Scientists find link between marine heatwaves and population crash
By Anastasia Goelitz
Photo Credit: Two mature snow crabs by Chris Long via NOAA
Since 2018, huge numbers of Bering Sea snow crabs (Chionoecetes opilio) have disappeared from Alaskan waters, and scientists have now linked the decline to warmer ocean temperatures. The findings of the recently published study mark the loss as one of the largest collapses of a mobile marine species due to oceanic heat waves globally.
Snow crabs are at home on the muddy and sandy seafloors of the Northern Atlantic, Pacific and Arctic, where they can hide from predators in burrows. Their range includes the Bering Sea off the coast of Alaska, where they can occur up to 1400m deep, but they are hardly ever found beneath 300m. These brownish crabs, with their white bellies and green to blue eyes, can grow to more than 13cm in size and have a life expectancy of 20 years. Both as prey and predator, they play an essential ecological role. Thanks to their economic importance in Alaska, their biology and population dynamics are exceptionally well studied through annual trawling surveys conducted by the National Marine Fisheries Service on the Bering Shelf since 1975.
Alaskan fisheries account for over half of US fishing industries and a third of crab catches. Snow crabs, being one of the seven commercially important species, bring in around US$150 million per year. However, the Alaska Department of Fish and Game cancelled the harvest season for the second consecutive year, just this month, following the overwhelming lack of crabs on the shelf.
After a historical high in their abundance, 10 billion crabs disappeared from the Bering Sea during marine heat waves between 2018 and 2021, a population decline of 80%. Following the survey in 2021, the National Oceanic and Atmospheric Administration (NOAA) changed their status to overfished and implemented a rebuilding plan to restore the population by 2029. However, scientists suspected that something other than fishing rates was at play. In a study published only days after the season cancellation, the potential drivers impacting mortality were modelled. Both warming ocean temperatures and a restricted distribution were found to be responsible for their drastic die-off. A recently discovered cause of destruction could explain the limited range. Midwater trawls, mainly used to catch pollock in Alaska, frequently touch the seabed. As a result, organisms, including crabs, and their habitats are being crushed, and the survivors are left to migrate into safer waters.
In contrast, the strong impact of warmer temperatures suggested by the data stumped the researchers initially, as the warming did not exceed the species’ thermal limits. However, it does increase their metabolic rate, and therefore, their requirement for more food inside reduced foraging grounds. This has led ten billion snow crabs to starve.
These shocking findings may link the crab decline to a constellation of factors. Still, they tie into the trend that all Alaskan fisheries and the 15% of the state’s population, which rely directly on the ocean for sustenance, face. Most economically important species (like salmon and crab) are declining in numbers - if they have not already experienced severe crashes. While factors like overfishing or disease are certainly underlying stressors further impacting mortality, they have not been identified as strong drivers in this study. Most importantly, global warming, like other human-impacted environmental conditions, is becoming increasingly apparent as a driver of population dynamics and a variable that must be accounted for in management strategies. Millions of people rely on functioning ecosystems in rural Alaska and the rest of the world, and the future of biodiversity depends on fighting climate change’s impact on them.
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