The ocean sequesters about 22% of global anthropogenic CO2 emissions. Marine vertebrates contribute to the ocean’s carbon sink capacity in various ways, such as by fertilizing coastal vegetated habitats, and (through the work of marine predators) protecting this vegetation from overgrazing. Additionally, fish sequester carbon in the deep sea when they sink to the bottom after their natural death, whereas fishing releases the carbon embodied in fish back into the atmosphere when the catch is processed and consumed. Large fish (tuna, mackerel, shark, and billfish) that die in the ocean particularly contribute to “blue carbon” because these species are more likely to sink than be eaten near the surface. Unlike the CO2 released by terrestrial animals after death, the embodied carbon in marine corpses remains in the deep ocean.
This study estimates the extent to which fisheries have obstructed blue carbon sequestration. Mariani et al. report that fishing prevented 21.8 ± 4.4 Mt C (million metric tons of carbon) between 1950 and 2014 from being sequestered in the deep ocean. Industrial fisheries (as opposed to smaller, artisanal fisheries) are responsible for 85% of this extraction.
The amount of blue carbon extracted from the ocean through the harvest of large fish increased by almost one order of magnitude in 65 years (from 0.13 Mt C in 1950 to 1.09 Mt C in 2015). Combining CO2 emissions from fishing fleet transport and that of the fish removal itself amounts to 20.4 MtCO2 emitted in 2014, which is equivalent to the annual emission of 4.5 million cars.
Moreover, the authors found that government subsidies are encouraging overfishing. Almost half of the blue carbon extracted from the world’s oceans comes from areas that would be economically unprofitable without subsidies.
Our findings thus show that government subsidies, through supporting large-scale exploitation of large-bodied fish that are economically unviable, exacerbate the depletion of a natural carbon sink [Mariani 2010: 2].
Limiting and managing all fisheries on the unprofitable areas of the oceans could reduce CO2 emissions, rebuild fish stocks, and promote carbon sequestration by increasing the populations of large-bodied fish and the eventual deadfall of their carcasses to the depths.