Growth and oxygen consumption in normal and O2 supersaturated water, and interactive effects of O2 saturation and ammonia on growth in spotted wolffish (Anarhichas minor Olafsen)

Abstract

The effects of dissolved oxygen (DO) concentration combined with sublethal doses of ammonia on growth and food conversion efficiency (FCE) were investigated in juvenile spotted wolffish. Fish with a mean (S.D.) initial weight of 95.4 (18.0) g were reared in shallow raceways (initial stocking density 10 kg m −2 ) at normoxic (9.6 mg l −1 ) and hyperoxic (14.5 mg l −1 ) conditions, and also under both normoxia and hyperoxia with an additional sublethal concentration of unionised ammonia (0.17 mg NH 3 l −1 ) added, for 8 weeks at 8 °C. There was an interacting effect of oxygen saturation and ammonia level on growth, as growth rate was significantly higher in the hyperoxic/NH 3 group compared to the normoxic/NH 3 group ( P <0.001), suggesting that hyperoxic conditions may increase tolerance to unionised ammonia in spotted wolffish. At the end of the experiment, no difference in mean weight was found between fish reared at normoxic and hyperoxic conditions without added ammonia, whereas mean weight was reduced under normoxia with added ammonia as compared to normoxia without added ammonia. Mean daily oxygen consumption was investigated in the non-ammonia groups at both normoxic and hyperoxic conditions, and O 2 consumption was significantly higher ( P <0.001) under hyperoxic (100.5 mg O 2 kg −1 h −1 ) compared to normoxic (79.9 mg O 2 kg −1 h −1 ) conditions. A clear diurnal rhythm in the O 2 consumption pattern was seen at both O 2 saturations, with the most striking observation being a dramatic increase in the morning when the light was turned on. Overall, our results support earlier findings of a high capacity to adapt to variations in environmental water quality parameters, and to restore normal growth rate in the spotted wolffish.