Harvesting and marketing
As indicated earlier, harvesting schedules are dictated by culture practices and market requirements. The simplest system, which prawn culturists call ‘batch culture’, consists of stocking a pond and allowing the stock to grow until they attain marketable size, after which the whole stock is harvested. For harvesting the stock effectively in this system, the ponds have to be drained.
The other techniques involve ‘continuous culture’ and ‘continuous stocking and harvesting’. In continuous culture, the ponds are stocked generally once a year at a comparatively higher rate, and harvesting is done by seines on a continuing basis. After about five to seven months, marketsized prawns are culled at regular intervals. The ponds are never drained. In the continuous stocking and harvesting system, ponds are restocked up to six times a year after culling. Some farms try to combine the main features of the different systems. About five months after stocking the post-larvae, regular cull-harvesting is done until about eight months, when the pond is drained and the whole remaining stock is harvested.
Except in cases where it is possible to drain the whole stock into a harvest sump and remove it by dip nets or mechanical devices such as pumps, seining is the most common method of pond harvesting. Seine nets have to be operated with special care, so that the bottom of the seine rides on the sump bottom when in operation, to ensure that prawns do not escape beneath it. In continuous or multiple harvesting, sometimes only one half of the pond is seined at a time, every two or four weeks, in order to avoid disturbing the whole pond each time. Another precautionary measure is to catch only what can easily be removed from the net, to avoid the prawns being crushed to death when hauled out of the water.
Modified seine nets have been designed specifically for cull-harvesting (Hanson and Goodwin, 1977). The head rope is made of light polypropylene which does not sag, and the foot rope is made of soft nylon which rides the contours of the pond. Sufficient floats are used to keep the head rope stretched above water toprevent prawns from crawling over the net. The seine has a bag similar to that of a beach-seine to hold the catch. A mechanical harvesting system descried by Williamson and Wang (1982) is a modification of the traditional seine, but uses a tractor or truck to pull the net. The seining time is greatly reduced and harvesting efficiency is reported to be at least as great as in manual seining.
In many areas, live prawns fetch the highest price and so every effort is made to keep the harvested prawns alive. Besides sorting them according to size, soft-shelled (newly moulted) and egg-bearing prawns are separated out. Live prawns can be hauled to markets in trucks in live tanks with proper aeration. Dead prawns are transported on ice, but some producers chill-kill the prawns and blanch them in water at about 65°C for 15–30 seconds, before packing them on ice. This process seems to help in extending the shelf life of the prawn to four to six days.
Experience indicates that the fresh-water prawn can best be marketed in the fresh ‘shell-on’ form in domestic markets, or in export markets which can be reached by rapid means of transport such as air-freight. The tough exo-skeleton and long appendages make peeling difficult, and so it is generally sold whole. Further, the frozen prawn is reported to undergo a rapid deterioration in quality. According to Nip and Moy (1979), prawns frozen in still-air and brine solutions, as well as liquid nitrogen, lose elasticity and viable bacterial counts. However, no significant losses of flavour and texture were noticed, and they were of the view that carefully frozen prawns are of good and acceptable quality. Hale and Waters (1981) reported that tails as well as whole prawns can be frozen and stored, up to 10 months in the case of tails and seven months for whole prawns.