The Fiji Fisheries Division has embarked on a program to develop existing aquaculture products and also to venture into new products with market and economic potential. One of these products is milkfish, primarily for tuna bait and secondarily for food. With the success in establishing sites for fry collection, developing the techniques in collections, and organising villagers, a good supply of fry is expected to complement successful milkfish bait or fish farms. Fry surveys carried out by the project staff alongside staff from the Fisheries Division have shown that enough fry can be collected from the wild.
In March 1997, a demonstration of brackishwater milkfish stocking was done in one of the ponds in Montfort Boystown, Wainadoi. The fish reach an average weight of 60 grams in 60 days, a size that can be used for pole and line baits for tuna fishing. At the time, the weather was quite bad, resulting in a slow growth rate for the fish. Better results could be expected under more favourable conditions. The system applied in this culture is the “shallow pond” culture, or “benthic algae feeding” method.
In April 1997, a demonstration pond for freshwater milkfish stocking was done in one of Viti Corps' ponds in Navua. The 2,000 milkfish fry which were stocked in a nursery pond came from Kiribati Fisheries as part of a program to study the option of importing fry from outside Fiji. After 60 days, the fingerlings (2 to 3 grams in weight) were transferred to grow-out ponds. About 600 pieces were brought to Montfort Boystown, and 948 pieces to one of Viti Corps grow-out ponds. However, the weather was so bad that the fish stocked in the ponds grew very slowly.
It is characteristic of milkfish that given unfavourable living conditions such as crowding, insufficient food, low water temperature, or low pH, etc., their growth would be slow or non-existent. However, when given good living conditions, the fish would grow faster than their normal growth rate even after having been subjected to those previous unfavourable conditions. This process is called “stunting”. It is common that milkfish farmers buy the total number of fry required for the year during peak collection season, and stunt them until they can sell them at a good price when they are not as plentiful, before the next collection period. This way, the farmers can take advantage of low prices, and the volume they need is available at one time. This can be done in all areas where there is a pronounced peak season of fry collection.
Milkfish, (Figure 13, Page 23) being a euryhaline fish, can grow in any clean water environment; salinity is not a requirement when growing this fish. However, transferring from saline water to freshwater should be done gradually in a process of “acclimatisation”, which will be discussed later on. There are differences in the culture system which depend on the food available for the fish. It should be noted that natural food from saline water is different from food that can be found in freshwater; however, the principle of growing these natural foods is the same, and milkfish feed on both.
Figure 13. A marketable size milkfish weighing 250 grams.
(Source: C.S. Lee, Aquaculture of Milkfish, Title page)
2.1.1 Freshwater pond
In general, this system will require acclimatisation of the fry because they were collected in seawater. Freshwater ponds used for tilapia are good sites for milkfish; in fact, any flat area with good type of soil and ample supply of clean freshwater from a dam, reservoir or deep well can be a good site, as shown in Plates 14 and 15 (Page 32). Unused rice paddies or flat vegetable farms are usually fed with irrigated water canals, so these farms would need little renovation and access to them is usually easy. Bare, flat areas with lots of big old trees, and those prone to flooding or run-off from mountain slopes, should be avoided.
2.1.2 Brackishwater pond
Brackishwater areas are those that are reached by seawater during high tides or where water can be drawn from a river or sea with salinity from 5 to 30 ppt. Unused salt farms, prawn farms or swamps and marshlands can be developed into brackishwater ponds for milkfish. These kinds of areas are plentiful in Fiji.
The design and layout depend on local topography and available space. Sizes also vary according to mode of operation. In a simple extensive farm, direct stocking is usually the common practice where milkfish fingerlings are available from nurseries not far from the farm as shown in Figure 14 on Page 25. When fry are available in the area, a combined nursery and grow-out pond is more advisable. However, in both cases, the elevation relationship is very important in order to facilitate harvesting of the fish, as follows. Milkfish by nature have a unique tendency to swim against incoming water, and this behaviour can be used very effectively to concentrate the fish in a small area and then harvest them using a seine net. Therefore, gate construction and pond elevation in relation to tide level must be taken into consideration when designing the pond (Figure 15 on Page 25).
In general, the layout consists of these features:
Main dyke used for protecting the whole area and water confinement (usually 3.0–5.0 m high, 1:2.5–3.0 slope, 3.7–5.0 m wide on top); small dykes for separating ponds; and a water supply canal (1.7 m high, 1:1.5–2.0 slope, 1.5–3.7 m wide on top).
Water supply canal and drainage facilities that consist of a main canal and a catching pond connected to the sea or water source. The catching pond usually has an area of 100 m2, and is used for concentrating the fish either from the nursery or from the grow-out pond. Using the tidal fluctuation and the right elevation relationship, transferring fish from one pond to the other is done simply by opening the gate, and requires minimal use of a seine net. Experienced farmers who are quite adept at estimation can transfer fish without touching them.
Nursery ponds where fry are grown to fingerling size that can also be used as stock ponds. The fry required for the year are stocked here during the peak collection season to take advantage of the low price and volume that is available. The common practice is to slowly harvest fingerlings from the nursery pond with the number being estimated, or counting individually according to the area of the grow-out pond prepared. The nursery pond is normally 10% of the total area.
Grow-out ponds, sometimes called “production ponds”, take the biggest area of the farm: 85% of the total area. For easy management, grow-out ponds are better divided into 5,000 m2-1 ha areas.
Figure 14. Basic pond design
Figure 15. Design and construction of dykes, canal, water gates and ponds
Pond preparation for both the freshwater pond and brackishwater pond is the same except for the schedule of events. The common practice only differs according to the type of feeding to be used, which is determined by the season of culture. Feeding with benthic algae is the common practice, though it is only possible during the long dry season. Plankton feeding is used by some farmers and also shows good results. During the wet season, growing filamentous algae is common although some farmers are now using supplemental formulated feeds.
Since the main objective of milkfish culture in Fiji is to supply bait for tuna fishing, the culture system should be the most economical one possible, to better compete with imported bait. The growing of benthic algae or plankton feeding would be the most practical and economical way of culturing milkfish. The preparation procedure for an extensive pond culture is discussed in the following section, and will cover plankton feeding in freshwater ponds and growing benthic algae in brackishwater ponds.
Newly constructed or harvested ponds are drained and sun-dried until the bottom soil cracks, as seen in Plate 16 on Page 33. Obviously, it is essential that ponds be drainable in order for this drying process to occur. Dykes, water gates and other structures are inspected for necessary repair (Plate 17, Page 33). Leveling is done if necessary using a flat timber run horizontally on the soft pond bottom. Liming is sometimes necessary when the pond becomes acidic, and it also helps to kill pests in the pond.
Ponds that can be drained and dried until the soil is cracked are usually free from predators, so an application of agricultural lime at 1 ton/ha would be enough to sanitise the pond bottom and maintain the pH of the soil. Ponds expected to contain predators can be poisoned with derris roots, known locally as “duva” (Plate 18, Page 34). One kilogram of freshly cut duva is enough to poison 1,000 m2 of pond area with a water depth of 20 cm.
Duva is prepared by having the roots pounded in a hard plate or wooden box to extract the white juices. Pounded roots are then squeezed into a bucket with water from the pond to be poisoned, which is then emptied into the pond. (Plates 19 to 21, Pages 34 and 35). The pond is closed for 3 to 4 days, and dead fish can be removed. The pond is drained again and the gate closed.
4.4.1 Benthic algae or “lab-lab”
For growing benthic algae, dried chicken manure is applied at the rate of 2 tons/ha on the dried pond bottom as evenly as possible (Plate 22, Page 36). Chicken manure may vary in its composition depending on the source; the chicken manure discussed here is expected to come from a broiler farm and so should have better purity than that from a layer farm. The rate of application should be adjusted accordingly. The pond is then refilled with 10 to 15 cm of brackishwater. With good sunshine for 2 to 3 weeks, benthic algae or “lab-lab” grows to a thickness of about 1 to 3 mm as seen in Plate 23 (Page 36). Water is then reintroduced to the pond to a depth of about 30 to 45 cm. The pond is now ready for stocking.
4.4.2 Plankton feeding
After poisoning and draining the pond, dried chicken manure is applied in all corners of the pond at the rate of 500 kg/ha. Manure can also be put in tea sacks and tied on posts mounted at the pond bottom (Figure 16). Additional tea sacks are mounted if necessary to maintain the turbidity or color of pond water, which represents the availability of plankton as food for the fish. In some cases, inoculation is needed to start the growing of plankton. Some amount of plankton is transferred to the prepared pond as starters. Fertilisers are added if plankton growth is unsuccessful.
Figure 16. Chicken manure application in perforated sacks.
To maintain the growth of natural food, either benthic algae or plankton, enough sunlight is needed. Growing benthic algae in dry weather is perfectly good for the fish and supplies more than enough food to eat, and a little bit of rain may not affect the formation of benthic algae. But too much rain, or a sudden heavy rainfall, may cause the algal mat to disintegrate and float to the surface. Once afloat, it starts to die and decompose. The decomposition process consumes oxygen and also produces toxic substance that is harmful to cultured fish. Thus, mass mortality may occur a day or two after a heavy rain. Farmers have to be adept in opening drainage gates to remove surface rainwater during a downpour. Continuous heavy rain may totally wipe out the algal mat and supplement feeds such as rice bran. In cases of overgrazing in the pond, the farmer should prepare another pond where fish can be transferred, or fish could be harvested for sale.
The amount of pond productivity to support the cultured fish is measured by the colour, or in aquaculture terms, the “turbidity”, or a “Secchi's disk” reading (Figure 17 on Page 28). This is an instrument with a reading scale in centimeters, used to measure the density of plankton in the pond. A reading of 20 to 30 cm is sufficient to feed the number of fish in the pond, but if one reading shows that the measured parameters are above 30 cm, fertilisation is needed. A reading below 20 cm would mean the beginning of plankton die-off, which is dangerous. In a plankton die-off, the oxygen level is low during the critical early morning period, which may cause weakening or death of the fish.
Figure 17. Secchi's disk.
The normal size of fish for stocking is about 2.5 to 5 cm long, and about 1 to 2 grams in weight. The fish are usually cultured for a month in the nursery pond and are transferred by water current to the grow-out pond. In cases where the nursery operation is a separate business, fish are transported either in live tanks or in oxygenated plastic bags (Plates 25 and 26, Pages 37 and 38).
Water salinity is always checked in the pond to be stocked against the source water of the fingerlings. Nursery ponds are usually brackishwater, while grow-out ponds can be fresh, marine, or brackish water. The standard procedure is to acclimatise or condition newly arrived fish before stocking (Plate 24, Page 37). If oxygenated plastic bags are used, they are kept afloat in the pond water for a few minutes to balance water temperature. Afterwards, the plastic bags are opened and if acclimatisation is needed, the fish are released into plastic basins (Plate 27, Page 38). Water from the pond to be stocked is introduced slowly until its salinity is equal to that of the source pond. In live tanks, a portion of water in the tanks is drained and replenished with water from the pond to be stocked. This is done in intervals of a few minutes until both salinity and temperature are equal to that of the pond, then fish are slowly released into the pond. Stocking should be done during early in the mornings or late in the evenings when the air temperature is at its lowest (Plate 28, Page 39). Proper stocking increases the survival rate.
Stocking density usually depends on the quantity of available benthic algae or the density of plankton available to the fish. Experienced farmers can anticipate the number of days, at normal weather conditions, that a certain thickness of benthic algae would last for a certain number of fish. However, it is good to start from 2 fish/m2 and determine the farmer's ability to estimate the amount of food available for the fish. It is also a safe method to stock at 2 fish/m2. The amount of food for the fish can easily be adjusted later by adding an organic fertiliser like chicken manure during the culture period if necessary.
4.7.1 Monitoring water parameters
The important water parameters to be monitored are temperature, salinity, acidity, turbidity and ammonia or hydrogen sulfide content. Temperature affects the growth of the fish, as it slows metabolism and growth when it is low, and increases metabolism and growth when it is high. High temperature in brackishwater ponds is also necessary for better growth of natural foods like benthic algae (“lab-lab”), or good plankton growth in freshwater ponds.
Salinity is important in brackishwater ponds for maintaining good growth of benthic algae without affecting the milkfish, because it can survive and grow at any level of salinity. Acidity also affects both types of pond system in terms of food growth and fish survival. Acidic water can be treated with lime (CaCO3), and the rate depends on the acidity reading of the water. It is better to do the treatment during pond preparation, when soil is treated with the right amount of lime. Turbidity is the measurement of plankton suspended in a water column; a Secchi's disk (Figure 17 on Page 28) can be used to measure turbidity in centimetres. A reading of 20 to 30 cm shows good plankton growth. A reading of 5 to 10 cm would mean plankton die-off, suggesting that an immediate water exchange and perhaps fertilising the pond again are necessary to start plankton growth. A reading of 30 cm and above would mean that there is not enough plankton to support the fish, and suggests that more fertilisation or inoculation of plankton specie before fertilisation is required.
The ammonia and hydrogen sulfide content of water can be measured using simple laboratory equipment or portable meters. However, experienced farmers can more or less estimate these by smelling samples of water or soil from the pond bottom. Pond water kept for a long time practically always needs a change, and regular tidal change would be the best precaution to avoid high ammonia or hydrogen sulfide content in the pond, which may cause slow growth or even mortality. Excess feeding also causes ammonia build-up, so feeding should be carefully monitored. Proper pond preparation before stocking can also provide a good environment for the fish during the entire culture period, especially drying the pond under intense sunlight.
4.7.2 Water management
The amount of water needed and when it is needed have a big influence on management requirements, as it must be ascertained how long a pond will take to fill up to the desired level. In a freshwater pond system, the water exchange schedule and the amount of water required depends on the quality of the present water in terms of turbidity, acidity, and ammonia/hydrogen content, which can often be determined by a simple visual inspection of the pond water.
In a brackishwater pond system or the shallow-pond culture system, water management may be less demanding in terms of volume, but more demanding in terms of timing the exchange of water. Experienced fish farmers are careful when changing water not to disturb the growth of natural benthic algae while replenishing the old water. This is done every tidal cycle, or twice a month during new moon and full moon periods.
Regular sampling of the stock can be done by netting the fish with a fine “kuralon” type net to avoid bruising the fish. Their weight and length can be measured, and any changes recorded. However, experienced farmers usually look at the fish when it concentrates near the supply gate during water exchange and would be able to estimate the size and even the number of surviving fish, more or less. Milkfish can also be trained to feed on floating feeds like stale bread or floating pellets so that observation and monitoring of the fish are easily done.
The design and layout of the pond for milkfish culture are somewhat different from the other cultivable fishes. This is because of this fish's unique behaviour of swimming against the current, thereby allowing farmers to concentrate them in small areas called “catching ponds” (Figure 14 on Page 25). Before scheduled harvesting, pond water is drained as low as possible during low tide. All gates leading to the catching pond from the pond to be harvested are kept open and the incoming high tide flows slowly through the main gate which has been fitted with a screen. All the fish from the grow-out pond will slowly swim against the current until they reach the catching pond, and when they are inside the catching pond, the gate to the supply canal is closed and fish are harvested with a special seine net fitted with a bag big enough to hold the entire load. Harvested fish can be put directly in chilled or iced water for market as food fish or for tuna bait; a live transport tank should be ready.
Annual Report. Fiji Fisheries Division 1995. Ministry of Agriculture Fisheries and Forests.
Lee, C.S.; Gordon, M.S., and Watanabe, W.O. 1986. Aquaculture of Milkfish (Chanos chanos): State of the Art. The Oceanic Institute of Hawaii USA. pp. 209–240.
Lock, K. et al. 1993. Fish farming in ponds in the tropics. Book I. Foundation for the Development of Agricultural Education and Training. Wageningen, The Netherlands. pp. 20–22.
Fitzgerald, W.J. 1996. Preliminary Report on the Use of Live Milkfish for Tuna Longline Bait. SPC/Fisheries 26/Information Paper 14. Twenty-sixth Regional Technical Meeting on Fisheries. Noumea, New Caledonia.
My sincere appreciation and gratitude go to Mr. Hideyuki Tanaka, Chief Technical Adviser of FAO's South Pacific Aquaculture Development Project (Phase II), for giving me the opportunity to be involved in this program.
I acknowledge the efforts and commendable hard work rendered by the survey team from the Fisheries Division, namely Tavenisa Vereivalu, Sam Mario, Kalaveitivodo Rabu and Semesarailala Suka, whose time and perseverance greatly contributed to the fulfilment of our objectives.
I am also grateful for the assistance from the staff of the Fisheries Division in Lami, especially the Director for Fisheries, Mr. Maciu Lagibalavu, the able Mr. Samisoni Tuilaucala, the staff and workers of Nadroloulou Research Station headed by Mr. Satya Nand Lal, and all the Fisheries Officers from the stations we have visited.
I would like to thank also the chiefs of the villages we have visited for their warm welcome during our visits, and the people who have expressed interest in this endeavour.
And lastly to my officemates, Ms. Lana Evans and Ms. Premala Arulampalam, for editing this report, and Senimili Mataika and Samuelu Laloniu for their moral support.
Photographs: Set B
Plate 14. Ponds should be constructed with well-built dykes.
Plate 15. Overgrown grass should be cut and ponds should be cleaned.
Plate 16. In ponds like Viti Corps', drain and dry pond until the soil cracks.
Plate 17. Check and fix the gate with screens and slabs.
Plate 18. Derris root, “duva”, is used to poison unwanted fish and predators in ponds that could not be dried, such as Montfort Boystown's.
Plate 19. Derris roots are pounded to extract juice.
Plate 20. Squeezing derris roots in water to extract the juice.
Plate 21. Apply derris root extract at 2kg/m2, water area of 15 cm depth.
Plate 22. Apply chicken manure at 2 tons/ha evenly all over the pond bottom.
Plate 23. Good growth of benthic algae, or “lab-lab”, observed within 6 to 7 days in brackishwater, while plankton growth in freshwater ponds can be observed within 3 to 4 days, depending on sunlight intensity.
Plate 24. Fry to be stocked in the pond should be acclimatised to zero salinity for freshwater, and to the salinity of the brackishwater pond.
Plate 25. Fry are transported from the fry source to the pond using plastic bags of 10 litres capacity at 500 to 1000 pieces/bag.
Plate 26. Plastic bags are filled with oxygen for lengthy transport or during hot weather.
Plate 27. Plastic bag containing fry is emptied into a big basin to condition fish for a few minutes before release to the pond by slowly exchanging water in the basin with that of the pond.
Plate 28. Slowly releasing the fish into the pond by tilting the basin and letting the water enter where strong fish can be seen going against the current.