EFFECTS OF ENVIRONMENTAL SALINITY ON THE STRESS RESPONSE IN MOZAMBIQUE TILAPIA Oreachromis mossambicus

Abrupt environmental challenges, such as a change in salinity, are often associated with the induction of stress responses in fish. These responses are typically mediated by the endocrine system to rapidly enable the mobilization of energy stores that are used to cope with the stressor. Plasma glucose, therefore, is often used as an indicator of a stress response. In the case of salinity acclimation however, the stress response may vary according to acclimation history. While most studies on the stress response in fish have focused on one- way transfers between fresh water (FW) to seawater (SW) or vice versa, little is known about the stress response in species which may be continuously exposed to dynamically changing salinities. 

We have investigated the physiological responses  of Mozambique tilapia acclimated to both steady-state and dynamically changing salinity regimes. We replicated a tidal estuarine environment by rearing male tilapia in salinities that changed  between FW and SW every 6 hours. Glucose concentration was compared between fish reared in FW, SW, those transferred from FW to SW, SW to FW, or either of the salinities to a tidal regimen. The fish transferred to a tidal regimen were sampled at either the FW phase (TF) or the SW phase  (TS) of the tidal cycle. 

The transfer of fish from SW to FW induced an increase in plasma glucose by days 3 and 7. By contrast, transfer of fish from FW to SW did not elicit a change in plasma glucose. A transient increase in plasma glucose was observed in TF fish transferred from either FW or SW by 3 days. These results indicate that the transfer from SW to FW elicits the highest plasma glucose levels compared with other salinity transfers.  These findings suggest that fish exposed to dynamically changing salinities may not be as susceptible to stre ss as those transferred between steady-state salinities.  Further analyses of markers of stress, shall further inform on the stress effects in fish exposed to dynamically changing salinities. [Supported by HATCH (#HAW02051-H), NOAA/ UH-Sea Grant (#NA14OAR4170071, R/SS-12), NOAA (#NA18OAR4170347), NIH (1R21DK111775-01) and NSF (IOS-1755016)]