New research has cast a spotlight on a pressing environmental challenge in our oceans: the overheating crisis affecting warm-bodied fish like sharks and tuna. Published in the prestigious journal Science, the study reveals that these mesothermic species—capable of generating and retaining heat in their bodies—burn nearly four times more energy than their cold-blooded counterparts to maintain their elevated temperatures. As global oceans warm due to climate change, this heightened metabolic demand places these apex predators in a precarious position, forcing them to consume more food at a time when prey populations are shifting and declining.

The findings come from a collaborative effort led by scientists at Trinity College Dublin in Ireland, with contributions from the University of Pretoria in South Africa and other institutions. By developing a novel method to estimate metabolic rates using biologging data from tagged fish, researchers bridged a critical gap in understanding how body size and temperature influence energy use across fish species, from tiny larvae to massive 3-tonne basking sharks. This breakthrough not only explains why these fish dominate marine food webs but also underscores their vulnerability in a rapidly changing climate.

Understanding Mesothermic Fish: Nature's Warm-Bodied Marvels

Mesothermic fish, also known as regionally endothermic fish, represent a remarkable evolutionary adaptation. Unlike fully ectothermic fish that rely solely on ambient water temperature to regulate their bodies, mesotherms like tunas, billfishes, and lamnid sharks (including great whites, makos, porbeagles, and salmon sharks) have specialized vascular systems that retain heat produced by their red swimming muscles. This allows them to maintain warmer core temperatures, often 10°C or more above surrounding waters, enabling faster swimming speeds, extended migrations, and superior hunting efficiency.

The basking shark, one of the world's largest fish, exemplifies this trait with its massive size and slow cruising style, yet it too benefits from regional heating to process vast quantities of plankton. These adaptations make up less than 0.1% of all fish species but play outsized roles in ocean ecosystems as top predators and prey for humans. However, this heat retention comes at a steep energetic cost, as detailed in the recent study.

The Trinity College Dublin-Led Study: Revolutionizing Metabolic Research

At the helm of this research is Dr. Nicholas L. Payne from Trinity College Dublin's School of Natural Sciences, alongside senior author Professor Andrew L. Jackson and an international team. Their innovative approach integrates field data from biologged animals—devices that track body temperature, acceleration, and depth—with laboratory respirometry measurements from smaller species. This allowed them to scale up estimates of routine metabolic rates (RMR) across body sizes spanning six orders of magnitude, from 1-milligram larvae to gigantic sharks.

Previous studies struggled with this scale because lab tanks can't accommodate large ocean giants. By modeling unsteady-state swimming dynamics and heat budgets, the team quantified how mesotherms' energy expenditure soars: approximately 3.8 times higher than ectotherms of similar size. A 10°C rise in body temperature more than doubles their metabolic rate, demanding constant foraging.

Key Findings: A Fourfold Energy Surge and Scaling Mismatch

The study's core revelation is the metabolic multiplier: mesothermic fish expend nearly four times the energy of ectotherms. This stems from their active red muscle, which generates heat via blood counter-current exchange systems, much like a thermos flask. But as body size increases, heat production scales faster than dissipation due to surface-area-to-volume ratios—larger bodies trap more heat, creating an 'overheating predicament.'

For instance, a 1-tonne warm-bodied shark hits a heat-balance threshold around 17°C, beyond which it can't shed excess heat without behavioral changes like seeking cooler depths. Q10 values (metabolic rate doubling every 10°C) for these fish are elevated, amplifying risks in warming waters projected to rise 2-4°C by 2100 in many regions.

• Energy use: Mesotherms ~4x ectotherms
• Heat production scaling: Faster than loss with size
• Threshold temps: Lower for larger individuals (e.g., 17°C for 1t shark)
• Behavioral coping: Deeper dives or poleward shifts limited by prey

Climate Change's Double Jeopardy: Warmer Waters and Scarcer Prey

Ocean warming, driven by anthropogenic greenhouse gases, raises sea temperatures, pushing mesotherms' metabolic rates higher while compressing their thermal niches. The study highlights a 'double jeopardy': elevated energy needs coincide with prey redistribution to cooler poles or depths, and declining abundances from overfishing. Fossil records show mesotherms like the extinct megalodon vanished disproportionately during past warm periods, a warning for today.

In tropical hotspots, habitable ranges could shrink dramatically, forcing migrations that strain already pressured populations. Tuna fisheries, worth billions globally, face sustainability threats as bluefin and yellowfin struggle to meet caloric demands.

Species Spotlight: Iconic Sharks and Tuna Under Threat

Lamnid sharks—great white, shortfin mako, porbeagle, and salmon shark—exemplify the crisis. These fast predators rely on regional endothermy for burst speeds up to 74 km/h, but warming could force them into suboptimal cooler zones. Basking sharks, planktivores reaching 12m, face similar issues despite slower lifestyles.

Tunas (bluefin, yellowfin, albacore) and billfishes (swordfish, marlin) dominate commercial catches, powering industries from Japan to the US. Their high-energy lifestyles make them sensitive; models predict range contractions of 20-50% in equatorial waters by mid-century.

Ecological Ripple Effects and Fisheries Impacts

As top predators dwindle, food webs destabilize: smaller prey explode, mid-level predators decline, affecting biodiversity. Fisheries lose prime targets; tuna stocks already overexploited could crash, impacting $40B+ annual revenue and 4M jobs worldwide. Regions like the Mediterranean and Atlantic hotspots face acute risks.

Stakeholders from governments to NGOs urge reduced quotas, marine protected areas (MPAs) in cool refugia, and monitoring via biologging. Universities like Trinity College Dublin advocate integrated modeling for policy.

Expert Perspectives: Voices from the Research Team

Dr. Nicholas L. Payne notes, "These fish evolved heat retention for competitive edges, but physics now turns it against them in warmer oceans." Professor Andrew L. Jackson adds, "The scaling mismatch explains why giants like basking sharks prefer cold waters—overheating limits their size and range." Dr. Edward P. Snelling from University of Pretoria emphasizes physiological constraints: "Heat loss lags production in big bodies, a universal challenge."

Oceanographers warn of cascading effects, calling for urgent conservation aligned with Paris Agreement goals.

Solutions and Conservation Strategies

Mitigation demands multi-pronged action:

• Protected Areas: MPAs in thermal refugia, e.g., polar fronts.
• Fisheries Management: Science-based quotas, bycatch reduction.
• Monitoring Tech: Expanded biologging by universities.
• Climate Action: Global emissions cuts to slow warming.
• Research Investment: Funding for metabolic studies at institutions like Trinity College Dublin.

Innovations like aquaculture for tuna offer hope, but wild populations need safeguards now.

Future Outlook: Research Frontiers and Calls to Action

Projections paint a dire picture: without intervention, mesotherm declines could reshape oceans by 2050. Yet, university-led research provides tools for resilience. Trinity College Dublin's biologging advances enable real-time tracking, informing adaptive management.

As oceans warm 0.2°C/decade, collaboration between academia, governments, and industry is vital. Explore opportunities in marine biology research jobs or higher ed programs focusing on climate impacts—vital for safeguarding these ocean giants. Read the full Science paper for deeper insights.

Photo by Karl Callwood on Unsplash

University Research Driving Ocean Conservation

Institutions like Trinity College Dublin and University of Pretoria exemplify higher education's role in tackling global crises. Their interdisciplinary teams blend physiology, ecology, and data science, training next-gen scientists. Programs in marine biology and climate science offer hands-on research, preparing graduates for impactful careers amid environmental shifts.

This study underscores academia's imperative: translate findings into policy for sustainable oceans.