The effects of rearing Mozambique tilapia in a tidally-changing salinity on osmoregulation and growth

Abstract

The native distribution of Mozambique tilapia, Oreochromis mossambicus, is characterized by estuarine areas subject to salinity variations between fresh water (FW) and seawater (SW) with tidal frequency. This dissertation characterized the effects of rearing tilapia in a tidally-changing salinity environment on osmoregulation and growth. Osmoregulation in the face of changing environmental salinity is largely mediated through the neuroendocrine system and involves the activation of ion uptake and extrusion mechanisms in osmoregulatory tissues. The immunohistochemistry results of this study revealed that tilapia reared in tidally-changing salinities maintain SW-type ionocytes in both the FW and SW phases of the tidal cycle. The study also indicated that tilapia reared in tidally-changing salinities can compensate for large changes in external salinity while maintaining PRL expression and FW ion transporters, pumps, and channels within a narrow range in both phases of the tidal cycle that are closer to the levels observed in SW-acclimated tilapia. A second study characterized the differences in osmoregulatory capacity among Mozambique tilapia reared in FW, SW, or under a tidally-changing salinity by exposing them to abrupt changes in salinity. The results suggested that Mozambique tilapia reared in a tidally-changing salinity, a condition that more closely represents their natural history, gain an adaptive advantage compared with fish reared in FW when facing a hyperosmotic challenge. Lastly, a third study focused on the effect of rearing tilapia in a tidally-changing salinity on growth and the growth hormone (GH)/ insulin-like growth factor (IGF) axis. This study indicated that rearing tilapia in a tidally-changing salinity does not cause endocrine parameters associated with growth to fluctuate between levels seen in FW or SW fish. The study also showed that tilapia reared in a tidally-changing salinity had enhanced growth rates and at some time points had significantly higher feed conversion rates than FW or SW fish.