Fish are an important factor in the bio- and geochemical cycles of the oceans – including their corpses and excrement. Fish take up large amounts of carbon from the water with their food and excrete it again in their feces. The excrement and also the corpses of fish sink into the sea, and what is not eaten on the way to the seabed sinks into the mud. At a speed of up to one kilometer per day, this biomass reaches the sea floor faster than other, smaller particles. So less of it is broken down on the way down and the chunks from the fish intestine make a decisive contribution to the storage of carbon, which remains trapped in the sea floor for up to 600 years – and thus does not enter the atmosphere as CO₂.
A study by an international team led by the biologist is now showing the great impact that the amount of fish excrement has on the carbon sink in the sea Daniele Bianchi from the University of California, Los Angeles. They have considered the reduced fish faeces biomass under the influence of industrial fishing. This means that the biogeochemical cycle in the oceans can be better understood for the first time.
The biologists first used historical sources and DNA traces in the seabed to determine how many fish swam in the sea before industrialized fishing at the beginning of the 20th century – the catches are documented for important species such as cod and tuna and molecular traces in the The sea floor can still be detected after decades. On this basis, they reconstructed the biomass of the fished and non-fished species. By comparison with other estimates, they came to 3.3 gigatons of fish between 10 grams and 100 kilograms in weight that were living in the seas at the time. Their results confirm existing studies such as those of the fisheries statistician Daniel Pauly: From the beginning of industrialized fishing to the largest catches in the 1990s the fish biomass has halved.
Fish as part of the carbon pump and the carbon cycle
In the meantime, many research groups have analyzed and quantified the global processes of the carbon cycle in the oceans: The oceans store around 50 percent of man-made carbon dioxide and pump it down to the deep sea floor. A distinction is made between two types of pumps: the physical carbon pump, the CO2 dissolves in seawater and transported into the depths by currents, and the biological carbon pump, where living things take up and transport carbon.
Previous models of the biological pump mainly take into account the myriads of bacteria and plankton. Bianchi’s model is the first to show the interrelationships between fish biomass, its metabolism and excrement production. This makes it easier to understand the influence of fishing on the bio- and geochemical cycles in the sea.
Fish mostly feed on the nutrient-rich sea surface, where the photosynthetically active phytoplankton CO2 converted into biomass. The fish droppings transport the absorbed carbon from the surface to the depths. Below 1000 meters, carbon is sequestered; according to the new model, fish faeces make up 10 percent. Without this biological pump, the CO would be2-Concentration in the atmosphere estimated to be 150 to 200 parts per million (ppm) higher. For comparison: around 1800 the concentration was 280 ppm, In 2020 one calculated one Value of 412.5 ppm.
Fish droppings make for a better climate
According to estimates by the researchers, fish stocks consumed around two percent of global primary production, but produced ten percent of the biological material that sank to the sea floor in the form of excrement and was stored there for centuries. With the halving of fish stocks, their excrement has also been significantly reduced, which means that a significant carbon sink has shrunk considerably.
Even if the biomass of marine fish and their faeces are estimated values, the new model has a clear message: enormous amounts of carbon are bound in fish excrement for centuries or even longer. “The figures in the model suggest that the effects of industrial fisheries on the carbon cycle of the oceans are comparable in magnitude to the effects of climate change on the carbon content of the oceans,” explained to Bianchi New Scientist.
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“The new study is the first to provide figures that are already known,” says the German marine biologist and fish expert Rainer Froese from the Helmholtz Center for Ocean Research Kielwho was not involved in the study. “Overfishing not only harms stocks, ecosystems, fisheries and consumers, but also exacerbates the effects of climate change. A preliminary ecosystem model by GEOMAR for the western Baltic Sea confirms that sustainable fishing with correspondingly larger biomass contributes to significantly more carbon deposits on the ground than the current overfishing. “
It is therefore time not only to regard fish as food, but also to understand them and their metabolic products as an important factor in the structure of the bio- and geochemical cycles in the oceans.