Ambient salinity modulates the response of the tilapia, Oreochromis mossambicus (Peters), to net confinement

Abstract

The stress response of the euryhaline tilapia Oreochromis mossambicus adapted to either fresh water (FW) or salt water (Instant Ocean™, 950 mosm; (SW)) was investigated to establish the influences of ambient salinity in this species. The fish were considered to be adapted to the salinities as pre-confinement plasma cortisol and glucose levels were typical for unstressed fish. Two hour net confinement increased plasma cortisol and glucose to a similar extent in both FW and SW. Individual plasma sodium and chloride levels were unaffected by confinement, although plasma Na:Cl ratio increased in FW. Confinement increased intestinal Na +/K +-ATPase activity in FW, but not in SW. In contrast, kidney Na +/K +-ATPase activity increased in SW only. Branchial Na +/K +-ATPase activity decreased with confinement in SW, but not in FW. In SW, confinement reduced the numbers of opercular chloride cells. Increased aging of the branchial chloride cell (CC) population of SW-confined fish was indicated by large numbers of apoptotic CCs in the interlamellar areas. This effect on the CC population was absent in FW-confined fish. Overall, confinement in SW-adapted fish had a more profound impact than confinement in FW-adapted fish. This is likely to have associated energetic consequences in terms of branchial oxygen and ATP consumption. Therefore, results suggest the possibility of different effects of confinement on subsequent growth in FW and SW.