Fry nursing in rice-fish systems

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

Nurseries come in several varieties:

· small pond in or near the field
· small rice-fish, well-supplied with water
· nursery cage in larger pond

NURSERY PONDS

A small pond, usually 100 sq m or smaller, is most common. During the dry season, the pond is dry or dried. Lime and manure are commonly added at about 3 kg and 10 kg/sq m, 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 fertilizer may help clear this up. If water is very clear, similarly, fertilizer should be added. This fertilizing in clear water should lead to the establishment of plankton, which gives the water a brownish to greenish color preferably the former. 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 this 10 cm of the surface, the water Is too rich. Fertilization should be reduced or stopped and some new water added. if possible.

Poor plankton

Good plankton

Plankton bloom

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 sq m 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 1000 Sq m, a 10-20 sq m pond will be large enough.

Following stocking, the pond can be fertilized and fed with fine, available materials (rice bran, termites, left-over). 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. (Take note of this!)

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

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 rice field, 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, seedfish 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.

NURSING CAGES

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

REQUIREMENTS

· Access to nylon netting material and nylon string.
· Livestock concentrate and fine rice bran.

Seed should be given twice daily as a mixed dry mash of duck or pig concentrate (40% 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.

Mixing feed ingredients weekly and storing in a dry place.

Nile tilapia is best raised in monoculture.

Common carp, mrigal, grass carp and silver barb all grow well in monoculture or polyculture to reach 6-10 cm size in 6-8 weeks.

The amounts should increase and be equivalent to:

10% body weight/day - week 1 & 2
8% body weight/day - week 3 & 4
5% body weight/day - week 5, 6, 7, 8

Other feeds can be given after week 4.

Use of sardine cans as the unit for estimating feed inputs.

Termites and green fodders (cassava leaf, morning glory and Euphorbia sp)

finely chopped and fed after week 4.

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

MAKING THE HAPA

Nylon hapas can be made by hand but are usually stronger when a machine is used. Attention should be given to making the reinforced corners. Hapas of two sizes have been found suitable for small-scale farmers depending on their requirement for seedfish.

Making the hapa

The hapa 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.

After use, the hapa should be cleaned and dried before careful storage to avoid damage by rodents.

The AIT Outreach Project has developed with farmers a booklet explaining the hapa method. The AIT Outreach project is funded by the Overseas Development Administration (ODA), U.K.

Prepared by: DAVID LITTLE NICK INNES TAYLOR, DENNIS TURONGRUANG & JOHN SOLLOWS

FARMER-PROVEN INTEGRATED AGRICULTURE-AQUACULTURE
A TECHNOLOGY INFORMATION KIT (IIRR-ICLARM)