Interactive effects of ammonia and oxygen on growth and physiological status of juvenile Atlantic cod (Gadus morhua)

In order to investigate the interactive effects of sublethal ammonia concentrations and oxygen saturation on the growth and physiological status of Atlantic cod juveniles (Gadus morhua), 877 specimens (mean initial weight 20.8 g, SD = 5.6) were exposed to nine different combinations of ammonia and oxygen concentrations for 64 days. The combinations applied were: control (1–2 μg L− 1), low (31–34 μg L− 1) and high (115–120 μg L− 1) unionized ammonia-nitrogen (UIA-N) concentrations and hypoxia (57–69%), normoxia (83–88%) and mild hyperoxia (101–104%) oxygen saturations. High ammonia concentrations caused a significant decrease in growth throughout the experiment. An interactive effect of ammonia and oxygen on specific growth rates (SGR) was found in all periods, and was related to an increase and decrease in ammonia toxicity, respectively, in hypoxic and hyperoxic conditions. High ammonia concentrations caused some physiological disturbance in terms of an initial elevation of plasma K+ concentration, and a decrease in plasma Na+ and glucose concentration. Low ammonia concentrations did not cause any considerable change in weight, growth or physiological status, compared to controls. It is suggested that a mild hyperoxia may increase the tolerance of Atlantic cod juveniles to sublethal ammonia toxicity.