"Ocean Warming Poses Greater Threat to Survival of Mesothermic Fish Like Sharks and Tuna"

Mesothermic Fish Struggle to Maintain Thermal Balance
Require Cooler Waters to Dissipate Body Heat
Rising Ocean Temperatures Increase Extinction Risk

A recent study has found that ocean warming could threaten the survival of mesothermic fish, such as tuna and sharks, which are species capable of maintaining their body temperature above ambient levels. This is because, although these fish generate heat quickly within their bodies, they require cooler water to dissipate that heat.

Image of a shark to aid in understanding the article.

According to Yonhap News on April 17, a team led by Professor Nicholas Payne at Trinity College Dublin in Ireland reported in the journal Science that, based on estimates of metabolic rates using changes in fish body temperature and heat exchange, large mesothermic fish such as sharks and tuna require nearly four times more energy for maintaining body temperature and hunting compared to other fish species.

Mesothermic fish do not regulate their body temperature as strictly as mammals or birds, but they do keep it significantly higher than the surrounding environment. This ability, along with their large body size, enables them to dominate as apex predators in marine ecosystems. However, their metabolic rates—including energy consumption and thermoregulation, which influence growth, food requirements, migration abilities, and even extinction risks—remain insufficiently understood.

In this study, the researchers attached sensors to large fish such as bluntnose sixgill sharks, which weigh between 800 and 3,500 kilograms, to measure changes in body temperature and surrounding water temperature. They combined these data with existing respiration experiment results to analyze the routine metabolic rates (RMR) of a total of 137 species, ranging from larvae weighing just 1 milligram to sharks weighing up to three tons.

The results showed that mesothermic fish consume 3.8 times more energy than typical ectothermic fish, which rely on external temperatures to regulate their body heat. The study also found that as body weight increases, the rate of internal heat generation rises rapidly, while the rate at which heat escapes into the seawater increases more slowly. As a result, larger fish tend to have higher body temperatures, and this trend was especially pronounced among mesothermic species. Consequently, maintaining thermal balance becomes challenging above certain water temperatures.

For example, ectothermic fish weighing two tons can tolerate water temperatures up to around 27 degrees Celsius, but mesothermic fish weighing 500 kilograms struggle to maintain thermal balance at around 20 degrees Celsius, and those weighing one ton face difficulties even at 17 degrees Celsius. If water temperature exceeds these limits, the fish must either reduce their swimming speed to lower heat production or move to colder waters to bring down their body temperature; otherwise, their body heat will continue to rise, threatening their survival.

The research team stressed, "For large mesothermic fish, the rate of heat generation increases faster than the rate of heat loss as body size increases. This means that as ocean temperatures rise due to global warming, it will become harder for these fish to cool themselves, increasing their risk of extinction." They added, "The problem is that ocean temperatures continue to rise due to climate change. As seawater becomes warmer, mesothermic fish require even more energy and face greater risks of overheating, which could pose an even greater threat to already diminished populations of large fish due to overfishing."

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