In estuaries salinity oscillates between nearly 100% FW and 100% SW as a result of tidal cycles.
Estuaries are key habitats for aquatic life, with their high productivity helping them serve as nurseries for a variety of organisms, including commercially valuable fish species .
However, they are also highly variable environments, with fluctuations in a number of key physicochemical variables, including salinity.
This offers significant challenge to estuarine life as changes in osmotic gradients between the environment and the animal can impair basic physiological processes and potentially cause death .
However, estuarine fish species are capable of ionic regulation ensuring that the internal osmolality is maintained at a level approximately 30% that of SW .
In FW, fish usually maintain plasma osmolality at ~ 281–310 mOsm kg− 1 (Nordlie, 2009), significantly higher than that of their surroundings (< 50 mOsm kg− 1).
In SW, plasma osmolalities of ~ 400 mOsm kg− 1 are observed (Nordlie, 2009), significantly lower than the osmolality of the environment (~ 1050 mOsm kg− 1).
Therefore life in most waters requires fish to maintain an osmotic balance, pumping ions against the concentration gradient, either into the body in FW, or out of the body in SW (Marshall and Grosell, 2005).isosmotic point.