However, these findings may also be explained by the microbial interactions in the cultivation tanks. An important aspect for the health and growth of the fish is the microbial environment in the cultivation tanks. Fast growing opportunistic bacteria populations are known to be both harmful and detrimental to the cod larvae, and preventing such blooms through controlling the microbial environment is an important operating parameter. Bacterial counts andchange in bacteria concentrations over time were compared for the cRAS and mRAS systems. Results show that the number of bacteria in the mRAS system was generally lower (up to 80%) and with more stable concentrations compared to the cRAS system (Fig. 4).This applies to both the conditions in the cultivation tank as wellas in the recycled water from the biofilters (intake water). From an operational point of view, bacterial blooms are to be expected after a change in type of feed. The response to change in feeding regimes is apparent in Fig. 4 with diet changes occurring on day 3 dph (from rotifer and algae) on day 24 dph (change to Artemia),and on day 32 dph (change to formulated dry feed diet). The resulting blooms are not as pronounced in the mRAS compared to the cRAS systems. At day 27 the bloom is in fact totally absent in mRAS while still high in the cRAS following the change to the Artemia diet. The change to dry feed diet at day 32 had the most impact on bacteria growth in both systems. This is probably a result of a higher organic loading and more bacteria substrate available in dry feed compared to live rotifers and Artemia cells. However the bloom in mRAS is significantly lower than the bloom in cRAS. Stable concentration of substrate to bacteria is desired in cultivation systems and the findings shown in Fig. 4 correspond well with observations from growth rate and number of survival as discussed above.