The results of nursery operations underline the need for a well improved and manageable system for maximum efficiency, gauged in terms of quantity and quality of fingerlings produced against input and operational costs. Such a set-up should be able to provide sufficient suitable feed and tolerable conditions to enable it to accumulate fish at high stocking densities, produce healthy, vigorous stock with the least mortality, yet require normal efforts in attaining these objectives. These could only be achieved in ponds with good, stabilized soil, water tight dikes and which could easily be drained and dried, yet be able to hold a minimum of 20 cm of water when growing fish.
The nursery ponds used in the aforementioned trials fell short of these requirements. The difficulty of water management, permeable dikes and the impossibility of draining and drying the ponds, without the use of pumps, made what would have otherwise been standard and normal operating procedures a complicated process involving unorthodox methods. While stocking rates of up to 100 fry per m3 with recoveries of over 90 percent have become common in well developed and improved ponds, the maximum stocking rates in the Centre's nursery ponds could not approach this level.
Similarly, the condition of the fingerling ponds needs much to be desired, the situation being pretty well the same with the nursery ponds. Also, there will be need for much bigger areas, preferably 0.5 to 1.0 ha ponds for growing post fingerlings or for holding them if they are grown in production ponds where they are part of the stock in the stock manipulation scheme. Certain ponds may be used for this purpose and they do not necessarily have to be integrated with the nursery system. It is hoped that when the renovation and development of the experiment ponds are accomplished, all these problems will be eventually solved. Nevertheless, it still is imperative that high efficiency pumps be acquired for operation of the nursery pond units, the conflicting management requirements for the various experiments going on at the same time making them necessary. Until such equipment is acquired, it is suggested that the B, C and D series ponds be used solely for experiments on one culture species at a time. Difference in culture techniques needing different nets of conditions or treatments could adversely affect results of all the experiments.
The problem of chironomid larva infestation during the day period when maximum results could be much better attained will have to be met squarely. At the moment, there seems to be no solution other than to use chemical pesticides. Laboratory results at the Centre showing that advanced fry about 20 mm long were not affected by diazinon at a concentration of 0.08 ppm seems promising. Biological control with shrimps may seem possible but under conditions of high temperature and high salinity, results may seem doubtful. This possibility, nevertheless, should be studied. Chirnomid larva infestation of the nursery ponds, if unchecked, could seriously affect production by limiting stocking densities and competing with the fish for the food.
The trials conducted during the crop year starting July 1974 had shown that even with undrainable ponds and poor water exchange, net yield averaged 1 084 kg of fish and miscellaneous products per hectare, with milkfish constituting 885 kg. Compared to the 308.24 kg/ha/year production of the rest of Jepara's fishponds for the same period (source, Jepara Fishery Office) the difference, percentage wise, is 251.6 percent.
The increase was obtained at a seemingly high cost. A total of 1 710 kg of rice bran, 5 540 kg of cow dung (mostly wet), 780 kg of urea, ammonium sulfate and triple-superphosphate and 7 360 milkfish fingerlings per hectare, on the average, was used. The production tests, however, were run with two major objectives as targets: to find out (1) if improved techniques of milkfish culture will work under the then existing conditions and (2) the maximum results that may be attained with liberal use of inputs. Profit was a secondary consideration. Nevertheless excluding management costs and labour, the money value of fish and miscellaneous species produced, far exceeded the cost of the major inputs.
A comparison of the results obtained in the first eight months (corresponding to the first two crops of the two years, showed an improvement with net production averaging 834.4 kg/ha. Of this, 786 kg were milkfish. This is a good indication. However, ways and means of increasing production while reducing the amount of inputs or finding substitutes for the more expensive items should be sought. Even if such substitutes could not be found it is definite that once the condition of the ponds has been improved, the rate of production will be much increased.
Considering the prevailing market demand for small fish, the practice of harvesting short term multiple crops should be encouraged as a policy to increase production though this will mean an increase in fry and fingerling requirements.
Certain problems considered serious were met. They are, however, not without any possible solutions. The narrow tidal range for instance will always remain, but measures have been instituted to solve this and the problem of water exchange. The new renovation plan calls for deepening of the water canals, construction of a multi-opening (six 1-m wide doors) concrete main sluice and maintenance of a channel from this sluice to the deeper portion of the Bay. Installation of a 50 cm low lift pump at the sluice will offset difficulties of drainage and ensure available water even during neap tides. Additional portable low lift pumps as well as centrifugal pumps may have to be acquired to augment the present equipment to take care of the needs of the individual pond or series of ponds.
The lack of organic manures will in due time be solved by the utilization of factory by-products like filter press cake, molasses, etc. Copra cakes, peanut and castor bean cakes are quite expensive. Filter press cake may make a good substitute for cow dung; molasses for rice bran as a stimulant for lab-lab. Rice bran may not be as effective as it is thought to be, both as a major organic fertilizer and as artificial feed. Eventually, composts will have to be used, with the Centre doing its own composting or making arrangements with rice and sugarcane planters to do the process and sell the product to the Project.
The problem of chironomid larva infestation has already been mentioned. However, its seriousness becomes more evident in the production ponds where the real key to profit is to be found. Solution to this problem will depend upon existing policy on the use of pesticides and definite recommendations of the Pollution Expert, health hazards and gain incentives considered. Until such natural measures (which are effective and immediately available when needed) can be found, tests should be continued on chemical pesticides that are quickly degradable, less stressing to the fish and more economical to apply.
Snails (Conithidae) constitute one of the problems in brackishwater milkfish ponds of the Centre. While tri-phenyl-tin compounds have been found to be the most potent, more detailed investigations in open ponds should be conducted to ascertain any negative effects on milkfish, shrimps and other fishpond products. Meantime, other control measures for snails should be sought.
The utilization of filamentous green algae and even higher acquatic weeds like Ruppia spp. and Chara spp. for food of milkfish is not a new concept. Production of more than one ton/ ha/year in ponds where filamentous green algae are dominant has been achieved, with regular application of chemical fertilizers. Ruppia spp. and Chara spp. can be softened by drying. Later, by inducing periphyton and minute organisms to get attached on the decomposing leaves and stems, they provide adequate nutrition. The right combination of fertilizers and the correct dosage that will result in making this pond vegetation a more efficient food under conditions prevailing in the Centre's ponds should be investigated.
Since July 1975, when renovation started, almost half the number of ponds has been completed. The 6-door concrete main sluice and the other three sluices for the lateral canals have also been finished. Almost all the ponds have also been equipped with standaard wooden sluices, made of kalimantan wood and painted with coal tar. At the present rate of progress, the Centre's experiment ponds may be completely renovated by the end of 1976. Once in full operation, a wide range of experiments which would substantially increase the present knowledge on brackishwater fish culture could be undertaken. The annual fish production and income from the farm will also increase considerably.
Work, however, will not end with the completion of the renovation and development. The increase in the number of ponds has given rise to additional dikes and sluices, all of which have to be maintained and repaired as necessary.
To prolong the life span of the wooden sluices, barnacles, oysters and polychete tubes attached to the sides should be scraped regularly, at least once a month. The same should also be done with the concrete sluices, to minimize water friction and help maintain a better volume of flow.
It will also be necessary to seed the dikes with grasses, and to put back on the dikes soil materials that have eroded near its base. Canals should also be kept efficiently working to assure water adequacy. From time to time, flushing may have to be done and accumulated silt removed. Above all, the channel that connects the canal to the deeper portion of the sea should be maintained, even if it means construction of breakwater and barrier walls along the sides.
For operation and maintenance work, additional labourers may be required and should be provided, with sufficient funds allocated for the purpose. A competent mechanic who could take care of the pumps now in operation and at least two carpenters should be added to the labour force.
