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Marine Heat Waves Do Not Cause Major Adverse Effects on Fish Communities

Rutgers-led research found that marine heat waves -- prolonged periods of unusually warm ocean temperatures -- haven't had a lasting effect on the fish communities that feed most of the world.

The finding is in stark contrast to the devastating effects seen on other marine ecosystems cataloged by scientists after similar periods of warming, including widespread coral bleaching and harmful algal blooms.

"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 heat waves are wiping out fisheries," said Alexa Fredston, the lead author of the study who conducted the research as a postdoctoral associate in the Global Change Research Group, part of the Department of Ecology, Evolution and Natural Resources in the Rutgers School of Environmental and Biological Sciences (SEBS.)

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.

"The oceans are highly variable, and fish populations vary quite a lot," said Fredston, now an assistant professor of ocean sciences at University of California, Santa Cruz. "Marine heat waves can drive local change, but there have been hundreds of marine heat waves with no lasting impacts."

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.

The data included some notable examples of marine heat waves that did have profound impacts, such as the 2014-2016 marine heat wave in the Northeast Pacific known as "the Blob," one of the largest on record.

While "the Blob" led to a 22 percent loss of biomass in the Gulf of Alaska, a 2012 marine heat wave in the Northwest Atlantic led to a 70 percent biomass gain. The authors also noted that these weren't large changes compared to natural variability in biomass, and similar effects weren't seen after most other marine heat waves.

"We found that these negative impacts are unpredictable and that other heat waves had no strong impacts," said Malin Pinsky, an associate professor in the Department of Ecology, Evolution and Natural Resources and director of the Global Change Research Group at SEBS and a co-author of the study. "This means that each heat wave that hits is like rolling the dice: Will it be a bad one or not? We don't know until it happens."

Other Rutgers researchers who participated in the study include Zoë Kitchel, a doctoral student, and Aurore Maureaud, a postdoctoral associate, both with the Department of Ecology, Evolution and Natural Resources at SEBS.

Researchers from other institutions participated in the study, including the University of British Columbia, the University of Bern in Switzerland, the National Oceanic and Atmospheric Administration, the French Research Institute for Exploitation of the Sea, the University of Montpellier in France, the University of Tromsø in Norway and Fisheries and Oceans Canada.

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