Diurnal and water salinity-dependent metabolic activity of juvenile white steenbras Lithognathus lithognathus

Abstract

The euryhaline white steenbras Lithognathus lithognathus is an important estuarine-dependent fish species in the South African marine and estuarine environment. Its tolerance to water quality fluctuations, in particular its metabolic rate responses to salinity changes, has not been studied. In order to contribute to ecophysiogical data on this species, oxygen consumption by white steenbras was determined to quantify metabolic activity. White steenbras were most active during daylight hours when oxygen consumption was 28% higher than during the scotophase. Standard, routine and active metabolic rate estimates were 0.04, 0.09 and 0.12 mg O₂ g⁻¹ h⁻¹, respectively. In juveniles (body mass [W] = 19.8–85 g fish⁻¹), the metabolic rate (MR; mg O₂ g⁻¹ h⁻¹) was estimated as MR = 0.62W^−0.526. The MR was reduced in fish exposed to a fast drop in salinity within 10 min from 35 to 5, but not in fish exposed to a fast drop to 25 or a slow drop within 3 or 9 h to 25 or 5, respectively. At a salinity of 5, oxygen consumption rate was 51% lower than at salinities of 25 and 35.