Clearly, the feed requirements for some types of aquaculture systems place a strain on wild fish stocks. But does farmed fish production overall represent a net gain to global fish supplies? Our calculations indicate it does, but most of that gain in fish supplies from aquaculture comes from carps, marine mollusks, and other mostly herbivorous species.
Global harvest of wild fish and aquatic plants removes 123 Mt from seas and lakes each year, and 27 Mt of this is directly discarded as bycatch. Without the bycatch, fisheries landings amount to 96 Mt, of which 65 Mt of whole fish and 1 Mt of seaweeds are consumed by humans.
The remaining 30 Mt of fish catch plus another 2 Mt of processing scraps from aquaculture and fisheries are used for fish meal production.
(The fish meal industry has proposed that fishing vessels be encouraged to retain the currently discarded bycatch for sale to producers of fish meal and fish oil. Sale of bycatch could prove undesirable, however, if it undermines efforts to reduce bycatch rates or decreases the return of bycatch to the waters from which it was taken.)
One-third of the fish used to make fish meal, about 10 Mt, is currently converted to aquaculture feeds, while the remaining 22 Mt goes into fish meal for chicken, pig, and other livestock feeds. The use of these wild-caught fish for feeds reduces supplies of wild fish that could potentially be consumed directly by people. In Southeast Asia, for example, small open ocean fishes such as mackerel, anchovy, and sardines supply an important protein source for local people.
Although some fish utilized for fish meal and fish oil, such as menhaden, are distasteful to humans or are worth more as fish meal and oil than as food for consumers, the demand for small ocean fish for direct human consumption is likely to increase with population growth in the developing world.
Finally, total aquaculture production of finfish, crustaceans, and mollusks amounts to 29 Mt. However, after the 10 Mt of wild-caught fish going into fish feed is subtracted, the net volume of fish provided for human consumption via aquaculture is 19 Mt.
Carps and marine mollusks account for more than three-fourths of current global aquaculture output, and tilapia, milkfish, and catfish contribute another 5 percent. These species, fed mostly herbivorous diets, account for most of the 19 Mt gain in fish supplies from aquaculture.
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