Schuster (1952), Djajadiredja and Daulay (1982), and Bandie et al. (1982) have described designs of tambaks that have been used for milkfish farming in different regions of Indonesia. Though the basic principles of pond farm design have not changed, improved designs have been evolved in the Philippines and Taiwan, which are now being introduced by progressive farmers in Indonesia as well (Wardoyo et al., 1982). Basically, each farm has nursery, transition and rearing ponds, which can be independently drained or filled through a canal system. As mentioned earlier, Taiwanese transition ponds are also used as wintering ponds. The rearing ponds generally form 85–90 per cent of a farm, and in modern farms where intensive culture systems are employed indi-vidual ponds seldom measure over 4–5ha. Usually they are rectangular in shape and located on either side of the canal system for water supply and drainage.
The overall configuration and operation of the rearing ponds are very similar to those described for nursery ponds. The majority of farms depend on the production of benthic organisms for raising milkfish, and so basically the same pond management methods are followed. Even though in pen culture in eutrophic lakes milkfish have been grown to market size on plankton, in actual practice the farmers have not yet been able to obtain consistent results with plankton feeding in pond culture.
In the early days of milkfish farming in the Philippines, lumut or the algal complex dominated by filamentous algae such as Chaetomorpha, Cladophora and Enteromorphawasconsidered to be the best natural food to be raised in ponds. It was later observed that filamentous algae like Chaetomorpha are too coarse and fibrous to be suitable food for milk-fish fingerlings and it is only the decaying algae in the detritus that the fish are able to utilize. Because of this, ponds with lumut growth can be stocked with only 1000–1500 fingerlings per ha and the yield expected would be only 200–300kg/ha per crop. Experience seems to indicate that lablab is also the best natural food for growing milkfish to market size. Fertilized ponds with good growths of lablab can yield 500–700kg/ha per crop in a period of two to three months.
Procedures for preparing rearing ponds are generally the same as for nursery ponds. The ponds are drained and the bottom dried. If required, the soil pH may be adjusted by the application of lime (CaCO3). Initial fertilization is done with a combination of organic and inorganic fertilizers. After the ponds are stocked, fertilization with urea and NPK fertilizer is continued at about half the initial dose, at fortnightly intervals, taking care to exchange the water regularly.
In traditional milkfish ponds, stocking is carried out with milkfish only, but during the course of culture other species, especially grey mullets shrimps (mainly Penaeus andMetapenaeus spp.), gain access, converting itinto a polyculture system. Stocking density is left to chance. However, in recent times, with the increased demand for and price of shrimps, milkfish farmers are undertaking deliberate stocking of Penaeid shrimps and in some cases even converting milkfish ponds into shrimp ponds. The combination of milkfish with shrimps is not entirely based on compatible feeding habits, as there is obviously some overlap.
In view of the fluctuations in the benthic growth, the success of milkfish production in ponds is largely dependent on the timing and efficiency of stocking. Consequently, a number of systems have been developed for better utilization of the food resources and increased yields in milkfish ponds. The simplest system is to stock rearing ponds at rates that the food resources can sustain, and harvest them when they have reached the marketable size. Since marketable size can be reached in two to four months, three to four crops can be raised every year if fingerlings are available. The usual practice is to stock a single size group of fingerlings (10–15cm) at the rate of about 2000/ha, and completely harvest when they have grown to marketable size. The main disadvantage of this system is that there is a wastage of food when the fish are small, as they cannot utilize all the food produced; and when they have grown the food produced in the pond may be insufficient because of the increased food requirements of the larger biomass.
In order to avoid shortage of food at critical times in rearing ponds, a procedure known as the ‘progression method’ is practised by many farmers in the Philippines (fig. 21.10). Rearing is carried out in two stages. The fingerlings grow for a certain period in one pond and are then transferred to another pond where they grow to the market size. The food resources in both ponds are not exhausted and several crops can be raised through proper management. This method has been further improved to a so-called ‘modular method’, which involves a three-stage rearing. Three contiguous ponds form a series, progressively increasing in size at a ratio of 1:2:4. The first pond is stocked at a density of 15000/ha. After about six to seven weeks the stock is transferred to the second pond, and after about four to five weeks to the third pond, until they reach market size. As soon as a pond is emptied, it is prepared to receive the next stock.
A more intensive system of stocking developed in Taiwan consists of stocking different sized groups and repeated selective harvesting, and this is sometimes referred to as the ‘multisize stocking method’. Initial stocking may be with three size groups; for example, 3000 fingerlings of average length 5cm, 2000 of 15cm length, and 2000 of 18cm length. Subsequent stocking may be with smaller fingerlings (for example, of 5cm length) at about one to two month intervals, at the rate of 2000–3000 per ha each time. Repeated selective harvesting is performed three to five times to remove the market-sized fish.
As milkfish farming is largely based on the production of natural food, artificial feeding is resorted to only in special circumstances, when the natural food production is not adequate. Locally available feedstuffs like rice bran, peanut meal and soybean meal are used for supplementary feeding. The feeding rate in rearing ponds in Taiwan is generally 30kg rice bran or 25kg soybean or peanut meal per ha daily.
An important aspect of pond management consists of reducing or eradicating organisms that disturb or feed on benthic growths in ponds. Chironomid larvae, polychaete worms and snails are the most common pests. Taiwanese farmers use different types of pesticides to eradicate them. The application of lime and urea for the initial preparation of milkfish ponds usually helps to reduce the growth of these organisms.
There are very few known diseases of milk-fish. Schuster (1952) observed a condition described as ‘catching cold’ when there is a sudden lowering of temperature in shallow ponds. The symptoms are a milky discoloration of the skin and sluggish movements. After two or three days, portions of skin may drop off. No mortality has been observed.
Pen culture of milkfish is practised in the eutrophic lakes of the Philippines, especially in Laguna de Bay. As mentioned, most of these pens are enclosed by synthetic netting of suitable mesh size, installed on a framework of bamboo poles dug deep into the lake bottom (see fig. 6.34). The size of the pens varies considerably from 1.5ha to as much as 100ha. The most common size appears to be between 10 and 20ha. When fingerlings have to be reared on site, a nursery pen is constructed within the rearing pen. It is made of smaller-meshed netting and usually measures around 20m x 20m with a depth of at least 1.5m.
The stocking rate depends on the density of planktonic blooms in the lake. Generally, it varies between 10000 and 20000 fingerlings per ha in the main pen, and 100 fry per m3 in the nursery pen. Supplementary feeding with rice bran, copra meal, soybean meal, etc., is provided in the nursery, and it may take up to one month for the fry to grow to fingerlings of about 20g size. Fingerlings stocked in the rearing pen feed on natural food in the lake and no artificial feed is provided. Multiple stocking and harvesting can be practised in pens, as in ponds, if there is a dependable supply of fingerlings. Depending on local conditions, it may take four to five months for the fingerlings to reach marketable size in pens.
One of the major hazards in such eutrophic lakes is the occurrence of fish kills due to anoxia caused by death and decay of algal blooms.