Great white sharks face ‘double jeopardy’ from warming oceans

By Stephen Beech

Great white sharks face a "double jeopardy" extinction threat due to global warming, warns a new study.

The fearsome ocean predators have high "fuel" demands due to their lifestyle and physiology, say scientists.

But they and other warm-bodied fish, such as tuna and basking sharks, face the dual threat of warming oceans due to climate change and declining food resources.

The new study, led by Irish and South African scientists, shows that warm-bodied fish burn nearly four times more energy than their cold-blooded counterparts.

That means they are being increasingly pushed to their physical limits and 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 toward the poles.

The research team say their "sobering" findings should set "alarm bells ringing loudly" for shark conservation.

The study, published in the journal Science, focuses on "mesothermic" fish, a rare group comprising fewer than 0.1% of all fish species, which can retain metabolic heat and keep parts of their bodies warmer than the surrounding seawater.

Scientists say the ability has evolved independently several times in some sharks and tunas, enabling higher swimming speeds, long-distance migrations, and enhanced hunting skills.

To understand the cost of their "high-performance" lifestyle, the research team formulated a new way to estimate metabolic rate in free-swimming fish.

By analyzing biologging data — from tiny sensors that record body and water temperatures — the researchers calculated how much heat fish produce and lose in real time.

The team combined the new measurements, including data from huge basking sharks weighing up to 3.5 tonnes, with hundreds of lab measurements from smaller species.

Study first author Nicholas Payne, from Trinity College Dublin's School of Natural Sciences, said: "The results were really quite striking — after accounting for body size and temperature, we 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.

"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."

The researchers found that larger fish become increasingly "warm-bodied" simply because of this imbalance, and it is that scaling mismatch that creates an "overheating dilemma" with significant implications for these species.

Senior author Andrew Jackson, also from Trinity College, said: "Based on the data we 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 behavior or physiology.

"For example, a one-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."

The research team say their findings seemingly help to explain long-observed patterns in the ocean, where large fish tend to occur in cooler waters, at higher latitudes, or at greater depths.

They also migrate seasonally, tracking favorable temperatures.

The research team predict that under future warming scenarios suitable habitat for large mesotherms will shrink, and particularly so during summer months.

And 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.

Study co-author Ned Snelling, of the University of Pretoria in South Africa, said: "This research shows that being a high-performance predator in the ocean comes at a greater cost than we previously appreciated.

"As the oceans warm, these species are being pushed closer to their physiological limits, which could have consequences for where they can live and how they survive.

"What's particularly concerning is that these animals are already operating on a tight energy budget, and climate change is narrowing their options even further.

"Understanding these constraints is essential if we want to predict how marine ecosystems will shift in the coming decades."

Payne said: "The implications are really sobering as this new finding essentially places these animals in 'double jeopardy.'

"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."

He added: "Fossil evidence suggests that warm-bodied marine giants, like the infamous extinct megalodon shark, suffered disproportionately during past climate shifts when seas changed and today's oceans are changing at unprecedented speeds, so the alarm bells are ringing loudly at this point."