Approximately 4 million common murres in Alaska were killed by an unprecedented marine heatwave, nicknamed 'The Blob', scientists from the University of Washington have revealed. This is believed to be biggest mass die-off of a single species in the modern era.
'The Blob' was an unusually warm patch of surface water in the northeast Pacific Ocean from late 2014 through 2016, affecting marine ecosystems from California to Alaska.
Phytoplankton populations dwindled, which, in turn, triggered a collapse in fish stocks, the murres' primary food source. By 2016, starvation swept through common murre populations, and beaches from Alaska to California became littered with their emaciated carcasses.
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No recovery yet seen
The study led by Heather Renner, a wildlife biologist at the U.S. Fish and Wildlife Service, estimates that 4 million Alaska common murres died in total, about half the total population. No recovery has yet been seen, the authors write.
"This study shows clear and surprisingly long-lasting impacts of a marine heat wave on a top marine predator species," Julia Parrish, a UW professor of aquatic and fishery sciences and of biology and co-author, said in a statement. "Importantly, the effect of the heat wave wasn’t via thermal stress on the birds, but rather shifts in the food web leaving murres suddenly and fatally without enough food."
Before this marine heat wave, about a quarter of the world’s population, or about 8 million common murres, lived in Alaska. Authors estimate the population is now about half that size. While common murre populations have fluctuated before, the authors note the Alaska population has not recovered from this event like it did after previous, smaller die-offs.
More marine heat waves to come
While 'The Blob' appears to have been the most intense marine heat wave yet, persistent, warm conditions are becoming more common under climate change. A 2023 study led by the UW showed that a 1 degree Celsius increase in sea surface temperature for more than six months results in multiple seabird mass mortality events.
"Whether the warming comes from a heat wave, El Niño, Arctic sea ice loss or other forces, the message is clear: Warmer water means massive ecosystem change and widespread impacts on seabirds," Parrish said.
“The frequency and intensity of marine bird mortality events is ticking up in lockstep with ocean warming,” Parrish said.
The 2023 paper suggested seabird populations would take at least three years to recover after a marine heat wave. The fact that common murres in Alaska haven’t recovered even seven years after 'The Blob' is worrisome, Parrish said.
"We may now be at a tipping point of ecosystem rearrangement where recovery back to pre-die-off abundance is not possible."
What are guillemots?
Common murres, known as guillemots in Europe, are seabirds that look a bit like flying penguins. They are slightly larger than razorbills and have a brown head and upper body and a white lower body.
Guillemots come to land to breed and nest, spending the rest of its life at sea. Their diet consists of a wide variety of fish and crustaceans. They forage for food by diving from the sea surface and swimming underwater using their wings for propulsion.
The guillemot is the most common auk in the British Isles.