Raising Freshwater Shrimp – Spawning and Larval Rearing of Shrimps
Mating in shrimps take place immediately after premating moult in matured female.Spawning occurs a few hours after mating. The incubation period of eggs lasts between 10-14 days depending upon the water temperature. The optimal water temperature being 25-35°C. At lower temperatures, the incubation period tales longer is stretching to more than 21 days. Hatching takes place due to body stretching of the zoea, which breaks the eggshell and comes out from the egg and starts swimming.
Different larval rearing technologies such as static, clear, green water, closed or semi-closed, with or without circulation systems are practiced. The green water technique is generally reported to increase the post-larval production by 10-20°C over other techniques. But higher mortalities are encountered because of rise in pH and greater algal bloom. Also, there is enhanced increase in numbers of adult artemia, due to abundance of feed in green water, contributing to accumulation of ammonia in the culture medium. The production of post-larvae in large numbers is possible following bio-filter re-circulatory systems. The larvae passes through eleven zoeal stages before attaining post larval stage within a period of 45-60 days at salinity and temperature ranging from 18-24°C and 25-35°C, respectively.
Bio-filters equipped with re-circulation capability have shown encouraging results in maintaining favorable water quality in different rearing media with enhanced rate of post-larval production.The most important parameters of the rearing medium include temperature, pH, dissolved oxygen, total hardness, total alkalinity, salinity and ammonical nitrogen. These influence greatly the growth and survival of larvae in captive rearing. The respective parameters within the ranges of 25-35°C, 7.8-8.2, 3000-4500 ppm, 80-150 ppm, 18-20°C and 0.02-0.15 ppm are considered optimal during the larval rearing of shrimps.
Raising Freshwater Shrimps – Harvesting and Rearing of Post-larvae Shrimps
Harvesting of post-larvae of shrimps is slightly difficult due to their crawling habit. Therefore, siphoning of water are commonly used for harvesting. However due to longer duration for attaining post-larval stage the above method is not very safe because of possible deto-riation in water quality. Also, the presence of post-larvae in the larval tank affects the growth and survival of advanced larvae due to competition for food. Therefore there is a need for an suitable device for regular harvest of post-larvae from the rearing unit. A method called string shell has been devised and is being successfully used for phasewise harvesting of post-larvae during larval rearing.Post-larval survival and production rates, following bio-filter re-circulatory system, are in the range of 10-20 PL/l.
Optimum growth, production and survival of shrimps can be achieved in grow-out ponds on stocking with nursery reared juveniles rather than by stocking directly with the fresh post-larvae. The post-larvae slowly adopt themselves to the freshwater. It is observed that generally optimum growth and survival of healthy juveniles during post-larval rearing is achieved at salinity of around 10‰.
Post-larval rearing of shrimps can be done both in well-prepared earthen ponds with adequate aeration facility and in indoor hatchery following bio-filter re-circulatory system. Stocking density, feed and water quality management play the major role in raising healthy juveniles during rearing. Optimum stocking density is between 10-15 post-larvae/l. Water quality has to be continously monitored to ensure that all parameters are within desirable limits.
Raising Freshwater Shrimps – Growout Culture of Shrimps
Growout culture of shrimps is similar to that of other freshwater fishes. To prevent shrimps from migrating from one pond to another, it is necessary to have the pond embankment at least 0.5m to 1m higher from the water level. A sandy clay pond bottom is is well suited for better growth. A stocking density of 30,000 to 50,000/ha is maintained for semi-intensive farming. Intensive freshwater shrimp farming with stocking density upto to 1 lakh/ha is possible in ponds with adequate aeration and recirculation capabilty. Water temperature is the single most important factor which directly affects the growth and survival of prawns. Temperatures above 40°C or below 14°C are generally not conducive to shrimp growth. On the other hand temperatures between 22-33°C is optimal.
Males grow faster than females. A production of 500-1200 kg/ha in six months of rearing are achieved under shrimp monoculture with the stocking density of 30,000-50,000. In case of shrimp polyculture, at a stocking density of 10,000-20,000/ha along with carps at density of 2,500-3,500 nos/ha, a production of 300-400 kg prawn and 2000-3000 kg carps or tilapia can also be raised. A discussed in a previous post, freshwater shrimp farming in conjunction with tilapia is therefore always preferable.