In late June, whale watchers in southwest Western Australia sounded the alarm. They had seen far fewer humpback whales at a time when they normally migrate up from the Southern Ocean to calve and feed. Numbers were 60% below normal.

A week later, birdwatchers spotted a “once in a lifetime event” as large numbers of rarely seen albatrosses and petrels appeared off Australia’s east coast, thousands of kilometres from their usual foraging grounds deep in the Southern Ocean. Many birds were in poor condition.

The sightings came just weeks after a dying migratory seabird was confirmed as Australia’s first case of highly pathogenic H5N1 bird flu.

These sightings raise troubling questions. Seabirds and marine mammals such as whales use ocean currents and weather to travel long distances to feeding and breeding grounds. If these species are changing their behaviour, it could be a sign of altered conditions in the Southern Ocean.

We know the world’s oceans are running very hot. Marine heatwaves can affect any part of the ocean – even frigid Antarctic regions. If species are used to water temperatures around 2–6°C, a sudden boost of a couple of degrees can upend ecosystems. If the heatwave lingers, the cumulative effect can be much bigger.

We don’t yet know for sure, as monitoring in the Southern Ocean is limited. But we know marine heatwaves can alter........

© The Conversation