Nearly one million seabirds starved to death due to severe marine heatwave off the West Coast of USA
Unprecedented numbers of common murres died between 2015 and 2016 and 62,000 carcasses were discovered
Nearly one million common murres died at sea and washed ashore from California to Alaska in 2015 and 2016. It was unprecedented -- both for murres which are North Pacific seabirds -- and across all bird species worldwide. Scientists have now found a potential culprit. In a recently published analysis, experts have pointed to severely reduced food supplies brought on by a harsh and long-lasting marine heatwave known as "the blob".
Marine heatwaves are defined as prolonged periods when ocean temperatures are much higher than usual. According to scientists, they have become more prevalent and intense over the last century.
"The magnitude and scale of this failure have no precedent. It was astonishing and alarming, and a red-flag warning about the tremendous impact sustained ocean warming can have on the marine ecosystem,” says lead author of the analysis John Piatt, a research biologist at the US Geological Survey's Alaska Science Center and an affiliate professor in the University of Washington (UW) School of Aquatic and Fishery Sciences.
The common murre is a self-sufficient, resilient bird. They nest in colonies along cliffs and rocky ledges overlooking the ocean. The adult birds, about one foot in length, are mostly black with white bellies and can dive more than two football fields below the ocean's surface in search of prey.
Though the seabird must eat about half of its body weight in prey each day, common murres are experts at catching the small "forage fish" they need to survive. Herring, sardines, anchovies and even juvenile salmon are no match for a hungry murre.
Warmer surface water temperatures off the Pacific coast -- a phenomenon known as the blob -- first occurred in the fall and winter of 2013 and persisted through 2014 and 2015. Warming increased with the arrival of a powerful El Niño in 2015-2016. During that period, from summer 2015 to spring 2016, an unusual number of dead or dying common murres washed onto Alaska and West Coast beaches. Several other species experienced mass die-offs during this period, including tufted puffins, Cassin's auklets, sea lions and baleen whales. But the common murre die-off was by far the largest in recorded history, any way you measure it, says the research team.
To better understand the anomalous die-off, scientists analyzed data on dead and dying birds from West Coast bird rehabilitation centers, citizen science beach surveys, community reports, and studies conducted by universities, private organizations, and government entities. They also investigated reproduction rates of murres at breeding colonies.
The team included experts from the US Geological Survey, University of Washington, US Fish and Wildlife Service, Farallon Institute, International Bird Rescue, Humboldt State University, National Park Service, NOAA Fisheries, Moss Landing Marine Laboratories, NOAA Greater Farallones National Marine Sanctuary, and Point Blue Conservation Science.
The analysis, published in PLOS ONE, shows that from May 2015 to April 2016, about 62,000 murre carcasses were found on beaches from central California north through Alaska. Citizen scientists in Alaska, monitoring long-term sites, counted numbers that reached 1,000 times more than normal for their beaches. Scientists estimate that the actual number of deaths was likely close to one million, 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.
About two-thirds of murres killed were adult birds, a substantial blow to breeding populations. With massive shifts in food availability, murre breeding colonies across the entire region failed to produce chicks for the years during and after the marine heatwave event, the scientists found.
The research team concludes that persistent warm ocean temperatures associated with the blob increased the metabolism of cold-blooded organisms from zooplankton and small forage fish up through larger predatory fish like salmon and pollock. With predatory fish eating more than usual, the demand for food at the top of the food chain was unsustainable. As a result, the once-plentiful schools of forage fish that murres rely on became harder to find, says the study.
"Food demands of large commercial groundfish like cod, pollock, halibut, and hake were predicted to increase dramatically with the level of the 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," says Piatt.
"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. We believe that the smoking gun for common murres -- beyond the marine heatwave 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,” says second author Julia Parrish, a UW professor in the School of Aquatic and Fishery Sciences.
The researchers say the common murre event may help explain the other die-offs that occurred during the northeast Pacific marine heatwave, says the team. It could also serve as a warning for what could happen during future marine heatwaves, they add.
“Researchers are only beginning to understand the mechanisms and full magnitude of effects of the 2014-16 heatwave, and what it portends if such heatwaves become stronger and more frequent, as predicted. Further research could explore the potential for future warming events to cause similar die-offs,” says the study.