After oxygen consumption determination, water was drained from the respirometry chamber and replaced by filtered water (0.22 μm syringe filter, Millipore; to remove any bacterial contamination), at the corresponding salinity. After rinsing (~ 30 s) water was drained again and replaced with 40 ml of clean, filtered water and fish were left undisturbed for a 1 h incubation period. Three containers without fish were used as controls and chambers were submerged in a controlled-temperature bath at 14 ± 0.1 °C. At the beginning and at the end of the incubation duplicate 2 ml water samples were withdrawn and the ammonia concentration was quantified spectrophotometrically at 635 nm by the indophenol method