Effects of nitrogen gas supersaturation on growth and survival in juvenile Atlantic cod (Gadus morhua L.)

Gas supersaturation is suggested to be one possible explanation for the extensive mortality that is sometimes experienced during the early life stages in intensive production of cod (Gadus morhua). In the present study, cod (35 g) were exposed to nitrogen-supersaturated water with first rising levels of total dissolved gas (TDG) for two weeks until the TDG levels reached 103 and 105%, at which the treatment levels were maintained for five weeks. A third group was given vacuum-degassed water (control group, TDG < 100%). Water temperature, TDG, oxygen saturation and other water quality parameters were monitored daily in each tank. Eighty-five out of initially 330 fish in each tank were individually marked with floy tags (biomass was regularly adjusted to 21 kg m3). Before initiation of treatment and two, four and seven weeks thereafter, all individually marked fish in each tank were measured for total length (TL, cm) and weight (W, g). The weight of unmarked fish in each tank was measured by bulk weighing. And further, to study stress-related changes in the blood parameters due to supersaturation, a few unmarked fish were randomly sampled from each tank on the same days for blood analysis. Within both treatment groups some fish (1.75%) experienced exophthalmia (“pop eyes”), likely as a chronic effect of gas bubble trauma (GBT). No other effect of exposure to supersaturated water was evident with regard to mortality, growth rate, condition or any of the blood parameters. Juvenile cod are therefore suggested to be relatively tolerant to low to moderate levels of nitrogen gas supersaturated water.