RESEARCH ON WHETHER SEA FISH CAN WITHSTAND MARINE HEATWAVES

Long periods of unusually warm ocean temperatures, known as marine heat waves, can have devastating effects on the marine ecosystem and have been linked to widespread coral bleaching, harmful algal blooms and precipitous declines in commercially important fish species.

However, a new study has found that marine heat waves have generally not had lasting effects on the fishing communities that form the basis of many of the world’s largest and most productive fisheries.

The research was recently published in the scientific journal Nature .

“As of now, we see no effects of marine heat waves on the fishing communities. 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, says one of the researchers behind the study Laurene Pecuchet, associate professor at UiT Norway’s Arctic University .

The researchers have used data from long-term studies with bottom trawling of fish on the continental shelf in North America and Europe from 1993 to 2019. The analysis included 248 marine heat waves with extreme seabed temperatures during this period.

The trawl surveys were carried out by towing a net over the seabed, to look at the amount of bottom-dwelling species that include commercially important fish species such as flounder, pollock, haddock, redfish and halibut. Around  82,000 different trawls are included in the study with over 1,700 different fish species from both the Pacific and Atlantic oceans.

The researchers looked for effects on fish biomass and composition of species in the fish community the year after a marine heat wave. To their surprise, they found no evidence that marine heat waves generally have major effects on regional fish communities.

“It seems that the oceans have a certain resilience, and even if they change due to climate change, we see no evidence that marine heat waves wipe out the fisheries,” says Alexa Fredston, who has led the research.

She is an associate professor at the University of California at Santa Cruz.

Although declines in biomass occurred after some marine heat waves, the researchers found that these cases were the exception, not the rule. Overall, they found that the effects of marine heat waves cannot be separated from the natural variation in these ecosystems.

Laurene Pecuchet says that this is the first study on heat waves in the sea that has studied the effect on fish species that live far out to sea.

“Fish move and they can find hiding places where the sea can be a little colder, says Pecuchet.

She explains that in coastal areas it can be worse. There are species that do not move, such as kelp and coral, but also fish that live more in one place for most of the year. These areas and these species are more affected by such temperature changes.

“Skrei that wander may not be as affected by heat waves. While coastal cod may be more affected, because they do not migrate as much,” says Pecuchet.

She emphasizes that this has not been researched much yet.

As well as assessing the impact on biomass, the researchers looked at whether marine heat waves caused changes in the composition of fish communities. For example, it can lead to a loss of species associated with cold water and an increase in species associated with warm water. The researchers call this phenomenon tropicalisation.

“Tropicalisation has been associated with long-term warming of the oceans, but we saw no signs that this is the case with marine heat waves,” says Pecuchet.

The researchers defined marine heat waves as periods of more than five days of extreme seafloor temperatures for that region and time of year. By extracting the effects of long-term ocean warming trends, they were able to distinguish episodes of extreme warming on top of the long-term trends.

“The findings suggest that fish may be able to find safe harbors by moving to areas with cooler water during these extreme events, says the marine scientist.

In the so-called Paris Agreement, the countries have set a goal that we humans must work to ensure that the average global temperature is not increased by more than 1.5 degrees.

“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. 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,” concludes Pecuchet.

Link to the research article:  AL Fredston et al.: Marine heat waves are not a dominant driver of change in demersal fishes. Nature . Vol. 620, 2023.

Source: https://uit.no/nyheter/artikkel?p_document_id=821608