Study finds fish surprisingly resistant to marine heatwaves 

Prolonged periods of unusually warm ocean temperatures or marine heatwaves can have devastating effects on marine ecosystems, but the extent of the impact on fish was not known. However, a new study has found that fish are surprisingly resistant to heatwaves in the ocean. 

Marine heatwaves can lead to decay and bleaching of sponges and corals, seabirds dying in large numbers, waterbodies witnessing harmful algal blooms, decimation of seaweeds and increased marine mammal strandings.  

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Researchers in Canada, Europe and the United States collaborated on the study, titled Marine heatwaves are not a dominant driver of change in demersal fishes. It was published August 30, 2023 in journal Nature.

The study relied on data from long-term scientific trawl surveys of continental shelf ecosystems in North America and Europe conducted between 1993 and 2019. Trawl surveys are conducted by towing a net above the seafloor to assess the abundance of species at the ocean's bottom. 

During the survey period, 248 marine heatwaves with extreme sea bottom temperatures were included in the analysis.

The scientists also looked at the infamous “Blob” that hit the British Columbia coast from 2014 to 2016. The Blob was a large mass of relatively warm water in the Pacific Ocean off the coast of North America. 

The researchers studied how it affected populations of demersal fish or groundfish, which include some of the world’s largest fisheries like Alaskan pollock and Atlantic cod. 

The researchers investigated the effects of a marine heatwave on fish biomass and community composition in the year following. Surprisingly, they found no evidence that marine heatwaves in general have a significant impact on regional fish communities.

There is an emerging sense that the oceans do have some resilience, lead author Alexa Fredston, assistant professor of ocean sciences at the University of California, Santa Cruz, on the university website.

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“While oceans are changing in response to climate change, we don’t see evidence that marine heatwaves are wiping out fisheries,” Fredston said in the statement. 

Although some marine heatwaves caused biomass declines, the researchers said these were the exception rather than the rule. Overall, they discovered, the effects of marine heatwaves are indistinguishable from natural variability in these ecosystems and in the survey sampling process.

While “the Blob” resulted in a 22 per cent loss of groundfish biomass in the Gulf of Alaska, a 2012 marine heatwave resulted in a 70 per cent biomass gain in the Northwest Atlantic.

“The oceans are highly variable and fish populations vary quite a lot. Against that background, we didn’t see evidence of marine heatwaves dramatically reducing the abundance of fish in the temperate oceans,” the lead author stated. 

Marine heatwaves can drive local change, Fredston pointed out, adding that hundreds of marine heatwaves have occurred with no long-term consequences. 

The scientists also considered whether marine heatwaves were causing changes in the composition of fish communities, looking for losses of species associated with cold water and an increase in species associated with warm water (known as “tropicalisation”). 

They found no consistent signature for such losses caused by marine heatwaves.

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The findings are in contrast with his previous studies, said co-author William Cheung, professor at the University of British Columbia Institute for Oceans and Fisheries, stated on UBC website. 

“My previous studies projected that heatwaves under a high emissions scenario will have major effects this century, including a six per cent drop in potential catches per year per country — or hundreds of thousands of tonnes of fish — on top of projected decreases from climate change,” said Cheung.

The results highlighted the impacts of marine heatwaves on fish are sporadic, he added.

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