5.1 Culture Systems
5.2 Fertilization Practices
5.3 Feeding Practices
Aquaculture in the Philippines is predominantly based on brackishwater pond culture of milkfish. Other economically important farming systems include freshwater milkfish pen culture, freshwater tilapia cage culture, and the brackishwater pond culture of shrimp. Very recently, pen and cage culture of milkfish and tilapia in brackish and marine waters has been successfully initiated in several municipalities around the country. With the majority of the mangrove areas and inland waters suitable for aquaculture already in use, the development of pen and cage culture in protected coastal areas is seen as the next frontier for expanding finfish aquaculture production. Details of important management and production aspects of milkfish, tilapia, and shrimp culture relevant to feeding and fertilization strategies are summarized in the following section.
5.1.1 Pond culture
5.1.2 Pen and cage culture
Most of the countrys brackishwater ponds are used for extensive milkfish culture. These ponds are shallow, ranging from 30-50 cm, in order to maximize light penetration for benthic natural food production. Pond compartments are also large, usually between 5-10 ha in size with single gates. Productivity of extensive ponds relies on the supply of natural food which is enhanced through fertilization, control of competitors snails and fish (e.g. O. mossambicus), and the use of a modular pond system where stocks are moved to a newly prepared pond everytime natural food becomes limiting (for milkfish). Extensive milkfish ponds typically produce a gross yield of 500-750 kg/ha/crop although in certain provinces in the country, such as Iloilo and Capiz, gross yields approaching 1,000 kg/ha/crop are common.
Black tiger shrimp are also an important cultured species within brackishwater fishponds, but since the outbreak of diseases in the early 1990s, there has been a declining interest in their farming. Emerging brackishwater pond operations are semi-intensive and intensive culture of milkfish and to a lesser extent tilapia (usually O. Niloticus × O. mossambicus hybrid). In semi-intensive culture, yield increases of up to 200-400% are attained over that of extensive culture with supplemental feeding, deeper water, and more frequent water change. Pumps are used for maintaining a minimum pond depth and the desired water quality. In intensive culture, small ponds with at least 1 m water depth, supported by pumping and aeration facilities, allow yields in excess of ten fold that of extensive culture. Semi-intensive milkfish farms are mostly upgraded extensive ponds while intensive farms are evolving largely as an alternative for shrimp culture. Investing otherwise for the necessary pond structures and facilities for intensification is not economically attractive.
Typical pond culture aspects for milkfish, tilapia, and shrimp are summarized in Table 70. For details on shrimp culture practices, the reader is referred to Cruz (1993).
Milkfish pen culture has been practised for over two decades in Laguna Lake. Fish are fed almost exclusively on the plankton-rich water of the lake, with stocking density ranging between 10,000-20,000/ha. Pen culture of milkfish in brackish or marine waters is a very recent development. Commercial operations are being pioneered in Alaminos in Pangasinan, some parts of Manila Bay, Bantayan Island (Cebu), and Davao del Norte using stocking densities of 50,000-80,000 pieces/ha. Unlike Laguna Lake, milkfish are fed in these pen culture environments due to the limited supply of natural food.
Freshwater cage culture is popular in several small lakes around the country with Nile tilapia being the primary species farmed (Figure 12). Typically the stocking density is 20 pieces/m3. In Pansipit River which drains Taal Lake, tilapia cage culture in running water is practised at densities of 150-300 pieces/m3. Brackishwater or marine cage culture of milkfish and tilapia is starting to find commercial success in areas such as Bohol, Batan Bay in Aklan, and Davao Oriental. The archipelagos extensive coastal areas and tributaries provide a vast resource for pen and cage culture in the years to come. Table 71 summarizes important information concerning these culture systems.
Natural food types
There are three important natural foods raised in ponds with fertilization , namely: lab-lab, lumut (filamentous algae), and plankton (Table 72). Lab-lab grows as a mat on the pond bottom and is the most desired natural food for milkfish. It consists of a benthic community of cyanobacteria, diatoms, and associated invertebrates. It is the presence of associated organisms which gives lab-lab its high food value.
Lumut consist mostly of Chaetomorpha spp., with Cladophora and Enteromorpha as associated species. It is grown mainly as a food for milkfish especially during the rainy months and under low salinity conditions when growing lab-lab is difficult. Plankton on the other hand includes numerous species of microscopic diatoms, algae, and zooplankton. Plankton is the preferred natural food for tilapia in fresh and brackish water, and is also the natural food that abounds in lakes and dams. In extensive shrimp culture, two species of aquatic macrophytes, Ruppia maritima and Najas graminea, are specifically propagated as a source of natural food; shrimp feed on the small copepods and insects that find food and shelter on these plants.
Juliano (1985) surveyed the fertilization practices employed by 44 brackishwater milkfish farms in the provinces of Bataan, Pangasinan, Aklan, Capiz and Iloilo, and reported that 91.7% used inorganic fertilizers, 60% used organic fertilizers, and 50% used both inorganic and organic fertilizers. The average inorganic fertilizer used was 94.3 kg/ha/crop (1.9 bags) at a nitrogen-phosphorus ratio of 2-4:1, derived from urea and ammonium phosphate (16-20-0 or 18-46-0). Organic fertilizer, largely in the form of chicken manure was applied at a rate of 461.3 kg/ha/crop. In all the farms surveyed, the dominant natural food cultured was lab-lab.
Progressive milkfish farmers usually apply chicken manure on the pond bottom at a rate of 1 t/ha/crop, while inorganic fertilizers are applied at a rate of 150-200 kg/ha/crop (basal and side dressing) with urea and 16-20-0 combined at a ratio of 1-2:1. Organic fertilizer is first applied on the moist pond bottom prior to flooding, and inorganic fertilizers are then broadcasted a few days later when the natural food starts to grow. To sustain the bloom, a maintenance dose is normally continued every spring tide. An example of a pond preparation procedure for growing lab-lab is shown in Figure 39. During pond preparation, the pond is dried, limed, and tilled at least once a year to allow mineralization and release of nutrients from the soil. The practice also reduces the build-up of any harmful organic waste.
Plankton is the natural food propagated in brackishwater and freshwater tilapia culture where the ponds are relatively deep. Inorganic fertilizer (urea, 16-20-0, and/or 14-14-14) is generally applied using the tray method at 50 kg/ha every two weeks. However the daily or weekly application of manure during the growout period is not popularly practised in milkfish culture (see Section 6.3.1).
Milkfish and tilapia
When the natural food in extensive milkfish ponds is depleted and the fish have not yet attained the desired harvest size, single ingredient feeds (such as rice bran, bakery waste, and snackfood rejects) or commercial pelleted feeds may be used during the last few weeks of culture. This practice is sometimes referred to as modified-extensive culture. Feeding is decided when most of the natural food appears to be already consumed (estimated visually), when growth rate tapers, or when the fish starts to lose the fullness of its body. The amount of feed given in many cases is only estimated.
The majority of the commercial pelleted feeds are utilized in semi-intensive and intensive culture. Feed manufacturers and suppliers recommend a feeding programme for farmers through their technical representatives. Feeding milkfish and tilapia have commonly been carried out using feed trays located around the periphery of the pond or by hand broadcasting. Feeding is conducted twice or three times a day at around 8 am, 12 noon, and 4 pm. A few intensive pond farmers use feed blowers imported from Taiwan. Feeding by tray is now being discouraged due to feed losses resulting from nutrient leaching and feed disintegration caused by the low stability of fish feeds (normally under 1 5 minutes). Hand feeding on the other hand, has been found ideal for cages where the compartments are small, although for ponds, this feeding method is costly and laborious. Since 1994, a local company (Kinetic Feeding Systems, 158-C Singcang, Bacolod City) has pioneered the design and manufacture of fish feeding equipment. Demand feeders and automatic feed spreaders are now consequently starting to be used in semi-intensive and intensive pond culture operations. The comparative advantages and disadvantages of these various feeding methods and equipment are shown in Table 73 with the suppliers and manufacturers listed in Table 74.
Reported feed conversions obtained with commercial pellets in extensive culture are generally below 1, and range from 1.2-1.6 and 1.5-2.0 within semi-intensive and intensive culture, respectively. Higher FCRs are generally obtained with smaller stocking sizes, higher stocking rates, and increasing harvest size.
For many small tilapia cage farmers who are not comfortable in computing feed rations, feeding is based on a pre-computed guide provided by feed companies based on a projected growth and feeding rate. It is interesting to note that routine sampling of weight gain is not commonly practised; tilapia cage farmers preferring to monitor and evaluate their use of feeds in terms of bags consumed (rather than kilogrammes), and the days of culture that have elapsed. This practice has been possible since cage farmers tend to adopt uniform compartment sizes and stocking densities.
Extensive black tiger shrimp culture commonly uses protein-rich fresh feeds such as trash fish, bivalves, and snails, during the last one or two months of the growout operation, with feeding employed once or twice per day. In Panay island and in Negros Occidental, a 7-10 mm bivalve locally known as agihis is a much sought-after supplemental feed for shrimp; given whole (the shrimps are able to break open its shell), the mollusc survives in the pond for a few days. This allows the farmer to feed only once every few days without the threat of polluting the pond water. However, the supply of this bivalve is now already scarce due to excessive collection.
Shrimp farmers are generally knowledgeable concerning the technical aspects of using commercial shrimp feeds as feed expenses account for more than half of their production cost. Farmers compute their daily rations and are able to make the necessary adjustments on the prevailing weather and water quality condition in the pond. Feed trays are popularly used as a key management tool for evaluating feed consumption during the days feeding schedule which is normally five times starting at 7-8 am, and every four hours thereafter. Food conversion ratios range from 1.2-2.2, depending upon the stocking density and days of culture. For a detailed review of the shrimp feeding management practices in the Philippines, the reader is referred to Cruz (1991).