How does ocean warming affect cetaceans?

The rapid warming of the oceans is upending the life cycle of cetaceans at a scale and a speed without precedent in their evolutionary history. Surface temperatures have risen on average by 0.88 °C since the pre-industrial era, and the heat is now working its way down into the deeper layers. For animals that depend closely on their feeding grounds, their breeding grounds and their prey, these changes are abrupt, and already measurable.

A geographical redistribution under way

The most immediate effect is the shift of populations towards the poles. As the waters warm, cold water species move up in latitude to stay within their optimal thermal niche. Some sightings are spectacular: a grey whale, normally confined to the Pacific, was observed in the Mediterranean in 2010 after crossing the Northwest Passage, a geographical feat unthinkable a few decades ago, made possible by the melting of the summer Arctic sea ice.

Other species are extending their range: killer whales and sei whales now venture further north, into waters previously out of their reach. This advance into the Arctic has knock-on consequences: the newly arrived killer whales prey on white whales (belugas) and narwhals, which had not developed anti-predator strategies against this kind of threat.

The species trapped by the melting ice

Conversely, some species are seeing their habitat shrink dramatically. The beluga and the narwhal, perfectly adapted to cold water and to the presence of sea ice, have nowhere to flee. Climate projections indicate that more than half of their current habitat for breeding and feeding could have moved to higher latitudes by 2100, that is to say beyond the present continental limits.

Sea ice is not merely scenery: it structures the whole Arctic ecosystem. Its melting affects the lipid rich Arctic zooplankton (copepods of the genus Calanus), an essential food base for bowhead whales and other northern mysticetes. When the zooplankton declines, the entire chain is destabilised.

The collapse of the prey

Warming disturbs not only the location of prey but also their abundance and their nutritional quality. Antarctic krill, the food base of the great southern whales, is closely tied to the sea ice whose melting threatens it. Chinook salmon of the North-East Pacific, the almost exclusive prey of Southern Resident killer whales, have been declining for decades, partly under the effect of the warming of rivers and estuaries.

More subtly, phenological shifts compromise the synchrony between the arrival of migrating whales and the peak of prey production. A whale that arrives too early or too late on a feeding ground misses its energy window, and so reaches the tropical breeding ground in poorer condition. Over several cycles, this affects fertility, calf survival rates and population dynamics.

Direct mortality and extreme events

Marine heatwaves are becoming more frequent and more intense. The “Blob” in the North-East Pacific, between 2013 and 2016, saw temperatures rise by several degrees over millions of square kilometres. The consequences: local collapse of salmon, mass mortality of humpback whales stranded on Pacific coasts, documented malnutrition among resident killer whales.

Warming is also changing the range of infectious diseases. Morbilliviruses, which periodically cause epizootics among dolphins, appear to be gaining ground in regions where they were once absent. Thermal stress also weakens the animals’ immune system, leaving them more vulnerable to opportunistic infections.

A situation made worse by the other pressures

Warming does not come alone: it adds to growing noise pollution, to persistent chemical contamination (PCBs, microplastics), to bycatch in fishing gear, to the loss of coastal habitats, to the acidification of the oceans which threatens some prey. This accumulation of pressures, known as cumulative stressors, is probably more dangerous than each factor taken on its own.

Species with a restricted range are the most vulnerable: the North Atlantic right whale (fewer than 400 individuals), the vaquita of the Gulf of California (fewer than 10), the resident populations of European killer whales. For these populations, each additional degree may be the degree too many. Ocean warming is not a future problem: it is a crisis already visible, one that calls for coordinated action on a planetary scale.

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