Fish in warmer water grow larger, but at what cost?

A new study published in eLife has challenged the theory that global warming will cause aquatic animals like fish to shrink. Research indicates that warm water pollution can increase growth rates, but can also lead to higher mortality rates, resulting in a larger but younger fish population.

These findings contradict some general predictions about the impact of warming on natural ecosystems, underscoring the importance of large-scale experiments to test them.

Warming waters affect fish growth

The study was conducted over a 24-year period in an enclosed coastal bay that has received cooling water from a nuclear power plant, which made it 5-10°C warmer than surrounding waters.

The research found that female perch in the warm area grew faster and reached a larger size-at-age compared to the reference area. These findings contradict predictions based on the temperature-size rule and highlight the need for large-scale experiments to test theories regarding the impact of warming on natural ecosystems.

The study suggests that warming-induced differences in growth and death rates among a natural population of an unexploited temperate fish species exposed to water temperature increases over two decades largely counteract each other, leading to a population of younger, but larger fish.

Using a combination of catch data and back-calculated length-at-age measurements of hard structures, the researchers analyzed the effects of warm-water pollution on fish populations. Despite finding significant differences in growth rates, mortality rates, and sizes between heated and reference areas, not all changes were in line with their expectations.

Although female perch grew faster in the warm area as predicted, they continued to do so throughout their lives, resulting in an increased size at age of approximately 7-11% compared to the reference area.

Study reveals warming's impact on fish

Max Lindmark, lead author and researcher at the Department of Aquatic Resources at the Swedish University of Agricultural Sciences said that large-scale experiments in natural settings can provide unique insights into the effects of warming waters on fish, although they are rare.

In the study, the team used a unique survey system to investigate the impact of warm water pollution on growth rates, mortality rates, and fish size over many generations.

"Our 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 more than two decades. These effects largely, but not completely, counteract each other—while the fish are younger, they are also larger on average," says co-author Malin Karlsson, Water Manager at the Department of Nature and Environment, County Administrative board of Västmanland, Sweden.

Lead author Anna Gårdmark, Professor at the Swedish University of Agricultural Sciences, Department of Aquatic Resources, Uppsala, Sweden, concludes that these findings demonstrate the limited use of generalized predictions based on theories such as the temperature-size rule to predict changes in a population level.

The study highlights the importance of considering both death rates and growth rates when studying the effects of temperature. Although the study focused on a single species of fish, its results suggest the potential effects of ecosystem warming on a larger scale, making her findings highly relevant to the context of global warming.

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