Chapter 5 Discussion

Most penaeid shrimps are known to be euryhaline species growing in a wide range of salinities. P. monodon exhibits hyper-osmotic regulation at low salinity levels, and exhibits hypo-osmotic regulation at high salinity levels (Cheng and Liao, 1986). However, the low survival rate at 5 ppt found in this study, which was similar to the result obtained with postlarvae by Cawthorne et al. (1983). The authors argued that the adaptation ability of P. monodon to low salinity is relatively weak.

Abnormal molting may also increase the opportunities of cannibalism due to weakness during molting, and anyway would usually be lethal for shrimps. On the other hand, as shown in our results, although low salinity enhanced molting, it did not accelerate growth rate but had adverse effects on the growth of P. monodon. In fact, similar observations have been verified in other crustaceans (Allan and Maguire, 1992; Vuayan and Diwan, 1995; Romano and Zeng, 2006). Considering the fact, the mortality was found in a few tank. In this experiment, the salinity have a significant difference on the variation of combined dependent variable. (P< 0.05) (Table. 4.2). Although, all the experimental tanks were placed in the same laboratory, but this effect might be due to different salinities that might have effect on the psychological responses such as increasing or decreasing the dissolve oxygen consumption. However, different feeds, and feed-salinity interaction were found to have no significant effect on the variation of combined dependent variables (P>0.05) (Table 3). On the other hand, test of between subject effects demonstrated that different feeds, salinities, and interaction of feeds and salinities had no significant effect on the individual dependent variable in different tanks. (P>0.05) (Table 4.3). This phenomenon was observed might be due to lower adaptability of shrimp with newly formulated feed, different level of salinities, and off course due to environmental fluctuations as well as seasonal dynamics during the experimental period.

The costs of formulated feed and labor associated with feeding are a major component of the cost of cultured shrimp production (Lawrence and Lee, 1997). It is well established that the nutrient content of the feed will influence growth, survival and the amount of metabolic and excreted waste products entering the system. (Smith, Burford, & Tabrett, 2002)

In the present study, the specific growth rate was ranged from 1.144±1.19 to -0.364±0.53. In this experiment we found a significant effect of feeds and salinities on the %SGR in S10F1, S10F2, S10F3 and S10F4 compared to other treatment groups (P<0.05). In addition, according to test of between subjects effects, different feeds and salinities had significant effect on the % SGR in different tanks (Table 4.3). However, interaction of this two factors did not have any influence on the individual dependent variable, %SGR (P<0.05) (Table 4.3). Variation of %SGR in different tanks might be due to variations in salinities which might influenced the respiratory metabolism of Penaeus monodon.(Ye et al., 2009).

No significant differences was observed in DGR as well as in %RGR values. DGR was raged from 10.760±0.08 to -0.253±0.53 and RGR was raged 378.143±26.03 to -4.553±20.68. The results of the current experiments indicated that salinity can have an immediate and significant effect on survival, and growth. A salinity range outside 20–30 ppt will increase energy channeled to respiration, excretion and exuviae. Thus, a salinity range between 20 and 30 ppt is recommended for the culture of juvenile P. monodon. This conclusion has significant implications for P. monodon aquaculture, as it can be utilized in farm site selection and salinity maintenance to maximize commercial productivity.(Ye et al., 2009).