The hidden toll of heatwaves on fish: what the evidence shows about heat, oxygen and welfare

Europe's hottest June on record, and what it means for fish

In a river that has grown a few degrees too warm, a sockeye salmon reaches the final stretch of a journey she has made almost without eating, swimming from the open ocean back toward the gravel bed where she hatched. The warm water carries less oxygen than her labouring muscles need. Her heart can't keep pace, and she falters before she can spawn. Biologists studying salmon migration and temperature have traced this kind of failure to a limit written into the animal's physiology.

This June, that same limit was being tested in waters across Europe. Copernicus confirmed the month as western Europe's warmest June on record, averaging 3.06°C above the 1991 to 2020 norm, alongside the highest June sea surface temperatures ever measured and marine heatwaves across the western Mediterranean and along the Atlantic coasts. Heat is not a background discomfort for fish. It reaches into the way their bodies work, and for many of them it proves fatal.

Why heat hits cold-blooded animals so hard

Fish are cold-blooded, so their body temperature follows the water around them, and with it the pace of everything happening inside. When the water warms, their metabolism races and calls for more oxygen. The cruel twist is that warmer water holds less oxygen, not more. Supply drops away just as demand climbs. As the temperature passes a fish's comfort zone, the energy it keeps in reserve for swimming and escaping predators bleeds off and then gives out. Heat also puts fish under lasting stress. Faced with a threat, a fish releases the hormone cortisol, much as we do, which frees up energy to deal with an emergency. That response is useful in short bursts. When warm water keeps it switched on for weeks, the fish burns energy it would otherwise spend on growing and staying healthy, and it has less held back for whatever comes next.

The range of healthy temperature is slim. Early work on sockeye salmon found that their growth peaked at around 15°C, while water of roughly 24°C was lethal. Fewer than ten degrees separate a fish at its best from one that cannot survive, and unlike us it has no way to step out of the heat. A river that runs a few degrees above normal for a week has already used up much of that margin.

The damage isn't spread evenly. Studies of Fraser River sockeye reveal populations with markedly different tolerances for heat, each tuned over many generations to its own stretch of river. This is a marvel of adaptation, and it's also the trap. This adaptation takes generations to build, and rivers are warming way faster than their fish can change. Freshwater species are the most cornered of all, unable to slip away to cooler water offshore. One assessment of some 11,500 freshwater fishes found that if warming reaches 3.2°C, a third of them could lose more than half their habitat, while holding warming to 1.5°C cuts that danger roughly tenfold. Warm water helps disease take hold as well, by speeding up parasites that a healthy fish might otherwise shrug off. In warming rivers, the parasite behind proliferative kidney disease bears down harder on wild brown trout and Atlantic salmon. At sea, warming and sea lice together damage the internal organs of juvenile Atlantic salmon.

Trapped in the heat: why farmed fish can't escape

Farmed fish face the same heat, but they can't get away from it. A wild fish might find a cool, deep pocket of water and wait out the worst. A fish held in a sea cage is confined to whatever the heatwave delivers. The consequences are turning up in the mortality figures. A global study of salmon farms found that mass die-offs have grown larger and more frequent over the past decade, with hundreds of millions of fish lost, a hazard the authors describe as built into the way we farm. Disease follows the heat here too, in farmed cod and in the seabream and seabass of the Mediterranean. And warmth seldom acts alone. Stacked onto crowding and low oxygen, a temperature a fish might otherwise have weathered turns fatal.

The suffering is the heart of it, but there's waste here too. Farmed fish are raised on feed made partly from wild-caught fish, so months of growth draw steadily on the ocean. When a heatwave kills those fish before they reach harvest, the wild fish caught to feed them are lost as well.

Scientists are on firm ground about the basic mechanism. Warmer water means less oxygen and more stress, and past a threshold it harms and kills. Some of the longer-range numbers carry more doubt. One widely cited projection has fish reaching smaller adult sizes as the ocean warms, with the average maximum body weight across fish communities falling by 14 to 24 per cent by 2050 under high emissions. The reasoning is that gills grow more slowly than the body they have to supply, so in warm water holding little oxygen a fish stops growing sooner than it otherwise would. That mechanism is contested, and its authors have since had to defend it in print, so the number reads better as a direction than a forecast. A separate set of models points the same way for the ocean as a whole, with the total mass of marine animals dropping by about 5 per cent for every degree of warming and the largest predators thinning out fastest. The spread across those models is wide enough that the figure could sit several points either side. What isn't in question is that the heat pushes fish the wrong way, and that marine heatwaves keep growing longer and more frequent. This June was a glimpse of the norm to come.

Can fish adapt fast enough? A look at the limits

Can't fish simply adapt? To a point, yes. Some species shift their range or acclimate over time, and farmers can lower their cages or move to cooler sites. Those responses genuinely help. But acclimation has a physiological ceiling and evolution is slow, while the heat keeps arriving faster. That farm deaths keep climbing even as the industry works to prevent them suggests adaptation is losing the race.

What actually helps: shade, deep water and lower emissions

All of this is easy to read as a supply problem, a story about fewer fish in the sea and smaller ones on the plate. That framing steps around the animals at the centre of it. A mass mortality event, put plainly, is a vast number of creatures dying slowly of suffocation and stress in water they cannot leave. The more hopeful part is that protecting fish welfare and building a durable food system point the same way. Shielding fish from thermal stress, through lower emissions and more careful farming, serves the animals and the system at once.

The most useful step is also the hardest, which is limiting how far the water warms. For wild fish, holding warming near 1.5°C rather than letting it climb well past that is what decides whether whole populations survive. In the nearer term, they need cooler water to retreat into. That means protecting the deep, cold pockets that persist through a heatwave, and planting trees along riverbanks to shade the water. On farms, keeping fewer fish in each cage and choosing cooler sites both cut deaths, though the harder question is whether some species should be farmed at all in waters that are becoming too warm for them. And if you eat fish, ask where it was raised and how it was killed.

The heatwaves keep coming, the water keeps warming, and the fish are left to absorb it. The salmon who never reaches her gravel bed, and the caged fish with nowhere cooler to turn, are early signs of what a hotter ocean asks of the animals living in it. Recognising the heat below the surface is the first step. Acting on it is the next.