Fish, chitin, and chitosan production 269Therefore, feeding materials will vary with the different fish species and the
stages of development according to their feeding habits, so that fish can keep
growing at higher rate.
Fish to be reared in waste-fed ponds should have the following
characteristics:
- tolerant to low DO level which can occur during night time or at
dawn when photosynthetic oxygen production does not occur, - herbivorous or omnivorous in nature to feed on the waste-grown
phytoplankton, and - tolerant to diseases and other adverse environmental conditions.
Some species such as Tilapia (Figure 6.2 e), Chinese carp, and Indian carp
have been widely used in waste recycling practices. Organic wastes, including
septage, could provide sufficient nutrients to promote the growth of
phytoplankton and consequently of zooplankton which is natural food of these
fish (Polprasert et al. 1984). Among these species, Tilapia needs to be
particularly mentioned, for its being used widely in waste recycling in the
tropical and sub-tropical areas. It feeds directly on algae and other primary
aquatic vegetation (and on zooplankton as well). It grows rapidly and multiplies
abundantly. Furthermore, it has better tolerance to low DO level and resistance
to diseases (which often occur in fish ponds) than many other species such as
carp.
The culture of carnivores is also of interest in waste recycling if fish for
human consumption is desired. But they have to be raised in separate ponds fed
with pellets made from herbivores, omnivores, or trash fish (to allow the growth
of herbivores or omnivores), and to safeguard public health because the
carnivores will not be in direct contact with the organic wastes. Catfish,
snakehead and shrimp, which are of high market value, are the carnivores
commonly reared.
6.3 Biological food chains in waste-fed ponds
Food chain is the series of organisms existing in any natural community through
which energy is being transferred. Each link in such a chain feeds on and
obtains energy from the ones preceding it. In a pond ecosystem, there are
generally three major groups of organisms present in the food chain, similar to
the other marine and freshwater ecosystems. The three groups consist of: the
primary producers; the primary, secondary and tertiary consumers; and the
decomposer organisms (Figure 6.3).
At the beginning of the food chain, the primary producers (algae and aquatic
plants) represent the first trophic or energy level. They synthesize organic