The effect of elevated salinity on 'California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) metabolism

California Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) are extremely saline tolerant and have been previously shown to reduce whole-animal oxygen consumption rate (MO2) upon exposures to salinities greater than that of seawater (SW). In this study tilapia were acclimated to 15, 30, 45, 60 and 75 g/L salinity for 1, 5, 14, or 28 days. There was little change in plasma osmolality or muscle water content in salinities below 60 g/L, and branchial Na+, K+-ATPase (NKA) activity was low in 15 and 30 g/L relative to 60 and 75 g/L. When tilapia were exposed to 75 g/L, plasma osmolality and NKA activity were significantly increased within 5 days of exposure relative to those in 15 and 30 g/L, and remained elevated over the entire 28 days acclimation, indicating that short term salinity challenges (i.e., 5 days) are predictive of longer exposure durations in this species. MO2 following transfer to 15 and 30 g/L was elevated, reflecting the high energy demand required for switching from a hyper- to a hypo-osmoregulatory strategy. The MO2 of 60 g/L-exposed fish was significantly reduced at 1, 5, and 14 days, relative to 30 g/L-exposed fish; however by 28 days there were no significant differences. We investigated the potential for a metabolic basis for the salinity-induced MO2 reduction, using forward stepwise linear regression to correlate enzyme activities of brain, liver, and kidney with MO2. Brain NKA was correlated with MO2 after 5 days (p < 0.01, r2 = 0.944) and both brain NKA and hepatic total ATPase were correlated with the reduced MO2 at 14 days (p = 0.027, r2 = 0.980 and p = 0.025, r2 = 0.780, respectively). These results may indicate a tissue-level metabolic suppression, which has not been previously described as a response to hypersaline exposure in fishes.