To encounter a great white shark in the water is to witness evolutionary perfection in motion—power, precision and unquestioned authority. For millions of years, these animals have ruled the seas. But today, that same evolutionary advantage may be placing them in danger.
Basking Shark (Green Fire Productions / CC BY 2
A growing body of research, including a recent study, points to an unexpected vulnerability: Some of the ocean’s fastest and most capable predators may be at risk of overheating as the seas warm.
Built for speed
Unlike most fish, species such as the great white shark, basking shark and Atlantic bluefin tuna are mesothermic. That means that their bodies run warmer than the surrounding water, due to specialised adaptations that trap metabolic heat.
This internal warmth fuels their lifestyles. It allows for explosive bursts of speed, long-distance migrations and highly efficient predation. In cold seas, it gives them an edge over slower, cold-blooded prey.
But that advantage comes at a cost. Mesothermic fish burn energy at rates up to four times higher than typical fish. They must constantly feed to sustain their elevated metabolism. And as ocean temperatures rise, their bodies face an increasingly difficult challenge: shedding excess heat.
The overheating problem
Water absorbs heat efficiently, and warming oceans reduce the temperature difference these animals rely on to release body heat. Larger individuals are particularly vulnerable. Their bodies generate heat faster than they can lose it.
Researchers using advanced sensors have begun measuring these “hidden heat budgets” in real time. Their findings suggest that a large, warm-bodied shark—of approximately one ton—may struggle to remain in waters above about 17°C (62.6°F).
To cope, these animals may need to dive repeatedly into deeper, cooler ocean layers, slow down, or redirect their blood flow to manage the excess heat. But all of these strategies can be problematic, especially when food is scarce.
Heat and hunger—a double bind
The challenge is not just temperature. It’s timing.
As oceans warm, many marine species shift their ranges. Prey moves. Habitats change. And for predators that already operate on tight energy budgets, finding enough food becomes harder.
Overfishing compounds the problem. For animals that cannot simply “find more food”, declining fish stocks mean less fuel to maintain their high-performance physiology.
Species such as the porbeagle shark and thresher shark—already rare—may find the only habitats they can live in shrinking, as warm zones expand across the oceans. This is particularly true during the summer months.
Shifting oceans, shifting ecosystems
For divers in places like False Bay or Gansbaai, changes in shark presence have already been noticeable. These shifts are complex, driven by temperature, prey movement, fishing pressure and habitat disturbance.
But the implications go far beyond the lack of sightings. Mesothermic predators are often apex species. Their presence shapes entire ecosystems. When they decline or relocate, the effects ripple downward through the food web, altering the balance of marine life.
In South Africa, the great white has even become a kind of ecological “sentinel”. When it disappears, something deeper in the system is changing.
A familiar pattern from the past
The fossil record offers a warning. Warm-bodied giants have faced this challenge before. The extinct megalodon—one of the largest predators to ever live—likely struggled during past periods of ocean warming. Its high energy demands may have become impossible to meet as ecosystems shifted.
Today’s oceans are changing far more rapidly.
The most immediate threat
While climate change is tightening the physiological limits of these animals, researchers emphasise that the most urgent danger remains human activity, especially fishing.
Bycatch, unintentional capture in industrial nets and longlines, kills vast numbers of sharks and other marine animals every year. For species already under stress from warming waters, this added pressure can be decisive.
What divers are witnessing
For those who spend time underwater, these changes are not abstract. They are seen in altered migration patterns, unexpected absences and shifting encounters. The ocean is still full of life, but it is reorganising itself in response to pressures we are only beginning to understand.
The great predators are not disappearing quietly. They are moving, adapting and struggling at the edges of their limits. And for the first time in millions of years, the conditions that made them supreme may no longer be enough to sustain them.
