The study, published in Nature, assessed effects on commercially important fish such as flounder, pollock and rockfish based on data extracted from long-running scientific trawl surveys -- conducted by towing a net along the seafloor -- of continental shelf ecosystems in North America and Europe between 1993 and 2019. The analysis included 248 marine heat waves with extreme sea bottom temperatures during this period. The researchers were surprised to find that marine heat waves in general don't show major adverse effects on regional fish communities. Although declines in biomass did occur after some marine heat waves, the researchers said these cases were the exception, not the rule. Overall, they found that the effects of marine heat waves aren't distinguishable from the natural variability in these ecosystems.

 

In addition to assessing the impact on the total quantity of organisms in a given area, known as biomass, the researchers examined whether marine heat waves were causing changes in the variety of fish species composing fish communities. For example, evidence might show the loss of species associated with cold water and an increase in species associated with warm water, a phenomenon known as tropicalization. The findings suggest fish may be able to find safe havens by moving to areas with cooler water during marine heat waves, which the researchers defined as periods of more than five days with extreme sea bottom temperatures for that region and season.