Impact of increased fishing on long-term sequestration of carbon by cephalopods.

Fish and other metazoans play a major role in long-term sequestration of carbon in the oceans through the biological carbon pump. Recent studies estimate that fish can release about 1,200 to 1,500 million metric tons of carbon per year (MtC year) in the oceans through feces production, respiration, and deadfalls, with mesopelagic fish playing a major role. This carbon remains sequestered (stored) in the ocean for a period that largely depends on the depth at which it is released. Cephalopods (squid, octopus, and cuttlefish) have the potential to sequester carbon more effectively than fish because they grow on average five times faster than fish and they die after reproducing at an early age (usually 1-2 years), after which their carcasses sink rapidly to the sea floor. Deadfall of carcasses is particularly important for long-term sequestration because it rapidly transports carbon to depths where residence times are longest. We estimate that cephalopod carcasses transfer 11-22 MtC to the seafloor globally. While cephalopods represent less than 5% of global fisheries catch, fishing extirpates about 0.36 MtC year of cephalopod carbon that could otherwise have sunk to the seafloor, about half as much as that of fishing large fish.