In the early period, arrangements had been made with local fishermen to collect shrimp spawners from the sea through commercial catches and incentive prices were offered to ensure that they were brought to the Centre alive.
A limited number of spawners of P. merguiensis were obtained by this arrangement, always in ones or twos and never in adequate numbers for a large tank operation. During the period from May 1975 to December 1975 a total number of only 20 spawners of P. merguiensis and one of P. monodon were obtained from the fishermen. On request the Marine Fisheries Training Project at Tegal programmed one of their training cruises off Jepara and during two nights trawling brought five of each live spawners of P. monodon and P. merguiensis; this established the availability of P. monodon spawners off Jepara.
Regular collection of spawners by the Centre's boat, a 15 ton, K.M. WINDU was commissioned in April 1976, and the trawling operation was continued in Jepara waters. The trawling operation data and the number of ovigerous Penaeus females with ovarian stage III and IV, as classified by Tuma (1967), collected during the past 26 months (from May 1976 to the early part of June 1978) are tabulated in Table 1. The fishing effort and the catch of females have increased. The availability of ovigerous females of P. merguiensis was found to be “year-round” and the better catch months appeared to be in the months of March, April, October and November. The ovigerous females of P. monodon appeared to be caught in the early and the late part of the rainy season. For the improvement of ovigerous female catch, a consultation with commercial shrimp trawlers was arranged for the modification of gears and the fishing ground of P. monodon off Jepara.
In the past, in spite of considerable time and effort paid for induced maturation and the spawning of penaeid shrimp, only a few spawnings and the production of a small number of postlarvae were achieved. However, the adoption of a new induced maturation method mainly by one eye ablation produced a total of 67 spawnings from 39 treated females of P. merguiensis with the subsequent postlarval production of 850 000 in two months during the last period under report (Vide 2.8). Over 30 g size non-ovigerous female and male shrimps also collected during the trawling were used for the maturation treatment. Three P. monodon females also spawned viable eggs and the culture is under progress. This lead could easily be a solution for the large postlarvae production by controlling the simultaneous spawning of a large number of females.
Because of its demersal nature, it is difficult to estimate the number of spawned eggs. The number of the first nauplii is regarded as the initial population in the culture. Optimal initial nauplii population density must be controlled. The observed relative fecundity of P. merguiensis and P. monodon represented by the number of early nauplius stage is shown in Table 2.
The number of the early nauplial stage was mainly based on the aliquot counts of N-1 (the first nauplial stage) and may include N-2 in some cases. The large P. merguiensis (125 g size) spawned only partially. Almost all well-matured gravid females brought in by the fishermen spawned during the following night. The high spawning rate was probably due to fast delivery (within a few hours after catching) to the hatchery without a long period of transport handling.
The ovarian egg fecundity of P. merguiensis reported by Wongwinyutrakarn (1971) ranged from 134 700 to 383 800 for specimens measuring 141 mm to 185 mm and by Tuma (1967) ranged from 61 900 to 142 000 for the specimens measuring 31–40 mm in carapace length.
With the increased availability of gravid females (Section 2.1.1) and the development of larval culture techniques, the production of postlarvae by hatchery operation has been rapidly increased since 1975 (Table 3). The production figure of 1978 is for the first six months excluding the culture in progress by the time of this report. The 1978 production target is 5 million mainly on P. merguiensis and P. monodon, the major target species. The goal could be easily achieved with availability of spawners and using the developed induced maturation method (Section 2.1.2).
During the early period under report, since only stray specimens of gravid females were available at a time, inevitably small pools (2.5 m2 bottom area, 0.9 m depth; capacity 2.3 m3) were frequently used for individual specimen spawning and larval rearing. The limited capacity of such tanks makes it difficult to successfully adopt direct fertilization methods for larval rearing due to the limited surface area. As the capacity of the tank is only for 100 000–150 000 nauplii, it is necessary to utilize only medium size individual spawners or divide the population with other pools. During the period under report, 26 larval rearings of P. merguiensis were carried out in these small pools. The average survival rate at the fifth postlarval stage (PL-5) was 19 percent and the highest survival was 46 percent. Twenty-two spawners, ranging from 32 g to 125 g produced about five million early nauplii, yielding 540 500 postlarvae (PL 1–18).
Successful production operation of small pool hatcheries is difficult and laborious compared to the production obtained. Larval culture experiments on the diet of frozen and live freshwater cladoceran, Moina, were carried out using these small pools. These pools are only useful for the rearing experiments by formula feed with the installation of the recently arrived variable speed stirrer.
The following methods were used for the larval culture of penaeid shrimps, excluding P. monodon, using large plastic pools (10.5 m2 bottom area, depth 0.9 m; 9.5 m3) due to the limited supply of spawners. The surface area is wide enough to receive adequate solar illumination for the growth of phytoplankton together with shrimp larvae.
The culture seawater from the reservoir tank must be first filtered through 100 or 120 micron mesh filter screen. The initial water level of 30 cm is necessary for further adjustment as the culture develops. The salinity used for the larval culture of P. merguiensis ranged between 24 and 33 ppt; the optimal range is between 28 and 32 ppt. The initial salinity of the culture seawater, if higher, should be reduced by adding freshwater prior to placing spawners in the pool. The diatom bloom is always better in the reduced salinity and this is an essential practice in the dry season, when salinity will generally be high.
The number of gravid females required for the large pool naturally depends on the size and ovarian condition of females. The maximum effective culture water capacity of the pool is only 7.5 m3. Estimating 40 percent survival at the postlarval fifth day (PL-5), the optimal number of early nauplii that could be reared in the pool will be under 500 000. When the initial nauplii become overcrowded in the pool, the population has to be thinned out by siphoning or by using pails into another pool preferably during the night, congregating the nauplii by strong flashlight at the selected area from where they could easily be removed to the other prepared pools. If the new pools are well prepared with about 15 cm depth of water, the larvae can be directly transferred to the new pool manually by pails at any time of the day. Thinning out of population during the later nauplial and zoeal stages is not advisable. As a rule, the spawned females should be removed from the pool by the morning following spawning, lest they start eating the laid eggs.
The glass roofing of the greenhouse hatchery tends to reduce daytime ambient temperature. The bright solar illumination on the northern half of the greenhouse produces better diatom population in the larval culture pools, while the southern side was found to be unsuitable for the larval culture due to lower culture water temperature and poor illumination by the heavy rafter structure. Bright solar illumination results in dense diatom population in the culture water causing wide fluctuations in pH and dissolved oxygen, which generally are not tolerated by the zoeae. It is hard to determine the optimal diatom population in the direct ferilization culture method because various species of diatoms and phytoplankton are growing in the culture water. The culture water has to maintain a light brown shade in order to achieve best production. Control of the excessive diatom bloom was achieved by shading the culture pool surface with bamboo screen periodically during daytime and by regulating fertilization rate in relation to the density of population. Excessive control of diatom population could likewise result in the starvation of zoeae and delay metamorphosis, especially when dense population of zoeae is present.
The initial unit of fertilization was 2 ppm of KNO3 and 0.2 ppm of K2HPO4 in the morning hours. If the diatom bloom was not sufficient, additional dosage was applied in the afternoon, or the water level was increased by 10–15 cm with additional fertilizers. The fertilization rate should be reduced in the later part of the rainy season when the seawater is enriched. In a situation of poor growth of phytoplankton or a show of clear culture water due to heavy grazing by the zoeae, emergency feeding should be immediately provided by diatoms cultured in a separate pool or 2 g of dissolved baker's yeast, or particulated soybean cured (Section 2.2.6); repeated applications in small amounts are necessary by observing the faeces tailing conditions.
Due to the low ambient temperature at night during the dry season, the culture water temperature may decrease to 24°C level in the early morning hours. This can cause a delay in the larval development and some mortality. A large plastic sheet (6 × 6 m) can be used to cover the culture tank during the night to minimize the lowering of the temperature. A temperature regulated water heater (approximately 5–10 Kw) can be utilized.
Daily estimation of the larval population in the pool is essential to determine management measures ensuring high survival. Several litre aliquot samples were taken and the number of larvae counted to estimate population. Beakers painted outside with black enamel paint at the bottom and one-third of the area of the vertical wall were introduced for easy observation of larvae, the food organism population, and for rapid counting. The feeding condition of myses and postlarvae should be examined under the microscope.
In the large pool, ordinarily, 10 aeration stones along the periphery of the pool and two to four in the centre area are installed in the preparation of the pool. About five percent of the total air capacity is supplied during the egg stage in order to maintain feeble circulation of water and to prevent the egg damage. When the eggs hatch into nauplii, the air volume promptly increases to 70 percent of the total capacity so that sufficient air can be supplied into the water. As the culture progresses with the increase of the water level, the aeration rate can be increased to 100 percent. Additional aeration points need to be provided if the larvae are overpopulated. However, the aeration rate should be reduced when the culture is on a rotifer diet to avoid rotifer mortality. Sudden addition of new water into the culture tank should be avoided during the molting period. The water level was initially started around 30 cm in depth and gradually increased to the maximum level of 75 cm parallel to the larval development. A situation needing reduction of water level in the zoeal stage of the culture should be avoided.
In the course of a culture, the brown flagellates and ciliates which can surpress larval development may start to bloom. To solve this about 20–50 percent of the culture water should be gradually changed with fresh seawater daily depending on the flagellate population. It will also be necessary to remove dead larvae and phytoplankton debris accumulated on the pool bottom by siphoning.
A few cultures were attempted on the diet of Artemia nauplii, newly hatched in several separate containers in different hours. The amount of Artemia nauplii supplied into the tank depended on the hatching rate of the Artemia eggs. About 100 g of the eggs were given four rations (25 g) daily, from the first mysis stage population level of 400 000, and the dosage was adjusted by the population and growth. Occasionally one-half dosage of the Artemia eggs was directly inserted into the culture pool to insure the availability of food, but the accumulated egg shell debris would cause problems at harvesting of the early postlarvae (PL-5). Artemia diet, started at the end of the third zoeal stage, should be (Z-3) continued to PL-4 which is one day before harvesting. The five-day-old postlarvae (PL-5) were regarded as the best harvesting stage from the culture pool or tank, because the majority of larvae commence demersal life at PL-4 and can be reared readily in the nursery pond on natural food. The method of postlarval production of P. merguiensis using the large pools as detailed above has given a very satisfactory rate of survival from the early nauplius stage to PL-5. The five-day-old postlarvae production of 28 700–46 300 per m3 of the tank capacity represents one of the highest levels of postlarval production by the direct fertilization method (Table 4).
When P. monodon larvae were cultured by the general culture method (Section 2.2.2), the survival rate of the postlarvae (PL-5) from the nauplial stage was about 12 percent. Because of the importance of this species, the culture method was developed to improve the production and the average survival rate increased to 38 percent. The maximum survival rate was 72 percent which represents the highest survival rate in the culture of P. monodon, producing 375 000 (PL-5) from the large pool on a diet of natural diatom, rotifers and Artemia nauplii.
The larvae of P. monodon were extremely sensitive to the quality of the culture water. The following practices are important points for a successful culture:
The females, unlike P. merguiensis, occasionally spawned two or three nights late after placing in the culture tank. Therefore it is necessary to exchange the culture seawater daily by siphoning without disturbing the females.
The larvae required high salinity seawater, above 30 ppt, for better development; one of the best cultures was produced in seawater of 35–36 ppt salinity.
The initial nauplii population level should not exceed 500 000 and an early population dispersion was necessary in most cases because of the large number of laid eggs.
The initial fertilization of the culture water should start at a half dosage (1 ppm KNO3 and 0.1 ppm K2HPO4) and the necessary additional dosage should be applied, at the same time gradually increasing the quantity of new seawater. Good diatom culture is required for best results.
In case of shortage of natural diatoms, a small amount of yeast of fluid-free soybean curd could be frequently applied by observing feeding conditions (Section 2.2.6). Overdose would cause a bloom of flagellates or ciliates in the culture water. Any unknown mixed culture of phytoplankton should not be introduced as food in the pool.
Separately cultured clean rotifers with green waters could be fed from the second zoeal stage (Z-2), and even from Z-1 in case of the shortage of diatoms. Feeding should be four times daily each with about one million. Even in the later stage, one abrupt large dosage of over 20 million rotifers will affect the culture water. The aeration rate must be reduced with the rotifer diet. The use of rotifers collected from the ponds should be avoided to prevent the contamination of culture water.
The debris of the dead larvae and food organisms and the faeces accumulated on the bottom should be removed by diaphragm pump at Z-2. The pumped out zoeae should be immediately returned to the pool when separated from the debris.
An increase of about 5 cm of water level at Z-3 showed better results for the metamorphosis into the mysis.
When the culture population reaches over 300 000, the dispersion of the population must be practised at M-2, by distributing the larvae manually with pails into other prepared pools which were fertilized two days prior to the division. Then the water level is increased in both pools and this replenishes part of the culture water.
A daily 50 percent water culture exchange was necessary in the early postlarval stages to prevent flagellate blooms.
A small quantity of newly hatched Artemia nauplii could be fed even from Z-2, if California cysts were used because of their smaller nauplius size. Artemia hatching should be prepared for four separate rations daily immediately after each feeding interval.
In the culture of P. monodon, the use of Artemia nauplii is very beneficial considering the availability and that of the spawners. The market price of one natural postlarva is Rp.1 8. If one regards the price of the PL-5 as Rp. 4, the total value of 375 000 (PL-5) per culture is Rp. 1 500 000 (US $ 3 640). The amount of Artemia eggs used for the production was 980 g which costs only $ 60.
Due to the availability of spawners, the large cement tanks (62 m3 capacity; 40 m2 bottom area; 1.5 m deep) were seldom used for postlarvae production. During the early period under report, a culture experiment of P. monodon was once carried out. The sudden mortality of the larvae at Z-2 due to the fouling of culture water (Section 2.2.4) led to abandonment of the culture; only 5 800 postlarvae were harvested.
With the recent availability of P. merguiensis spawners from the improved method of induced gonad maturation, a preliminary culture was carried out using four spawners. The number of produced nauplii was 575 000 and the harvested postlarvae (PL-18) was 111 000 with 19 percent survival rate after 26 days culture. The initial water level was started at 35 cm depth and gradually increased to 50 cm, depth to the PL-1 stage, and this depth was maintained until harvesting due to the occasional low capacity of the aeration system. The culture water was replaced daily by 50 percent and from the postlarval stage only one half dosage of the fertilizer was applied (Section 2.2.2). Soybean curd was fed from Z-2 (started from 40 g to 200 g per day rate until PL-5; totalling 1 760 g). The formula feed (Section 2.4.5) was given from PL-1 (started from 25 g in powder form to 200 g of granules per day until PL-17; totalling 2 925 g). The average PL-5 size was 0.5 mg (4.8 mm long) which was smaller than those intensively reared in the large pools, and the PL-14 weighed 12 mg. The survival rate from PL-1 to PL-18 was 79 percent on the formula diet.
This culture was carried out based on the method of using the large pool. The application of the large scale culture method (Yang, 1975) and the further improvement of formula feed will increase the survival rate and the production.
During the early period under report, two larval rearing trials of P. merguiensis were performed exclusively providing rotifers (Brachionus sp.) as larval food along with fertilization in the culture pool. The rotifers, which were abundant at the end of the rainy season in the Centre's experimental ponds were collected using a plankton net and brought to the hatchery alive. Feeding on rotifer diet was started on the last day of the third zoeal stage (Z-3). One culture, starting from 695 000 nauplii, resulted in the production of 33 900 first postlarvae in a 2.5 m3 pool. The second culture in a similar pool started with initial N-1 population of 446 000, and produced 103 800 mixed population of M-3, and PL-1 from the pool (Table 5). The survival rate from N-1 was 23 percent. The average daily rotifer population in the pool was 6.0 ml of the culture water and the average food dosage was 52 rotifers larvae. The relatively high mortality in the culture was probably due to initial high larval population for the small pool. Hudinaga and Kittaka (1966) first reported that P. japonicus larvae could be reared on rotifer diet, without mentioning production number and survival rate. The use of rotifers as larval diet is a general practice in postlarval production (Cook, 1976).
1 Rp. Indonesian rupiah. Rp. 412 = US $ 1.00 (June 1978)
The culture, preparation, and feeding of rotifers were discussed in other sections (viz. 2.2.3, 2.2.6 and 2.3.3). The number of rotifers applied in the early preliminary experiment (Table 5) was dependent on the availability from the ponds. Rotifer as a substitute of Artemia nauplii can be fed normally from Z-3, but it also can be fed as early as Z-1, and the rotifers were fed from Z-2 for the P. monodon larvae. The opaque foam-like ingested rotifers in the stomach of zoeae and myses are easily detected under microscope. The optimal rotifer population per day per ml of culture water with the presence of phytoplankton was calculated as follows:
| Stage | No. of rotifer per day per ml of culture water | |
| Zoea | 1 | 0.6 |
| 2 | 3.0 | |
| 3 | 3.0 | |
| Mysis | 1 | 5.0 |
| 2 | 5.0 | |
| 3 | 7.0 |
In the larval rearing of P. merguiensis on rotifer diet, the survival rate from the nauplial stage to the last mysis (M-3) was normally 80 percent. The feeding rate was reduced when the larvae were fed with the soybean curd. Strong aeration was avoided to reduce the rotifer mortality which was indicated by the formation of protein bubbles on the surface of the culture water.
Soycake which is the byproduct of soysauce manufacture was used as the larval food of P. japonicus (Hirata et al., 1975). Due to the limited availability of Artemia and other feed materials, the daily locally available soybean curd (SBC) was used exclusively as larval food of P. merguiensis, and occasionally for the P. monodon along with the cultured rotifers and phytoplankton. The SBC contains 85 percent water, 6 protein, 3.5 fat and 1.9 sugar.
The SBC particles were prepared by different mesh size screen made of plankton net materials (Section 2.2.8). The following particle size SBC was used for different stages:
| Stage | Screen mesh size (micron) |
| Zoea 1 | 35 –48 |
| Zoea 2 and 3 | 48 – 75 |
| Mysis | 75 – 105 |
| Postlarvae | 105 up |
The following approximate daily dosage appeared to be optimal for 200 000P. merguiensis larvae in the large pool:
| Stage | Daily SBC dosage (g) | |||
| with diatom bloom | without diatom | |||
| Zoea | 1 | - | 15 | |
| 2 | - | 20 | ||
| 3 | 30 | 30 | ||
| Mysis | 1 | 30 | 30 | |
| 2 | 40 | 40 | ||
| 3 | 40 | 40 | ||
| Postlarval | 1 | 60 | 60 | |
| 2 | 80 | 80 | ||
| 3 | 100 | 100 | ||
| 4 | 120 | 120 | ||
The SBC diet usually commenced at late Z-2 or at Z-3 with the fertilization method. The above daily feeding rate was adjusted depending on the feeding conditions and other available food in the culture water. The daily dosage was supplied in four rations.
It was an essential practice to eliminate the water portion of SBC (emulsion) produced during the screening process. The SBC emulsion and excessive feeding should be avoided as it can cause heavy blooms of brown flagellates and ciliates. These can be fatal to the, growth of the larvae. For the finer particles intended for the early larvae, the prepared food with water had to stand for a while to await precipitation and the particles were only fed after decanting the upper liquid portion.
When the suspended SBC particles in the culture water with diatom bloom were examined under the microscope, numerous diatoms were attached to the SBC particle surface which would have higher nutritional value. The larvae could be reared only on the diet of SBC without diatoms although the survival rate was low.
The larvae reared on the diet of SBC with or without rotifers were smaller (average weight 0.8–0.9 mg in weight) than those reared on the Artemia nauplii diet (average weight of over 1.0 mg).
The average larval survival rate from the early nauplial stage to the postlarvae (PL-5) was 14 percent on the non-Artemia nauplii diet (Table 4 and Section 2.2.3). The mortality usually occurred in the early postlarval stage.
Experiments on rearing the larvae on the diet of Moina (Section 2.3.4) were carried out in the small pools and the live cladocerans were able to stay in the larval culture seawater for about ten minutes. One batch cultured on the diet of live Moina and SBC plus fertilization showed a survival rate of 36 percent while the culture solely on a diet of frozen Moina showed rather poor survival rate of 11 percent. The cultures needed stirring and siphoning of the debris in the culture pool.
The local market price of one SBC cake (50 g size) is only Rp. 10 (US $ 2.4). As shown above, one culture needed 600 g of SBC which cost only Rp. 120 (US $ 0.29).
Another exploratory larval culture of P. merguiensis designed to reduce dependence of feeding with Artemia nauplii was tested by feeding larvae on a diet of klekap (lablab or benthic algal mass). A 2.5 m3 pool was used without the mechanical bottom stirring system, but the bottom was manually stirred several times daily to suspend the klekap particle in the culture water. The klekap was obtained from a 10 m3 pool and had diatoms (Nitzschia, Navicula, and Pleurosigma), with some unidentified Monas spp. as major constituents. Initially only 10 g of Artemia eggs were supplied at the first day of Z-3. Then about 3–6 litres of klekap collected by fine mesh dip-net were fed daily from the Z-3 stage to the third day of the postlarvae. Fresh klekap was sieved through 360 micron mesh plankton net material by washing through with seawater. The estimated PL-1 population was 16 000 with the survival rate of 36 percent from N-1.
Semi-automatic water draining U-shape siphons were made for large cement culture tanks, which can drain the water to a desired level by adjusting the height of the outlet hose (Figure 1). Several similar siphons for the pools were made with adjustable outside standpipe.
Upon completion of the Centre's new greenhouse hatchery rearing facility, the aeration system for the entire complex to house a series of plastic pool hatcheries was designed and installation supervised.
A set of different mesh plankton net sieves (35, 48, 75, 105, 180, 350, 530 and 1 000 micron) mounted on 6 inch diameter PVC pipe cutting was constructed to screen and wash soybean curd, and rotifer, and other food material. Efficient filtering of seawater, “GAF Extenda-life” filter bags (5, 50 and 100 micron) were introduced for the food organism culture, media preparation. For the culture, a water exchange system with different mesh (120 and 350 micron) filters and a controlled water supply system for the pools, were designed and constructed.
A manual diaphragm pump was introduced for removing the bottom debris in the larval culture pools during the culture to reduce mechanical damage of larvae. Several sets of flow regulated water faucets were constructed in order to carry out gradual culture water exchange. A set of experimental air-lift systems for the large pool was designed and constructed to test feeding of prepared or frozen food materials.
Bamboo screens (4 × 4 m) were made to control the excessive bloom of phytoplankton by covering the culture pools. A hatchery log book was set up, in addition to the larval rearing record. Variable speed control stirrers for the small culture pools, and automatic submersible pumps were ordered and arrived recently for larval culture experiments on formulated feed diet.
To determine the optimal postlarval shipping population in ambient temperature, an experiment was carried out by using five-day-old P. merguiensis postlarvae (1.2 mg size) packed in oxygen sealed plastic bags. The duration of the experiment was about 18 hours; from 18.00 to 12.00 hours of the following day. The starting seawater temperature was 28.5°C, and it was 28.4°C when unpacked. The salinity of seawater used was 32 ppt. The harvested postlarvae were crowded in a small plastic pool with a bottom area 0.64 m3 and depth 0.6 m with aeration, and the specified numbers for each bag were collected by aliquots (Table 6). The larvae were gently placed into a clean 25-litre plastic bag with 6 litres of clean seawater. The air was removed from the bags and replaced with about 15 litres of oxygen. The bags were securely tied and left in the laboratory overnight.
The higher mortality rate in the package No. 3 was probably over-aeration of the batch prior to packing. The shipping population overnight or under shade should be under 5 000 postlarvae (PL-5) per bag as tested above. However, further detailed experiments are required for the safe shipment of postlarvae.
Developing substitute food organisms and feed materials to replace Artemia nauplii is fundamental to the development of a dependable shrimp larval mass-culture, particularly in this region where Artemia cysts are not readily available. Experiments were initiated on the improvement of the mass-culture of food organisms and subsequent rearing of the larvae on the multiple diet for better production.
In order to use food for the larval food organisms, facilities for the uni-algal massproduction system were set up in the Centre's new food organism culture room. Initially the stock culture of four marine species was obtained from the Woods Hole Oceanographic Institution; Chlorella sp. A, Chlorella 580, Chaetoceros calcitrans and Chroomonas spp. Later 13 additional species of phytoplankton were introduced from the South East Asia Fishery Development Centre (SEAFDEC)1 Laboratories. The stock cultures were maintained in test tubes with the media sterilized by autclaving. The intermediate cultures were set up in cylindrical containers of clear glass specimen bottles (ranging 10–15 litre capacity) or home-made containers from clear plastic-bag materials. The culture containers were placed or hanged from a distance on an adjustable rack along the wall on which eight 40 watt flourescent lamps were vertically mounted. The light intensity for the cultures was adjusted to 6 000–8 000 lux and a 24 hour photoperiod was employed. The culture seawater was prepared by chlorine sterilization method at 10 ppm level, using commercial bleach solution, and later dechlorination by adding sodium thiosulfate. A seawater sterilization device was constructed by using two 40 watt germicidal flourescent lamps for the culture media preparation.
A series of experiments for the intermediate culture was made to determine inexpensive media for the Chlorella culture, by using the plastic containers. Among the tested media, the Miquel-Allen-Nelson medium was found to be effective and inexpensive; the population of Chlorella reached to over 38 million cells per ml in 10 days culture.
Large plastic pools (10 m3 capacity) were used for the large-scale Chlorella culture. The seawater used in the process was sterilized by the chlorine method (Section 2.3.1) or was filtered through 5 micron filter (GAF Extenda-life filter bag) to minimize contamination by other species. Then freshwater was added to adjust salinity approximately to 15–17 ppt keeping water depth at 40–50 cm, and strong aeration was supplied with five aeration stones. The initial fertilization was urea (600 g) and ammonium sulfate (120 g), and then 100 g of urea weekly. Usually heavy innoculum was recommended, and the population reached nearly five million cells/ml in three days from the initial population of 300 000 cells/ml. Application of 1/10 strength of Allen-Miquel-Nelson solution improved the culture when the growth was sluggish. The lower water temperature (below 26°C), adequate illumination and aeration were recommended to maintain good culture. An addition of EDTA, a chelating agent, to improve culture may be necessary due to the seasonal changes of the seawater. An experiment was performed with the soil extract to substitute EDTA but the results were not confirmed.
To obtain seed rotifers in the early dry season, experiments on hatching dormant eggs were carried out. Four pools (2.3 m3) were filled with seawater having salinity adjusted to about 12.5 ppt, and were fertilized with cowdung (100 ppm), coconut oil cake (20 ppm), and urea and superphosphate (2 ppm). In addition to the fertilizer, Na2EDTA, a chelating agent was added at 10, 20, 30 ppm respectively to the first three pools and the fourth was kept as a control. The following day, about 320 g of surface portion of the bottom mud from a pond, where rotifers had bloomed previously, were inoculated into each pool. After six days of incubation, rotifers started to appear in each tank and a few individuals in the 20 ppm EDTA treated pool-produced eggs. This rotifer stock was used in the subsequent culture experiments by using larger pools (10 m3).
1 Southeast Asian Fisheries Development Centre, Aquaculture Department, Iloilo, Philippines
The past rotifer culture method by manuring with cowdung, coconut oil cake, urea and superphosphate could only produce a maximum population of about 30 rotifers/ml. The other limitation of this method was the population succession by copepod and contamination by competitors, especially ciliates (Euplotes), due to insufficient treatment of the seawater culture.
Modified culture experiments were performed to develop a new method for high density culture and for continuous harvesting. The seawater was sterilized with the chlorine at 5 ppm level, and 20 ppm of Na2EDTA was added. The diet was on the mass-cultured Chlorella, and yeast, and molasses which were locally available. Two glass containers with about 8 litres of media were used without aeration. The population reached respectively 327 and 215 rotifers/ml in a five-day period from the initial population of 10 rotifers/ml. The culture was inoculated to 60 litre containers, and gradually expanded to the large pools. This system resulted in the elimination of the competitors.
A further experiment was performed to determine optimal baker's yeast dosage per water volume, rather than the feeding based on the rotifer biomass, using 2.5 litre capacity plastic boxes with feeble aeration. The tested dosages were 0.03, 0.04, 0.05 and 0.06 mg/ml of culture water per day. The cultures also started in 2 litres of media with Chlorella (1 million cells/ml) and 10 ppm Na2EDTA. The initial Brachionus inoculum was 8.4/ml, and the population reached over 400 rotifers/ml in each culture in four days. There were no significant differences in the dosage of the yeast. The number of egg-carrying females was checked daily. The maximum number of eggs in some females in the best culture was five. These cultures were inoculated into three small pools (380 litre capacity) and the rotifers were cultured in the 15 ppt salinity seawater treated with EDTA on a diet of yeast (0.02 mg/ml/day). The initial population of 5.8 rotifer/ml reached 89 in one week.
The intermediate cultures were expanded into larger pools (10 m3) where Chlorella was already growing (Section 2.3.2) and the aeration rate was reduced. As the Chlorella was grazed down, about 30 g of dissolved baker's yeast were supplied per pool daily as supplementary food. Approximately 20–40 million rotifers were harvested daily per pool, depending on the culture conditions, by plankton nets (35–47 micron mesh), and carefully washed with Chlorella water through a 75 micron sieve in a large pail with seawater. The small size newly hatched rotifers screened through were returned to the culture pool. Difficulties were evident when handling large-scale outdoor culture; there had been a bloom of Synchaeta rotifers introduced by the replenishment seawater. Cultures were renewed after one week of use. An extra large size pool (50 m3 capacity) is being planned to set up a large-scale Chlorella mass-culture to supply sufficient food for rotifers and to replenish the rotifer culture water after daily harvesting, for a continuous culture system.
In one experiment, about 99 million rotifers were harvested during a five-day period. The harvested rotifers were frozen in small ice cube trays and stored in a freezer compartment. Each cube contained about 10 million rotifer organisms.
To explore the possibility of using locally available rotifer food material, “Terasi”, a salted cake of Acetes shrimp, was tried at the feeding rate of 10 mg per litre of culture water. The population reached nearly 150–170 rotifers/ml in a week, but the problem was the explosion of Euplotes ciliates in the culture water due to the outdoor exposure.
In order to eliminate ciliates from the rotifer culture, an experiment was conducted by using different strengths (2, 5, 8 ppm) of chemicals; metronidazole, tetracycline and clindamycin HC1. None of the chemicals were able to eliminate the ciliates.
To evaluate Moina, a freshwater cladoceran, as a food organism, the culture of Moina was experimented on by using two pools with 1.5 m3 of freshwater. Chicken manure (100 ppm), coconut oil cake (10 ppm) and superphosphate (2 ppm) were utilized as the nutrient source in the culture. Only one strong aeration was placed in each pool without an air-stone for the circulation of the culture water. In one pool 20 ppm of Na2EDTA was added. The fertilizers were wrapped in a fine-meshed cloth bag to allow a slow dissolution. A half dosage of the fertilizer was applied every five-day period. The inoculum of Moina placed in each pool was 2.6 per litre and the population reach 3 750 per litre in a pool with Na2EDTA, and 2 725 in the other pool without EDTA after nine days of culture.
The general method of penaeid postlarval culture largely relies on the diet of chopped clam, squid, fish and/or formulated artificial feed, which requires a large amount of labour and expenses. The daily feeding rate is 100 percent of body weight, and the postlarvae are usually cultured to 0.02 g size, which requires about 20–30 days of rearing after PL-1 depending on the feed and management. Based on the previous larval rearing of P. merguiensis by feeding klekap (Section 2.2.7), this experiment was carried out to determine the food value of klekap for the postlarvae. The postlarvae (PL-5) were transferred into a large 62 m3 cement tank which had a culture of diatoms produced as emergency larval food. The tank had been fertilized three times with commercial fertilizers, ammonium sulfate (180 g) and K2HPO4 (20 g) for diatom culture for a ten-day period prior to the rearing. The water depth was maintained at around 60 cm level, with aeration at 6 points. The salinity was about 32 ppt and the temperature ranged between 27 and 30°C during the culture period. One-third of the culture water was replenished daily with new seawater, just before feeding klekap, and the tank bottom was stirred up manually twice a day. Fresh klekap was collected daily from ponds, and sieved through a mosquito screen. The prepared klekap was braodcasted along the periphery of the tank where the postlarvae generally congregate. The klekap with benthic diatoms was the first choice, and if that was not available, then the klekap of blue green algae was fed. A small amount of finely ground and washed fish flesh was fed daily in the early morning hours when the postlarvae needed supplementary food. The collected and washed chironomid larvae were fed when the larvae had grown to the 25 mg size (Section 2.7).
Initially about 55 000 (PL-5) was stocked in the tank at stocking rate of 1 370/m2, and later an additional 9 500 postlarvae (PL-20) of the same brood from another feeding experiment were added into the tank after 15 days of culture. After 37 days of culture, a total number of 47 000 (PL-42) was harvested at the survival rate of 74 percent. The growth of P. merguiensis postlarvae in the early period was noted in the tank and was almost comparable to the post larvae reared on formula diet (Section 2.4.3); and the postlarvae reached the 20 mg size, by PL-11 and 16, despite the high stocking rate (Table 7).
In order to determine optimal stocking, growth and survival rates of P. merguiensis postlarvae in milkfish nursery ponds, rearing experiments were carried out using hatchery-produced postlarvae (PL-5). Three nursery ponds were prepared, stocked at different rates and the postlarvae were reared for a period of 41 days. The results are summarized in Table 8. The ponds were drained by pumping, and fishes were exterminated with derris root extract. The primary fertilization was applied at 10 cm depth of water. A bag net (350 micron opening mesh) was installed at each sluice to prevent the admission of predators and competitors. A heavy zooplankton bloom was observed in all ponds during the early period of the culture. The major groups were Brachionus and copepods plus some Sagitta, a few polychaete larvae and fish larvae.
The dissolved oxygen level in the early morning in the three ponds was considerably low. In pond No. 1, the DO at 00.40 hours, on the day after the weekly fertilization, was never over the 3 ppm level, and on two occasions it was observed to be approximately at 1.5 ppm level. The water in pond No. 1 was clear during the early period of culture because of the presence of mats of well grown klekap. The postlarvae were able to grow under these conditions.
Seepage from the canal to the ponds, and from one pond to the other, a mass migration of population in No. 2 to the neighbouring pond affected the survival rates in spite of all normal precautions. The growth rates of the postlarvae were remarkable and the postlarvae in the two low stocking ponds reached over 100 mg size in 16 days of rearing.
Another nursery experiment was carried out during the dry season using all six nursery ponds with double the stocking rates of 60, 80 and 100 (PL-5) per m2. The pond bottom was also improved by sloping gradually to the gate for easy harvest and the dike was also lined with the plastic sheets to prevent seepage. However, the survival rates ranged from 5 to 23 percent due to the high salinity attained, which was up to 52 ppt, despite pumping of seawater into the ponds. There was also the problem of capturing reared juveniles.
The postlarvae (PL-5) of P. merguiensis were reared on formula feed with or without cobalt chloride (0.01 mg/day/larva) mixture in the water for a 15-day period. The composition of the artificial feed was:
| Dried Acetes shrimp powder | 50% |
| Rice bran | 20% |
| Ground peanut cake | 20% |
| Cassava powder | 10% |
The plastic pools of 2.5 m2 bottom area were used, and the stocking rate was 1 000/m2. The feed was prepared in dry fine powder form and was broadcasted over the water surface. The feeding schedule was once for the first two days, and twice on the third day, and three times from the fourth day onwards. The culture water depth was maintained at 60 cm without aeration. The daily water replenishment rate was one-fourth during the first 10 days, and one-third for the last five days. The rearing temperature ranged between 27.5° and 30.5°C, and the salinity was about 32 ppt.
The survival rate showed little differences, but on the whole the conversion ratio was a little better in the treated pool than in the control pool (Table 9). The postlarvae were very active in feeding when the food was braodcasted on the surface. The postlarvae reared in the treated tanks reached respectively 40 and 34 mg size in a 15-day culture. The growth was nearly the same as the postlarvae reared on the mixed diet (Section 2.4.1).
The postlarvae of P. monodon were reared in large pools (18.5 m2) to estimate the growth and survival rate under different stocking densities on formula diet for 15 days.
Six pools for rearing the postlarvae on artificial diet were provided with 5 cm depth of garden soil at the bottom, and fresh seawater was introduced to a depth of 60 cm. The primary fertilization was applied with cowdung (100 ppm), coconut oil cake (20 ppm), urea (2 ppm) and superphosphate (2 ppm). About 30 litres of water containing zooplankton from the extra large pool were introduced into each pool three days before the stocking. Two days later, the pools were stocked with P. monodon postlarvae (PL-5) at 500 (Pool 1 and 2), 750 (Pool 3 and 4), and 1 000 (Pool 5 and 6) per m2. The rearing water temperature ranged between 27.5° and 30.5°C. The salinity of the seawater used was around 32 ppt. No aeration was provided, but one-fourth of the water was replenished daily. The same formula feed as for P. merguiensis was given (Section 2.4.3).
The water of the pools with old garden soil (Pool 2 and 4) was clear during most part of the rearing period, and Chaetomorpha had grown on the bottom. Heavy scum was present on the water surface in the early part of the rearing period, and the scum was heavier in the two pools. The general DO level in all these pools was considerably low without aeration (Table 10).
The growth rate was more than doubled or nearly tripled in each 5-day period (Table 10). The postlarvae of P. monodon directly fed on the formula feed particles, but feeding was less extensive than that of P. merguiensis (Section 2.4.3). Despite the differences in the stocking rate, the survival rate was nearly the same in Pool 1 (500 shrimp/m2) and Pool 5–6 (1 000/m2). Although the postlarvae growth rate of P. monodon was lower than that of P. merguiensis (Section 2.4.3), the postlarvae reached to over 20 mg size, between the tenth and fifteenth day (PL-15 to PL-20) of rearing in the pools.
Due to the limitations on seasonal and harvesting problems in the nursery ponds, postlarvae were also reared in cement tanks (42 m2 bottom area), plastic sheet lined experimental wooden boxes (8 m2 bottom area), and extra large plastic pools (42 m2 bottom area). The rearing water was usually fertilized 3–4 days prior to the stocking, and aeration was provided for the high density culture. The culture water was changed with fresh seawater and the bottom debris was siphoned regularly.
The rearing of postlarvae on the diet of minced fresh fish or clam was not possible due to the limited seawater supply and the availability of a good quantity of clams. The diet of postlarvae was on the SBC and/or gradually switched to powdered formula feed and then to the granulated form. Due to the availability of Acetes (Sections 2.4.3 and 2.4.4), the formulation of the diet was changed with regularly available raw materials. The following new composition feed cost was Rp. 104 (US $ 0.25) per kg:
| Rice bran | 20% |
| Dried shrimp head | 20% |
| Coconut oil cake | 20% |
| Fish meal | 20% |
| SBC (dried weight) | 10% |
| Cassava powder | 10% |
An eight-set of the same condition postlarval rearing of P. merguiensis (PL-5) was carried out using wooden boxes at the stocking rate of 625/m2 and 30 percent daily water exchange. After 38 days of rearing, the survival rate ranged from 80 to 98 percent with an average of 87 percent; the formula feed conversion ratio ranged between 5.6 and 4.6 and the average was 5.1. The average body weight was 150 mg which was ideal for stocking in ponds. The growth rate was better than the nursery pond rearing at the rate of 320/m2 (Section 2.4.2).
The average survival rate of P. monodon postlarvae in the cement tanks and the extra large plastic pool was 18 percent with the maximum of 49 percent, and 42 percent in P. merguiensis with the maximum of 74 percent. The low survival rates were considered to be due to over population, and the seawater supply problems.
In order to increase the survival rate in the early postlarval stages, reared on the diet of SBC in the larval culture pools (Section 2.2.6), about 320 000 P. merguiensis (PL-1 and 2) from the larval culture pool were carefully transferred into the cement tank. The early postlarvae were reared on the diet of powdered formula feed. After 24 days of rearing, approximately 150 000 (PL-25) were harvested at a 42 percent survival rate with an average weight of 30 mg. the results were close to the postlarvae reared on the mixed diet despite the high stocking rate (Section 2.4.1). This suggested a lead to future improvement of the culture methods for postlarvae.
The tidal range at Jepara area, in Central Java, is considerably narrow. According to Schuster (1952), the mean spring tide range at Semarang was 60 cm, and 30 cm during the mean neap tide range. At the experimental ponds of the Centre, some pond bottoms become partially exposed during low tides. Adverse conditions affecting growth in shallow shrimp ponds are not only due to rapid thermal fluctuation but also to bird predation. The experimental ponds at the Centre were renovated, covering a total pond area of 54 ha (Figure 2). Ponds for shrimp farming were excavated to provide at least 0.7 m water below mean high water level. The inevitable condition of a deepening pond bottom below the water supply canal level caused difficulties in drying the pond bottom in order to produce klekap properly. The soil composition in some ponds used for the shrimp farming are sandy and porous. The problems were seepage and poor growth and fastening of the klekap. As far as practicable the water supply canals and sluices were also being arranged in such a manner as to avoid water stagnation. To use certain small ponds, the sluice filter screens have been redesigned to prevent effectively the entry of unwanted fish or other organisms with the tidal water. Height adjusting by dam boards will also enable the maintenance of the desirable water depth and permit outflow of rain water (Figure 3). For sampling and harvesting, traps were made of an iron frame with nylon netting material (mesh opening about 3 mm), and 4 m long bamboo screens were used as leaders (Figure 4).
To evaluate the growth and production rates of P. merguiensis at different stocking rates, but at the same manuring rate, experiments were conducted under controlled conditions. Two glass-roofed cement hatchery tanks (40 m2m2 bottom area; 1.5 m depth) were used at respective stocking rates of 2.5 and 5.0 shrimp per m2. Approximately 10 cm depth of garden soil was provided on the bottom. Gravel filtered seawater was supplied and the water depth was maintained at about 70 cm. The stocked juvenile shrimps averaged 0.1 g.
Initial and weekly fertilization was applied with cowdung, coconut oil cake, urea and triple superphosphate. From the second week of the culture, about 20 percent of seawater had been replenished just before the weekly application of manures in both tanks. Light green algal scum covered the water surface in both tanks.
The shrimps were sampled at the forty-fifth day after stocking for an interim checkup. The survival rate in both tanks was over 95 percent. At the lower stocking rate, growth was satisfactory (average 4.6 g), while at the tank with 5 per m2, the shrimps remained relatively smaller (average 2.2 g) and the production was also less. Net production in both tanks was quite satisfactory for the first 45-day period of the culture. The experiment was modified for the next 38 days by providing 25 percent more fertilizer input in the heavily stocked tank and also by assuring a better water change. Survival rate was again remarkable in the second phase (38 days) with over 80 percent survival in both tanks. The results indicated that no cannibalism took place in this high density population. On the eighty-third day, final net production in tank No. 1 was 620 g with average 7.9 g size while that of tank No. 2 was 510 g with average of 3.2 g. The net production gain in tank No. 2 was still lower than that of tank No. 1 despite the higher fertilization. The net production in tank No. 1 and 2 was respectively 436 and 400 g in the first phase, while it was 184 and 110 g respectively in the last phase, i.e., 70 percent of the growth was achieved in the first phase of the culture. The space might have been a limiting factor. Better yields may be obtained by thinning out the population during culture.
Along with the previously mentioned growth and production experiments, two new ponds (0.2 ha each) were used to compare P. merguiensis production in ponds with the culture under controlled condition in tanks. Pond No. 1 had a sandy bottom while the second had a mud bottom. The stocking rate was one shrimp per m2 (average 0.10 g size) in pond No. 1 and 2.7 shrimps per m2 in pond No. 2; and nearly the same amount of fertilization rate was applied.
Compared with the experiments in cement tanks, survival of shrimp in both ponds was low in ponds by 27 and 33 percent respectively. In pond No. 1 where the survival was only one shrimp per 4 m2, the maximum size attained was 20–23 g in two months culture with an average of 13.1 g. The weight composition at harvesting was as follows:
| Range of weight (g) | Composition percentage |
| 7–9 | 6 |
| 10–14 | 69 |
| 15–19 | 20 |
| 20–23 | 5 |
Although the total production was low in spite of regular fertilizer inputs, the experiment revealed the extremely rapid growth of P. merguiensis and the possibility of high production when mortality is reduced, as was the case in the cement tank experiment (Section 2.5.3). The nearly equal production in both ponds indicated that the lower stocking was favourable by producing larger size shrimps. The net production for two months in the two ponds was 34 kg and 31 kg respectively.
If four months of the year are allowed for the preparation of ponds and other interval activities between harvests and only eight months of actual culture are used, then at least four two-month crops can be raised. This will be equivalent to about 124 and 136 kg/ha/year for ponds No. 1 and 2 respectively.
The programme was originally designed to determine the growth and production rates of P. merguiensis at different stocking rates, types of bottom (soil composition and with or without the provision of peripheral furrow (1.0 m wide and 0.5 m deep) in 0.2 ha ponds, but the same manuring rate and simultaneous water change in triplicate. However, the plan was changed by stocking late postlarvae in the first three ponds (No. 1– 2 –3) and by the addition of 50 milkfish, Chanos, in each pond. The culture duration was 82–90 days.
Due to the unavailability of derris roots, tobacco dust was applied at 300–400 kg/ha rate which was not effective to eliminate all predators and competitor fishes. Considerable numbers of Tilapia, catfish, Lates, mullets, crabs and numerous small gobies were harvested. A number of small fishes and eggs may have migrated into the ponds through the sluice gate screen (mosquito screen) during the water replenishment. Therefore, it was difficult to conclude that the shrimp production figures were based on the different stocking rates. The sandy bottom ponds without peripheral furrow showed low production of both shrimp and fish with a total gross production of less than 100 kg/ha/crop for the period of about three months, whereas the pond with peripheral furrow produced over 650 kg/ha/crop. However, if this production is projected to at least a three crop year, which is highly possible, the annual gross production can reach 300 kg and 1 950 kg/ha/year respectively. The ponds with high Tilapia production showed low production of shrimp. For better results, this experiment should be repeated with complete eradication of extraneous fish.
In order to determine the production of mixed culture of penaeid shrimps, at the same fertilization and stocking rates, experiments were performed in combinations P. monodon with P. semisulcatus or P. merguiensis, which were produced in the hatchery. The six 0.5 ha series ponds used have their own canals with two sluice gates and the maximum water depth is about 52 cm at high tide which is still shallow for Penaeus culture. Efforts were made for frequent water replenishment during the high tide periods and occasional pumping of seawater through the main sluice gate. After stocking the shrimp, palm leaves were planted in the ponds at approximately 8 m apart distance to provide shadow, and clinging substrata for P. monodon. The weekly fertilization rate had been reduced due to high salinity during the dry season. The stocking and harvesting data are summarized in Table 11.
Although the average size of P. merguiensis was smaller, in 2 to 2-½ months culture, the harvesting had been started by intensive trapping. As shown in the table, the survival rate of P. merguiensis in the ponds was remarkable with a range of 85–93 percent in pond No. 3, 4, 5 and 6 (an almost similar stocking ratio of 20:80) except pond No. 2 where the stocking rate of P. merguiensis was lower (50:50 stocking ratio with P. monodon). The general production of shrimp in the ponds was much higher than that of the previous experiments in the small ponds (Table 11). The production of P. merguiensis and M. monoceros in pond No. 3 was 142.2 kg/ha excluding the unharvested P. monodon then. The projection for production per year showed that 600 kg/ha/year is attainable considering four crops or 240–270 days culture, by stocking large postlarvae (0.1 g size) and avoiding the end of dry season and the peak of the rainy season.
The high production of P. merguiensis, P. monodon, and the natural stock M. monoceros in pond No. 3 was due to the interim fish eradication after 20 days of stocking by an application of derris root based on previous experiments (Secion 2.5.8), since the fish extermination by the tobacco dusts (300 kg/ha) was not sufficient to kill all fishes. Over 900 dead fish weighing 8.4 kg were collected along the periphery of the pond. In final harvest the total weight of edible fish collected from pond No. 3 was only 28.8 kg/ha whereas the fish collected in the other ponds ranged from 116 to 345 kg.
M. monoceros harvested were natural stock which migrated into the ponds during the water replenishment through the sluice gate screens. The migrated number was greater in pond No. 1, 2 and 6 near the canal sluice gates than the inside ponds. Referring to the catch of P. merguiensis natural stock in G-1, the migration of the species in the other ponds was regarded as negligible during the water replenishment. Culture of P. monodon was continued up to over 150 days expecting further growth. The average weight of P. monodon in pond No. 3 was 12 g two weeks before the harvest of P. merguiensis.
Two rainy season P. merguiensis production experiments were performed. The growth and survival rates of the shrimp in the ponds were generally poor during the rainy season, due to the violent fluctuation in salinity resulting in low survival rates ranging from 3–11 percent. The lowest salinity which continued for some time observed during the rainy season was 2.5 ppt.
A two-month mixed culture rearing second rainy season experimental stock was carried out in two (0.5 ha) ponds with routine clearing of the fish extermination with derris roots and manuring of organic and inorganic fertilizers. One pond was stocked with P. monodon (at 12 000 per ha) and P. merguiensis (at 8 000 per ha). The survival rate of the former species was 8.9 percent and 35 percent for the latter, and the total production was 34 kg per ha which is equivalent to 130–170 kg/ha/year. The other control monoculture pond stocked with only P. merguiensis (at 20 000 per ha) produced 25.2 kg per ha per crop or equivalent to about 100–130 kg/ha/year. The major problem confronted was the floating of the benthic algae due to heavy rainfall and influx of low salinity seawater during routine pond water changes, and subsequent decomposition on the pond bottom.
Also during the same two-month period, an experiment on the effects of the pesticides applied for the extermination of Cerithidea snails, which were food competitors for shrimp, was performed using four ponds, each pond stocked with P. merguiensis (20 000 per ha) and with the routine fertilization. Three ponds heavily infested by snails were treated with tobacco dusts (300 kg/ha) and then with molluscide “Baylucide” (AI 25 percent, 2 litre/ha) but no effects were observed. Then “Brestan-60” (1 kg/ha) was applied and the snails were exterminated and the pond water was flushed twice by water changes. However, the production in the two months was low in the treated three ponds with poor growth of the benthic algae. On the contrary, the production of the control pond without snails and treatment produced 46.6 kg/ha with a survival rate of 42 percent (equivalent to 180–230 kg/ha/year).
The application of tobacco dusts (300 kg/ha rate) was effective to nexterminate polychaetes prior to the stocking of shrimp at low water level, preferably with less than 10 cm water depth. However, it did not eradicate some fishes like Tilapia. Average nicotine concentration of the tobacco dust used was to be 0.12 percent.
An experiment to determine optimal concentration of derris for interim extermination of fish without harming shrimp was carried out. The rotenone concentration in the derris roots varies according to the origin of the plant, freshness and size of the root. The fresh roots collected from Jepara area are processed into extract form. The containers used were about 40 litre capacity with 20 litres of seawater of approximately 32 ppt salinity. Five of each P. merguiensis (30 mm 0.2 g size) and Tilapia (60 mm, 3–4 g) were used for the bioassay. Shrimp and Tilapia were tested in the following concentration:
| Concentration of derris root (ppm) | Tilapia ave. size (mm)/(g) | Average time required to death (hours) |
| 2 | 58/3 | 25 |
| 5 | 62/4 | 7 |
| 10 | 57/3 | 5 |
| 15 | 59/3 | 3 |
All shrimps died in 10 and 15 ppm derris concentration in 15 hours. For safe interim extermination in ponds, 7 ppm concentration at above 15 cm of water depth was determined to be suitable considering the water replenishing time after application. The application should be performed in the early morning hours considering salinity and tides.
During the dry season, the brown colour bat-shaped epibionts were growing on the body surface of the larvae, as early as from Z-1 immediately after metamorphosing from the nauplius. The heavily infected zoeae had as many as 30 on the body, mainly on antenna and caudal area. No significant mortality was observed in the cultures. The epibiont was identified as Liomorpha sp., (a sessile diatom), by Dr E. Cox through Dr S.K. Johnson of Texas A and M University, Texas, USA.
The juveniles of P. monodon in the nursery tank were infected by peritrichous ciliates (species unidentified). A treatment was attempted with chloroquin diphosphate locally available. The culture water was lowered to 40 cm depth level and 1.1 ppm of chloroquin diphosphate was applied for two days. Some ciliates appeared to be dead after three treatments. The heavily infected shrimp were unable to molt, and new infection was started on the molted individuals despite daily water changes. This infection was not observed in P. merguiensis juveniles. Similar infection was observed on the gill surface of pond-grown species. The ectocommensal ciliates infection was caused by the deterioration or stagnancy of the culture water. It was difficult to make repeated changes of the culture water.
Chironomid larvae occur in heavy swarms in milkfish ponds during the dry season taking a very high toll of fish food. While chemical methods of controlling chironomid larvae are known, such methods often become lethal to shrimp. A biological control method, if effective should therefore be preferable in the brackishwater habitat.
Under aquarium conditions P. monodon and P. merguiensis, particularly eyestalk extirpated specimens, have been reported to feed avidly on mysid shrimp (Alikunhi et al.,1975). A laboratory experiment was therefore carried out in glass bowls (diameter 34 cm; depth 14.5 cm) with 10 litres of seawater and a known number of chironomid larvae. To determine the feeding rate of P. merguiensis on burrowed chironomid larvae, a different depth of mud with seawater was placed in each glass bowl and a small amount of klekap was added as chironomid food. Then 200 chironomid larvae were placed in each bowl and allowed to burrow. Shrimp were introduced into each bowl on the following morning. After 24 hours of feeding the shrimps were removed and the number of survived chironomids was counted by screening the mud (Table 12). The intake rate by 0.06 g size shrimp was much higher than that of the smaller size. Shrimps of this size should therefore be able to catch chironomid larvae in the pond also. Feeding of juvenile P. merguiensis in cement tanks on collected live chironomid larvae has been reported (Section 2.2.1).
From the early period, the project has been involved with the induced gonad maturation and spawning of penaeid shrimps. A good number of females of P. monodon, P. merguiensis and M. monoceros were able to attain gonad maturation by both eyestalk ablation. However, the major problems were the induction of spawning despite various shock trials on temperature, salinity, chemical stimulation and even injection of crustaecdyson.
In the last period under report, the new eyestalk ablation method developed by the Aquaculture Team of the Centre oceanologique du Pacifique (Aquacop, 1977) was introduced by the Centre through the hatchery counterpart who received training in the Southeast Asian Fisheries Development Centre, Philippines. The method involves the ablation of one eyestalk by pinching and extruding. The brood stock (39 females and 25 males) of P. merguiensis (ranging 30–50 g) were collected when trawling took place off Jepara, along with the regular ovigerous female collection during the early part of March 1978.
Among the 39 treated females, some rematured three times during a two-month period. Besides the 67 spawnings used for the larval rearing, many spent females in the brood stock tank were observed in the routine check-ups. The occurrence of the matured females appeared to be low in the first one-month period after the treatment. Two peaks of spawners were produced between the thirty-eighth and forty-eighth day after the ablation, and then the number decreased at the end of two months.
No female died immediately after ablation through the eyestalk pinching operation. Matured females placed in the culture pool or tank spawned in the same night without undue delay. The spawning was induced by reducing the illumination covering the surface of the pool with bamboo screen.
The number of natural spawners collected by trawling by the Centre's research boat off Jepara is 387 with 1 114 (thrity-minute) hauls covering 225 operation days during the last 26 months. This indicates that approximately one spawner was caught per three hauls or 0.58 operation days. The 67 spawnings produced by induced maturation from the non-gravid females obtained from the wild are equivalent to approximately 39 boat operation days, although the average nauplii produced per spawner was less than from natural spawners (average 91 000).
Anaesthesia had been attempted to ease eyestalk ablation or ligation. Locally available clove oil (Minyak Cengkeh) was used to anaesthetize the shrimps. P. merguiensis were easily anaesthetized at the rate of 0.1 ml per 1 litre of seawater and also recovered when placed in the new seawater.
In addition to the regular training of the staff of the Breeding Section of the Centre, Mr. Hamdan Habib from Sumatra, was trained in shrimp culture at the Centre for two months. Mr Soewardi from Rembang, alsomade a one-week visit to learn culture methods. Both persons are staff of the shrimp hatcheries of local governments. Mr Intizam, a student from Diponegoro University at Semarang, was trained in unialgal mass-culture for three months. The training included the preparation of media and practical mass-culture from the stock culture.
The expert also served as the technical supervisor of Mr Marc Talloen, Associate Expert, and supervised his work on the culture of Moina, Artemia, Diaphanosoma and phytoplankton.
