Back in 2015–16, about 62,000 dead or dying common murres — a North Pacific seabird — washed ashore between Alaska and California.

Key points: Food for the birds disappeared in the warmer conditions while competition increased

Food for the birds disappeared in the warmer conditions while competition increased Warm conditions have returned to waters off Alaska and the Bering Sea

Warm conditions have returned to waters off Alaska and the Bering Sea It's too early to know whether Australia's mutton bird populations have crashed

Only a fraction of the dead birds made it to shore, and the total number of deaths was estimated to be close to a million birds.

Researchers think it was the largest seabird die-off in recorded history.

Compounding the deaths, at least 22 colonies completely failed to produce any offspring over several breeding seasons.

Now a major study has concluded that the die-off was the result of a huge disruption to energy flow through food webs, precipitated by "the blob" — an unprecedented mass of warm, nutrient-poor water that emerged off the Pacific coast of the US from 2013.

The blob was up to 6 degrees Celsius above typical maximum temperatures in places and extended to a depth of 200 metres, and more than 3,000 kilometres up the US coastline into Alaska.

As well as the huge seabird die-off, the researchers believe the marine heatwave caused the mass mortality of a suite of other fish, mammal and bird species during 2014–17.

During the period that the blob persisted off the coast of the US, production of phytoplankton or microscopic algae dropped, and "the largest harmful algal bloom in recorded history" stretched from California to the Gulf of Alaska in 2015, the researchers said.

"A massive die-off of planktivorous Cassin's auklets [seabirds] occurred from Central California to British Columbia in the winter of 2014-15, a marked increase in mortality of [sea lions] was noted in Southern California, and an unusually large die-off of baleen whales occurred in the Gulf of Alaska in 2015–16," they wrote in their paper, published today in PLOS ONE.

An 'ectothermic vice' squeezed the birds' food supply

Many seabirds feed around Alaska during the northern summer. ( Supplied: Sara Germain )

Their study used a combination of data gathered by citizen scientists, government, university and private organisations, and wildlife rehabilitation centres, to conclude that the common murres were caught in an "ectothermic vice" — basically, a squeezing of the murre's food supply from above and below.

On the one side, the murre's preferred prey species, fish like anchovies, juvenile salmon, capelin and sardines, are ectothermic or cold-blooded.

Warmer ocean temperatures increased their metabolic rate, said lead author John Piatt from the US Geological Survey.

"If you crank up the temperature a few degrees, these ectothermic fish, their metabolic rate cranks up and so they had to eat more," he said.

But at the same time, food for the fish like zooplankton, had been diminished by the warmer temperatures, and the fish suffered accordingly.

"A lot of forage [fish] species did poorly. Juvenile salmon lost body condition because as it turns out their diets were impacted," Dr Piatt said.

"The quality of the food they wanted to eat was getting poor."

Sea surface temperature imagery shows warm waters returned off the US west coast in 2019. ( Supplied: NOAA )

Not only were there fewer fish for the murre to eat, but because the fish were smaller and in worse physical condition, the murre needed to catch and eat more to survive.

On the other side of the "vice" were the murre's competitors — ectothermic or cold-blooded fish like Pacific cod and halibut, which compete with the murre for food.

"The big fish, the big cod, the flounders and the pollock, these large predatory fish, their metabolic rates went up too," Dr Piatt said.

Increased competition for food meant the murre — which has been recorded diving to depths of up to 180 metres — had to work extra hard to sustain themselves.

Murres are highly energetic, and need to consume around 56 per cent of their own body mass every day to meet their own energy demands.

"These are high-energy birds with high-energy demands. If they don't eat for three to four days they're dead," Dr Piatt said.

The result was the biggest seabird die-off or "wreck" that scientists know of.

Did Australia's mutton birds suffer the same fate?

Mutton birds migrate from Alaska to southern Australia every year. ( Supplied: Eric Woehler )

Late last year, as only trickles of mutton birds showed up at their regular southern Australian roosting sites, ecologists feared the worst.

For some reason, many had failed to make the annual migration from Alaska.

Birdwatcher Peter Barrand told the ABC at the time that we could be "looking at an extinction event".

Because seabirds have large natural population fluctuations year on year, it's too early to say the drop in mutton bird numbers last year was anything more than a blip, said John Arnould of Deakin University, who wasn't involved in this study.

Puffin numbers crashed in 2014. ( Supplied: Andrew Peacock )

"We'll have a better idea in March when we do the annual monitoring," Professor Arnould said.

"There were visibly fewer animals but that varies from year to year. I'm cautious about saying the sky is falling, but there's no doubt that fewer animals came back."

Because of the difficulty in detecting rapid declines in species that have big natural population variability, Australia needs to increase its monitoring, he said.

"To understand what's happening in the environment you have to monitor regularly," he said.

"Long-term monitoring in Australia could be better funded. You have to factor in natural environmental variability overlaid with changing climate impacts."

A study published this week showed that 2019 was the hottest year in recorded history for our oceans, a trend that is predicted to continue as climate change intensifies.

And ocean warming has been greatest in the Atlantic and Southern Oceans.

In late 2019, the National Oceanic and Atmospheric Administration (NOAA) reported that the blob had reformed off the west coast of the US.

While seabird wrecks have occurred historically, Dr Piatt discovered during his research records of just two other mass die-offs comparable in size to the event he was studying.

"As we looked we discovered that there were actually two very large die-offs in the 2010s — in this decade," he said.

"One of them was off the coast of New Zealand. Then there was the die-off of puffins off the coast of France in 2014. They lost 50 to 60 per cent of puffins.

"What do they have in common? They all happened in the 2010s and that's been the warmest decade.

"It's not me saying it, it's oceanographers saying it.

"[Climate change] gives rise to these heatwave events and it makes them more frequent and of greater magnitude."