Issues in Ecology Number 8 Winter 2001 if it takes several kilograms of wild-caught fish to grow one bycatch,fisheries landings amount to 9 Mt,of which 65 kilogram of salmon or cod in captivity.these and other car- Mt of whole fish and 1 mt of seaweeds are consumed by nivorous fish species would consume at least that amount of humans.The remaining 30 Mt of fish catch plus another 2 smaller fish if they grew to maturity in the wild.Whether Mt of processing scraps from aquaculture and fisheries are natural predation or captive feeding is more energy efficient used for fish meal production. s an unsettled scientific question that involves calculations of (The fish meal industry has proposed that fishing energy flows in wild food webs.It is reasonable to believe vessels be encouraged to retain the currently discarded that farmed fish operations are somewhat more efficient since bycatch for sale to producers of fish meal and fish oil.Saleof bycatch could prove undesirable,however,if it undermines they grow. the outcome of the efficiency de efforts to reduce bycatch rates or decreases the return of tu wa en.) ely on fin ught used to make abou as fully exploite 10 Mt, ds,while th fish fre or dep goe 1t0 for cke ans to he comm b ch 0 d fish fo tha counter oductive to fa eal and fish oil.such as unts of wild-caught fish are distasteful to bum or are worth mor that form the foundation of the ocean food chain fish meal and oil than as food for consumer s the demand for small ocean fish for NET INCREASE IN FISH SUPPLIES FROM AQUACULTURE increase with population are Finally.total aquaculture production of finfish,crus Clearly,the feed requirements for some types of aqua taceans and mollusks amounts to 29 Mt However afte culture systems place a strain on wild fish stocks But does the 10 Mt of wild-caught fish going into fish feed is sub farmed fish production overall represent a net gain to global tracted,the net volume of fish provided for human consump fish supplies?Our calculations indicate it does.but most of tion via aquaculture is 19 Mt that gain in fish supplies from aquaculture comes from carps Carps and marine mollusks account for more thar marine mollusks,and other mostly herbivorous species. three-fourths of current global aquaculture output,and tila Global harvest of wild fish and aquatic plants re- pia,milkfish,and catfish contribute another 5 percent.These moves 123 Mt from seas and lakes each year,and 27 Mt of species,fed mostly herbivorous diets,account for most of the this is directly discarded as bycatch(Figure 5).Without the 19 Mt gain in fish supplies from aquaculture salmon species far worldwide are com monly fed compound rich in fish oils and fish meal from wild fish. Roughly.three kilograms of wild fish are required for each kilogram of salmor produced (Photos courtesy the New Brunswick Depart- ment of Fisheries and Oceans). 5 Issues in Ecology Number 8 Winter 2001 Figure 4 — Atlantic salmon, the dominant salmon species farmed worldwide, are com￾monly fed compound feeds rich in fish oils and fish meal from wild fish. Roughly, three kilograms of wild fish are required for each kilogram of salmon produced (Photos courtesy the New Brunswick Depart￾ment of Fisheries and Oceans). if it takes several kilograms of wild-caught fish to grow one kilogram of salmon or cod in captivity, these and other car￾nivorous fish species would consume at least that amount of smaller fish if they grew to maturity in the wild. Whether natural predation or captive feeding is more energy efficient is an unsettled scientific question that involves calculations of energy flows in wild food webs. It is reasonable to believe that farmed fish operations are somewhat more efficient since captive fish are protected from some types of mortality as they grow. Regardless of the outcome of the efficiency de￾bate, however, it is clear that the growing aquaculture in￾dustry cannot continue to rely on finite stocks of wild-caught fish, many of which are already classified as fully exploited, overexploited, or depleted. Taking ever-increasing amounts of small fish from the oceans to expand the total supply of commercially valuable fish would clearly be disastrous for marine ecosystems and, in the long term, for the aquaculture industry itself. If the goal of aquaculture is to produce more fish for consumers than can be produced naturally, then it will become increasingly counterproductive to farm carni￾vores that must be fed large amounts of wild-caught fish that form the foundation of the ocean food chain. NET INCREASE IN FISH SUPPLIES FROM AQUACULTURE Clearly, the feed requirements for some types of aqua￾culture 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 re￾moves 123 Mt from seas and lakes each year, and 27 Mt of this is directly discarded as bycatch (Figure 5). 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 sar￾dines 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, crus￾taceans, and mollusks amounts to 29 Mt. However, after the 10 Mt of wild-caught fish going into fish feed is sub￾tracted, the net volume of fish provided for human consump￾tion via aquaculture is 19 Mt. Carps and marine mollusks account for more than three-fourths of current global aquaculture output, and tila￾pia, 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