Warm-Blooded Fish are hit Double by Warming Oceans
New research shows that larger warm-blooded fish species, such as sharks and tuna—known as mesotherms—are at risk of overheating as sea temperatures rise.
Text: Marika Griehsel
Mesothermic species, which account for just under 0.1 percent of all fish, rapidly convert food into energy and heat. This makes them exceptionally agile and is likely the reason they are apex predators in the ocean.
However, according to the study, their higher body temperature makes them vulnerable.
Mesotherms can retain metabolic (self-generated) heat and keep parts of their bodies warmer than the surrounding water.
This means they generate heat faster than they can dissipate it and therefore prefer to live in cold water currents and at ocean depths.
But as seawater warms due to climate change, these fish also become warmer. They are forced to migrate to more suitable habitats, such as the poles, both in search of food and for their well-being.
Energy flows examined in detail
In the study, researchers have, for the first time, examined in detail how energy flows work—from the tiniest creatures, such as larvae in the ocean, to large fish—in rapidly warming marine environments.
The findings have been published, among other places, in the scientific journal Science, where the international research team describes the striking results.
“For example, a 1-ton warm-blooded shark may have difficulty maintaining its thermal balance in water above about 17 °C,” explains senior author Andrew Jackson of Trinity College in Ireland in the article in Science. Above such thresholds, fish must slow down, alter their blood flow, or dive to cooler depths to avoid dangerous overheating. But this also comes at a cost. For example, it can become harder to find food or catch it, especially if your primary weapon is speed and power.
The researchers developed a method to estimate the basal metabolic rate of larvae and fish by analyzing the heat exchange of tagged individuals and combining the results with published respiration data for the species.
Using this framework, the authors then evaluated how body size, ambient temperature, and heat-conserving physiological adaptations affect energy requirements.
The results show that mesothermic fish require nearly four times more energy than their cold-blooded counterparts.
Furthermore, the analysis revealed a scaling imbalance between heat production and heat loss, where the rate of heat production increases faster than heat loss as fish species grow larger, meaning that larger mesothermic fish become increasingly warm-blooded.
The vast majority of fish are cold-blooded and cannot regulate their own body temperature.
“After accounting for body size and temperature, we found that mesothermic fish use about 3.8 times more energy than cold-blooded fish of similar size,” explains senior author Andrew Jackson of Trinity College in Ireland in the Science article. “Based on the data, we were able to establish theoretical ‘heat balance limits’—that is, the water temperatures above which large fish cannot dissipate heat quickly enough to maintain stable body temperatures without altering their behavior or physiology.”
Warm-blooded fish face dual threats
The Earth’s oceans have absorbed about 90% of the excess heat trapped in the atmosphere due to increased concentrations of greenhouse gases resulting from human activity.
“As the oceans warm, these warm-blooded species are pushed closer to their physiological limits, which can have consequences for where they can live and how they survive,” says Edward Snelling.
“What is particularly concerning is that these animals already live on a tight energy budget, and climate change limits their options even further. Understanding these constraints is crucial if we want to predict how marine ecosystems will change in the coming decades.”
The consequences are truly serious, as this new discovery effectively exposes these animals to a “double threat, according to the research team.
“Many mesothermic fish are already heavily impacted by overfishing of both themselves and their prey, so their increased energy needs make them particularly vulnerable when food becomes scarce,” says Edward Snelling.
Fossil findings suggest that warm-blooded marine giants, such as the infamous extinct shark Megalodon, were disproportionately affected during past climate changes when the oceans shifted. Today’s oceans are changing at an unprecedented rate, so the alarm bells are ringing loudly right now, according to the study.
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