The physical, chemical and biological factors of the brackish water fish pond have an individual as well as synergic effect on the dynamics of brackishwater aquaculture. Unlike the freshwater system, the ecology of a brackishwater pond is subjected to abrupt changes primarily due to changes in salinity at the onset of monsoon. Fish food organisms, benthic flora and fauna are also observed to keep pace with the changed conditions and thereby directly affect the stocked population.
Factors like, temperature, depth, turbidity and organic matters have direct bearing on the productive process of the system. Regular monitoring of the physico-chemical parameters like salinity, pH, dissolved oxygen, turbidity etc. of the stocked pond is very much essential, since abrupt change in any of these parameters may lead to large scale fish mortality, Secondly, accumulated unused feed and other waste products may create unfavourable conditions. Strees and strain of which reduce the resistance power of the stocks and they become prone to the attack of diseases and pests.
The best method to overcome all such problems is to flush the pond water at regular intervals with fresh tidal water. At the time of flushing the pond, care should be taken to see that neither unwanted organism enters the fish pond nor the stocked materials escape out. In cases where tidal exchange is not possible, the circulation and aeration of the pond water may be affected by pumping in water from outside or by some other mechanical means.
Depth of water column in brackishwater impoundments is a limiting factor for the growth of benthos and zoophyte matrix. To ensure maximum photosynthesis and also to provide a congenial ecological environment at the pond bottom, a depth varying from 30 to 80 cm should be maintained. Salinity plays a vital role in the brackish water organisms but its monitoring is very difficult. Salinity in the range of 10 to 30 ppt, particularly in its raising phase is suitable for optimum growth of penaeid prawns and fishes. Hense immediately after monsoon is over, aquaculture practices should be initiated for better yield of prawns.
Dissolved oxygen is a vital factor affecting the culture of P. monodon. Survival of prawns is greatly influenced by dissolved oxygen concentration. Judicious application of feed and fertilizers, timely removal of metabolites through water exchange method and application of lime for decomposition of organic matter to maintain positive oxygen level over 4 ppm at early morning hours will help to keep the system congenial for prawn culture. Crawling of prawns at the edge of the fish pond during morning hours is an indication of oxygen depletion. The growth of P. monodon is more or less uniform during the first two months attaining 110-120 mm in length. At this time utmost care is necessary to obviate sudden fluctuation in physico-chemical parameters of stock ponds by suitable management measures. Shifting prawn at this stage to another well-prepared pond generally gives very encouraging growth.
- Central Inland Fisheries Research Institute, Barrackpore