332 Organic waste recycling: technology and management
Potamogeton spp., Ceratophyllum demursum, and most other submerged plants
(Mehta et al. 1976; NAS 1976). Overhanging terrestrial plants and bank grasses
are consumed when preferred plants are not available. Small fish, less than 1.2
kg, may eat several times their body weight of plant material daily, while large
fish consume their body mass daily (NAS 1976). Thus, rapid weed control is
achieved if more than 75 fish/ha are present. Stocking rates lower than 75
fish/ha reduce the rate of weed control, but result in larger, more valuable food
fish. Grass carp cultured for food yields 164 kg/ha in temperate zones and 1,500
kg/ha in tropical weed infested waters (NAS 1976).
The main difficulty with grass carp is that it is restricted to southern China
and does not reproduce in captivity. Recent studies, however, have shown that
spawning can be induced by injecting fish pituitary hormone, and in Arkansas,
USA, artificial spawning is routine. Another problem is that water hyacinth,
Salvinia, and other floating weeds that are prevalent in the tropics are not
preferred by the grass carp, and it is therefore not a satisfactory agent for
controlling them. Neither is it good for controlling tall, hard, emergent aquatic
weeds like cattails.
Other herbivorous fish
Besides the grass carp, there are several other species of fish which are used in
the dual role of a source of food and as weed-control agents (Edwards 1980;
Mitchell 1974; NAS 1976). Of the different species, Tilapia spp. is the most
commonly used. This tropical lowland fish is common in Africa and Asia, cheap
and easy to breed, grows rapidly, and is a voracious feeder of aquatic weeds.
Since its flesh is much valued as food, peasant farmers in Africa and Asia
culture them in small-scale subsistence and commercial enterprises. Several
species of Tilapia fish have been grown in different parts of the world,
especially Africa and Southeast Asia.
Two South American species, Metynnis roosevelti and Mylossoma
argenteum, both known as silver dollar, attack a variety of submerged weeds,
especially pondweeds. Dense growths of weed are rapidly removed at stocking
densities of 1,200 to 2,500 fish/ha. Little is known of their potential yield or
value as food, although they occur in large numbers and are sought and relished
by people along the Amazon River (NAS 1976).
In addition, a number of other species namely silver carp, tawes, common
carp, tambaqui, pirapitinga, American flagfish, and goldfish are being tested for
use as weed control agents.
Considerable research is required to identify techniques for their spawning,
culturing and managing; their value as food; and their sensitivity to adverse
water quality.