A marine heatwave has led to a devastating loss in Alaska's common murre population, resulting in the largest recorded die-off of a single species in modern history, according to research. This catastrophic event underscores the broader changes occurring in marine environments due to rising ocean temperatures, which are rapidly altering ecosystems and affecting the survival of species.

The Northeast Pacific heatwave, dubbed "the Blob," spanned from California to the Gulf of Alaska between 2014 and 2016. This event is the largest and longest known marine heatwave, with temperatures increasing by 2.5 to 3 degrees Celsius (4.5 to 5.4 degrees Fahrenheit) above the norm, as stated by Brie Drummond, co-author of the study published in the journal Science on December 12.

Common murres, or Uria aalge, with their distinctive black-and-white feathers, play a crucial role in the marine food web of the Northern Hemisphere. While they have experienced smaller die-offs due to environmental and human factors, they typically recover quickly with the return of favorable conditions. However, the scale and speed of the die-off during the heatwave were particularly alarming.

Researchers determined the extent of this population loss by tracking extreme declines at 13 colonies across the Gulf of Alaska and Bering Sea that have been monitored over the long term. By the end of the 2016 heatwave, Drummond and her team counted over 62,000 common murre carcasses, a fraction of the total loss since most dead seabirds are never found on land. Biologists monitored the death and reproduction rates of common murres and found no signs of the colonies returning to their previous size.

"The only reason we had this data and were able to detect this event was that we had these long-term datasets and long-term monitoring" said Drummond, a wildlife biologist at the Alaska Maritime National Wildlife Refuge. “Monitoring is the only way we’ll be able to continue to look at what happens in the future.”

As temperatures in Alaska rose, the murres' food supply dwindled, with their primary prey, Pacific cod, declining by about 80% between 2013 and 2017. With the collapse of this key food source, an estimated 4 million common murres died in Alaska from 2014 to 2016. "There are about 8 million people in New York City, so it would be like losing half of the population... in a single winter," Drummond said.

Before the 2014 heatwave, Alaska's murre population constituted 25% of the global population of the seabird species. However, comparing the seven-year period before the heatwave (2008 to 2014) with the seven-year span following (2016 to 2022), the study found the murre population in 13 colonies between the Gulf of Alaska and Bering Sea declined by 52% to 78%.

Drummond and her colleagues continued monitoring the murres from 2016 to 2022 after the heatwave but found no signs of recovery. While further research is necessary to understand why murres are not bouncing back, the team believes the changes are driven by shifts in the marine ecosystem, particularly those related to food supply. Reproductive challenges and relocation difficulties may also be contributing to the species' lack of recovery.

Unlike some species, seabirds like murres take longer to reproduce, making repopulation a slower process. Additionally, murres are bound to their colonies, and relocating can be challenging as they adjust to new conditions. As temperatures continue to rise in areas like Alaska, tropical or subtropical waters are moving into different areas, creating conditions for an entirely new ecosystem. Animals will either adapt or fail to survive in the new climate.

Murres are not the only species in Alaskan waters undergoing significant changes. The tufted puffin, a sensitive marine bird, has been seen migrating north due to poor conditions in southern areas of the North Pacific but is struggling to adapt to its new home. King salmon, whales, and crabs are also grappling with finding their place.

While heatwaves have affected many species, other populations are not substantially impacted. Half of the data collected from organisms such as phytoplankton and even homeothermic top predators showed "neutral" responses to the heatwave. Twenty percent of these apex predators even responded positively to the abnormal heat exposure.

Homeothermic animals, including birds and mammals, maintain stable internal body temperatures regardless of the environmental temperature. "That gives us perspective on which species might more readily adapt to these kinds of warming water events in the future and which will not," Drummond said.

Although rising temperatures are the primary factor affecting animals like murres, other elements may also be contributing to marine life changes. "From an ecological perspective... microplastics, ocean acidification, sea levels rising, and chronic oil spills... are other massive mortality factors at play," said Dr. Falk Huettmann, an associate professor of wildlife ecology at the University of Alaska, Fairbanks.

However, studies tracking the long-term effects of climate events on marine life are limited, so scientists are still uncertain about how these animals will continue to be impacted in the future.