Research Product

The sensitivity of juvenile spot, Leiostomus xanthurus, to total residual chlorine (TRC) in flowing sea-water was investigated. Incipient LC50 bioassays, histopathology, avoidance tests and the combined effect of thermal stress and TRC were used to assess sensitivity. Estimated incipient LC50 values were 0.12 mg1-1 TRC at 10 degrees C and 0.06 mg1-1 TRC at 15 degrees C. Histological examination of spot used in the incipient LC50 bioassay at 15 degrees C and sacrificed while alive indicated pseudobranch and gill damage occurred in individuals exposed to a measured TRC concentration of 1.57 mg1-1. Spot exposed to lower concentrations of TRC, 0.02-0.06 mg1-1 at 15 degrees C and sacrificed alive showed no consistent tissue damage.Spot demonstrated temperature dependent avoidance responses to TRC. At 10 degrees C, a concentration of 0.18 mg1-1 was required for significant (X2; P is less than 0.05) avoidance; at 15 and 20 degrees C, spot showed significant avoidance of TRC concentrations as low as 0.05 mg1-1. Simultaneous exposure of spot to thermal stress (5, 10 or 13 degrees C above the acclimation temperature of 15 degrees C) at measured TRC concentrations of 0.05-0.07 and 0.34-0.52 mg1-1 demonstrated a significant, (X2 with Yates correction, P is less than 0.05) increase in sensitivity to TRC with increased temperature and exposure times for some of the groups tested.