Reevaluating the Impact of Global Warming on Fish Size

by François Dupont
Global warming's impact on fish size

A recent scientific study challenges the prevailing notion that global warming will reduce the size of water-breathing creatures, particularly fish. Contrary to expectations, the research suggests that warm water pollution actually leads to larger fish, albeit at younger ages, due to accelerated growth and mortality rates. This study raises doubts about generalized predictions related to climate change and emphasizes the importance of conducting large-scale, semi-controlled experiments in natural environments.

A groundbreaking 24-year investigation on freshwater fish exposed to warm water pollution has uncovered changes in growth, mortality rates, and size, but the findings do not align entirely with initial hypotheses.

Recently published in the journal eLife, a new study refutes the theory that global warming will result in a decrease in the size of aquatic organisms such as fish.

The research reveals that while warmer water pollution increases both growth and mortality rates, it leads to a population of younger yet larger fish. This discovery challenges conventional predictions about the effects of global warming on natural ecosystems and underscores the necessity for rigorous experimental validation on a large scale.

As aquatic ecosystems continue to warm, it has been predicted that fish and other animals will experience accelerated growth during their early stages but reach smaller sizes as adults. This pattern has been primarily observed in small-scale experiments, and though some studies have examined this prediction in natural settings, they have often focused on fish species that are subject to fishing, a process that can itself influence growth rates and body size.

“Studies investigating the impact of warming waters on fish through large-scale, semi-controlled experiments in natural environments are rare, yet they provide invaluable insights,” explains lead author Max Lindmark, a researcher at the Swedish University of Agricultural Sciences, Department of Aquatic Resources in Lysekil, Sweden. “We utilized a unique study system to examine how warm water pollution has altered the death rates, growth rates, and size of fish over numerous generations.”

The study was conducted in a confined coastal bay that receives heated water from a nuclear power plant, resulting in temperatures 5-10°C higher than the surrounding waters. The researchers compared populations of Eurasian perch from the heated bay with those from a reference area in the nearby archipelago over a span of 24 years. They combined catch data with measurements of the fish’s length at different ages, calculated using “age rings” in hard structures. Statistical models were then employed to analyze these data and investigate the effects of warm water pollution on the age, size, growth, and death rates of the fish populations.

Although the researchers observed statistically significant differences in growth rates, death rates, and sizes of fish populations between the heated and reference areas, not all of these changes aligned with their initial expectations. While female perch in the warm area exhibited faster growth, as predicted, this growth persisted throughout their lifespan.

Consequently, these fish attained larger sizes at each age, approximately 7-11% larger than their counterparts in the reference area. Moreover, the study’s authors note that the increase in the growth rate of younger fish due to warm water was so significant that even with higher death rates caused by warming, resulting in an overall younger fish population, the average size and relative abundance of larger fish remained greater in the heated area. This trend contradicts the prediction that global warming would gradually reduce fish size, especially among larger, older individuals. Essentially, ecosystem warming led to younger yet larger fish in this study.

“Our study provides compelling evidence of warming-induced disparities in growth and death rates within a natural population of an unexploited temperate fish species exposed to 5-10°C increases in water temperature over two decades. These effects largely offset 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.

“These findings emphasize that generalized predictions based on theories like the temperature-size rule may have limited applicability when predicting changes at the population level. Both death rates and growth rates are important factors to consider when studying the effects of temperature,” concludes senior author Anna Gårdmark, Professor at the Swedish University of Agricultural Sciences, Department of Aquatic Resources in Uppsala, Sweden.

“Although we focused on a single species, this exceptional climate change experiment provides insights into the effects of warming at the scale of an entire ecosystem, making its findings highly relevant in the context of global warming.”

Reference: “Larger but younger fish when growth outpaces mortality in a heated ecosystem” by Max Lindmark, Malin Karlsson, and Anna Gårdmark, 9 May 2023, eLife.
DOI: 10.7554/eLife.82996

Frequently Asked Questions (FAQs) about Global warming’s impact on fish size

What does the recent study suggest about the impact of global warming on fish size?

The recent study suggests that warm water pollution, caused by global warming, leads to larger but younger fish. It challenges the conventional belief that global warming will shrink fish size.

How does warm water pollution affect fish growth and mortality rates?

Warm water pollution increases both the growth and mortality rates of fish. While fish grow faster in warmer waters, resulting in larger sizes, the accelerated mortality rates lead to a younger overall population of fish.

What are the implications of the study for climate change predictions?

The study calls into question generalized climate change predictions. It highlights the need for large-scale, semi-controlled experiments in natural settings to better understand the effects of warming waters on fish populations.

How was the study conducted?

The study was conducted over a 24-year period, comparing populations of Eurasian perch in a heated coastal bay receiving warm water from a nuclear power plant to those in a reference area. The researchers analyzed catch data and measured the length-at-age of the fish to investigate the effects of warm water pollution on growth, mortality rates, and size.

Are the study findings applicable to other species and ecosystems?

While the study focused on Eurasian perch in a specific ecosystem, the findings provide insights into the effects of warming waters on fish populations and may have implications for other species and ecosystems impacted by global warming.

More about Global warming’s impact on fish size

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FishWhisperer July 12, 2023 - 1:48 pm

okay, this study blows my mind. warm water pollution makes fish bigger but younger? that’s cray cray! but wait, what about other fish species and ecosystems? do they do the same thing or nah? more research needed!

FishLover123 July 12, 2023 - 2:45 pm

wow this study is mindblowing it says global warming might make fish bigger but younger this is so cool! i mean we always thought warming would shrink fish but this says noooo! so interesting!!!

ClimateWarrior22 July 12, 2023 - 6:13 pm

this research really messes up all the predictions about climate change and fish size like whoa! it’s so important to do big experiments in nature and not just tiny ones in labs. we gotta rethink everything!

SciEnthusiast July 12, 2023 - 7:32 pm

this study is like a big red flag! it shows that warm water pollution makes fish grow faster and die quicker. that’s a big deal for the whole ecosystem. we need to pay attention and take action!

NatureLover77 July 12, 2023 - 9:51 pm

love how this study challenges what we thought we knew about the impact of global warming on fish size. it’s time to think bigger, do more experiments in real-world settings, and really understand what’s happening in our changing environment.


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