The 'Blob' That Formed Off the West Coast in 2015 Killed a Million Seabirds, Study Finds

Scientists believe an intense marine heatwave (nicknamed "the Blob") triggered a mass die-off of seabirds in the North Pacific.

New estimates suggest around a million common murres succumbed to starvation between 2015 and 2016, when tens of thousands of carcasses washed ashore in Alaska and along the West Coast—an event scientists have called "unprecedented" and "astonishing."

Drawing from data collected in citizen science beach surveys, rehabilitation centers and community reports, researchers from the U.S. Geological Survey say that the die-off was caused by depleted food supplies, triggered by above-average sea temperatures between 2013 and 2016.

The marine heatwave started in late 2013, when a patch of warm water later nicknamed "the Blob" formed off the Alaskan coast. The "Blob" swelled to incorporate parts of the Pacific Coast as far south as Mexico.

Common Murres on a Nesting Island in Kachemak Bay in Alaska
Common murres on a nesting island in Kachemak Bay in Alaska. Scientists attribute a mass die-off of common murres to a marine heatwave from 2013 to 2016. Wildnerdpix/iStock

In 2015 and 2016, approximately 62,000 bird carcasses were found on beaches along the West Coast, particularly in Alaska, where there were 1,000 times as many bodies as usual.

However, as most carcasses never make it to land, scientists believe the true number of dead birds is much higher. By their estimates, 530,000 to 1.2 million murres perished during the heatwave.

In contrast, the Exxon Valdez oil spill (1989) caused the death of an estimated 300,000 to 645,000 murres. According to the scientists, only a handful of die-offs caused by natural events (as opposed to human-caused events, like oil spills) achieve numbers higher than the hundreds or low thousands.

"Die-offs and breeding failures occur sporadically in murres but the magnitude, duration and spatial extent of this die-off, associated with multi-colony and multi-year reproductive failures, is unprecedented and astonishing," said the researchers.

According to a paper published in PLOS ONE, there are two factors that, combined, caused murre populations to plummet.

First, high ocean temperatures reduced the quantity and quality of phytoplankton—an impact that has significant ramificationS right up the food chain, reducing the quantity and quality of prey murres rely on to survive (namely, fish like cod).

Second, higher ocean temperatures speed up the metabolism of larger fish who compete with the murre for food. This ups their appetite, increasing the amount of food they need for themselves and reducing the amount of fish available for other predators.

The die-off will have long-term implications on the health of common murre populations, the researchers say. Two-thirds of the murres that died were adults—a "substantial blow" to breeding populations.

The scientists have already observed steep declines in the species' reproduction rate. Seven of eight annually monitored colonies failed to produce a single chick in 2016. The effects were felt as late as 2018, when four colonies were just as unsuccessful at producing chicks.

The team call it a "red-flag warning," highlighting the plight of marine ecosystems around the West Coast.

It comes the same week a report revealed 2019 experienced the highest ocean temperatures in recorded history, completing a decade of unparalleled ocean warming. The paper's authors estimate that the oceans absorbed enough heat energy to fuel 3.6 billion Hiroshima atomic bombs in a period of 25 years.