The limitations, costs and possible side-effects of the methods of weed control have naturally led to searches for acceptable biological control measures that can be adopted in aquaculture farms. The use of several herbivorous fish and other aquatic animals has been considered and some experimental work carried out. Some of the reports are listed in the references. Besides herbivorous fish, nutria (Myocaster coypus), muskrats, manatee or seacow, ducks and geese, beetles or their larvae and snails have all been considered. Though in theory any aquatic herbivore would be useful in reducing the growth of aquatic plants, their selective feeding at different stages of life, the population density needed to exert an effective control on the plant growth and the current aquaculture techniques that also use artificial feeds make it much more difficult to use biological control methods in aquaculture farms. The situation will, of course, be different in open bodies of water, including irrigation channels, small reservoirs, lakes and swamps.
Among fish that have a high cultural potential, the grass carp, certain species of tilapia and the tawes (Puntius javanicus) are the ones that have proved useful in controlling dense growths of vegetation. The practice of feeding grass carp with cut grass and aquatic vegetation is described. It has been estimated that 19.9 metric tons of water weed would be consumed by grass carp to produce about 195kg fish flesh. At such a consumption rate, the grass carp can effectively keep weed growths in fish ponds under check. Depending on their size and age, fish show preferences for certain types of weeds, and consequently their effectiveness would vary to some extent in a fish farm. For example, fingerlings of 30–50g feed on aquatic weeds like duckweed, but larger fish prefer larger vegetation. It is estimated that a population of at least 100–200 grass carp per hectare will be required to control the growth of weeds. Tawes feed on a variety of weeds, such as Chara, Hydrilla, Nechamandra and Azolla. Ponds choked withHydrilla could be cleared within a month bystocking the fish at 300–375 per hectare. Stocked at levels of 125–150 fish per hectare, they consume duckweeds at a rate of 1.8kg per fish per day. Tilapia rendalii and Sarotherodon (= Tilapia) mossambica have been found useful in controlling the growth of filamentous algae and soft submerged vegetation. Childers and
Bennett (1967) reported that when more than 1000 S. mossambica per acre (2470 per ha) were present in a pond, they were able to eliminate algae and rooted submerged vegetation.
Sarotherodon (= Tilapia) nilotica and T. zillii
are also reported to be useful in weed control. Biological control measures may involve the use of exotic species. In such cases, it will be advisable to use sterile hybrids to avoid their multiplication in aquaculture farms.
Periodic fertilization of ponds with inorganic fertilizers has been recommended to produce phytoplankton blooms to shade and kill submerged vegetation. This method is useful under certain situations, such as soon after filling a pond. But an algal screen may not be effective in preventing penetration of light, particularly in tropical areas where intense sunlight is available for long periods of time.
Despite the limitations of weed growth as a result of discharge of wastes from aquaculture farms, integrated fish–sea weed culture has been tried as a biofilter for regulating water quality (Neori et al., 1996). Constructed wet lands have been proposed as an option for the disposal of fish farm effluents. Macrophytes can be grown in the waste discharge area, and these may serve to clean waste water by direct assimilation (Negroni, 2000). Culture of bivalves and macrophytes, including seaweed, are in general practice included as a means of removal of aquatic farm wastes, particularly dissolved wastes in the case of macrophytes, and particulate wastes in the case of bivalves, in modern aquaculture though its effectiveness is restricted to relatively less weed-infested sites.
Plant (seaweeds) and animal crop (finfishes, crustaceans and molluscs) integration in aquaculture has not been researched adequately (FAO/NACA, 1996), but there are some positive reports on using the seaweed Gracilaria spp., in reclaiming shrimp pond effluents (NICA, 1992; Chandrakrachang et al., 1991; Chaiyakam, 1996) from Thailand and polyculture of oysters with shrimps in Hawaii (Wang, 1990), and mussels and shrimps in Thailand (NICA, 1992).
There are also reports on using mangroves as biological filter to reclaim shrimp pond effluents (Phillips, 1995; Macintosh, 1996; New, 1999).