The present study showed that inanga successfully acclimated to salinities ranging from FW to 43, proving that it is an excellent osmoregulator. At the physiological level minor changes in plasma osmolality, coupled with no significant changes in the metabolic rate, suggest that salinities within the range used here do not impose a significant stress on inanga. Ammonia excretion was lowest at 20 salinity, which together with the O:N data suggests that there may be reduced costs and higher reliance on lipids as energy substrate at these intermediate salinities. The lack of metabolic rate changes suggests that this excess energy is diverted to anabolic processes and changes in nitrogen excretion patterns suggests that it is accompanied by a protein sparing behaviour that reduces protein breakdown for energy. These strategies might be common for fish species that use estuaries as nurseries or as habitat, enhancing our understanding of these complex habitats. Therefore, besides acting as a refuge for predators and high food availability, estuaries might also have bioenergetics advantages for its inhabitants. These findings might also have some important implications for inanga aquaculture , as they suggest that rearing fish at intermediate salinities is energetically efficient and may therefore produce higher growth rates. Although the extra-branchial epithelia have significant importance for oxygen uptake and ammonia excretion, the main locus of these functions is the gill, and remains so irrespective of elevated salinity and the potential compromises in physiological function associated with this environment.