(CN) – A severe and long-lasting marine heatwave known as "the blob" is to blame for the mass death of the common murres, totaling nearly 1 million seabirds, according to a study released Wednesday.
The murre is an exceptionally self-sufficient, resilient bird and an avid hunter. They must eat roughly half their body weight in prey every day, making them some of the best experts at catching the small "forage fish" they need to survive, including herring, sardines, anchovies and even juvenile salmon.
Common murres nest in colonies along cliffs and rocky ledges overlooking the ocean, giving them a great view of potential prey. An adult bird is characterized by black feathers and white bellies, reaching about one foot in length. In adulthood, these hunters can dive deeper than two football fields below the ocean's surface in search of prey.
This is why the death of nearly 1 million common murres shocked the world, starving out at sea and washing ashore from California to Alaska in 2015 and 2016. This event was unprecedented not only for murres, but for any bird species in general. Scientists from the University of Washington, the U.S. Geological Survey and others point to the unexpected squeeze on the ecosystem’s food supply brought on by a long-lasting and devastating marine heat wave known as “the blob.”
They published their findings Wednesday in the journal PLOS ONE.
"Think of it as a run on the grocery stores at the same time that the delivery trucks to the stores stopped coming so often," explained second author Julia Parrish, a professor in the School of Aquatic and Fishery Sciences at the University of Washington. "We believe that the smoking gun for common murres – beyond the marine heat wave itself – was an ecosystem squeeze: fewer forage fish and smaller prey in general, at the same time that competition from big fish predators like walleye, pollock and Pacific cod greatly increased."
“The blob” first struck in the fall and winter of 2013 and persisted through 2014 and 2015, bringing on warmer surface water temperatures off the Pacific coast. This warming became a larger problem with the arrival of a powerful El Niño in 2015-2016. This phenomenon caused several other species to experience mass die-offs, including tufted puffins, Cassin's auklets, sea lions and baleen whales, with the common murres suffering the largest.
From May 2015 to April 2016, about 62,000 murre carcasses were discovered on beaches spanning from central California to Alaska. Local scientists in Alaska monitoring long-term sites counted numbers that were roughly 1,000 times more than normal for their beaches. Scientists estimate that since only a fraction of birds that die will wash to shore, and only a fraction of those will be in places that people can access, the actual number of deaths was likely closer to a grave 1 million.
This massive shift in food availability caused murre breeding colonies across the entire region to fail to produce chicks for the years during and after the marine heat wave. The authors found that most of the deceased birds were breeding-age adults.
"The magnitude and scale of this failure has no precedent," said lead author John Piatt, a research biologist at the U.S. Geological Survey's Alaska Science Center and an affiliate professor in the UW School of Aquatic and Fishery Sciences. "It was astonishing and alarming, and a red-flag warning about the tremendous impact sustained ocean warming can have on the marine ecosystem."
By looking at fishery studies conducted during the blob period, the research team concluded the metabolism of cold-blooded organisms increased due to warm ocean temperatures. This includes species from zooplankton and small forage fish up through larger predatory fish like salmon and pollock. Because the predatory fish ate more than usual, the food chain was thrown off and there wasn’t enough food for those at the top – leaving the once-plentiful schools of forage fish that murres rely on harder to find.
"Food demands of large commercial groundfish like cod, pollock, halibut and hake were predicted to increase dramatically with the level of warming observed with the blob, and since they eat many of the same prey as murres, this competition likely compounded the food supply problem for murres, leading to mass mortality events from starvation," Piatt said.
This common murre event is the largest mass die-off of seabirds in recorded history, but the authors note it could help explain the other die-offs that occurred during the northeast Pacific marine heat wave and serve as a warning for what could happen during future marine heat waves. UW scientists have recently identified another marine heatwave beginning to form off the coast of Washington state stretching up into the Gulf of Alaska.
"All of this – as with the Cassin's auklet mass mortality and the tufted puffin mass mortality – demonstrates that a warmer ocean world is a very different environment and a very different coastal ecosystem for many marine species," said Parrish, who is also the executive director of the Coastal Observation and Seabird Survey Team (COASST). "Seabirds, as highly visible members of that system, are bellwethers of that change."
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