Warming waters have an unexpected impact on fish size

A study published today in the journal eLife challenges the theory that global warming will cause water-breathing animals, such as fish, to shrink in size. 

The researchers found that although warm water pollution did increase growth rates and death rates, it ultimately led to a population of younger and larger fish. 

This finding is somewhat at odds with general predictions of the effects of warming on natural ecosystems, emphasizing the need for large-scale experiments to test these predictions.

Previous research has suggested that as aquatic ecosystems warm up, animals like fish will experience faster growth at a young age but reach smaller body sizes as adults. 

This pattern has primarily been observed in small-scale experiments, and while some studies have tested this prediction in natural environments, these investigations have mostly focused on fish species subjected to fishing. This is significant because the process of fishing itself can influence growth rates and body size.

“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,” said lead author Max Lindmark, a researcher at the Swedish University of Agricultural Sciences. “We used a unique study system to investigate how warm water pollution has changed the death rates, growth rates, and size of fish over many generations.”

How the study was done

To conduct their study, Lindmark and his team examined an enclosed coastal bay that has been receiving cooling water from a nuclear power plant for years, making it 5-10°C warmer than the surrounding waters. 

The researchers compared the Eurasian perch from this heated bay to those from a reference area in the adjacent archipelago over a 24-year period. 

They combined data on catches with measurements of the fishes’ length-at-age (back-calculated throughout their life from “age rings” in hard structures) and used statistical models to investigate how warm water pollution affected the age and size of the fish populations, as well as their growth and death rates.

What the researchers learned

The researchers discovered statistically significant differences in estimated growth rates, death rates, and sizes of the fish populations between the heated and reference areas. However, not all of these changes aligned with their expectations. 

Although female perch in the warm area grew faster, as anticipated, they continued to do so throughout their lives. As a result, these fish reached a larger size-at-age, approximately 7-11% larger in the heated area at any age when compared with the reference area.

Moreover, the authors found that the increase in growth rate of younger fish due to warm water was so pronounced that even if death rates were higher because of warming, the average size and relative abundance of larger fish were still higher in the heated area. 

Why this outcome is disturbing

This outcome conflicts with the prediction that global warming would shrink fish over time, particularly the large and old ones. In this study, ecosystem warming led to younger, but larger fish, casting doubt on the widely accepted theory and stressing the need for more extensive research in natural settings.

“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,” said co-author Malin Karlsson. “These effects largely, but not completely, counteract each other – while the fish are younger, they are also larger on average.”

“These findings highlight that generalised 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,” said study senior author Professor Anna Gårdmark.

“Although we only studied a single species, this unique climate change experiment suggests the effects of heating at the scale of a whole ecosystem, making its findings highly relevant in the context of global warming.”

More about climate change and fish populations

Climate change and warming waters have significant impacts on fish populations globally. These impacts can manifest in various ways, including changes in distribution, abundance, growth rates, reproductive success, and species interactions. Some of the key ways in which climate change affects fish populations are:

Shifts in distribution

As ocean temperatures rise, many fish species are moving towards cooler waters, either by migrating to higher latitudes or deeper depths. This shift in distribution can lead to changes in local and regional fish populations, potentially affecting the ecosystems and communities that rely on these species.

Altered growth rates and body sizes

Warming waters can influence the growth rates of fish, with some species growing faster at a younger age, as seen in the eLife study. However, the long-term effects of these changes on adult body sizes are still debated and may vary among different species and ecosystems.

Changes in reproductive success

Warmer temperatures can impact fish reproduction in various ways, such as affecting the timing and success of spawning, the development of eggs and larvae, and the survival of juvenile fish. These changes can ultimately impact the recruitment and abundance of fish populations.

Species interactions

Climate change can also affect the interactions between fish species and their predators, prey, and competitors, leading to changes in food webs and ecosystem dynamics. The potential for new species interactions can emerge as fish shift their distributions, which can create complex cascades of effects on ecosystems.

Ocean acidification

As the oceans absorb more carbon dioxide from the atmosphere, their pH level decreases, causing ocean acidification. This process can have harmful effects on fish, particularly on the early life stages, by impairing their ability to form shells or skeletons and impacting their sensory systems.

Deoxygenation

Warmer waters can hold less dissolved oxygen, and this, combined with increased stratification of the water column, can lead to the development of low-oxygen zones. These conditions, known as hypoxia, can be detrimental to fish populations, as they limit the available habitat and can cause physiological stress or even death.

Habitat loss and degradation

Climate change can also contribute to the loss and degradation of critical fish habitats, such as coral reefs, seagrass beds, and mangroves, which provide important breeding, nursery, and feeding grounds for many fish species.

These various impacts of climate change on fish populations can have significant consequences for global food security, livelihoods, and ecosystem health. It is essential for scientists, policymakers, and resource managers to continue studying and monitoring these changes to develop effective strategies for mitigating and adapting to the effects of climate change on fish populations and the broader marine environment.

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