Marine heat waves can have devastating effects on ocean and coastal ecosystems, and have been linked to mass die-offs of fish and wildlife, as well as widespread coral bleaching, and harmful algal blooms.
However, new research published this week suggests that marine heatwaves have no lasting effects on demersal fish, the bottom-dwelling fish populations that support many of the world's largest and most productive fisheries.
The study, published on 30 August in the scientific journal Nature, investigated the effects of 248 sea-bottom heatwaves from 1993 to 2019 on marine fish.
Researchers from California and Norway analysed 82,322 samples from long-term scientific surveys of continental shelf ecosystems in North America and Europe, spanning the subtropics to the Arctic.
What they found surprised them: the results showed that in general, marine heat waves had no lasting effects on bottom-dwelling species that include commercially important fish species such as flounder, pollock, haddock, redfish and halibut.
"As of now, we see no effects of marine heat waves on fish communities," said Laurene Pechuchet of UiT Norway's Arctic University, in a Norwegian language statement.
"But we know that fish communities are affected by temperature, so that does not mean that heat waves will not affect fish in the future," she said.
The researchers looked for effects on fish biomass and composition of species in demersal fish communities the year following a marine heat wave, defined as a period of more than five days of extreme seafloor temperatrues for that region and time of year.
To their surprise, they found no evidence that marine heat waves had major effects on regional fish communities.
"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," said lead author Alexa Fredston, assistant professor of ocean sciences at UC Santa Cruz.
Although declines in biomass did occur after some marine heatwaves, the researchers say such cases were the exception, not the rule. Overall, the effects of marine heatwaves could not be distinguished from natural variability in these ecosystems and in the sampling process of the surveys, the researchers said.
"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."
However, Pecuchet notes that temperatures in the North Atlantic during 2023 were the warmest ever recorded, and that this was not included in the data the researchers used in their study, which looked at the years 1993-2019.
"If anything, this study shows the importance of staying below 1.5 degrees. Today we are about 1 degree warmer than pre-industrial levels, and it seems that the fish are dealing with the effects of the heat waves at the present time," Pechuchet said.
"But we do not know what the effect will be if the temperature continues to rise. As long as we stay below a 1.5 degree increase, there is still hope, but that means reducing emissions drastically," she added.
The full research article is available here: Fredston, AL et al. Marine heat waves are not a dominant driver of change in demersal fishes. Nature. Vol. 620 (2023)