Over 600,000 seabirds killed in single marine heatwave

Towards the end of 2023, dead seabirds started washing up on Australia’s golden beaches.

While it’s not unusual for some birds to die at sea, this wasn’t just a handful of unlucky individuals. Thousands of shearwaters were washing up across the country’s east coast, stretching thousands of kilometres from Queensland down to Tasmania.

With the help of concerned local residents-turned-community scientists from across Australia, a team of scientists from the Adrift Lab has managed to piece together the full picture. They linked the deaths to a marine heatwave in the middle of the shearwater breeding season when the birds are at their most vulnerable.

Their research suggests that the shearwaters washing up on the beaches were only a tiny fraction of the overall number that died during the heatwave. In total, the researchers estimate over 629,000 seabirds died, with the short-tailed shearwater making up 96% of the casualties.

Dr Alex Bond, a member of the Adrift Lab and our Principal Curator of Birds, says that this figure means over 5% of all short-tailed shearwaters died in just a few months. If heatwaves continue to strike, then the future of these birds is increasingly uncertain.

“These events are happening more frequently, and while seabirds have some ability to bounce back from them their resilience is being exhausted,” Alex says.

“Before, these events happened once in a generation. Now, they’re happening faster and faster, and they won’t slow down. They come on top of everything else seabirds have to deal with, from pollution to persecution, and they can’t cope with it.”

The findings of the study were published in the journal Conservation Biology.

Marine heatwaves are becoming more common

As our greenhouse gas emissions cause the Earth to grow increasingly hot, much of the excess energy is being funnelled into the oceans. Since the 1980s, the temperature increase has risen from 0.06°C per decade to 0.27°C per decade now.

This is making marine heatwaves increasingly common, with the numbers of days of abnormal heat rising by over half in the past century. They’re also becoming more powerful, with eight of the 10 most extreme marine heatwaves happening since 2010.

As a result, ocean ecosystems are rapidly changing. Fish are shifting their ranges to adapt to the increasing water temperatures, while blooms of seaweed are expanding. Coral reefs are experiencing widespread bleaching and may disappear almost entirely by 2100.

As a major part of marine food chains, seabirds are also affected by climate change. Bycatch, shifts in prey and rising temperatures are among the many factors adding to the burden of these animals, which already includes plastic pollution and disease.

To understand how seabirds are being affected by marine heatwaves, it’s important to establish a baseline to compare future events to. However, this is easier said than done.

Understanding seabird mortality

While some of the seabirds killed in marine heatwaves wash up on beaches, the vast majority don’t. Instead, most sink without a trace as their carcasses become waterlogged and grow heavier.

Even if the birds do reach the shore, the chance that they are then seen by anyone is also remote. Temperature, wave action and scavengers can lead to carcasses disappearing within hours, or degrading to the point that they can’t be recognised. In total, it’s estimated that less than 1% of seabirds that die in a marine heatwave will be reported.

Using models developed to estimate the death of birds and bats around terrestrial windfarms, Alex and the rest of the Adrift Lab were able to use the roughly 5,000 seabirds found on Australia’s beaches to work out the overall number that died in the marine heatwave.

In total, they estimated that around 610,000 short-tailed shearwaters were killed in the 2023/2024 marine heatwave, as well as about 14,000 sable shearwaters and thousands of other seabirds.

While these numbers are significant, they pale in comparison with a heatwave that hit a decade earlier. During this event over 200,000 seabirds washed up on beaches, which suggests that millions of shearwaters died overall.

Though the team weren’t able to look at exactly how the birds were being killed by the heatwave due to the possible risk of infection, there are some likely causes. Rising levels of heat can cause the birds’ prey to die or move, leaving the seabirds to starve. Heat stress can also overwhelm the shearwaters, as can toxic algal blooms.

With more heatwaves in quick succession, these birds are facing the very real threat of extinction. But with the current limited monitoring of their populations it’s likely that we won’t know until it’s too late.

Alex hopes that the research acts as a wake-up call for more funding and action to be devoted to climate change action and conservation in Australia.

“For years, people have tended to explain away these deaths on stormy conditions or birds tired from migrations,” Alex says. “We’ve known for a long time that doesn’t make sense – seabirds thrive in storms – but now we have the data to prove it.”

“We have the science and we have the resources to be able to give these seabirds a chance at survival, but we have to act now. This isn’t an issue that will go away, and without taking real action to tackle the climate, biodiversity and pollution crises it’s only going to get worse.”