Direct feeding

Feeding types among fishes range from predatory gulpers to sifters of organic materials in mud, to zooplankton feeders, and to herbivores that eat algae or even leafy plants. As already intimated, the rationale of polyculture is the selection of compatible species with different feeding patterns. In addition, because fish learn to feed on almost anything, it is relatively easy to develop pelleted food for fish culture, dietary quality considerations aside. At the same time, such catholic feeding habits permit the use of plant materials, especially cheap or nearly valueless crop residues such as bran, etc. Table 3 (22) illustrates this, as does the practice of building very wide pond margins to the fish ponds in China for cultivating grasses where leafy plant-feeding grass carp (Ctenophryngodon idella) comprise about 20 per cent of the stock in the pond (12).

TABLE 3. Proximate Composition of Feedstuffs Used in Fish Culture

Source: Bardach 122).

All sorts of other wastes, even sludge, are fed to fish (23 - 25) with very low conversion efficiencies, to be sure, but presumably favouring cheap production costs just the same (Table 4).

TABLE 4. Yields of Fish for Various Residues Used in China

Based on information given to mission members; after Tapiador (12); conversion efficiencies are our estimates

Carnivores make up a certain portion of the polyculture components; in fact, various traditional aquaculture schemes incorporate a few voracious predators, albeit under intensively supervised management conditions, for example, pike in common carp ponds and catfish (Silurus glands) in polyculture carp ponds (1) and (A. Ruttkay, persona) communication, 1978). Sometimes pure carnivore culture is practiced depending on the availability of the so-called trash fish; that is, species that are too small to be eaten directly or not acceptable as table fare. The culture of groupers in various parts of the Pacific and of yellow tail in the Inland Sea of Japan are based on the availability of this kind of high-protein feed; it is mentioned here because one sometimes hears the argument that such practices are ecologically unsound. They may appear so at first glance, but these comments usually do not take into consideration that aquaculture is pursued to gain a livelihood, providing its practitioner with income first and foremost. It usually also supplies fish for the table, but hardly as its prime purpose. The rearing of carnivores relying on "waste" species, or for that matter on slaughterhouse wastes and/or blood meal, can be a sound practice, even from the ecological vantage point.