Tens of thousands of common murre carcasses washed ashore DAVID B. IRONS

A million seabirds that died along the US west coast were probably the victims of an unprecedented marine heatwave in the Pacific. Such events are expected to become more frequent due to climate change.

About 62,000 common murres (Uria aalge) washed ashore from summer 2015 to spring 2016 between Alaska and California, most having apparently starved. Researchers extrapolate that this means around a million died in total.

“The amazing question is, how could a million die over 6000 kilometres, pretty much all at the same time, and what could cause it,” says John Piatt at the US Geological Survey.


Members of the species have died en masse in the past, but in far smaller numbers and only at a local level. The magnitude of deaths from 2015 to 2016 is also unusual because the murres are so well adapted to its environment: they are fast and capable of diving deep for their main diet of fish.

The killer appears to have been ‘the blob’, a vast, record-breaking patch of warm water that occurred off the west coast of North America between late 2013 and 2016.

Looking at surveys of the beached murres, sea surface temperatures and fisheries data, Piatt and his colleagues say the most plausible explanation is that the birds were outcompeted, as the warmer waters caused cold-blooded species such as cod to eat far more fish in an effort to regulate their temperature. Murres need to eat half their body mass each day, while cod only need to eat 1 per cent of their body mass, and the birds die after 3 to 5 days without food.

Read more: Mystery blob in the Pacific messes up US weather and ecosystems

“It’s very convincing, and I would actually say it’s fairly conclusive. There’s very little else that could have caused the extensive effects they document,” says Andrew Leising at the US National Oceanic and Atmospheric Administration.

Piatt and his colleagues ruled out the idea the birds were killed by some of the toxic algae blooms that the blob spawned, after toxin levels in the birds were found to be below those that have caused past die-offs.

Last year, the UN climate science panel warned that marine heatwaves will become more common because of climate change. Leising says there are indications that the blob has returned, but over a smaller area, so the effect on birds is likely to be less severe this time.

Piatt says that while murres are in no danger of extinction, the US west-coast populations could take decades to recover. “These heatwaves are going to have jarring effects on our ecosystems,” he says.

Journal reference: PLOS One, DOI: 10.1371/journal.pone.0226087