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Fry nursing in rice-fish systems

by David Little, Nick Innes-Taylor, Dennis Turongruang and John Sollows

Most northeast Thai rice-fish culturist cannot control predators in their fields and finding seed fish over 7 cm in length is difficult, if not impossible Therefore, culturing small fry in a nursery where they can grow, safe from predation, to a size when most of them can escape predation, is often advisable.

Nurseries come in several varieties:

Nursery ponds

A small pond, usually 100 m2 smaller, is most common. During dry season, the pond is dry or cried. Lime and manure are commonly added at about 3 kg and 10 kg/100 m2, respectively. With the first rain, in new ponds, these rates should often be increased.

Once water begins accumulating, depth and color should be monitored. Is the water turbid? Adding more manure, straw or other fertilizers may help clear the pond. If water is very clear, similarly, fertilizer should be added. This fertilizing in preferably clear water should lead to the growth of plankton, which gives the water a brownish to greenish (preferably) color. Checking the amount of plankton in the water is easily done by observing at what depth the palm of the hand disappears. Ideally, the palm will become invisible around elbow depth. If the palm disappears in 10 cm of the surface, the water is too rich. Fertilization should be reduced or stopped and some new water added if possible.

The depth of water should preferably reach 70-80 cm prior to stocking. The farmer should feel reasonably sure that water should remain near this depth, as well. Two to three thousand 2-cm fry can be stocked in a 100 m2 pond, for culture up to fingerling size (5 cm). This is enough for a 1 ha field when fish are not fed. If the field is about 1 000 m2, a 10-20 m2 pond will be large enough.

Following stocking, the pond can be fertilized and fed with fine, available materials (rice bran, termites, leftovers). Feeding will be especially important in turbid ponds.

Early every morning, the pond should be checked to see if fish are gaping. This is a sign of insufficient oxygen and, if noticed, should be dealt with according to the situation.

Fish are usually held in the pond until the rice is well-established in the field (with 2-3 new tillers) and the fish have reached a length of around 5 cm. This usually takes about six weeks. If there is stable, standing water in the field at this time, fish can be released. If large fingerlings are stocked before rice is well-established, they will damage the rice. (See paper on stocking for rice-fish culture, this volume.)

A nursery allows the farmer to stock fish earlier, thereby prolonging the growing season for the fish and possibly allowing purchase of a wider choice of fish than later in the season. A good nursery also assures higher survival for fry than would be the case in a ricefield, when predation is uncontrollable. The farmer who is used to buying fingerlings will save money by investing in smaller fry.

A bad nursery, however, is worse than no nursery at all. If predators are present, seed fish cannot escape and mortalities will be very high. For similar reasons, pollution due to overfeeding or toxic chemicals can be dangerous. Overheating, particularly in very shallow water, can be another problem. A small patch of shade over the water may be needed. In this case, this should cover only a little of the water surface, since sunlight is needed to produce oxygen and natural feed. When an existing pond is used to hold water or fish all year round, it should not be used as a nursery. The farmer will do better to dig a small, shallower pond or cage to set a nursing cage of fine mesh in the existing pond. Fish stocked in such a cage will need daily or twice-daily feeding with good quality food.

The hapa net cage should be suspended using enough bamboo
poles and nylon string. The bottom of the net is kept down
using a rock attached to a string for easy removal.

Nylon hapas can be made by hand but are usually stronger when a machine is used. Attention should be given to making the reinforced corner.

bamboo poles for peg

nylon thread

thin nylon twine
thick nylon twine
fine nylon material

The corners of the hapa should be folded over and double sewn for extra strength. The net material is 90 cm wide.

The corners and hoops need to be strengthened by using 3x4 in nylon pieces.

Cut and fold a piece of nylon 3x4 in before sewing and using as a hoop.

Scrubbing hapa net
After use the hapa should be cleaned and dried to avoid damage by rodents.

Hoops are required to attach the hapa at the bottom and top of the bamboo pole.

Storage of hapa net

Nursery cages (hapa nets)

The Aquaculture Outreach project of the Asian Institute of Technology (AIT) has developed with farmers a nursery cage technology which is becoming popular in northeast Thailand. These fine mesh cages assure an absence of predators, make management easy and give the farmers a chance to become more familiar with their fish. Feeding, however, become, more expensive.

Seed should be given twice daily as a mixed dry mash of duck or pig concentrate (40 percent crude protein) and fine rice bran (at a ratio of 2:1 by weight). This may appear rich, but has been found appropriate in trials with farmers. Feed can be mixed for 1 week and kept in a dry place.

Fertilization of the pond or ricefield using urea and buffalo manure will improve growth and allow some reduction in quantity of concentrate given.

Change feeding rate weekly. Amount of feed should increase and be equivalent to:

Other feeds can be given after week 4.

The feeding unit should be readily available to all farmers. In this case, a sardine can was used.

Feed supplement materials

mixing container

Mix feed ingredients weekly and store in a dry place.

sardine can to measure feed input

Termites and finely chopped green fodders
(cassava leaves, morning glory and Euphorbia sp.) can be fed after week 4.

Green fodders are finely chopped.

Fish reach this size in 6-8 weeks.

Nile tilapia is best raised in monoculture,
while common carp, mrigal, grass carp and s
ilver barb all grow well in monoculture or polyculture;
they reach 6-10 cm in 6-8 weeks.

Issues for further consideration

After the hapa method was developed in northeast Thailand, it has been promoted in areas such as the Lao PDR where there is little access to concentrate feed. Instead, a whole range of supplementary feeds have been used with success.

Hapas are best used in waterbodies that already have fish in them as they help control fouling of the net that can build up on the outside of the cage in the absence of fish. The hapa can also be used for spawning tilapias and small carp, and for holding food fish before marketing them alive.

Local seed production in hatcheries is desirable to both the nursing farmer and his customer, the food fish farmer. Farmers with hapas may also nurse enough fish for their own purposes. Small hapa units ensure that entry costs and risk are low. Hapas have been used successfully to breed and nurse local and improved tilapia strains in northwest Bangladesh and the Lao PDR. In the former, the technology, when promoted for use in backyard ponds, has allowed women to be much more involved than in other forms of aquaculture.

The economics need to be considered, based on the local situation, aside from the risks and constraints.

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