Many intensive and semi-intensive aquaculture systems use two to five times more fish protein, in the form of fish meal and fish oil, to feed the farmed animals than is produced in the form of farmed fish.
By contrast, so-called extensive or traditional aquaculture systems use little or no fish meal or fish oil, although operators often add nutrient-rich materials such as crop wastes to the water to stimulate growth of algae and other naturally available organisms on which the fish feed.
Worldwide, about 80 percent of carp and 65 percent of tilapia are farmed without the use of modern compound feeds – that is, feeds formulated from multiple ingredients. In China, however, farmed production of carp and other omnivorous species is intensifying, and new commercial feed mills are being developed to serve this industry.
China is also the largest importer of fish meal in the world. Such intensive systems, including U.S. catfish farms, must rely heavily on added feeds because fish are stocked at higher densities than can be supported by natural food sources.
Generally these operations use compound feeds that contain high percentages of protein supplements from soybean meal, cottonseed meal, and peanut meal. But compound feeds for herbivorous and omnivorous fish can also contain low to moderate levels of protein obtained from fish and terrestrial animals.
By contrast, fish meal and fish oil are dominant ingredients in compound feeds for carnivorous fish and shrimp. These two ingredients supply essential amino acids (that is, lysine and methionine) that are deficient in plant proteins and fatty acids (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA], known as n-3 fatty acids) not present in vegetable oils. The fish oil and protein also provide energy, which is important because fish tend to be poor at using carbohydrates for energy.
All fish, whether omnivorous, herbivorous, or carnivorous, require about the same quantity of dietary protein per kilogram. But freshwater herbivores and omnivores such as carp, tilapia, and catfish are better than carnivores at using plant-based proteins and oils, and consequently, they need only minimal quantities of fish meal to supply essential amino acids.
Nevertheless, compound feeds for tilapia and other omnivorous fish often contain about 15 percent fish meal — much more than required. Indeed, manufacturers often over-formulate feeds, in part because information on the dietary requirements for particular fish species is inadequate.
Because of these high levels of fish meal and fish oil in aquaculture feeds, it takes more fish biomass to raise some farmed species than those species produce. For the ten types of fish most commonly farmed, for instance, an average of 1.9 kilograms of wild fish are required for every kilogram of farmed fish produced using compound feeds (Figure 3). The highest inputs of wild-caught fish — more than five kilograms for each kilogram produced — are used in raising marine fish such as flounder, halibut, sole, cod, hake, haddock, redfish, seabass, congers, tuna, bonito, and billfish.
Many salmon and shrimp operations use roughly three kilograms of fish biomass for each one produced (Figure 4). Only three of the ten types of fish most commonly farmed — catfish, milkfish, and carp — use less fish as inputs than is ultimately harvested. (Marine mollusks and many filter-feeding carp are not fed compound feeds at all.) Aquaculture is not the world’s largest consumer of fish meal. That distinction belongs to the poultry and swine industries.
Aquaculture, however, has the fastest growing demand for fish meal and fish oil. Its share of fish meal supplies rose from 10 percent in 1988 to 17 percent in 1994 and 33 percent in 1997. Also, the proportion of fish meal in aquaculture feeds is much higher than in poultry and livestock feeds, which contain an average of only 2 to 3 percent fish meal as a protein supplement. The production of a kilogram of pork or poultry typically uses large amounts of plant proteins, but only a few hundred grams of fish, whereas production of a kilogram of carnivorous fish can use up to five kilograms of wild fish.
Some aquaculture proponents argue that even if farmed fish production requires more wild fish biomass than is ultimately harvested, it is still more efficient than the making of big fish from little fish in the wild. In other words, even if it takes several kilograms of wild-caught fish to grow one kilogram of salmon or cod in captivity, these and other carnivorous 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 debate, however, it is clear that the growing aquaculture industry 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 carnivores that must be fed large amounts of wild-caught fish that form the foundation of the ocean food chain.
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