Common murres are arguably the most successful seabirds in the Northern Hemisphere. The penguinlike seafarers can crisscross vast expanses of ocean faster than any other northern seabird, and can dive the length of two American football fields to snatch small fish.
But from 2015 to 2016, this superstar bird experienced an unprecedented die-off.
Over that period, about 62,000 emaciated, dead or dying murres (Uria aalge) washed onto beaches from Southern California to the Aleutian Islands of Alaska, a new study finds. What’s more, colonies throughout this range failed to reproduce during and shortly after the same time. All together, an estimated 10 to 20 percent of the region’s total population was wiped out, researchers report January 15 in PLOS ONE.
The cause? A gargantuan, extended marine heat wave nicknamed the Blob whose impact reverberated throughout the food web, the scientists say. Warmer ocean temperatures shifted the range and makeup of plankton communities and amped up the metabolic demands of all fish, shrinking one of the ecosystem’s key food supplies and starving out murres.
“This study leaves no stone unturned to see what might be affecting these birds,” says Andrew Leising, a research scientist at the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in La Jolla, Calif., who wasn’t involved in the study. The team synthesized a diverse range of data to reveal “the stressors that resulted from the heat wave that combined to really put the smackdown on the forage fish these birds rely on,” he says.
When John Piatt, a biologist at the U.S. Geological Survey in Anchorage, Alaska, first heard reports of large numbers of starving or dead murres washing ashore in Northern California and Washington in the summer of 2015, he wasn’t sure if the events were connected. Occasional die-offs of murres aren’t unusual. But within months, citizen scientists all along the U.S. and Canadian coast began encountering dead murres 10 to 1,000 times as often as normal. Piatt recalls thinking “this is too coincidental not to be related.”
These reports came hot on the heels of the largest and most powerful marine heat wave ever recorded: the Blob. This patch of warm water formed in late 2013 and grew to stretch more than 4 million square kilometers, from the Baja peninsula to the Aleutian Islands, by the summer of 2015. The Blob, which scientists have directly tied to human-caused climate change, languished until late 2016, heating many parts of the Pacific Ocean 2 to 3 degrees Celsius above normal temperatures and disrupting many marine ecosystems (SN: 12/14/17).
To begin connecting these dots, Piatt and his colleagues first assessed the extent of the die-off. Observations from citizen scientists at over 700 sites revealed that about 62,000 dead or dying murres washed ashore from 2015 to 2016. Since only a fraction of dead murres drift onto monitored beaches, researchers estimate that all together 530,000 to 1.2 million murres died.
“The magnitude of this die-off is without precedent,” Piatt says. “It likely represents about 10 to 20 percent of all the murres in this region.”
Those deaths were coupled with widespread reproductive failure. From 2015 to 2017, there were 22 instances of murre colonies completely failing to produce any chicks, while many other colonies produced fewer chicks than normal, the researchers found. “If these birds aren’t reproducing, it means they aren’t finding enough food,” Piatt says. “And if any bird can find food in the ocean, it’s the common murre.”
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Murres can dive up to 200 meters to snatch up sardines, anchovies and other small prey, broadly labelled “forage fish” by ecologists. To survive, murres must eat more than half their body weight each day. Normally, Piatt says, they meet these demands easily. But the Blob disrupted this ecosystem in ways that made forage fish harder to come by.
In the oceans, energy flows up the food web from the hordes of phytoplankton that convert sunlight into carbohydrates. The Blob lowered phytoplankton biomass to levels lower than any year measured since 1997, as the flow of nutrients to these regions diminished in the warmer waters. This, in turn, caused reductions in fat-packed zooplankton that forage fish eat, thinning out a key ecosystem resource. One study found that the whole-body energy content of the sand lance, a common forage fish, declined by 89 percent on average in 2016, compared with cooler years.
The heat wave pinched murres’ food supply in other ways, too. When waters warm, the pace of life also increases for cold-blooded fish. Both tiny anchovies and large Pacific cod that eat them need to eat more to sustain their amped-up metabolism. The researchers used simulations of how temperature affects metabolism to calculate that an increase of 2 degrees C above normal temperatures in the Gulf of Alaska would have increased the food-consumption needs of predatory fish like Pacific cod by an average of 63 percent.
“You do the math, and almost overnight these large predatory fish needed to eat a lot more forage fish,” Piatt says, in total about 1.5 times as many as they would have without the heat wave. That change meant murres were facing much stiffer competition over fewer, less nutritious forage fish. Ultimately, Piatt says there just weren’t enough forage fish to sustain the murres.
Whether the murres will bounce back remains to be seen, says Julia Parrish, a marine scientist at the University of Washington in Seattle. Birds can recover from a bad year or two, she says. But scientists expect massive marine heat waves like the Blob to become more frequent and intense in the near future (SN: 9/25/19), which could overwhelm the birds. Already, researchers reported last September the emergence of a similarly massive marine heat wave growing along the Pacific coast of North America that they’re monitoring. “Our study offers a window into what that future might hold,” Parrish says, “and it’s not pretty.”