A new study published in eLife challenges the theory that aquatic animals, such as fish, will shrink due to global warming. The study reveals that warm water pollution actually increases growth rates but also death rates, leading to a population of younger but larger fish. These findings contradict earlier predictions regarding the impact of warming on natural ecosystems and highlight the need for large-scale experiments to test such predictions.
While it has been predicted that warmer aquatic ecosystems would result in faster growth rates for young fish, leading to smaller adult body sizes, such patterns have mainly been observed in small-scale experiments. Furthermore, previous studies that tested this prediction in natural environments have mostly been carried out on fish species subjected to fishing, where the fishing process itself can influence growth rates and body size.
According to Max Lindmark, lead author of the study and a researcher at the Swedish University of Agricultural Sciences, “studies into the effects of warming waters on fish from large-scale, semi-controlled experiments in natural settings are rare, yet they can provide unique insights.” To investigate the impact of warm water pollution, the team conducted their study in an enclosed coastal bay that received cooling water from a nuclear power plant, making it 5-10°C warmer than the surrounding waters. They compared Eurasian perch fish species from the enclosed bay and a reference area in the adjacent archipelago over a 24-year period.
The researchers combined data on fish catches with measurements of the fishes’ length-at-age (calculated from “age rings” in hard structures) to investigate how warm water pollution affected the age, size, growth, and death rates of fish populations. The study was conducted in an enclosed coastal bay that received cooling water from a nuclear power plant, making it 5-10°C warmer than the surrounding waters. The team compared Eurasian perch fish species from the heated and reference areas over a 24-year period.
The researchers found significant differences in growth rates, death rates, and sizes of fish populations between the heated and reference areas. However, not all changes were as expected. While female perch in the warm area grew faster throughout their lives, they also reached a larger size-at-age (around 7-11% larger) than those in the reference area. The increase in growth rate due to warm water was so pronounced that it led to a younger overall population of fish but with a higher average size and relative abundance of larger fish, which contradicts the prediction that global warming would shrink fish over time, especially the large and old ones.
According to co-author Malin Karlsson, the study provides strong evidence for warming-induced differences in growth and death rates among a natural population of an unexploited temperate fish species exposed to 5-10°C water temperature increases for over two decades. The findings suggest that generalized predictions based on theories such as the temperature-size rule may have limited use for predicting changes at a population level, and that both death rates and growth rates are important when studying temperature effects.
In conclusion, the study provides insights into the effects of heating at the scale of a whole ecosystem and its findings are highly relevant in the context of global warming. Although the researchers only studied a single species, their unique climate change experiment can be useful for predicting changes in other aquatic species.