Ocean-wide biomass declines projected due to climate change

Climate change will cause fish biomass to decline 5 percent for every one degree Celsius of warming, according to the most comprehensive analysis of marine ecosystem models to date.

The study, which was authored by 35 researchers from 12 countries, combines multiple climate and ecosystem models to create an ensemble model that estimates future global marine biomass — the total weight of all the fish, invertebrates, and marine mammals in the ocean. 

The effects of the decline are projected to be more severe for creatures at the top of the food chain, including fish species popular with fishermen.

The decline in biomass will be more severe if humanity continues to emit high amounts of greenhouse gases, but even a certain amount of decline is already locked in under low emissions scenarios. If current trends continue, marine biomass will decline 17 percent by 2100, while it will decline only 5 percent if humanity implements strong mitigation programs. 

The study finds that it didn’t make a difference whether fishing pressure remains or disappears entirely — the effect is driven entirely by the warming waters that are leading the oceans to be less-effective producers of life. 

“Reduction of greenhouse gas emissions will undoubtedly help to safeguard marine life as much as possible against further losses,” Heike Lotze, the lead author of the study and a professor at Dalhousie University in Halifax, Canada, said in a statement. 

Fishermen might not notice the declines much, because of the long timelines and the fact that they already remove large quantities of fish from the sea every year, according to study co-author Manuel Barange, the director of the Fisheries and Aquaculture Policy and Resources Division at the United Nations Food and Agriculture Organization. Rather, the actions of fisheries managers — setting catch limits and other management schemes — will ultimately influence the health of the oceans more. 

“We’re talking 80 years into the future. The changes year-on-year are very minimal,” Barange told SeafoodSource. “Fishing activity bears no way in the results of the study. The level of fishing is not affecting the overall message that there will be an overall decline in biomass of 10 percent. Fishing is not causing it. Fishers may see this as an opportunity to not be blamed.”

The study, which was compiled over six years by a group of experts and modelers on marine ecosystem dynamics, uses an ensemble of six global marine ecosystem models combined with two Earth system models. The researchers apply the models to four carbon emissions scenarios, both with and without fishing.

While modelers of climate change have been creating ensemble projections for years, this study is the first time the ensemble method has been applied to the entire marine ecosystem.

The results of the individual models diverge widely, with some showing barely any difference in ocean biomass by 2100, and others showing as much as 35 percent. The differing results occur because each model has its own processes, algorithms, and assumptions, Barange said. But few show more than 10 percent biomass reduction, and the majority tend to converge around that number.

The effects of climate change on fish biomass will differ geographically, the study finds, with biomass declining in lower latitudes near the tropics even as it increases in the far north and south. The growth of marine life in those upper latitudes is limited by temperature and a short growing season, but those limits will be raised with warming waters. 

“It’s not as though there will be a 5 percent decline per degree of warming across the ocean. It will be a rebalancing. Some places will have increases. Other places will have decreases. But if you average it out, then it’s an overall decline,” researcher and study co-author Tyler Eddy told SeafoodSource. 

Generally, models that take into account more food web linkages yield a more moderate decline, while those that take into account the size of species and individual species project greater declines, Eddy added.

Any time target species move, fisheries management strategies change or allocations are reapportioned, fishermen and their livelihoods are affected. In the U.S., shifting stocks are likely to weigh especially heavily on fishermen in the South, where no new species are coming in behind stocks that are migrating north, according to Leigh Habegger, executive director of the Seafood Harvesters of America, a group that represents fishermen. 

“Climate change is scary and I don’t think there’s many people in our group or in the fishing industry at large who are not witnessing the changes happening in our ocean,” Habegger told SeafoodSource. “Our fishermen are already seeing shifting stocks and warming ocean temperatures and ocean acidification impacts.”

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