MFSc

Salinity tolerance of the juveniles of the prawn Metapenaeusmonoceros (Fabricius) was found out by abrupt exposure to high and low salinity levels after acclimation at 25 ppt salinity. 100% mortality was recorded both at zero and 50 ppt and no mortality between 5 and 35 ppt. The lower and higher LC50 values for 120 Hr. period were 1.03 + 0.51 and 42.67 + 0.52 ppt respectively. The juveniles with mean wet weight of 0.4728 + 0.0893 g (41.52 + 6.48 mm TL) were grown at 5, 15, 25 and 35 ppt salinities for 35 days (first experiment), to find out the effect of salinity on food intake, growth, conversion efficiency and assimilation efficiency. Food intake was found uniform throughout the test salinity levels. Growth and assimilation efficiency were comparable between 25 and 35 ppt but conversion efficiency was higher at 25 ppt salinity. Similarly, growth and conversion efficiency were higher at 25 ppt than at the lower test salinities (5 and 15 ppt), even with a significantly lower assimilation efficiency. Growth, conversion efficiency and assimilation efficiency were not significantly different between 5 and 15 ppt as well as between 5 and 35 ppt salinities. Prawns at 15 ppt showed growth and conversion efficiency comparable to that at 35 ppt though assimilation efficiency was more at 15 ppt salinity.
In the second growth experiment M.monocerosjuverniles of mean wet weight of 0.4996 + 0.0695 g (42.13 + 5.62 mm TL) and of 1.5483 + 0.1875 g (63.19 + 5.16 mm TL) designated as group I and group II respectively were grown at 20, 25, 30 and 35 ppt salinities for 30 days, to compare food intake, growth, conversion efficiency and assimilation efficiency between the two size groups. In the group I prawns, food intake, growth, conversion efficiency and assimilation efficiency were comparable among the four test salinities. This was the case with group II prawns also with respect to food intake and growth. In group Ii prawns conversion efficiency was significantly higher at 25 ppt than at 30 and 35 ppt, and significantly lower at 35 ppt than at 20 and 25 ppt salinities. Comparable conversion efficiency was obtained between 20 and 25 ppt, 20 and 30 ppt and between 30 and 35 ppt salinities. The lowest assimilation efficiency was recorded at 35 ppt while at all other test salinities, it was more or less uniform.
The comparison between group I and group Ii prawns showed that the percentage food intake and percentage weight increase were higher in group I prawns. They also showed higher conversion efficiency at 20, 30 and 35 ppt salinities, but the value for 25 ppt was comparable to that of group Ii prawns. Assimilation efficiency of the two size groups were not different among 25, 30 and 35 ppt salinities but at 20 ppt, group I prawns had lesser assimilation efficiency than group Ii prawns. The dry weight balance, in both growth experiments, showed high assimilation efficiency and comparatively low growth efficiency (K1 and K2) values in all treatments, indicating higher protein requirement in the diet for M. monoceros.