Spawning and larval rearing
The minimum age of spawning females varies between species and according to the environmental conditions, as can be seen from the following:
In general, spawners from captive brood stock are smaller than those from the wild. Farmers usually believe that spawners from wild stock are superior to captive ablated spawners, and that the quality and quantity of their eggs are higher.
The maturity of the males can be determined by examination of the petasma on the first pair of pleopods. In mature males these accessory organs are joined together by means of inter-locking hooks. Swelling and whitish coloration of the terminal ampoules near the fifth pair of pereiopods indicate gonadal maturity.
The spawning season in nature varies according to species and location. When larvae have to be collected from the wild, or when wild spawners are used for spawning and larval rearing, it is essential to know beforehand the period and locations of their occurrence. On the other hand, captive stocks can be matured and spawned almost throughout the year under controlled conditions. Though tropical species spawn throughout the year, most Penaeids have peak periods of spawning.
In closed thelycum species (i.e. species with lateral plates that lead to a seminal receptacle, where the spermatophores can be inserted), the mating occurs soon after the females have moulted. In species with an open thelycum (with only ridges and protuberances for spermatophore attachment) mating can occur soon
after the eggs become mature. In the latter group of species, spermatophores can easily be lost or fail to be affixed before spawning. The spermatophores deposited during a single moulting are generally enough, irrespective of the moult cycle, to fertilize up to three successive spawns.
For controlled spawning, gravid females and males in advanced stages of maturity are stocked in spawning tanks. Spawners obtained from commercial catches during the winter are likely to be infected and are therefore usually treated with 3ppm KMnO4, 25ppm formalin or the commercial product Treflan®(trifuralin) at concentrations of 3–5ppm. In large tank systems used for community culture, several spawners are introduced into the community tank, whereas in the other systems individual spawners or batches of spawners are placed in separate spawning tanks each time. In large tanks, the density of spawners are generally:
P. japonicus: 1 spawner/2m3
P. monodon: 1 spawner/5m3
P. indicus: 1 spawner/1m3
P. merguiensis: 1 spawner/1m3
Generally a 1:1 sex ratio is maintained in spawning tanks, but a ratio of two females to one male has produced higher spawning rates and egg production. There is usually a time lag between mating and spawning, as the eggs may still not be fully mature at the time of mating.
Penaeus japonicus and P. indicus females havebeen observed to eat their own spawned eggs and so it is advisable to install mesh trays or plates on the bottom of the spawning tanks to protect the eggs. The salinity in the tanks gen-erally ranges from 28 to 35ppt and the temperature from 23 to 33°C. Spawning usually takes place at night. Fertilization is external and at the above temperature range the embryonic development is rapid.
The nauplius passes through three to six sub-stages (N1–N6) and subsists on its own yolk material. In about two to three days it metamorphoses into protozoea with three substages (PZ1–PZ3) during which period the larva starts feeding on unicellular algae. This stage, which lasts for three to six days is succeeded by the mysis stage with three sub-stages (M1–M3). During this stage the larva retains the filteringmechanism for feeding on algal cells. The mysis metamorphoses into post-larva in about three to five days. At this stage it ceases to be a filter-feeder and becomes capable of capturing and eating zooplankton. Development from the post-larval stage to the juvenile stage is very gradual and a PL5 (P1–P5 or PL1–PL5 denotes the post-larval age in days) may take 15–20 days to reach a size of 20–25mm, suitable for stocking production ponds.
In larval culture of most Penaeids, the main difficulty is in rearing the protozoeal stage when they start feeding. At this stage the larva is highly light-sensitive, and so the tanks should be properly covered to ensure darkness. The key to the success of the pioneer experiments of Hudinaga in Japan was the method developed for the culture of the diatom Skeletonemacostatum, which formed a suitable food for thelarvae. Since then several other types of live foods and feedstuffs have been tried, but cultured phytoplankton appears to be still the most efficient food for larvae at this stage. Since the size of larvae of different species of Penaeids are not the same, they require phyto-plankton of different sizes. They start feeding on zooplankton when they reach the last sub-stage of protozoea. Both mysis and post-larvae up to the fifth day prefer zooplankton, but after that stage they will consume larger food and may feed at the bottom. They can then be fed on polychaetes, chopped mussels, clams, cockles and artificial compound diets.
The more important phytoplankters suitable as food for shrimp larvae are species of Chaetoceros, Skeletonema and Tetraselmis.Algal culture methods have been described. Among the zooplanktonic organisms, the rotifer Brachionus plicatilis is probably the most important as larval food. Many hatcheries depend largely on the brine shrimp, Artemia salina, the nauplii of which form excellent food for shrimp larvae. Methods of culturing Brachionus and hatching Artemia cysts have been described. In large tank hatcheries practising community culture, the tanks are fertilized soon after hatching, at the rate of 3ppm KNO3 and 0.3ppm Na2HPO4 to produce the phytoplankton needed to feed the larvae when they reach the protozoea stage. In some hatcheries pure cultures of diatoms are inoculated before fertilizers are applied. The
production of phytoplankton is maintained through additional fertilization if needed. If the density of plankton is inadequate, supplementary feed in the form of soybean cake, soybean curd, egg yolk of fertilized eggs of oysters may be given.When the larvae reach the mysis stage, they are fed on Brachionus or brine shrimp nauplii. In most hatcheries, post-larval stages are fed on brine shrimp and after the P6 stage on minced mussels, clam meat or formulated larval feeds, partly replacing brine shrimp nauplii until they reach the P7 stage. Beyond this stage, the post-larvae are fed only minced mussel, clam meat or artificial diets, three or four times daily.
As indicated earlier, in hatcheries with separate small hatchery tanks, algae (Skeletonemacostatum and Tetraselmis spp.) are cultured inseparate algal tanks or in plastic bags and the required quantities are introduced daily during the protozoea stage. Artemia nauplii hatched in special tanks are fed to mysis and early post-larval stages. In hatcheries with intermediate size tanks, fertilization of the tank water and introduction of pure algal cultures are combined, and the post-larvae are reared up to the P25 stage.
Hatcheries that produce only P5 or P6 stage post-larvae use concrete tanks, earthen ponds or net cages for larval rearing. Small tanks with a filtered sea-water supply and aeration are stocked at a density of up to 150/l. Diatom cultures are introduced to feed the larvae and often a substrate such as polyethylene netting is provided for the larvae to rest on. Early post-larvae are fed with chopped mussel and cockle meat together with young and adult Artemia. Daily exchange of water is maintained for the duration of culture (about 30 days).
Earthen nursery ponds range in area from 500 to 2000m2 with an average depth of 40– 70cm. The larvae are stocked at the P9–P10 stage at densities of 100–150 per m2. The ponds are prepared by eradicating predators and fertilizing with a combination of organic manures (such as 1000kg/ha of chicken manure) and inorganic fertilizer (such as 50kg/ha of ammonium sulphate). Supplementary feeding is done with chopped mussel or cockle meat, at about 10 per cent of the total biomass. The larvae can be reared in such ponds up to the P40 or P60 stages.
Twenty-one- to twenty-five-day-old post-larvae are suitable for stocking grow-out ponds. In some farms, particularly in Ecuador and Taiwan, post-larvae are grown (sometimes referred to as pre-growing) for 30–60 days at densities of 50–200 per m2. With daily exchange of water (10–40 per cent) and supplementary feeding with compound feeds, they reach a mean weight of 0.5–2g, with a survival of 80 per cent depending on species and pond conditions. These fry are then stocked in grow-out facilities.
Nursery cages are used only rarely, as the very small mesh sizes required can become rapidly choked by bio-fouling. The cages, when used, are rectangular in shape (1–2m x 5m x 1m) and are of the floating or stationary type installed in protected bays, lagoons or ponds. Post larvae (P6–P7) are stocked at higher densities of 1000–2000 per m3. Feeding is carried out in the same manner as in earthen ponds.
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