Although spawning occurs throughout the year among tropical species of shrimps, there are distinct periods of the year when majority of the shrimps spawn. In the case of P. monodon in Southeast Asian waters, there are two pronounced spawning season, from December to March and June to September with only one pronounced period, viz: June-September in the case of P. merguiensis and P. indicus.
While spawners of these species of shrimps are supposedly available all year round, they are abundantly caught during the spawning season.
A gravid female can be easily recognized by the presence of a very large and distinct diamond-shaped dark green mass of ovary between the first and second abdominal segment below the dorsal shell. Selection of spawners can be easily done by holding the animal with its ventral body facing a light source (Fig. 27).
The criteria used for selecting spawners from the wild are:
Fig. 25. Filter box for draining in big tank.
Fig. 26. Filter box for small tank.
Fig. 27. Spawner with mature gonad
Spawners are usually collected by professional fishermen. In Japan, hatchery operators can easily procure spawners from fish markets because fishermen bring them back alive as live shrimps are preferred by consumers over dead ones. In other Southeast Asian countries, however, hatchery operators must deal directly with fishermen, provide them with the necessary facilities (e.g. containers, battery operated air pumps, etc.) and teach them handling techniques to ensure getting quality live spawners. In order to do this, hatchery operators must have comprehensive knowledge of the operating gears and the means of transporting spawners. In cases where spawners are available only during a certain spawning period (e.g. 3–4 months in P. orientalis) hatchery operators should carefully plan the hatchery operation to meet their requirements.
The major fishing gears in collecting spawners can be broadly grouped into three categories, viz: stationary trap, mobile and entangling net.
Stationary traps - There are many types of stationary traps used in shrimp fishing. The more common ones used in Southeast Asia are fish corrals, net traps and filter nets. A shrimp trap usually consists of two major components; the leads or wing and the terminal trap. The leads are made up of a series of closely arrayed bamboo or wood which serve as guideposts or reference points to lead the shrimp or fishes intothe trap. The terminal trap contains a non-return device and the trapped animals can be harvested directly using a scoop net at low tide. Some fish traps are installed with a filter net at the terminal trap whenthe cod end of the filter net canbe lifted periodically to collect the harvest. In the case of net traps, the lead or wing can be made up of one or more compartments of net-wall vertically suspended about 45° against the receding tides.
Mobile gears - Trawlers are the most popular and efficient gears in shrimp farming. Most other trawls used are designed for shrimp fishing and some of them are rather small but efficient enough to operate close to the shore. These small trawlers are also known as baby trawl. In countries such as Indonesia where trawling has been completely banned, the supply of spawners largely depends on other fishing gears. In collecting shrimp spaweners, especially P. monodon and P. semisulcatus, the trawl operation should not exceed two hours per haul while for P. indicus and P. merguiensis, 45 minutes per haul is considered sufficient. Trawling is usually operated in the night especially P. monodon and P. semisulcatus, when the shrimps are most active. For the P. merguiensis and P. indicus however, fishing can be done anytime of the day.
Entangling nets - The gill net used for shrimp fishing consists of one three layers of netting, cork floats, lead sinkers, head and foot ropes and a pair of anchors. The mesh sizes of netting used are usually 1.5 cm for single layered gill net and 1.5 cm to 4 or 5 cm for three layered gill nets. The height of the net is about 5–10 meters.
Gill nets are operated both during night and day. The nets are suspended vertically and anchored to the bottom from the end. The net is periodically lifted at 4–5 hours interval to collect the harvest.
There are several ways by which spawners are transported from the field to the hatcheries. They can be transported in:
live fish holding compartment in the boat with running water system (very convenient for hatcheries close to the fishing ground).
holding tank with aerated seawater at controlled temperature (22–24°C) using ice in plastic bags and transported by trucks.
plastic bags injected with oxygen and packed in styrofoam boxes. The water temperature can be controlled by using ice mixed with sawdust. In this case, the rostrums of shrimps could be covered with plastic caps to prevent puncturing of plastic bags, and
bamboo or PVC tubes: the spaweners are immobilized in separate tube without overstraining them. While transporting in boats, the shrimps are held in cool (22–24°C) aerated water in tanks but are transferred to plastic bags with oxygenated water while transporting on land. Tube transport along with reduced temperature reduces the frequency of untimely spawning in transit and/or injuring themselves.
Upon arrival, each spawner is nowmally placed directly in a spawning tank without any further treatments. However, during winter or when there is known spread of disease, spawners are usually treated with either (a) Treflan (trade name), 0.5–1 ppm (b) KMnO4, 3ppm or (c) Formalin, 25 ppm for 10–15 minutes.
In nature, adult shrimps mate after the females have molted. Spawning usually occurs while swimming with the spermatophore in the thylecum and eggs are released from the genital pore which is located at the base of the third pereiopod; sperms are likewise discharged into the water through an apperture at the base of the fourth pereiopod. Fertilization is external. Spawning usually occurs between 0200 to 0300 hours at water temperature and salinity ranging from 25 to 30°C and 28 to 32 ppt, respectively.
After spawning, the animal is removed from the tank the following morning. The eggs are then cleaned either by siphoning into egg collectors (Fig. 28A) or draining 2/3 of the water through a filter net that effectively retains the eggs within the tank (Fig. 28B). When drainaing is completed, the scum is then removed using a scoop net with a mesh size bigger that the shrimp eggs. The tank is then filled up with new seawater (Fig. 29).
During the cold season, fungus and bacteria are likely to infect the eggs during incubration. Preventive treatment normally consists of dipping the eggs in 1 ppm of methylm ene blue or 0.5 ppm of malachite green for 10 minutes or 3 ppm KMnO4 for 30 minutes. After that, the eggs are transferred to a cleaner tank for further incubation and subsequent hatching. From the incubation/hatching tank, samples of eggs are counted to determine the number of eggs spawned per female.