In intensive production systems with high water temperatures and a varying degree of recirculation, there will be profound changes in several water parameters. We have carried out a series of experiments where cod has been exposed to different levels of nitrogen, oxygen, ammonia and nitrite over long periods.
In one experiment growth in cod fry that were ex posed to ammonia levels be tween 0.0006 and 0.18 mg/l was studied. The results show a clear tendency towards reduced growth with increas ing ammonia levels, even though the cod appears to adapt to the high levels and the differences between the groups become smaller over time (see fi gure).
Supersaturation with nitrogen gas is another problem that can cause reduced growth, health and survival. Our research indicates, however, that cod fry weighing between 30- 55 gm are relatively tolerant to moderate nitrogen gas saturations up towards 105%. Cod larvae are more sensitive. We found a tendency towards increased mortality in groups that received 105 and 107% nitrogen supersaturation in relation to the control group.
In intensive farming, there can be periods with both undersaturation and supersaturation of oxygen in the production water. Our studies indicate that cod weighing between 20-40 gm have a much poorer sur vival rate, growth and health when they are kept in water with only 46% oxygen saturation compared with cod that are kept at 63 and 76% saturation. We also experienced that long-term, moderate oversaturation (120%) does not have unfavourable effects on survival and weight of cod, but that oxygen levels over 210% may result in acute mortality and signs of gas bubble disease. We also observed slightly increased mortality and a number of fi sh with gas bubbles in the group that received 150% saturation.
Our results indicate that cod are sensitive to poor water quality, and we are continuing our studies of effects of other water parameters and combinations of these.