Freshwater aquaculture (2+1)

Unit 4 - Cultivable fish and their culture methods

4.3. Freshwater prawn cutlure
The giant freshwater prawn Macrobrachium rosenbergii is one of the highly priced species cultured in freshwater.
It is distributed in the major river systems of the country. Although the freshwater prawns live in freshwater their larvae require brackish water to survive and grow. Adults migrate to estuaries to breed and post larvae migrate back to fresh waters. Freshwater prawns are omnivores, their feed includes decaying plants and animals at the bottom. The food in the stomach can be viewed through the transparent carapace. They are cannibalistic in nature and attack newly molted and weak prawns. Therefore provision of adequate feed and shelters need is essential in their culture systems.

The Biology and management of size variation
Size variation in the population, particularly of males – a typical characteristic of Macrobrachium rosenbergi - is a major obstacle to prawn culture and its profitability. The prize of the prawn depends on its size and those below minimum size are discarded by growers. The harvest consists of a large fraction of small unmarketable individuals and a fraction of large prawns. However, removal of large prawns leads to a rapid compensatory growth of the smaller individuals. Size variation is the result of heterogeneous individual growth (HIG) which is a complicated biological characteristic of the giant freshwater prawn. It reflects a complex population structure composed of three sexually mature morphotypes (small male, orange claw male and blue claw male), which differ in their morphology, physiology and behavior and transform from one morphotype to another. The growth regulation of prawns – suppression as well as enhancement - is achieved by means of social interactions among individuals.
The blue claw (BC) male is characterized by its extremely long claws. The stunted small male (SM) is differentiated from younger juveniles by their greater age. The intermediate orange clawed (OC) male has golden colored claws which are 30 to 70% shorter than those of the blue claw males. Harvested mature prawn populations are composed of three distinct male morphotypes (sM, OC and BC) which represents the normal male developmental pathway from SM to BC through OC.
Prawn populations display a disproportionate increase in size variation with time because individual prawns grow at different rates i.e., they show heterogeneous individual growth. The fastest growing individuals are called “Jumpers” and slowest growing individuals are called “laggards”. When jumpers are remove from the population some laggards will transform into jumpers. In other words presence of blue claw male suppresses the growth of SM and OC individuals. When BC males are removed some OC males transform to BC Males and some SMs become OC males. Then harvesting of larger prawns will result in faster growth of small individuals resulting in higher yields.

Grow – out systems
The management of size variation is an extremely important and complicated aspect of culture of freshwater prawns, because of the uneven growth rate of individual prawns especially males.