Effects of Hypoxia on the Behaviour, Mortality and Plasma Electrolyte Concentrations of Goldlined Seabream, Rhabdosargus sarba

The behaviour, mortality rates and plasma electrolyte concentrations of goldlined seabream Rhabdosargus sarba challenged with low dissolved oxygen (DO) conditions was studied in an experimental setup, comprising a control (7.9 ml/l DO) and two hypoxic (2 ml/l and 1 ml/l DO) treatments. Increased ventilation rates and decreased swimming activity were observed in hypoxic treatments, but some fish exhibited strenuous avoidance actions. No mortalities were observed after 3 h, 6 h, or 24 h, but 50% of males and 18% of females died in the 48 h treatment at 1 ml/l DO. The mean size of surviving fish (305± 32.1 g total weight) was significantly smaller than those that died (425 ± 33.1 g). The plasma concentrations of Na+, Cl- and Mg2+ did not vary significantly relative to treatment, exposure time, fish size and gender, or interactions among treatment, time and gender. Ca2+ concentrations increased significantly after 48 h at 1 ml/l, but this result may be artificial because of the small sample size. The results suggest that R. sarba is comparatively tolerant of the low oxygen or hypoxic conditions that often occur in the coastal waters of Oman, where seasonal upwellings and  high primary productivity have in the past caused mass mortalities of demersal fishes.