Temperatures up to 50 degrees Celsius and a string of records tumbled: The heat wave that hit the US Northwest and the west coast of Canada in June 2021 was tough. During this time, some scientists and beach-walkers observed a proliferation of dead and dying shellfish on the shores of the Salish Sea and other regions. An analysis of the data quickly collected at the time shows that the animals fell victim to a “perfect storm,” as Wendel Raymond of the University of Washington and his team report in “Ecology.”
Temperature extremes for the region coincided with the lowest tide levels of the year, which also occurred during the warmest parts of the day. “A worse scenario for the creatures of the intertidal zone could not have been imagined,” says Raymond. Not every species was hit equally hard: Mussels that burrow deeper into the sediment died less often than species that want to survive the ebb near the surface.
Mussels on the inner coasts were particularly badly affected, where the tide came in at the hottest time and where the air temperatures were significantly higher than on the outer coasts without offshore islands. Right on the open Pacific, waves and generally stronger water movement ensured that the conditions remained more bearable and fewer animals died.
Thanks to the large number of recording points, the working group was even able to identify small-scale differences: where trees shaded the bank or streams and rivers flowed into the sea, more mussels survived than a few hundred meters away, where the sun could shine unhindered and no other cooling took place.
Since the heat wave coincided with the period when many mussels reproduce, the mass die-off could affect stocks for several years, the researchers fear. And because climate change is leading to more frequent extreme heat events, such mass extinction events could also occur more frequently and endanger species accordingly. A long-term study is to be used to further investigate the development of the mussel stocks.