By Peter Rüegg-ETH Zurich for Futurity.

A new study highlights how achieving the Paris Agreement—global warming of no more than 1.5º Celsius over pre-industrial levels—would affect global fisheries.

It is currently unclear how the world will achieve the climate target, but the fisheries support the diets, livelihoods, and cultures of billions of people.

According to simulations from computer models, the fishing industry would strongly benefit from meeting the Paris Agreement target. The maximum catch potential of fish would increase by three percent for every degree Celsius decrease in surface temperature, report the researchers in Science.

Fishing catches in the tropical Pacific Ocean would shrink by 12 percent if the 1.5 degree rise in temperature is met, while a 3.5 degree increase would lead to a reduction of 47 percent. The benefits for vulnerable tropical countries are therefore a strong support for the 1.5° C goal.

“People living in these regions rely heavily on fish as their primary protein source from animals,” says Thomas Frölicher, coauthor of the study and a senior scientist at the ETH Zurich Institute of Biogeochemistry and Pollutant Dynamics.

By contrast, Arctic fishing boats could see their catch increase by a fifth for every degree Celsius rise in surface temperature. The warming will have a positive effect on fisheries in the Arctic region, because it reduces the amount of sea ice and allows more light and heat to penetrate the ocean. This promotes the growth of phytoplankton, which in turn boosts fish stocks.

In addition, fish stocks in the Arctic region will increase due to the invasion of species from warmer low latitude regions. If the 1.5° C target agreed in Paris could be met, Arctic fishermen would see their catch increase by about 30 percent. A rise in surface temperature of 3.5° C would boost their catch by 55 percent. In extreme cases, the local catch could even quadruple.

Fish stocks in the high northern latitudes will not necessarily always benefit from the ocean’s increased warming and productivity, however. A tipping point would eventually arrive, after which the increase in temperature has negative consequences for Arctic fisheries.

For example, fish stocks in the North Sea will start to decrease again as soon as the average temperature rise exceeds about 3.5 degrees. This is because high water temperatures result in more stable stratification of the ocean, which in turn inhibits the growth of phytoplankton.

“The correlation between the average global temperature and the cumulative manmade carbon-dioxide emissions into the atmosphere is linear, just like the connection between global fishing yields and temperature,” explains Frölicher. This makes it possible to directly calculate the impact of one tonne of CO2 emissions on the global fishing catch.

At present, the resolution of the climate models is to some extent not high enough to make robust predictions for individual coastal regions. Frölicher and his colleagues are therefore working on a follow-up study that will use climate models with much higher resolution.

Frölicher is part of a group of fish experts called the Nippon Foundation-Nereus Program, which lead author of the study, Professor William Cheung at the University of British Columbia, heads. The Canadian researchers have developed a computer program that lets them model the population of 900 different species of fish. It includes environmental conditions for the fish, such as water temperature, and the levels of nutrients and oxygen in the water. The team has combined these potential populations with existing climate simulations to determine the living conditions of fish under different climate scenarios.

Depending on the future climate change scenario, tropical fish currently living in waters between 27 and 29 degrees Celsius will be faced with water that is one to 2 degrees higher. However, many species will be unable to adapt to these conditions, also because warm water contains less oxygen. As a result, the fishes have to search for a new habitat.

“Once the temperature reaches a certain threshold, these fish will be forced to migrate to cooler waters in order to survive,” warns Frölicher.

Source: ETH Zurich