New research published in the journal Nature finds that despite the devastating impact marine heatwaves may have on marine ecosystems, that these heatwaves generally have not had lasting effects on the ocean waters that are home to many of the world’s most robust fisheries.
The study by a team of researchers funded through FISHGLOB, an international consortium of scientists who collect, curate, share and use date from scientific bottom trawl surveys, looked at how fish biomass and community composition was affected in the year following a marine heatwave.
Researchers found that in general, these marine heatwaves do not have a major impact on regional fish communities.
“This is the year of marine heatwaves, which now cover nearly half of the world’s ocean,” said Malin Pinsky, a study co-author, who’s an associate professor with the Department of Ecology and Evolutionary Biology at the University of California, Santa Cruz.
“Past heatwaves have been tied to mass die offs of ocean life and economic disasters costing billions of dollars,” he stated. “Our paper is the first comprehensive test of heatwave impacts on the ecosystems most important to global fisheries and blue food security.”
In stark contrast to the current concern about heatwaves, “our analysis reveals a silent majority of heatwaves that had no or even positive impacts on these ecosystems,” Pinsky explained. “While this ocean resilience is good news, it also means that negative impacts are unpredictable.”
“Each heat wave that hits is like rolling the dice,” he continued. “Will it be a bad one or not? We don’t know until it happens, and this uncertainty is difficult for fisheries and coastal economies.”
Pinsky added that to reach these conclusions, the researchers assessed the impacts of 248 marine heatwaves along the coasts of North America and Europe from 1993 to 2019, using 82,322 samples from scientific surveys of marine fishes.
“There is an emerging sense that the oceans do have some resilience, and while they are changing in response to climate change, we don’t see evidence that marine heatwaves are wiping out fisheries,” lead author Alexa Fredston, assistant professor of ocean science at UC Santa Cruz, noted in comments on the study published by UC Santa Cruz.
“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,” Fredston said. “Marine heatwaves can drive local change, but there have been hundreds of marine heatwaves with no lasting impacts.”
Researchers also studied whether marine heatwaves were resulting in loss of species associated with cold water and an increase in species associated with warm water, known as “tropicalization.” Fredston said that while tropicalization has been associated with long-term warming of oceans they saw no consistent signature of that associated with marine heatwaves.
For the study, researchers defined marine heatwaves as periods of over five days with extreme sea bottom temperatures for that region and season. By subtracting the effects of long-term ocean-warming trends, they were able to focus on episodes of extreme warming on top of those long-term trends, the researchers said.
The data set employed for the study included the 2014-2016 marine heatwave in the Northeast Pacific known as “the Blob.” Fredston said the magnitude of impact of “the Blob” was quite unusual.
“There were some really problematic outcomes, including huge declines in the Gulf of Alaska, where commercial fisheries really suffered, but other West Coast fisheries did well because a lot of species came up from the south,” he remarked.
The findings suggest that fish may be able to find safe haven by swimming to areas with cooler water during extreme heat events, and that other factors in addition to temperatures may be important in determining the response of an ecosystem to a marine heatwave.
The research was funded by the French Foundation for Biodiversity Research (FRB), the Centre for the Synthesis and Analysis of Biodiversity (CESAB), the Canadian Institute of Ecology and Evolution (CIEE), and the French Embassy in Canada.
The study was also supported by the Lenfest Oceans Program and the U.S. National Science Foundation.