Fish, chitin, and chitosan production 267Marketing and public acceptance
Unless the public is convinced that waste-grown fish are safe for consumption,
the fish production is bound to fail. In those areas where fish from sea or river
origins are available in sufficient quantity and cheap, marketing of waste-grown
fish will be difficult for human consumption. However, other methods of use
(i.e. as animal feed) or other recycling options have to be considered.
6.2 HERBIVORES, CARNIVORES AND OMNIVORES
Feeding is one of the most essential factors that strongly influence the growth
rate of fish. Moreover, similar to other animals, different fish species have their
own special feeding habits. Fish can be classified into three groups on the basis
of feeding habits. Their morphologies are also different accordingly, mainly in
the aspect of the specialized type of alimentary system. Thus, the functional
morphology of fish species can often indicate the type of food they eat and those
they are unable to eat. These three groups are characterized below:
6.2.1 Herbivorous fish
Herbivorous fish lack teeth, but possess fine gill rakers that can sieve
microscopic plant from the water; they also lack a true stomach (i.e. a highly
muscular, acid secreting) but possess a long, thin-walled intestine. They mainly
feed on plant including algae and higher plants. Typical examples are grass carp
(Ctenopharyngodon idella) (unlike other herbivores, its gut is very short), and
silver carp (Hypophthalmichthys molitrix) (Figure 6.2 a, b).
Under optimum conditions, small grass carp, less than 1.2 kg, may eat
several times their body weight of plant material daily (NAS 1976). Thus, in
addition to being as a source of protein, many herbivorous species, such as grass
carp and silver carp, are often grown in the weed-infested water for biological
control of phytoplankton blooms and some aquatic weeds.
6.2.2 Carnivorous fish
Carnivorous fish have teeth well developed to seize, hold and tear, and gill-
rakers modified to grasp, retain, rasp and crush prey. There is a true flask-like
stomach and a short intestine, elastic and thick walled. These fish mainly eat
zooplankton, insects, bacteria, trash fish and other animals. Snakehead
(Ophicephalus striatus) (Figure 6.2c) is mainly carnivorous, feeding on small
fish, crustacean, insects and worms. Usually carnivorous fish are preferred to
other fish by people for their high nutritive value and taste. But much higher
investment is needed for carnivores' feed and fertilizer than other fish.