Murres, a common seabird, look a little like flying penguins. These stout, tuxedo-styled birds dive and swim in the ocean to eat small fish and then fly back to islands or coastal cliffs where they nest in large colonies. But their hardy physiques disguise how vulnerable these birds are to changing ocean conditions.
A University of Washington citizen science program — which trains coastal residents to search local beaches and document dead birds — has contributed to a new study, led by federal scientists, documenting the devastating effect of warming waters on common murre s in Alaska.
In 2020, participants of the UW-led Coastal Observation and Seabird Survey Team, or COASST , and other observers first identified the massive mortality event affecting common murres along the West Coast and Alaska. That study documented 62,000 carcasses, mostly in Alaska, in one year. In some places, beachings were more than 1,000 times normal rates. But the 2020 study did not estimate the total size of the die-off after the 2014-16 marine heat wave known as "the blob."
In this new paper, published Dec. 12 in Science, a team led by the U.S. Fish and Wildlife Service analyzed years of colony-based surveys to estimate total mortality and later impacts. The analysis of 13 colonies surveyed between 2008 and 2022 finds that colony size in the Gulf of Alaska, east of the Alaska Peninsula, dropped by half after the marine heat wave. In colonies along the eastern Bering Sea, west of the peninsula, the decline was even steeper, at 75% loss.
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," said Julia Parrish , a UW professor of aquatic and fishery sciences and of biology, who was a co-author on both the 2020 paper and the new study. "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."
The "warm blob" was an unusually warm and long-lasting patch of surface water in the northeast Pacific Ocean from late 2014 through 2016, affecting weather and coastal marine ecosystems from California to Alaska. As ocean productivity decreased, it affected food supply for top predators including seabirds, marine mammals and commercially important fish. Based on the condition of the murre carcasses, authors of the 2020 study concluded that the most likely cause of the mass mortality event was starvation.
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.
While the "warm 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, including many of the same authors, 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."
Other co-authors are Brie Drummond and Jared Laufenberg at the U.S. Fish and Wildlife Service offices in Alaska; John Piatt , a former federal scientist now with the World Puffin Congress in Port Townsend; and Martin Renner at Tern Again Consulting in Homer.
