There are several behavioural characteristics of shrimp which can be used to advantage during harvest. They move around the pond at night looking for food. They are attracted to light. They are stimulated by movement of water. When water is let into a pond, the shrimp become active, swimming around the pond and often gathering near the sluice gate. Larger shrimp have a natural tendency to migrate to deeper water offshore, so they swim out of a pond with the water when water is discharged. Most species of shrimp are more active during the new moon and full moon. The periods of greatest activity are normally shortly after sunset and again shortly before sunrise.
Partial harvesting is useful in some types of management systems where only large shrimp are to be caught and smaller shrimp left in the pond to grow larger, and in polyculture where a farmer wants to harvest shrimp but not fish. Shrimp typically have different growth rates with some individuals growing much faster than others. It is possible to selectively harvest these large individuals before the main crop is harvested. In a programme of selectively cropping the larger shrimp from a pond, the pond must be totally harvested occasionally, or the pond will end up with mostly small males. This is especially true for species of Metapenaeus with a large size difference between sexes.
Barrier traps (Fig. 13) set around the edge of a pond are very effective. The traditional type trap made of bamboo screens is good, but it has two disadvantages. First, most of these traps has no bottom, so a worker must catch the shrimp with a net. Second, it is difficult to provide the correct size mouth opening. If the mouth is too large, crabs can enter the trap and they eat or damage many shrimp. If the opening is too small, the shrimp will not go into the trap. It is better if a rigid frame trap with a bottom is made. In Indonesia fyke type shrimp traps made of bamboo screens with built-in bottoms at their catching ends are used. It should be of a size that one or two men could lift it, empty the shrimp out, and set it back in the pond. The openings in the walls of the trap should be large enough so small shrimp can escape if a partial harvest is desired. The mouth opening should be 4 cm wide. Wire or string should be placed at 4 cm intervals across the mouth to prevent crabs from going into the trap.
This type of trap should be fished at night. No bait is needed. A small kerosene lamp placed on top of the trap will attract more shrimp. If the number of shrimp in the pond is large, the trap should be emptied periodically during the night. If too many shrimp are caught in the trap, a portion of them will die and they will be spoiled and in poor condition by morning.
Baited traps are not recommended, but if they are used, fish with a high oil content make the best bait.
Cast nets, lift nets and seine nets can be used to harvest shrimp. Their use can be made more effective by setting out bait or food in the area to be fished. If fishing is done at night, lights can be used to attract the shrimp.
In Taiwan, China, a hand-held electric gear is used to harvest shrimp. The gear is composed of an accumulator and two bamboo poles. One of the poles is equipped with a metal tip and the other has a steel ring with a net attached. The metal tip is connected by wires to the anode of the accumulator and the steel ring to the cathode. The accumulator is carried in a backpack or on a small raft and the operator holds one pole in each hand as he wades through the pond. When the gear is switched on, an electric field is formed between the two poles. On receiving an electric stimulation, the shrimp jump out of the water and are caught in the net. The gear has been used to totally harvest large ponds (Liao, personal communication).
Most species of shrimp can be harvested effectively by using a bagnet placed in the sluice gate and catching the shrimp as they swim from the pond with the outflowing water. The best time to do this type of harvesting is at night during the new moon or full moon.
However, many species of shrimp molt during the full moon period and the soft shell shrimp bring a lower price, so, if possible, harvest is during new moon. It is useful to let some water into the pond before harvesting is started to stimulate the shrimp and get them swimming around the pond. A light placed near the mouth of the sluice gate will attract the shrimp toward the gate and aid in their capture.
The bagnet must be long enough so it extends far enough from the sluice gate to make emptying the bag easy. The same type of “lazy line” used on shrimp trawl nets can be used to pull the bag to shore for emptying (Fig. 14). During harvest, the water is drained from the surface. Initially, the top two closure boards are removed. Additional boards are removed as needed to keep water discharge at about the same amount.
P. monodon is particularly hard to harvest with a bagnet as it does not swim out of the pond readily. Some farmers have reported that about 90 percent of P. monodon can be harvested from a pond if the draining procedure is repeated on three successive nights. The remaining shrimp have to harvested by hand after the pond is totally drained.
A method of harvest particularly useful for P. monodon is to drain water from the pond very slowly until water remains only in the peripheral canals or a harvest basin. With the slow lowering of the water level, the shrimp move to the deeper canals. Shrimp can be removed from the canals by dragging a seine net around the peripheral canal. A bamboo screen can be pushed around the peripheral canal to concentrate the shrimp in a restricted area. They are then caught with scoop nets.
A modification of this type of harvest is to let water in slowly after the shrimp have been concentrated in the canals. This stimulates the shrimp into moving and they enter traps which are set around the pond.
Some farmers build large traps in the water outlet canal outside the sluice gate. Shrimp are caught in the traps as the pond is drained. Traps are more costly than bagnets and cannot be moved from pond to pond. Their use should be discouraged.
In all cases, final harvest is by hand after all the water has been drained from the pond.