Modular Farm for All-Male Prawn Production
In the last decade, with the adoption of semi-intensive farming practices, freshwater prawn farming (primarily Macrobrachium rosenbergii) has experienced a remarkable expansion.
This paper discusses the requirements and procedures for enhancing prawn production through size grading and mono-sex culture. It describes a theoretical modular farm consisting of two nursery ponds, three transition ponds and one growout pond, occupying a total area of 1.5 hectares.
Size Grading: Size grading of both sexes, which can be done with finfish bar graders or seine nets, of nursery-reared juveniles at about one gram has been found to increase both mean harvest size and total pond production. Size grading is done to separate the fast growing individuals in the nursery population from the slow growers because the fast growers suppress the growth of the slow growers. Removal of the fast growers gives the smaller prawns a chance to achieve their growth potential. It also reduces socially induced differential growth rates. Juveniles from the graded upper fraction of the population develop into larger orange-clawed males and blue-clawed males at a much higher frequency than non-graded populations stocked at the same density. Moreover, in a short growout period, the income from the fast-growing prawns has been found to be nine times higher than the income from the slow-growing prawns.
All-Male Growout: M. rosenbergii is a dimorphic species (males grow faster than females), making the production potential of an all-male crop economically attractive. In fact, when males and females are cultured in the same pond, the slower-growing females slow down the growth of males. A recent report has indicated that all-male culture is 63.31% more profitable than mixed growout, and other studies have found all-male production to be 30% greater than mixed growout.
Prawns can be separated manually based on a gonopore at the base of the 5th walking leg. The presence of a petasma at the 2nd swimming leg is also an identification character for males. The stage at which segregation is carried out and the method of manual sexing greatly affect the total production from mono-sex growout. Many farmers in Andhra Pradesh, India, segregate the sexes six to seven months after stocking. They remove the females and larger males from the pond and sell them, allowing the remaining males to grow for two to three months.
The Production Cycle
Nursery Ponds: Both sexes of postlarvae are stocked (30 per m2) into two, 1,000-m2-nursery ponds, which may or may not have protective substrates, and reared for 45 days. Approximately 60-80% of them survive to the juvenile stage.
Transition Ponds: The juveniles are graded into small, medium and large groups and stocked into separate transition ponds (2,500 m2) at a stocking density of five per square meter and reared for 60 days to around 10 to 15 grams. Next, the prawns are collected using seine nets and the male prawns are segregated and stocked into the growout pond and the females are returned to the transition ponds. Therefore, the transition ponds become all-female ponds. Culture of females continues for approximately 90 days. After 60 days of growout, however, females larger than 40 grams can be harvested and marketed, while the remainders grow for another month, which ensures the availability of large females for breeding. Because there are no males in these ponds, there is no breeding and, consequently, no “berried” or egg bearing females. Females without eggs sell for a higher price.
Growout Ponds: The males obtained after segregation from the transition ponds are hand sexed and stocked in the growout pond (around 5,000 m2) at a density of two individuals per square meter. After 60 to 75 days of culture the pond is seined and the large blue-clawed prawns are removed and marketed. The smaller orange-clawed males remain in the pond for another 75 days, until they become more valuable blue-clawed prawns. Another partial harvest may be carried out before the final harvest, which is done by draining the pond.
Artificial Substrates: Using artificial substrates and hiding places can increase the production and survival of prawns. Farmers in Andhra Pradesh erect triangular shaped hideouts using coconut leaves in the nursery ponds. These hideouts help increase survivals and allow higher stocking densities. They also reduce fighting and provide more space for each prawn, reducing the adverse impacts of territorial fighting. In a pond trial, artificial substrates were found to enhance production by 24% and survivals by 10%.
Advantages of a Modular Farm: The most important advantage of a modular farm is that it can run two or more modules with continuous harvesting and seed stocking year round, opening the possibility of producing two crops of prawns a year with minimum investment. Moreover, a modular farm provides sufficient time intervals between crops to perform pond management like drying, liming and fertilization. This system also facilitates production of large females that can be sold to hatcheries or used in selective breeding programs.
Schedule of Production for Two Prawn Crops a Year
Sources: Aquaculture Asia Magazine. Editor, Simon Wilkinson. A Model Modular Farm for Size Grading and Monosex Culture of Freshwater Prawn. G. Venugopal, P.P. Suresh Babu and P. Srinivasa Rao (Central Institute of Fisheries Education, Kakinada Centre, Beach Road, Kakinada, Andhra Pradesh, India - 533 007). Volume 18, Number 1, Page 29, January-March 2013. 2. Bob Rosenberry, Shrimp News International, August 4, 2013.