Up until now, discussions about the effects of industrial fishing have mostly focused on the impact on marine ecosystems. In a model study, researchers are now showing that the massive removal of fish is also of great importance for global geochemical material cycles. Accordingly, sinking fish faeces are an important carbon sink and also play a role in the availability of oxygen at great depths. Although the results are fraught with great uncertainty, they provide a new perspective for future climate models.
Oceans play a crucial role in the global carbon cycle. They store 45 times more CO2 than the atmosphere and 20 times as much as vegetation and soil on land. Two carbon pumps are responsible for this: On the one hand, CO2 dissolves in cold sea water and is transported into deeper water layers by the circulation. On the other hand, photosynthetically active organisms in surface water convert CO2 into biomass. In turn, they are consumed by other living beings. Through the excretions and carcasses of these animals, carbon sinks to the sea floor, where it is withdrawn from the cycle for several centuries. Without this biological pump, it is estimated that the concentration of CO2 in the atmosphere would be 150 to 200 parts per million (ppm) higher. For comparison: pre-industrial the concentration was 280 ppm, currently 415 ppm.
Global model of marine ecosystems
When researchers have dealt with the biological carbon pump so far, they have primarily focused on the interplay between phytoplankton, zooplankton and bacteria. Fish, on the other hand, which have a lower overall share in terms of their biomass, have so far received little attention in this context. A team led by Daniele Bianchi from the University of California in Los Angeles is now using a model study to indicate that fish probably have a greater influence on material cycles than previously assumed.
The researchers created a global model of marine ecosystems before and after industrial fishing began. In doing so, they included various estimates of global fish stocks today and in the past and modeled the influence that fish, their food intake, their excretions and their sinking carcasses have on carbon storage and oxygen availability in the oceans.
Fish droppings as carbon stores
“Our approach necessarily makes many assumptions, and the nature of the data available requires a number of simplifications,” the researchers write. The mere question of how much biomass the fish make up worldwide is subject to great uncertainty. Estimates range from less than a gigaton to 50 gigatons. By comparing different estimates and combining them with other available data on fish catches and marine food webs, they came to the conclusion that before industrial fishing began, around 3.3 gigatons of fish weighing between ten grams and 100 kilograms lived in the oceans.
According to estimates by the researchers, fish stocks consumed around two percent of global primary production at the time, but produced ten percent of the biological material that sank to the sea floor in the form of feces and was stored there for centuries. “Since fish faeces sink orders of magnitude faster than small particles, the expected contribution from fish faeces is becoming increasingly important at depth,” the researchers said. With a sink rate of up to one kilometer per day, the fish droppings reach the seabed faster, so that less is broken down on the way there – a decisive contribution to the storage of carbon.
Fish biomass halved
Bianchi and his colleagues compared this pre-industrial period to the 1990s, which had the highest catch quotas ever before they were limited by regulations in many countries around the world. “By the 1990s, biomass and cycle rates were almost halved,” they write. “This suggests that the biogeochemical effects of fishing are comparable to those of anthropogenic climate change.” As a result of the massive removal of fish, fewer fish faeces and carcasses sank to the sea floor. An important carbon sink was thus partially eliminated.
Industrial fishing probably also had an impact on the oxygen content in the deep sea. When microorganisms on the seabed metabolize fish faeces, they need oxygen for this. With a smaller amount of feces, they use less oxygen. The rising temperatures caused by climate change have ensured that less oxygen is dissolved in the seawater – and without fishing it might have had an even greater impact on the availability of oxygen: “The decline in fish biomass and thus in respiration could have masked a significant part of the effect.” the researchers said.
“Stimulating Hypotheses”
“The results of the study show for the first time what contribution fish make to the biogeochemical cycles in the ocean,” comments Nicolas Gruber, head of the Institute for Biogeochemistry and Pollutant Dynamics at ETH Zurich, who was not involved in the research. The data basis is fraught with many uncertainties, but the fundamental implications are still exciting and important. “I consider these estimates to be well-founded and stimulating hypotheses that now have to be verified or falsified in further studies,” says Gruber.
Source: Daniele Bianchi (University of California, Los Angeles) et al., Science Advances, doi: 10.1126 / sciadv.abd7554