Leaving more big fish in the sea reduces CO2 emissions

An international team of scientists has found leaving more big fish in the sea reduces the amount of carbon dioxide (CO2) released into the Earth's atmosphere.

Leaving more big fish in the sea--especially where fishing is not economically profitable in the
Central Pacific, South Atlantic, and North Indian Oceans --reduces the amount of
carbon dioxide (CO2) released into the Earth's atmosphere [Credit: Enric Sala]

When a fish dies in the ocean it sinks to the depths, sequestrating all the carbon it contains with it. This is a form of 'blue carbon'--carbon captured and stored by the world's ocean and coastal ecosystems.

"But when a fish is caught, the carbon it contains is partly emitted into the atmosphere as CO2 a few days or weeks after," said Gael Mariani, a PhD student at the University of Montpellier in France.

Mr Mariani led a world-first study showing how ocean fisheries have released at least 730 million metric tons of CO2 into the atmosphere since 1950. An estimated 20.4 million metric tons of CO2 was emitted in 2014--equivalent to the annual emissions of 4.5 million cars.

Co-author Professor David Mouillot from the ARC Centre of Excellence for Coral Reef Studies at James Cook University (CoralCoE at JCU) and the University of Montpellier said the carbon footprint of fisheries is 25 percent higher than previous industry estimates.

"Fishing boats produce greenhouse gases by consuming fuel," Prof Mouillot said. "And now we know that extracting fish releases additional CO2 that would otherwise remain captive in the ocean."

Large fish such as tuna, sharks, mackerel and swordfish are about 10 to 15 percent carbon.

"When these fish die, they sink rapidly," Prof Mouillot said. "As a result, most of the carbon they contain is sequestered at the bottom of the sea for thousands or even millions of years. They are therefore carbon sinks--the size of which has never been estimated before."

He says this natural phenomenon--a blue carbon pump--has been increasingly and greatly disrupted by industrial fishing.

The authors also say the phenomenon has not only been overlooked until now, but it happens in areas where fishing is not economically profitable: in the Central Pacific, South Atlantic, and North Indian Oceans.

"Fishing boats sometimes go to very remote areas--with enormous fuel consumption--even though the fish caught in these areas are not profitable and fishing is only viable thanks to subsidies," Mr Mariani said.

For the authors of the study, the new data strongly supports more reasoned fishing.

"The annihilation of the blue carbon pump represented by large fish suggests new protection and management measures must be put in place, so that more large fish can remain a carbon sink and no longer become an additional CO2 source," Mr Mariani said. "And in doing so we further reduce CO2 emissions by burning less fuel."

"We need to fish better," Prof Mouillot said.

The study is published in Science Advances.

Source: ARC Centre of Excellence for Coral Reef Studies [October 28, 2020]

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The uncertain future of the oceans

The ocean plays a key role in the current climate change, as it absorbs a considerable part of the atmospheric carbon dioxide emitted by mankind. On the one hand, this slows down the heating of the climate, and on the other hand, the dissolution of CO2 in seawater leads to acidification of the oceans. This has far-reaching consequences for many marine organisms and thus also for the oceanic carbon cycle. One of the most important mechanisms in this cycle, is called the biological carbon pump. Part of the biomass that phytoplankton forms in the surface ocean through photosynthesis sinks to the depths in the form of small carbonaceous particles. As a result, the carbon is stored for a long time in the deep sea. The ocean thus acts as a carbon sink in the climate system. How strongly this biological pump acts varies greatly from region to region and depends on the composition of species in the ecosystem.

One of the mesocosm experiments evaluated in the current study took place
in 2010 in Kongsfjord, Spitsbergen [Credit: Kerstin Nachtigall]

The study, which has now been published in the journal Nature Climate Change, is one of the most comprehensive studies so far on the effects of ocean acidification on marine ecosystems. Scientists at the GEOMAR Helmholtz Centre for Ocean Research in Kiel have now been able to show for the first time that ocean acidification influences the carbon content of sinking organic material, and thus the biological pump. Surprisingly, the observed changes were highly variable. The carbon content of sinking particles increased or decreased significantly with increasing CO2, depending on the composition of species and the structure of the food web. Since the underlying data cover a wide range of ocean regions, this seems to be a global phenomenon. These findings allow a completely new assessment of the effects of ocean acidification.

Dr. Jan Taucher, marine biologist and main author of the study, says: "Interestingly, we found that bacterial and animal plankton, such as small crustaceans, play a key role in how the carbon cycle and biological pump respond to ocean acidification. Until now, it has been widely held that biogeochemical changes are mainly driven by reactions of phytoplankton. Therefore, even modern Earth system models do not take into account the interactions we observe between the marine food web and the carbon cycle. Our findings thus help to make climate models more realistic and improve climate projections".

Up to now, most of the knowledge on this topic has been based on idealized laboratory experiments, which only represent ecological interactions and the dynamics of the complex marine food web in a highly simplified way. This makes it difficult to transfer such results to real ocean conditions and project them into the future. In order to gain a more realistic insight, the study summarizes several field experiments that were conducted with large-volume test facilities, so-called mesocosms, in different ocean regions, from arctic to subtropical waters.

Mesocosms are, so to speak, oversized test tubes in the ocean, in which changes in environmental conditions in a closed but otherwise natural ecosystem can be studied. For the present study, a large amount of data from five mesocosm experiments was synthesized to provide a more precise picture of plankton communities and biogeochemical processes within the ecosystem. A total of over ten thousand data points were included in the analysis.

The newly gained knowledge can now be used to implement the complex ecological interactions in Earth system models, thus contributing to further improve climate projections.

Source: GEOMAR Helmholtz Centre for Ocean Research Kiel [October 26, 2020]

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