1. Fish and Prawn Production, Growth, Survival Rate and Food Conversion Rate (F.C.R)
The average of fish production (kg per ha) in L.D and H.D were similar, which presumably might be higher, since the production of B.C and S.C were lower, due to the less fertile of the pond water, compared to other studies done by Malecha et al., 1981 and Wohlfarth et al. (1985). Although there was no significant difference of prawn production (kg per ha) between L.D and H.D, which is unsatisfactory data of prawn production from the second replicate of L.D was much higher (Table 2), due to second restocking of prawn. However, there is an indication that higher in stocking density would yielding higher as well by the average production of prawn in L.D and H.D amounted to 721 and 908 respectively. At this condition of experiment, we might conclude that there was no effect of different stocking rate of prawn to the fish production. This result was also similar to other studies (Cohen et a.., 1983 and Wohlfarth et al., 1985).
The growth of C.C was the highest compared to other species, because they have a good ability to utilize pellet, additionally they also utilizing the natural food especially benthos. It seems, however, the stocking rate of C.C at this condition should have been lower, since C.C stirring up pond bottom too much causing the turbidity recorded only 6–7 cm. This situation was not favourable for plankton proliferation even ponds were manured, which was proved with the chlorophyll a mostly ranging 51–129 μg/1 during culturing period considered low. Thus, consequently plankton availability was not sufficient for the S.C and B.C. Other reason for a low production of S.C and B.C was that they cannot utilize pellet well (Boonyaratpalin, 1986, pers. comm.), although from their stomach consent were mostly consist of pellet. Presumably the G.G can utilize an aquatic plant as well as algae in case there was a shortage of pellet supply, whereas resulting the percentage weight gained (% per day) still increasing during growing period.
The survival rate of fish were considered moderate compared to other study, this might be due to no extreme water quality fluctuation recorded during culturing period which may allows mortality. It seems that the mortality of fish mainly due to seining when prawns were harvested, whereas it obviously observed that after each time partial harvest, some of fish died or got injured especially S.C and G.G which they are very sensitive to handling. Wohlfarth et al. 1985 also mentioned that the presense of fish makes selective partial harvesting of prawns very difficult with presently known techniques. The survival rate of prawn range from 49–61% except for one replicate of L.D in which the survival rate up to 95%, this mighe be due to the second restocking which based on the previous partial harvest. The number of prawn restocked amounted to 1,944 compared to other replicate of L.D was only 666, and this number probably causing high number of unmarketable prawn at the end of culturing period which comprised 7,846, almost double to other replicate amounted to 3,911.
F.C.R of fish were calculated out based on the F.C.R of prawn (2.5) from the previous study (Boonyaratpalin, 1986 pers. comm.), thus resulting the F.C.R of fish only ranging from 0.78 – 1.20. Presumably the fish mainly C.C and G.G utilizing the natural food as well.
2. Prawn Yield Characteristics
Dur to the second restocking in L.D replicate causing unsatisfactory result to be analyzed statisticaly, as shown in the result that both marketable yield and number of prawn were not significant between in L.D and H.D. However, from Table 7 and 8 shows there was an indication that in H.D yielding more (328 kg and 383 kg) compared to the yield in L.D (187 kg and 294 kg).
Percentage in weight of female was lower in L.D, while in H.D it was higher, and the percentage in number of female both in L.D and H.D were higher. The predomination of female prawn probably because of female only mate just after moulting, therefore they will not be attacted by male. In contradiction, when males are being moulting, they might be attacted by either male or female (Lin, 1986, pers. comm.). Furthermore, Peebles (1978) in Karplus (1986) mentioned that prawns are vulnerable to limb damage and cannibalism during moulting even under condition of excess food, female prawn avoid this hazards at least during premating moult by seeking refuge between the claws of blue claws prawn before and after moulting. This reason also might explains the relationship between the stocking density and the percentage in weight as well as the percentage in number of females, which from the result it shows there is a positive relationship on this matter concern.
From Fig. 6 and 7 shows that the size distribution of male are more wide range compare to female size distribution, since after getting matured, which is occurred in a relatively small size (30–40 g) they grow very slow (Lin et al., 1986).
Unmarketable sized prawn were remained in such a big amount at the last partial harvest, indicate that the time duration between the second restocking and the last harvest, which was about two months, did not allow enough time for growing out the prawn to reach marketable size. It shows that higher in stocking rate resulting more number of marketable sized prawn (Table 9). Thus, using the stocking rate of 5 tails per sq.m (H.D) gives better production.
3. Production Economics
So far the economical study on Macrobrachium culture, especially on the polyculture with fish is scarce. The studies on production economics of monoculture of Macrobrachium had been conducted in Hawaii, South Carolina (U.S.A.) and in Thailand (Shang and Fujimura, 1977; Gibson and Wang, 1977; Robert and Bauer, 1978; Pinnoy and Onchan, 1983). The results presented here cannot be casually compared to economics study because of its culture system is different.
Feed was the major item among the operating cost, which comprised of 48.2% (L.D) and 44% (H.D). This was also occurred in South Carolina and in Thailand previous study whereas amounted to 42% and 37% respectively of the total operating cost. Weber (1973) suggested that since the feed bill is likely to be the largest single cost item in the oeprating cost, improvement in feeding efficiency, such as in feed conversion ratio, or by substitution of lower cost, but nutritionally proper, components in the ration, will materially reduced the business risk. The second major item was the juveniles cost (26%), this was happen because of the prawn stocking size used in the experiment relatively big already (3–4 g), which the price was estimated to 85 baht per kg. Compared to the previous study conducted in Thailand in the same area (Nakhon Pathom Province), the juveniles cost also lead to a second major item which amounted to 22.7% of total operation cost.
Total operating cost in H.D was 34% higher than that in L.D, mainly due to the juveniles cost, which was almost three, times compared to L.D, this resulting the production cost in H.D was higher and also it caused the rate of return on operating cost was lower in H.D. Since the capital cost in both L.D and H.D were the same, while the net revenues or profit per pond was higher in L.D, consequently the rate of return on capital cost in H.D became higher. However, the rate of return on both operating cost and capital cost in this study were considered high already either in L.D or H.D (Shang, 1986, pers. comm.). Thus, from the economic point of view, the polyculture of Macrobrachium with fish assumed in this farm size is such profitable. It will be more profitable, however, in the location where the price of fish are better, for instance in Indonesia. The main reason probably why in Thailand monoculture of prawn is carried out rather than polyculture of prawn with fish, is because of the price of fish are considered low relatively.
