The ‘Blob,’ an unprecedented marine heat wave, killed 4 million seabirds

The tall, stony coastlines of the northeast Pacific Ocean are much quieter than they were just a decade ago. Following a punishing marine heat wave in the region, the raucous seabird colonies that once crowded the sea cliffs are now greatly thinned, to a quarter of their former size in some places.

This abrupt loss of millions of birds, and probably many other animals, may be the largest wildlife mortality event recorded in modern times, researchers report in the Dec. 13 Science.

“We knew [the population decline] was big, but the numbers are a gut punch,” says Heather Renner, a wildlife biologist with U.S. Fish and Wildlife at the Alaska Maritime National Wildlife Refuge in Homer.

The story begins in late 2014, when a brutal marine heat wave nicknamed the “Blob” parked itself over the Pacific northeast, raising ocean temperatures far above normal for almost two years. The colossal cauldron cooked up an ecological chain reaction, slashing phytoplankton populations and in turn the forage fish that seabirds like common murres (Uria aalge) eat (SN: 1/15/20). In 2015 and 2016, the birds starved to death en masse.

Renner runs a monitoring program spanning the region that has been collecting data on seabirds for the last 50 years. The scale of the toll was immediately obvious.

“We knew right away that it was a big catastrophe,” Renner says. “There were 62,000 carcasses that washed up on the beaches, all over the Gulf of Alaska, all the way down to California. It was clearly a big deal, but we couldn’t really quantify the size of the mortality very well.”

To get a better idea of the full impact on the murre population, the team used colony count data from 1995 to 2022, gathered across 13 colonies along the margins of the Bering Sea and the Gulf of Alaska. After getting bird counts before and after the heat wave, the researchers then extrapolated those results to the entire Alaskan murre population.

Renner and her colleagues estimate that the heat wave killed 4 million murres between the Gulf of Alaska and Eastern Bering Sea. Roughly half the region’s murres died during a single winter.

“It was just so much worse than we thought,” Renner says.

She and her team suggest the loss is the largest die-off of wildlife, specifically nonfish vertebrates, yet reported in the modern era. In the same heat wave, some 10 billion snow crabs in the Bering Sea also died from starvation (SN: 10/19/23).

The sheer scale and speed of the common murre population collapse is shocking, says Simon Tye, an ecologist at the University of Arkansas in Fayetteville who wasn’t involved with the research. “The before and after pictures [of the colonies] are pretty heartbreaking.”

The fact that the birds hadn’t rebounded even seven years later helps reject an earlier hypothesis that the birds were just temporarily delaying breeding to wait out the hostile climatic conditions. The stubbornly sparse colonies may mean that something fundamental has changed in the ecosystem, and it can’t support a return to past murre numbers.

Renner says she doesn’t think climatic impacts spurring such a dramatic, swift shift has been previously documented. The findings show such intense changes can occur on the scale of years.

Tye and Renner both point out that with continued climate warming, heat waves like the Blob are expected to occur more frequently. This could imperil already vulnerable populations of many animals that have yet to recover in an ocean ecosystem reeling from the previous heat wave.

While there’s little immediate human control over marine heat waves, Renner says the findings underscore the importance of other conservation efforts for seabirds. This may include removing invasive predators or other species that — alongside climatic swings — create an additive stress on seabird populations.

“The top of the food web going away, I think that’s really important,” Tye says. “It should be an ominous thing for everybody.”