RAS and microbial maturation as tools for K-selection of microbial communities improve survival in cod larvae

• K-selection of bacteria increases microbial stability of the water going in to fish tanks.
• K-selection of bacteria increases the survival of marine larvae.
• Recirculation increases the microbial stability of the water in fish tanks.
• K-selection of bacteria is most efficient at a stable system carrying capacity.

A high share of opportunistic and fast-growing microbes is generally negative for the performance of marine fish larvae. It is hypothesised that larvae that are reared in water dominated by K-strategists (mature microbial communities) will perform better, because they are less likely to encounter opportunistic (r-selected) microbes and develop detrimental host–microbe interactions. K-selection can be set up by securing competition for resources by maintaining a low nutrient supply per microbe in a maturing biofilter in a flow through system or in a recirculation system without disinfection. In an experiment with Atlantic cod larvae (Gadus morhua L.) the development of the microbial community in the inflowing water to tanks as well as in the tank water was studied in a conventional flow through system (FTS) and two hypothesised K-selective systems: a microbially matured flow through system (MMS) and a recirculation aquaculture system (RAS). As predicted, the K-selective treatments resulted in a significantly more mature and stable microbial community composition in the water going in to the fish tanks compared to the FTS. The larvae in the K-selective treatments showed 72% higher survival than the larvae in the FTS. In K-selective systems where the inflowing water to the fish tanks is matured at a microbial carrying capacity that is significantly lower than the carrying capacity in the rearing tank (i.e. MMS) there is hypothetically an opening for proliferation of opportunists to match the higher carrying capacity in the tank water. As predicted, the RAS showed K-selection of the incoming water at a carrying capacity similar to that of the tank water, which resulted in a significantly more stable microbial community composition in the tank water of the RAS than in the MMS and FTS. The results of this experiment showed a high potential for increasing fish survival by using K-selection of bacteria, which is a cheap and easy method that can be used in all kinds of new or existing aquaculture systems.