The theory that water-breathing animals like fish are caused by global warming has been called into question by a study published today in eLife.
The study found that warmer water pollution increased growth rates but also mortality rates, resulting in more, younger fish. This finding partially contradicts general expectations of the effect of warming on natural ecosystems and highlights the need to test them in large-scale experiments.
As aquatic ecosystems become warmer, animals such as fish can be expected to grow faster at a young age but reach smaller body sizes as adults. This pattern has been mainly observed in small experiments, and although some studies have tested this prediction in natural settings, these have mostly been performed on fish species that are subject to fishing, as the fishing process itself can affect growth rates and body size.
says lead author Max Lindmark, a researcher at the Swedish University of Agricultural Sciences Department of Water Resources, Lysekil, Sweden. “We used a unique study system to investigate how pollution of warmer waters has altered mortality rates, growth rates, and fish size over many generations.”
The team conducted their study in an enclosed coastal bay that received cooling water from a nuclear power plant, making it 5-10 degrees Celsius warmer than the surrounding waters. They compared species of Eurasian perch from the enclosed bay and from a reference area in the neighboring archipelago over a 24-year time period. They combined data on catches with measurements of fish length at age (calculated retrospectively over their lives from ‘age rings’ in scleral structures), and then analyzed them using statistical models to investigate how warm water pollution affects lifespan, population size, and their rates of growth and death.
While the researchers found statistically significant differences in estimated growth rates, mortality rates, and fish population sizes between the hotspot and reference areas, not all of these changes were what they expected. Although female perch in the warm region grew faster, as the team expected, they continued to do so throughout life. So these fish reached significant size in lifetime – about 7-11% larger in the heated area at any age, than in the reference area. Furthermore, the authors say that the increase in the growth rate of younger fish due to warming water was so pronounced that even if mortality rates were higher due to warming, and the population of younger fish increased, the average size and relative abundance of larger fish . It was still higher in the hot zone. This trend runs counter to the prediction that global warming will shrink fish over time, especially large and old fish. Essentially, warming the ecosystem instead resulted in smaller but larger fish in this study.
“Our study provides strong evidence for warming-induced differences in growth and mortality rates among natural populations of unexploited temperate fish species exposed to increases in water temperature of 5–10°C for more than two decades. These effects are largely, but not entirely, inconsistent. fish with each other – while the fish are younger, they are also older on average,” says co-author Malin Carlson, Director of Water in the Department of Nature and Environment, Västmanland County Board, Sweden.
“These findings highlight that generalized predictions based on theories such as the temperature magnitude rule may have limited use for predicting changes at the population level, and that mortality rates and growth rates are important when examining the effects of temperature,” concludes senior author Anna Gardmark, Professor at Swedish University of Agricultural Sciences, Department of Water Resources, Uppsala, Sweden. “Although we only studied one species, this unique climate change experiment indicates the effects of heating at the scale of an entire ecosystem, making its findings highly relevant to the context of global warming.”