ICYMI

Warm-bodied sharks and tunas face “double jeopardy” in warming seas – new research

Trinity College Dublin

Facebook

XEmail

 

image: 

The large and warm-bodied great white shark (Carcharodon carcharias) has high fuel demands and risks overheating in warm oceans”.

view more 

Credit: Andrew Fox

A new study reveals that some of the ocean’s most powerful predators are running hotter, and that they are likely paying an increasingly steep price for it. The significance of this headline finding is the “double jeopardy” in which it places these iconic animals, which have high fuel demands due to their lifestyle and physiology, as they now face a future of warming oceans and declining food resources.

The research, led by scientists at Trinity College Dublin in collaboration with the University of Pretoria’s (UP) Faculty of Veterinary Science, shows that warm-bodied fish such as tunas and some sharks, including the legendary Great White and Ireland’s iconic basking shark, burn nearly four times more energy than their cold-blooded 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 study, published today in leading international journal Science, focuses on “mesothermic” fishes, 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. This ability has evolved independently several times in some sharks and tunas, enabling higher swimming speeds, long-distance migrations, and enhanced predatory performance.

To understand the cost of this high-performance lifestyle, the Trinity and UP scientists developed a novel way to estimate metabolic rate in free-swimming fish. By analysing biologging data—from tiny sensors that record body and water temperatures—the team 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.

Dr Nicholas Payne, from Trinity’s School of Natural Sciences, is first author of the research paper. He 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,” adds Dr 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.” 

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

Professor Andrew Jackson from Trinity’s School of Natural Sciences is senior author of the research paper. He 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 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.” 

These findings seemingly help to explain long-observed patterns in the ocean, where large fishes tend to occur in cooler waters, at higher latitudes, or at greater depths. They also migrate seasonally, tracking favourable temperatures. 

Unsurprisingly, the scientists 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.

Dr Snelling, UP, says: “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.”

“The implications are really sobering as this new finding essentially places these animals in ‘double jeopardy,” adds Dr Payne. “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.” 

“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.”

What are the potential implications of this research?

Ultimately this crucial research provides a new framework for predicting which species are most at risk in a warming world and shows that many of the ocean’s fastest and most formidable predators may also be among its most physiologically constrained. As climate change accelerates, understanding the hidden heat budgets of marine giants could prove critical to conserving them.

Basking sharks feeding off the Irish coast [VIDEO] | 

---

Journal

Science

DOI

10.1126/science.adt2981 

Warm-Bodied Sharks And Tunas Face ‘Double Jeopardy’ In Warming Seas

April 18, 2026

By Eurasia Review


A new study reveals that some of the ocean’s most powerful predators are running hotter, and that they are likely paying an increasingly steep price for it. The significance of this headline finding is the “double jeopardy” in which it places these iconic animals, which have high fuel demands due to their lifestyle and physiology, as they now face a future of warming oceans and declining food resources.

The research, led by scientists at Trinity College Dublin in collaboration with the University of Pretoria’s (UP) Faculty of Veterinary Science, shows that warm-bodied fish such as tunas and some sharks, including the legendary Great White and Ireland’s iconic basking shark, burn nearly four times more energy than their cold-blooded 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 study, published in leading international journal Science, focuses on “mesothermic” fishes, 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. This ability has evolved independently several times in some sharks and tunas, enabling higher swimming speeds, long-distance migrations, and enhanced predatory performance.

To understand the cost of this high-performance lifestyle, the Trinity and UP scientists developed a novel way to estimate metabolic rate in free-swimming fish. By analysing biologging data—from tiny sensors that record body and water temperatures—the team 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.

Dr Nicholas Payne, from Trinity’s School of Natural Sciences, is first author of the research paper. He 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,” adds Dr 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.”

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

Professor Andrew Jackson from Trinity’s School of Natural Sciences is senior author of the research paper. He 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 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.”


These findings seemingly help to explain long-observed patterns in the ocean, where large fishes tend to occur in cooler waters, at higher latitudes, or at greater depths. They also migrate seasonally, tracking favourable temperatures.

Unsurprisingly, the scientists 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.

Dr Snelling, UP, says: “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.”

“The implications are really sobering as this new finding essentially places these animals in ‘double jeopardy,” adds Dr Payne. “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.”

“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.”

What are the potential implications of this research?

Ultimately this crucial research provides a new framework for predicting which species are most at risk in a warming world and shows that many of the ocean’s fastest and most formidable predators may also be among its most physiologically constrained. As climate change accelerates, understanding the hidden heat budgets of marine giants could prove critical to conserving them.

A Civilization May Yet Die

Laura Flanders

 April 13, 2026

FacebookTwitterRedditBlueskyEmail

A Dugong near Marsa Alam in Egypt. Photo by Camille Ménard, CC BY-SA 3.0 via Wikimedia Commons.

“A whole civilization will die tonight, never to be brought back again.”

As our power-mad president tapped out that genocidal threat, a whale shark in the Persian Gulf was navigating waters it has known for millennia. It is the largest fish alive¹ — the size of a school bus, spotted like a galaxy, moving with a gentleness that is hard to reconcile with its scale. If a diver comes too close, it curves its huge body to avoid them. Its wide-grin mouth is fangless. It feeds on plankton, with no capacity for threat, and yet its lineage survived the dinosaurs.

It may not survive us.

The ceasefire announced this week between the United States and Iran is scheduled to last two weeks — the duration of a single pay period, or the bloom of a rose. The timeline of the creatures that swim the Gulf waters is measured in millennia. Sixty million years in the case of the endangered whale shark. Fifty million in the case of the dugong.

I’d never heard of a whale shark or a dugong until I looked up the Gulf’s most precious species. Like large, bristled manatees, the dugongs have more in common with elephants than any whale. The Persian Gulf holds the world’s second-largest population of them. In the fragile waters now crammed with oil tankers brimming with billions of liters² of oil and war waste from 40 days of reckless destruction, the dugongs graze on seagrass in the shallows. Indeed, biologists report that they eat in such deliberate, complex patterns that they prevent monocultures from forming and maintain nursery habitat for dozens of other species.

Share

The mother dugong nurses her young, not for two weeks — but for two whole years. She is almost certainly the origin of the mermaid myth. Sailors glimpsed a large, warm-bodied creature nursing its young at the surface — and carried that image home through every maritime culture on Earth.

Robin Wall Kimmerer, who appeared on Laura Flanders & Friends earlier this year, might call what the dugong does reciprocity — the kind of tending that extractive capitalists have spent centuries refusing to see or value. In her book “Braiding Sweetgrass”, and in our conversation, she teaches us that the natural world is in constant reciprocal exchange. Care is not a human invention. The dugong and the seagrass meadow have been in that exchange for longer than our species has had language to describe it.

Civilizations come in many forms. A city, like a reputation, can be rebuilt; a warmonger defeated or unseated (please!) But a species — a multi million-year evolutionary experiment, a creature that has been tending its ocean since before our genus existed — cannot be negotiated back into existence.Thinking about all that we — certainly I — do not know about the whale shark and the dugong, I remember my conversation with Alexis Pauline Gumbs about her book “Undrowned: Black Feminist Lessons from Marine Mammals”. In that conversation (which podcast subscribers receive in full) Gumbs asks us to look at marine mammals not as curiosities but as teachers; beings whose ways of breathing, moving, and tending their young carry a wisdom we have barely begun to hear. She writes:

I don’t know what that will look like, but I do know that our marine mammal kindred are amazing at not drowning. So I call on them as teachers, mentors, guides, and I call on you as breathing kindred souls, may we evolve.

I have learned very little about the whale shark and the dugong. Dugongs communicate in chirps, whistles, and trills that scientists don’t fully understand. Biologists are bewildered by whale shark reproduction. No whale shark has ever been observed giving birth in the wild. They are, in a real sense, still mysterious to us. Extinction would end that mystery permanently.

Now, at least four oil spills have been confirmed in the Gulf since hostilities began.³ Our necrophile president bragged about torpedoing the Iranian frigate Dena⁴ early in the war. The Dena sank close to the coast of Sri Lanka, sending an 20 km oil slick towards that ecologically-important coastline. That same hot-headed killer and his Beirut-bombing pal launched multiple missile strikes at Iran’s Bushehr nuclear plant, where a direct hit could have released radioactivity across the entire region.⁵

The same dance with death is playing out in “our” Gulf, too. The Persian Gulf is a body of water that is semi-enclosed, shallow, with almost no natural flushing mechanism — meaning that what goes in, stays. The looney logic that concentrated the world’s fossil fuel infrastructure in these ecologically fragile waters is being replicated along coastal Louisiana, where liquefied natural gas export terminals are being built into wetlands already vulnerable to storm surge.

We reported on those risks last summer when we looked at LNG expansion on the same coast that, twenty years ago, was ravaged by Hurricane Katrina. By disrupting Gulf-based LNG supplies, the US/Israeli escapade is accelerating madcap LNG production on US shores.

There’s no truce – however shaky – in our war on the climate or the rising tides.

April is Earth Month. All month, on our podcast and our radio broadcasts, we will be airing our best Earth-related episodes.

A whale shark can live for 100 to 150 years. Their species has been around for 60,000 millennia. The dugong tends her meadow (and her young) and communicates in a language we haven’t learned yet. The ceasefire is two weeks long. The least we can do — the very least — is notice what is at stake in those waters beyond the oil price.

Laura Flanders interviews forward-thinking people about the key questions of our time on Laura Flanders & Friends, a nationally-syndicated radio and television program also available as a podcast. A contributing writer to The Nation, Flanders is the author of several books, as well as a column on Substack.  

 

High levels of forever chemicals in Svalbard reindeer

Heavy metals levels are stable or have dropped, but newer varieties of PFAS have skyrocketed in the animals.

Peer-Reviewed Publication

Norwegian University of Science and Technology

Facebook

XLinkedInWeChatBluesky
Email

 

image: 

So you want to study contaminants in Svalbard reindeer? Then you have to be willing to find and collect reindeer poop. 

view more 

Credit: Photo: Malin Andersson Stavridis/NTNU/UNIS

Researchers have long known that heavy metals and persistent organic pollutants such as DDT concentrate in the Arctic, in top predator animals like polar bears.

But what happens with animals like Svalbard’s reindeer, which graze on arctic plants?

How contaminated are these animals? How do they compare to other reindeer species, such as caribou, across the Arctic? What kinds of effects might those contaminants have?  And lastly, people eat Svalbard reindeer. Do they need to cut back?

PFAS levels were among the highest of all the reindeer across the Arctic and had increased dramatically over the past decade.

That knowledge gap led Malin Andersson Stavridis, who recently defended her PhD at NTNU, to spend four years in the northern archipelago of Svalbard to find the answers.

What she found was that levels of toxic metals in the animals – such as cadmium, lead and mercury – have been relatively stable for the last decade.

[caption id="attachment_66254" align="aligncenter" width="640"] Svalbard reindeer grazing on an open patch of vegetation surrounded by snow. Photo: Brage B. Hansen[/caption]

'What the heck?'

But what really shocked her was that levels of so-called forever chemicals, PFAS, have skyrocketed over that same period.

PFAS levels had spiked so precipitously that “I thought they must be wrong,” Stavridis said.

“To see that the concentrations are so high, maybe even the highest measured in reindeer, that was a not an ah-ha moment, but more like ‘what the heck?’” she said.

Her latest study, on exposure and accumulation of heavy metals and PFAS, or “forever chemicals,” in Svalbard reindeer, has just been published in Environmental Science and Technology.

From poop and fur to more comprehensive sampling

One of the challenges Stavridis faced was how to measure meaningful contaminant levels in the animals.

Earlier studies had relied on testing reindeer poop and hair to get this information. That’s an easy, non-invasive way to collect samples. But these sampling methods don't necessarily give a full picture of how pollutants may be affecting the animals.

“I was asking myself, can we say anything about the contaminants inside? Can we determine the contaminants in liver or soft tissues just based on what's in the fur? And I felt like that link was missing, especially since Svalbard reindeer are also consumed by people living on the island,” she said.

[caption id="attachment_87292" align="aligncenter" width="640"] Malin Stavridis at work, looking for reindeer to collect fresh reindeer droppings. Note the rifle on her backpack. It's required equipment, along with a flare gun, to scare away any polar bears that might cross their paths. Photo: Private[/caption]

She was lucky enough to piggyback on a huge international, interdisciplinary research project funded in part by the Research Council of Norway and the Svalbard Environmental Protection Fund and approved by the Governor of Svalbard in which 68 female reindeer were culled over three years.

That’s how in October, 2021, she found herself taking brain, kidney, liver, faeces and thigh samples from humanely culled Svalbard reindeer.

The following two years she would analyse fur, faeces, muscle and liver samples to determine contaminant concentrations.

Baseline mercury measurements

Mercury in the Arctic comes from natural sources, such as from forest fires or volcanoes. It’s also emitted as fossil fuel is burned.

How will we know what is happening to the mercury in the Arctic if we don't know what levels look like now?

Mercury can be airborne and ride the wind northward, or it may arrive in the bodies of fish and mammals that consume it in their food and concentrate it. Airborne mercury can be taken up by plants through the openings in their leaves, which is why the Arctic’s frozen soils contain a vast pool of the toxic metal, Stavridis said.

“Permafrost soils are just old plants that have been slowly building up over time. So that's how we have this very big accumulation of mercury in Arctic soils,” she said.

As their name suggests, these soils are permanently frozen, with a top layer that can freeze and thaw every year.

Of course, global warming is causing permafrost to thaw.

“So in that context, I just wanted to lay the foreground for how we can determine whether mercury concentrations are becoming bioavailable?,” she said. “How will we know what is happening to the mercury in the Arctic if we don't know what levels look like now?”

In the end, by comparing the different types of measurements, she was able to confirm that faeces could be used to monitor mercury levels in the animals. That’s good news, because for this one element in particular, there was no need to cull animals to keep tabs on this important contaminant.

Some toxic metals drop

Stavridis didn’t just want to take a snapshot of contaminants in Svalbard reindeer, she also wanted to know how these levels changed over the summer. Summertime is when reindeer are actively grazing and building up body fat so they can survive Svalbard’s long, dark, icy winters.

Levels of two toxic metals, cadmium and lead, had decreased compared to levels reported in the 1980s.

After that first sampling session in 2021, she took samples in August 2022 and October 2023.  October is key because that’s when the animals are reaching their highest weights. They’re putting on fat to survive the long, dark arctic winter.

That matters for the reindeer themselves, but it also matters for hunters on Svalbard. The hunting season starts in mid-August and ends in late September. Hunters are limited to one animal a year.

Not surprisingly, she found that contaminants such as mercury, cadmium and PFAS were higher in October than in August.

But levels of two other toxic metals, cadmium and lead, had decreased compared to levels reported in the 1980s.

The PFAS mystery

Most of these contaminants were similar to, or lower than what has been found in other reindeer subspecies across the Arctic. But not PFAS levels.

These were among the highest of all the reindeer across the Arctic and had increased dramatically over the past decade, she said.

In the last decade, she found, PFAS levels increased by more than 900 per cent, from roughly 0.6 nanograms per gram to 5.48 nanograms per gram.

The chemical “fingerprint” of PFAS from previous studies suggested the previous source of forever chemicals was from firefighting training in Longyearbyen.

But the latest measurements, in addition to being so high, also showed a different chemical fingerprint.

“Now we see a profile that's dominated by another type of PFAS, and that the concentrations are so high, maybe even the highest measured in reindeer,” she said.

The reason behind this dramatic increase remains a mystery, however.

“I only have my samples from the reindeer to go from. I can say that there is something that has affected their exposure. It could be as simple as they have a different diet,” she said.

But she isn’t satisfied with that answer.

“Even if they have a different diet, the levels shouldn't change that much in 10 years,” she said.

Hints of biological effects

There are limits as to what kinds of genetic studies Stavridis could do, because of the way samples she collected had to be handled.

Nevertheless she was able to screen for 20 different genes to see if they were upregulated, meaning that they might produce more of the substances they play a role in making, or downregulated, meaning that they would produce less.

Many of the genes she studied were related to lipid – or fat – metabolism, and appeared to be downregulated with increasing contaminant concentrations.

“Building fat and using fat is very important for these animals because they are undergoing such large fluctuations in body mass throughout the year,” she said. “This is such a vital function to have in an Arctic animal.”

Similar studies in polar bears and orcas have shown much the same, she said.

In this case, she found this association when the animals contained higher levels of a cocktail of contaminants. This particular cocktail contained a family of PFAS that has been found more recently in reindeer, plus mercury and cadmium.

Good news and not so good news

In the end, Stavridis found that each of the individual contaminant concentrations in Svalbard reindeer are below current wildlife toxicity thresholds. But the genes she studied suggested that even if individual contaminants are low, the combination can have an effect.

She also found that the animals can be eaten, but with limits. Svalbard allows hunters to take one animal per hunter per season. That means the overall exposure isn't that high.

However, “if a person wants to eat Svalbard reindeer, they can only consume 11.5 grams of liver per week over a year so as to not exceed the PFAS threshold,” she said.

Svalbard reindeer are “out there. They’re wild. They’re in the middle of nowhere,” she said. “So this is one pathway of exposure from something that you would assume is completely safe. And yet here we are.”

Reference:
Seasonal Shift in Exposure and Accumulation of PFAS and Heavy Metals in High Arctic Reindeer. Malin Andersson Stavridis, Tove Petersson, Görkem Deniz Kendir, Shannen Sait, Øyvind Mikkelsen, Vebjørn Veiberg, Tomasz Maciej Ciesielski, and Bjørn Munro Jenssen. Environmental Science & Technology 2026 60 (4), 3449-3458
DOI: 10.1021/acs.est.5c11066

Malin Stavridis at work, looking for reindeer to collect fresh reindeer droppings. Note the rifle on her backpack. It's required equipment, along with a flare gun, to scare away any polar bears that might cross their paths. 

Credit

Photo: Malin Andreasson Stravridis/NTNU/UNIS

---

Journal

Environmental Science & Technology

DOI

10.1021/acs.est.5c11066 

Method of Research

Experimental study

Subject of Research

Animals

Article Title

Seasonal Shift in Exposure and Accumulation of PFAS and Heavy Metals in High Arctic Reindeer