Seawater acclimation of tilapia, Oreochromis spilurus spilurus Günter, fry and fingerlings

The effect of size and acclimation period to full-strength sea water was investigated using the mortality, growth, chloride cell proliferation and plasma Na concentration in four size groups (0.03-1.6 g mean initial weight) of mixed-parentage, sex-reversed, all-male fry and fingerlings of Oreochromis spilurus spilurus Günter obtained from University of Stirling. The fish were transferred from fresh water to sea water (36.6%₀) through a continuous salinity increase during a 48, 72 or 120 h acclimation period. Survival was high in all groups, although the smallest fish (0.03 g) showed significantly higher mortality than the larger fish. The acclimation regime had no effect on growth rate, but 120 h acclimation gave highest final weight in the newly released group. Regardless of acclimation period, size and plasma Na concentration during the acclimation period, the plasma Na level stabilized within 12 h after full salinity in the two largest size groups. Ovoid voluminous chloride cells proliferated in all groups during the acclimation period, and numbers stabilized with stable salinity. It is concluded in this study that O. spilurus spilurus can successfully be acclimated to sea water with a gradual continuous acclimation period of 48 h, even as newly released fry (0.03 g). However, an acclimation period of 120 h gave the highest final weight for this size group.