Seawater (SW)-acclimated Nile tilapia, Oreochromis niloticus, can grow and survive but rarely have offspring under an elevated salinity environment. To find out the cause(s) of this problem, the size and histology of the gonads of the SWacclimated Nile tilapia were determined, which revealed that the gonadosomatic index and histology of the gonads of the SWacclimated fish did not differ from those of the freshwater (FW)-acclimated ones. However, the embryos of the SW-acclimated O. niloticus could tolerate salinity from 0 to 15 ppt, with a survival rate of embryos more than 80%, but 0% under 20-ppt. However, when the embryos were incubated under 10 ppt up to the stage of eye formation, and thereafter the salinity was abruptly raised to 20 ppt, >90% of them survived. The results suggested that SW-acclimated O. niloticus have normal oogenesis and spermatogenesis, and their embryos can survive salinity above 15 ppt if the salinity is raised stepwise.
