MAKING AQUATIC WEEDS USEFUL

Tilapia rendalU (C. Phillppy)

Limpopo, Congo (Zaire), and in Angola. The fish is now cultured commer-cially for food throughout Africa. It is a herbivore that prefers submerged aquatic plants, and, like the grass carp, is an efficient grazer.

Tilapia rendalli will consume an established stand of vegetation in one season if stocking rates of 2,500-5,000 young fish per hectare are used.

A lesser number of fish are used to keep regrowth suppressed. These fish are prolific breeders, capable of reproducing when 2-4 months old and, if the water is warm, they spawn every 3-6 weeks after that. The large numbers of fish produced convert large masses of plant material into fish flesh. Indeed, they often become excessively stunted by lack of food and may never reach a size suitable for human consumption. However, if the population is controlled, Tilapia rendalli may reach 30 cm and weigh 0.7 kg.

Tilapia guinnnsis

This estuarine tilapia from West Africa (originally classified incorrectly as Tilapia melanopleura) is one of the few salt-tolerant fish that feed largely on plants (macrophytes). Although it is most at home in brackish waters (25- 75 percent seawater) it can be found in the sea and can be kept in freshwater for considerable periods. Above 5 cm in length Tilapia guineensis feed predomi-nantly on terrestrial vegetation washed into estuarine areas. This fish may be

OTHER HERBIVOROUS FISH 27

Female Java tilapia (Sarotherodon mossambicus), which eats large quantities of filamentous algae. The flesh is edible. (R.R. Yeo)

a valuable and under-recognized species for estuarine and brackish-water culture.•

Sarotherodon mossambicus

This species (formerly called Tilapia mossambicus, and commonly known as

"Mozambique tilapia" or "Java tilapia") consumes filamentous algae and plankton, a useful characteristic because fdamentous algae (such as Enteromorpha spp.) provide a prime mosquito-breeding habitat by sheltering mosquito larvae from predators. The Java tilapia can be stocked with a larvae-eating fish (e.g., the mosquito fish Gambusia affinis) in an integrated algae-mosquito control program. It can also be stocked in rice fields (after the rice has emerged) to feed on the red alga (Euglena sanguinea) and other algae that rob rice plants of nutrients.

Sarotherodon mossambicus are edible and, in Puerto Rico, males have grown to weigh as much as 2.7 kg.t

In 1957 it was reported that on one 1,400-ha sugar plantation in Hawaii the costs to control aquatic weeds were reduced to virtually nothing by using Sarotherodon mossambicus. Seventy-five thousand fish, each 7-10 cm long, were released into the irrigation channels. Before they were released, algae and weeds were removed by using chemicals and, given this start, the fish

*Infonnation supplied by I. Payne. (See Research Contacts.) tinfonnation supplied by R. R. Yeo. (See Research Contacts.)

28 MAKING AQUATIC WEEDS USEFUL

Sliver Dollar Fish. Mylo1wma argenteum eat large quantities of aquatic plants (such as Potamogeton spp. shown). They often graze the stems close to the root zone, cutting off the bulk of the plant, which then decays and dies. (R. R. Yeo)

were able to suppress the regrowth. The initial cost was $3,000, compared with a previous $5,000 annual cost for control using herbicides. The annual cost for weed clearance in the subsequent years was quoted as $25.*

Sarotherodon niloticus

The Nile tilapia (formerly known as Tilapia niloticus) consumes large amounts of fdamentous algae, such as Pithophora spp. Stocking rates of2,500-5,000 per hectare in ponds will control this algae. They will consume some higher plants, but not enough to control them.

Silver Dollar Fish

The small, tropical, herbivorous, South American fishMetynnis roosevelti and Mylossoma argenteum (commonly referred to as "silver dollars") can control a variety of submerged aquatic weeds. They are aggressive eaters and, when traveling in schools, consume large quantities of submerged aquatic vegeta-tion, including many pondweeds. Silver dollars are mowers; they frequently bite off the weeds at the base of the stem close tQ the roots. The mass of

*H. M. Hee. (See Research Contacts.)

OTHER HERBIVOROUS FISH 29

loose vegetation floats to the surface; there, the silver dollars usually graze it later or leave it to accumulate along the shoreline where it decays. Dense growths of vegetation are rapidly removed from ponds when the fish are stocked at rates of 1,200-2,500 fish per hectare. Little is known of their potential yield or value as food, other than that they occur in large numbers and are sought after and relished by the people along the Amazon River. . temperate countries heated facilities are needed for over-wintering;

tempera-tures less than 16° C are fatal.

Considerable research is needed on silver dollars to identify the weeds they consume or shun; the techniques for spawning, culturing, and managing them;

their value as food; and their sensitivity to adverse water quality.

Silver Carp

A silvery, shad-like minnow from China, the silver carp (Hypophtha/michthys molitrix Val.) is a short-bodied fish that consumes only microscopic algae. It has extremely fine-meshed gill rakers, which strain its planktonic food from the water. It will also eat such processed foods as bean meal, rice bran, and flour. A large, rapidly growing fish, silver carp can grow· to weigh 15 kg.

This species is an important source of protein in the People's Republic of China and other Asian nations. As such, it is bred extensively for food.

Several thousand fJSh per hectare can be grown in highly enriched ponds or lakes. The silver carp does not reproduce in lakes or ponds but may spawn in rivers that meet its specific requirements. It is easily spawned artificially using hormone injections.

Silver carp, a hardy fish that filters and consumes large quantities of microscopic algae from the water. (W. M. Bailey)

30 MAKING AQUATIC WEEDS USEFUL