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Weed problems in aquaculture farms
Weed infestation of aquaculture farms is a problem of varying intensity in almost all systems of aquaculture all over the world. But it assumes very severe proportions in tropical and semitropical pond farms, especially in ‘undrainable’ ponds, such as those in use in South Asia. Limited growth of aquatic plants may be useful in maintaining water quality and may serve as shelter and substrates for food organisms in ponds, but profuse and uncontrolled growth affects aquaculture operations in several ways. Besides restricting the movements of fish and other aquaculture species, dense growths of vegetation, particularly floating plants, prevent adequate light penetration into the water and thus affect their productivity. Photosynthesis and oxygen production will be reduced when pond surfaces are covered by vegetation and this may cause oxygen depletion and consequently anoxia of the cultured species. Considerable amounts of nutrients from the water and those introduced into ponds through fertilization will be used up by the weeds and consequently the growth of food organisms will be reduced, resulting in low yields of the cultured species. Blooms of algae in ponds and enclosures often lead to oxygen depletion as a result of dead and decaying algal mass. Mass mortality of fish can occur under these conditions. Dense growths of aquatic weeds will make fishing with nets extremely difficult in ponds. Weed-infested stagnant ponds provide favourable conditions for mosquito breeding and thus become a public health hazard. In cage culture, in both fresh- and sea- water, thick growths of algae on the net cages reduce water exchange and thus affect the water quality within the cages.
Control of weed growth is not so difficult in small farms, when labour is not too expensive. As discussed, in countries like China aquatic weeds are effectively used as feed or fertilizer in fish ponds. However, in large farms in most tropical countries weed control is a formidable problem. It adds sub-stantially to the operational cost as control measures have to be adopted at frequent intervals to prevent reinfestation. Seeds or spores may be brought in through water intake, blown in by wind or carried by birds or other animals or inadvertently by workers. If the control measures employed do not include removal and disposal of dead weeds from the pond area, the weeds decay and add to the fertility of the soil and water and thereby promote further growth of dense weeds. This problem is not unique to aquaculture farms: shallow lakes, reservoirs and irrigation channels can also be choked by dense growths of weeds, which are very difficult to control.
Several factors, individually or jointly, influence the growth of particular species of weeds. Besides the geographical and climatic conditions, topography, depth of water, extent of bottom sediments, clarity and fertility of the water, access to sources of infestation, and occurrence of floods are some of the factors that are of importance. Aquaculture farms which cannot be drained and dried regularly, and where there are thick deposits of silt at the bottom, are more likely to have recurring weed problems. Persistent blooms of certain algae have been variously attributed to their ability to store nutrients for use when they are not available or to produce and liberate certain metabolites which help in the exclusion of other algae.
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