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The planet’s fast-warming oceans will deal a double blow to some of their most iconic fish like the Great White Shark. Not only will such fish have to find areas of the sea which are cooler, there is no guarantee that they will be able to find suitable prey there, according to a new study.
Researchers from Trinity College Dublin in Ireland and the University of Pretoria in South Africa, conducted the study on what are known as ‘mesothermic’ or warm-bodied fishes like the Great White, the Basking Shark and tunas.
Mesothermic fishes are a rare group, comprising less than 0.1 per cent of all fish species. They can retain metabolic heat and keep parts of their bodies warmer than the surrounding seawater.
This ability, according to a statement by Trinity, has evolved independently several times in some sharks and tunas, enabling higher swimming speeds, long-distance migrations, and enhanced predatory performance.
The Great White and basking shark burn nearly four times more energy than their cold-blooded or ‘ectothermic’ counterparts. This means they are likely to face an increasing risk of overheating as oceans warm, which may result in a reduction of suitable habitat and an enforced relocation towards the poles.
The researchers analysed biologging data—from tiny sensors that record body and water temperatures— and calculated how much heat fish produce and lose in real time. They combined these new measurements, including data from huge basking sharks weighing up to 3.5 tonnes, with hundreds of lab measurements from smaller species.
According to Nicholas Payne from Trinity’s School of Natural Sciences, who is first author of the research paper, after accounting for body size and temperature, the team found that mesothermic fishes use about 3.8 times more energy than similarly sized ‘ectothermic’, or ‘cold-blooded’ fishes.
In addition, a 10°C increase in body temperature more than doubles a fish’s routine metabolic rate which, in practical terms, means warm-bodied predators must consume far more food to fuel their lifestyle.”
“But that heighted energy demand is only part of the story because as fish grow larger their bodies generate heat faster than they can lose it,” added Payne.
“This creates a mismatch driven by basic geometry and physics because bigger bodies retain heat more effectively, and in mesotherms, high metabolic rates amplify this effect.”
Andrew Jackson from Trinity’s School of Natural Sciences and a senior author of the research paper, noted that based on the data they were able to create theoretical ‘heat-balance thresholds’, which are the water temperatures above which large fish cannot shed heat quickly enough to maintain stable body temperatures without changing their behaviour or physiology.
“For example, a 1-tonne warm-bodied shark may struggle to remain in heat balance in waters above about 17°C. Above such thresholds, fish must slow down, alter blood flow, or dive into cooler depths to avoid dangerous warming but that comes at a cost too; it might be harder to find food, or catch it, for example — especially if your main weapon is speed and power,” said Jackson.
Under future warming scenarios suitable habitat for large mesotherms will shrink, and particularly so during summer months, according to the scientists. While some species, such as Atlantic bluefin tuna, can temporarily increase heat loss or dive to cooler waters, even they may be pushed to their limits if surface waters continue to warm.
The implications, said Payne, are really sobering. Many mesothermic fishes are already heavily impacted by overfishing of themselves and also their prey species, so their elevated energy needs make them especially vulnerable when their food becomes scarce.
The study has been published in the journal Science.