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

Gas supersaturation is one suggested cause of the extensive mortality that is often found during intensive production of Atlantic cod larvae (Gadus morhua L.). In the present study, cod larvae (32 to 64 days post-hatch (dph)) were exposed to nitrogen supersaturated water with levels of total dissolved gas (TDG) of 103 and 106%. A control group was given vacuum-degassed water (TDG < 100%). Larvae were sampled weekly and standard length and dry weight were measured. Larval mortality was estimated regularly and at the termination of the experiment, the numbers of survivors were counted. As regards larval mortality, no effect of exposure to supersaturated water was evident. Towards the end of the experiment some larvae were found floating on the surface most likely as an effect of gas supersaturation. Growth rate was not affected during the first three weeks of treatment. By the time the larvae reached metamorphosis, a chronic effect of gas bubble trauma (GBT) was expressed as a reduction in growth. Even though not significant, the effect seemed to appear even at the lowest saturation level used (103%). Thus, even a low level of gas supersaturation should be avoided in intensive culture of cod larvae.