter Flood Lab at SEAS
- Community Science Digitization Project
- Largemouth Bass Abundance Study
- Mass Mortality Timing Study
- UM Museum of Zoology, Division of Fishes
- DOI Link to Published Paper
References:
Flood, P. J., Alofs, K., King, K., Wehrly, K., Schiller, K., Runyon, A. (2025). Long-term and regional-scale data reveal divergent trends of different climate variables on fish body size over 75 years. Global Change Biology. DOI: 10.1111/gcb.70584
Image Credits: Peter Flood
Keywords: Climate change, fish body size, inland lakes, Michigan, fisheries management, long-term ecological data, community science, fish aging, predator-prey interactions, aquatic ecosystems, biodiversity, museum specimens
