Study says cold-blooded animals grow larger, not smaller, in warmer waters

Scientists expected cold-blooded animals would grow to have smaller bodies as adults. This may not be true.

May 20, 2023 03:30 pm | Updated 08:50 pm IST - Chennai

An illustration of European perch (Perca fluviatilis), c. 1879.

An illustration of European perch (Perca fluviatilis), c. 1879. | Photo Credit: Alexander Francis Lydon, public domain

Scientists have called into question an idea that the bodies of cold-blooded marine animals, such as fish, will shrink in a warming world.

A study published last week said that aquatic animals such as fish would actually grow larger due to warm water pollution, i.e. water heated by warm pollutants.

As the planet warms, scientists had expected that cold-blooded animals, or ectotherms, would grow faster at a younger age but reach smaller body sizes as adults. They had observed this pattern in small-scale studies, including in natural settings.

Eurasian perch

The new study, published in the journal eLife, was conducted by researchers from the Swedish University of Agricultural Sciences, Uppsala. They studied samples of fish called Eurasian perch (Perca fluviatilis) collected over 24 years, to investigate the effects of warming on the ‘body sizes’ of ectotherms.

These fish were taken from an enclosed bay into which a nuclear power plant nearby discharged water. This water heated the bay by 5-10° C over its surroundings.

They collated older data to estimate the ‘normal’ death rates, population size, and distribution for this fish, at various ages. Then they compared this to the corresponding data for perch from the bay as well as perch from a water-body nearby that wasn’t being heated by nuclear discharge. Finally, they combined this with knowledge of the fishes’ sizes at different ages.

According to the paper, the researchers studied only female fish because male and female perch usually have different body sizes, making direct comparisons harder.

Study findings

The researchers found that all the female perch in the bay grew faster and were significantly larger than those from the nearby water-body. So their body sizes were larger than they ought to have been at all ages, by 7-11%. The affected fish also had higher growth and death rates than the unaffected ones.

And while the death rates were higher in the warmer areas – perch here lived for 0.4 fewer years – their bodies were nonetheless 2 cm larger than the perch elsewhere.

The team also reported that although the female perch in the warmer area grew faster, as expected, they continued to do so throughout their lives, which was surprising.

The researchers also report that even if the water was so warm as to be inhospitable to the fish, the boost to growth was so pronounced that there were more young-and-large fish in the area.

Faster, larger

“The primary significance I think is that fish grow faster when it gets warmer,” the paper’s Max Lindmark told The Hindu in an email. Dr. Linkmark is a researcher at the Department of Aquatic Resources, Institute of Marine Research, SUAS.

“This is well known for small and young fish, but we don’t observe that this positive effect disappears when they grow bigger and older. In fact, it’s maintained throughout life: in contrast to common predictions, not only do fish reach their maximum body size faster, they also reach a larger maximum body size.”

But since the rate at which the fish die is also higher, even the better growth potential doesn’t translate to the average size being much bigger, he added.

The researchers concluded that their findings contradict the prediction that global warming shrinks fish over time.

The author is an intern at The Hindu.

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