A shifting food web, driven by the impact of climate change and one of the most notorious marine heatwaves on record, has been determined as the cause of one of the largest and “most devastating” mass die-offs of a single seabird population recorded in modern history. 

While murres – a seabird also known as guillemots across Europe – are not considered to be a threatened species, they have been found to be among those highly susceptible to ocean pollution and the wider reaching impacts of warming waters and climate change.

As details behind a 2020 study led by the University of Washington emerge, seabird specialists are only now beginning to understand the scale of such impact; one that led to the death of some four million guillemots along the US West Coast and Alaska – almost half the species’ population within the region.

It started back in 2020, when the University of Washington-led citizen science programme, ‘Coastal Observation and Seabird Survey Team’ (COASST), which trains coastal residents to search local beaches and document dead birds, first noticed a large mortality event affecting common murres along the US West Coast and Alaska.

In that year alone, the survey documented around 62,000 dead murres, while in some places the estimates were more than 1,000 times the normal rates.

Yet, while these numbers already seem steep, the 2020 study did not take into account the lasting impact of ‘the blob’ – a notorious marine heat wave that spanned 2014 and 2016. The event – described as an unusually warm and long-lasting patch of surface water in the northeast Pacific – went on to impact weather and marine ecosystems in such a way that ocean productivity decreased and, ultimately, the food web’s supply of sustenance for seabirds an other predators was shifted.

In a new paper, published in the journal Science on December 12, a team of researchers from the U.S. Fish and Wildlife Service analysed total mortality of 13 murre colonies between 2008 and 2022 to find that the average colony size in the Gulf of Alaska dropped by half after the marine heat wave, while the decline was even steeper, at 75% loss, along the eastern Bering Sea.

In total, around 4 million common murres died in Alaska, about half of the population as approximately around 8 million common murres lived in the area before the marine heat wave. No recovery of the population has been seen yet.

Julia Parrish, a professor of aquatic and fishery sciences and of biology at the University of Washington, said: “This study shows clear and surprisingly long-lasting impacts of a marine heat wave on a top marine predator species.

“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.”

While ‘the blob’ has been the most intense marine heat wave yet, the effects of climate change already make warm conditions more common. Another study, led by the University of Washington, published in 2023, argued that a 1 degree Celsius increase in sea surface temperature for more than six months results in multiple seabird mass mortality events.

Parrish added: “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.

“The frequency and intensity of marine bird mortality events is ticking up in lockstep with ocean warming.”

The 2023 paper suggested that it would take at least three years for seabird populations to recover after a marine heat wave. However, the populations of common murres in Alaska haven’t yet recovered after seven years – a worrisome observation, according to Parrish. “We may now be at a tipping point of ecosystem rearrangement where recovery back to pre-die-off abundance is not possible,” she said.

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