Harvesting drainable ponds
Harvesting from drainable ponds is relatively easy, if there is a harvesting sump or similar device. In a nursery or fry-rearing pond it is almost essential to have a harvesting sump to avoid injury to fry or fingerlings during harvest. Drainage is performed at a rate suited to the size of the outlet and the drainage channels, and the fish are concentrated in the harvesting sump. From the harvesting sump, the fish can be collected by loading equipment, if necessary with the help of a net. In case the harvesting sump is considered too small for the quantity of fish, it may be necessary to combine seining and draining to harvest the fish in good condition. Some of the fish may be seined and the rest caught in the sump. When live fish are marketed, it is useful to spray fresh water or aerate the water in the sump, to avoid weakening or mortality of the fish.
In Asian brackish-water ponds, where special catching ponds or canals are provided, fish are concentrated in the catching areas by taking advantage of their habit of swimming against currents. At high tide, tidal water is allowed to flow into the pond system through the catching ponds (see fig. 6.7) and by opening the sluice gates the fish are allowed to swim from the rearing ponds to the catching ponds. From there they can easily be fished out, when the tide turns, after full or partial drainage.
In shrimp ponds in Asia, long bag nets are set in the sluice gates to catch the shrimps as they swim out from the pond with the outflow of water at low tide (see fig. 6.10). The high tide stimulates them to move around the pond and when the tide changes they move with the current and are caught in the nets.The best time for such total harvest is during the new moon or full moon periods, as at this time the shrimps are more active. But at the full moon, many of the shrimps may be moulting and the soft-shell shrimps caught will not fetch a good price in the market. Catches are best at night and a light placed over the sluice gate will serve to lure shrimps to swim towards it.
Some species of shrimps, such as Penaeusmonodon, do not swim out of the pond easilyand repeated draining is necessary in order to harvest a good percentage of the stock. Another method of harvest is by concentrating them in peripheral canals in the pond by partial draining, and then seining them from the canals.