Effect of salinity on survival, growth and breeding of the shrimp

Abstract

The present study has been undertaken with the major objective of elucidating the effect of salinity on survival, growth, ovarian development, fecundity, incubation period and batch hatching of the freshwater prawn Macrobrachium idella. Mortality of juveniles of M. idella which were acclimated at 0 and 20 ppt and treanferred abruptly to varying salinities, was found to start at 27.5 and 32.5 ppt respectively, whereas in those which were gradually acclimated from 0 to 45 ppt, it was found to start only at a higher salinity level of 37.5 ppt. The 120 Hr LC50 values were 30.90 + 0.7107, 35.89 + 0.7842 and 40.74 + 0.7402 ppt for the prawns acclimated at 0 and 20 ppt and for those gradually acclimated respectively. Thus it could be seen that this prawn is a highly euryhaline one and that acclimation at a higher salinity level and gradual acclimation have got advantage in extending the upper tolerance level. Growth was found to be significantly influenced by salinity, being highest at 0 – 10 ppt, lower at 15 ppt and almost nil at 20 ppt and beyond. Sporozoan infection was noticed in the prawns reared at 15, 20 and 25 ppt. Salinity was found to have no significant influence on fecundity, while ovarian development is being influenced by it. The rate of ovarian development was high at 6 and 12 ppt and low at 0 ppt. Incubation period, which takes about 13 – 15 days was not found to be influenced by salinity, while there was some effect on batch hatching, the rate of which was more at 0 ppt and less at 6 ppt. During incubation, mortality of eggs due to infection was observed only at 0 ppt. Thus, it could be seen that a salinity level of around 6 ppt is ideal for ovarian development, maximum fecundity and synchronous hatching.