If a fish dies in the ocean, the carbon it contains sinks with the carcass to the sea floor instead of escaping into the atmosphere as carbon dioxide. However, if the fish is caught, the greenhouse gas is released into the atmosphere. In this context, the analysis of fisheries data since 1950 has shown that industrial fishing is responsible for significantly higher CO2 emissions than previously assumed. In addition to the fuel for the ships, this is because the removal of large fish reduces the natural carbon storage of the oceans – even though many of the areas particularly affected are not even economically profitable.
The oceans act as a huge reservoir for carbon dioxide (CO2). Fish play a part in this, especially large specimens such as tuna, mackerel, sharks and swordfish. After their death, their carcasses sink particularly quickly to the sea floor, so that the carbon contained in the animals does not get into the atmosphere as part of the decomposition. However, industrial fishing brings large quantities of these fish out of the water every year. The animals are processed and consumed on land. The carbon stored in their bodies is released into the atmosphere through direct emissions, excrement and breathing of consumers.
Emissions from fuel and dead fish
Researchers led by Gaël Mariani from the University of Montpellier in France have now quantified how high CO2 emissions are caused by catching large fish. To this end, they have evaluated fishery data since 1950. In order to provide a solid, conservative estimate, they only included fish in the calculation that were caught in regions with a water depth of over 200 meters and whose body size was at least 30 centimeters. “When these fish die, they sink quickly,” explains Mariani’s colleague David Mouillot. “As a result, most of the carbon they contain is often confiscated from the ocean floor for thousands or even millions of years. However, it is a carbon sink the size of which has never been estimated before. “
The researchers have now included this estimate in their study. Their result: Since 1950, marine fishing has released at least 730 million tons of CO2 into the atmosphere. This makes the fisheries’ carbon footprint 25 percent larger than previous industry estimates, Mouillot said. The emissions have increased significantly over the decades. Japan, Indonesia and Taiwan, which together are responsible for around a third of global fishing emissions, have a particularly large share. “Fishing boats produce greenhouse gases by using fuel,” says Mouillot. “And now it is becoming clear that the production of fish releases additional CO2 that would otherwise remain trapped in the ocean.”
Not economically viable
Some of the hardest hit areas are the Central Pacific, the South Atlantic and the North Indian Ocean. As the researchers showed, fishing in many of these regions is not even profitable: the fuel costs for the ships are hardly offset by the profit from the fish caught. Without government subsidies, 43.5 percent of carbon withdrawals from the high seas would no longer be worthwhile. “Our results show that government subsidies aggravate the disappearance of a natural carbon sink by promoting the economically unprofitable catch of large fish to a large extent,” the researchers write in their publication.
If fishing were restricted to areas where it is economically viable, the study found that both biodiversity and the climate could benefit. Less fuel would be used to get to remote trapping sites. This would reduce the resulting CO2 emissions. The biomass of living fish would increase and would bind more carbon in the short term. In the long run, the carcasses of dead fish would store additional carbon.
Sustainable fishing
The researchers also recommend this approach in terms of sustainable development. You advocate responsible, sustainable fishing. “The destruction of the blue carbon pump that large fish represent suggests that new conservation and management measures must be put in place so that more large fish can remain a carbon sink rather than an additional source of CO2,” says Mariani. “And since we burn less fuel this way, we further reduce CO2 emissions.”
It is important, however, not to look at fishing in isolation, but to include other social effects. For example, if the catch quotas are reduced, other, more climate-damaging foods would be consumed in larger quantities, such as the meat of cattle, pigs or lambs. In this case, CO2 emissions would have been saved in one place, but increased in another. In view of the current overfishing, the researchers believe that it makes sense, in particular, to limit fishing in unprofitable areas – also in order to preserve fish as food for humans in the long term. “We have to fish better,” said Mouillot.
Source: ARC Center of Excellence for Coral Reef Studies, specialist article: Science Advances, doi: 10.1126 / sciadv.abb4848