Microbial maturation of intake water at different carrying capacities affects microbial control in rearing tanks for marine fish larvae

• By adding organic matter during microbial maturation the potential for regrowth of bacteria in the fish tanks can be reduced
• Maturation of the incoming water at a high microbial carrying capacity increased microbial stability in the rearing water
• Reducing the regrowth of bacteria in fish tanks increased control of the development of the microbial community of the tank

The selection pressure can be used to reduce the opportunities for proliferation of potentially harmful bacteria. Hypothetically the chance of proliferation of opportunistic bacteria in the fish tank can be minimized by microbial maturation of the incoming water at a microbial carrying capacity similar to that in the rearing tanks. In a start feeding experiment with Ballan wrasse (Labrus bergylta) we compared the bacterial environment in two flow through systems: a microbially matured system (MMS), where the water presented to the fish was matured in a biofilter, and a fed microbially matured system (F-MMS), where the biofilter was fed organic matter to increase the microbial carrying capacity. As predicted, the MMS showed a more variable and often high microbial growth potential in the tank water. The microbial community composition of the tank water was more stable, diverse and species rich in the F-MMS than in the MMS. The results are promising for controlling the microbiota of the rearing water by competent use of water treatment and selection regimes.Statement of relevanceThe experiment shows that small changes in management (organic load and maturation of water) of water treatment give significant different microbiota in fish tanks.The experiment also shows that by increasing the microbial carrying capacity in a maturation unit to the level of the tank microbial carrying capacity, the microbial community in the fish tank becomes more stable and less open for opportunistic proliferation.This work reveals promising possibilities for controlling the microbiota of the rearing water in land based aquaculture by competent use of water treatment and selection regimes.Improved control and understanding of microbial control is very relevant for the aquaculture industry.