Harvesting forage fish can prevent fishing-induced population collapses of large piscivorous fish.

Fisheries have reduced the abundances of large piscivores-such as gadids (cod, pollock, etc.) and tunas-in ecosystems around the world. Fisheries also target smaller species-such as herring, capelin, and sprat-that are important parts of the piscivores' diets. It has been suggested that harvesting of these so-called forage fish will harm piscivores. Multispecies models used for fisheries assessments typically ignore important facets of fish community dynamics, such as individual-level bioenergetics and/or size structure. We test the effects of fishing for both forage fish and piscivores using a dynamic, multitrophic, size-structured, bioenergetics model of the Baltic Sea. In addition, we analyze historical patterns in piscivore-biomass declines and fishing mortalities of piscivores and forage fish using global fish-stock assessment data. Our community-dynamics model shows that piscivores benefit from harvesting of their forage fish when piscivore fishing mortality is high. With substantial harvesting of forage fish, the piscivores can withstand higher fishing mortality. On the other hand, when piscivore fishing mortality is low, piscivore biomass decreases with more fishing of the forage fish. In accordance with these predictions, our statistical analysis of global fisheries data shows a positive interaction between the fishing mortalities of forage-fish stocks and piscivore stocks on the strength of piscivore-biomass declines. While overfishing of forage fish must be prevented, our study shows that reducing fishing pressures on forage fish may have unwanted negative side effects on piscivores. In some cases, decreasing forage-fish exploitation could cause declines, or even collapses, of piscivore stocks.

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 pump1. Recent studies estimate that fish can release about 1,200 to 1,500 million metric tons of carbon per year (MtC year-1) in the oceans through feces production, respiration, and deadfalls, with mesopelagic fish playing a major role1,2. 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 fish3,4 and they die after reproducing at an early age4,5 (usually 1-2 years), after which their carcasses sink rapidly to the sea floor6. Deadfall of carcasses is particularly important for long-term sequestration because it rapidly transports carbon to depths where residence times are longest1,6. We estimate that cephalopod carcasses transfer 11-22 MtC to the seafloor globally. While cephalopods represent less than 5% of global fisheries catch7, fishing extirpates about 0.36 MtC year-1 of cephalopod carbon that could otherwise have sunk to the seafloor, about half as much as that of fishing large fish8.