An ocean heat wave in 2015-16 produced the biggest-ever mass mortality event for an avian species, disrupting food supplies for common murres in the North Pacific and killing as many as 1.2 million of them.
The increased frequency of ocean heat waves appears to be a “relatively new” phenomenon linked to climate change, said University of Washington ecologist Julia Parrish, co-author of a paper published earlier this month in the journal PLOS ONE.
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“During the period from summer 2015 to spring 2016, around 62,000 emaciated murres washed ashore dead or dying along the North American Pacific coastline—in Alaska alone the rate of die-off was a thousand times higher than normal,” Agence France-Presse reports , in a dispatched published by France 24. “Since only a tiny fraction of birds that die wash ashore in areas that are accessible, they estimated the total number of deaths was between half a million to 1.2 million.”
By comparison, said lead author John Piatt, a research biologist at the U.S. Geological Survey’s Alaska Science Center, 30,000 bird carcasses were discovered after the Exxon Valdez oil tanker disaster in 1989, leading researchers to estimate total mortality in the 300,000 to 600,000 range.
The news story traces the heat wave back to a combination of factors—a mass of warm seawater nicknamed the “blob” combining with a strong El Niño effect to produce a marine heat wave stretching along North America’s west coast from California to Alaska.
“Parrish said the effects of the heat wave were two-fold,” AFP writes. “First, elevated temperatures reduced the quality and quantity of phytoplankton, which in turn reduced the quantity and quality of the fish eaten by common murres—prey like herring, sardines, and anchovies.”
And then, “as waters warmed, it increased the energy requirements of larger fish like salmon and Pacific cod that have the same prey as the murres.”
Those changes collided badly with what Piatt called the murres’ Achille’s heel—their need to consume half their body mass in food every day.
“Everything they do depends on that breast muscle,” he said. “And when they can’t eat three or four days, they burn up all that muscle,” and can no longer fly or dive.
The loss of murre population was that much worse because many of the birds it affected were of breeding age. The same heat wave caused mass die-offs in species like tufted puffins, Cassin’s auklets, sea lions, and baleen whales.
The cluster of mass mortalities showed that “a warmer ocean world is a very different environment and a very different coastal ecosystem for many marine species,” Parrish said, and seabirds are “bellwethers of that change.”