Role of mangroves in brackish water fish culture

Abstract

Mangroves is a specialized marine ecosystem consisting of a group of plants growing in muddy, loose and wet soils in tropical and subtropical areas, comprising of shallow, coastal waters, deltas, estuaries or lagoons. Besides ecological importance in trapping and accreting sediment material to reduce the erosion, mangrove ecosystem plays a significant role in Aquaculture. A linear relationship exists between shrimp production and the size of the mangrove forest area. The fishery potential of these areas are tremendous providing the livelihood to the majority of the coastal population. The fishes lay their eggs in tangled roots of mangrove trees and later hatch and grow with needed nutrients available (Leh and Sasekumar, 1984). Thus mangroves act as natural nursery grounds. Mangroves afford shelter to the juveniles of a wide variety of marine organisms, notable among them being certain species of penaeid prawns and species of Metapenaeus. The prawns after spawning migrate towards nearby mangroves where salinity is lower. In fact, low salinity of estuaries had been reported to be one of the main factors influencing the larval migration of Penaeid prawns in this specialized ecosystem. Mangrove ecosystem and the neighbouring water masses receive the input from two basic sources i.e. inorganic from surface runoffs and organic material from mangrove vegetation itself in the form of mangrove litter. It is estimated that in India mangroves export an average of 12 tonnes cf litter per hectare annually, representing a nutrient influx having an average of carbon, nitrogen and phosphorus as 5076 kg ha-1 yr-1 carbon yield, 97.63 kg ha-1 yr-1 and 7.28 kg ha-1 yr-1 (Water 1989). Mangrove swamps and other low-lying areas along the estuaries are generally preferred for brackishwater fish farming. This is an age old practice in India and is called "filtration" or "trapping" in the intertidal zones of coastal waters. The species cultivated are Liza parsia, L. nade, Mugil cephalus, Chanos chanos, Penaeus monodon and P. indicus. This brackishwater aquaculture is callEd. by different names in different states like "Bheris" or "Bhasabada" in West Bengal. "Cheemankettu" in Kerala, "Gazani" in Karnataka and "Khajan" in Goa. Till recently the wild stocking during highest high tide water was used to get seed mixtures. Dwivedi 1982 has introduced the concept of hatchery earning Rs. 20,000 or more by fish and prawn culture. Majority of coastal population depend for their livelihood on the fishery resources of mangrove estuaries. Thus, it becomes very obvious that any kind of damage or destruction for mangroves may affect the capture or culture fishery resources. Aquaculture development is recently becoming the more fashionable landuse of mangroves. Prawn culture is being practiced throughout the world but there is a potential risk of area expansion for prawn culture and it has been clearly seen in Indonesia, Malaysia, Philippines, etc. Here large mangrove areas were converted into prawn culture ponds, which has resulted into reduced capture fishery resources. Mangroves in India are at present in a degraded condition as a result of increased population pressure, modern developments and industrialization. Mangroves have also suffered a loss form reclamation, deforestation and pollution which in turn has adversely affected coastal and estuarine fisheries. This paper highlights the rate of mangroves in brackishwater fish culture