A common challenge in landbased systems is the loss of fish due to unfavourable conditions and disease outbreaks linked to opportunistic bacteria. A popular approach to prevent this is to attempt to reduce the load of bacteria in the systems by disinfection. This is however, not possible or sufficient in the majority of systems, because disinfection has a non-lasting effect on the numbers and a destabilising effect on the composition of bacteria. In most systems, the water exchange rates and organic loading allow for microbial regrowth in the rearing tanks. This project pursued the concept of establishing and maintaining stable microbial systems.
We developed methods for characterizing the fraction of r-strategist in a community. The most promising method was published in a scientific paper. We developed an experimental small-scale system for hypothesis testing. We carried out an experiment to investigate the effect of available biofilm surface area and water exchange rate on the development of the microbial environment in seawater and fresh water at 12 and 20°C. Generally, we found a higher share of r-strategists in the water coming out of the reactors without biofilm carriers. We quantified the release/migration of bacteria cells per time unit to the water phase from biofilm grown at low and high nutrient loading. Biofilm that had been formed at high nutrient loading contributed more bacteria cells per hour to the water phase than biofilm formed at lower loading, whereas temperature did not seem to influence the amount of cells released much. We also carried out an experiment to document the development of the community composition of the microbiota in the biofilm and water phase from start with new, clean biofilm carriers. Available surface for biofilm, water exchange rate of tanks and the water exchange rate of the total system were factors that affected the number of bacteria in the tank. We carried out experiments to document the numbers and composition of the microbiota after several weeks of operation with different systems designs (flow through, reuse of water, recirculation (including biofilter) and recirculation with UV-disinfection in the treatment loop) and with different water exchange rates in the tanks (0.5, 2 and 8 hours hydraulic retention time). For low water exchange rates, the number of bacteria was similar in the fish tanks in the recirculating systems and the flow through system, but for high water exchange rates in the fish tanks the number of free-living bacteria was reduced to the level of the intake water in the flow through system. Whereas UV-treatment did not significantly affect the number of bacteria in the fish tanks in the recirculating system, having a biofilter reduced the number of bacteria in the fish tanks compared to reuse of water. With reuse of water, the number of bacteria in tanks increased compared to the flow through system. For the recirculating systems and the flow through system, increased water exchange rate of tanks gave an increased share of opportunistic bacteria, whereas for the reuse system a low water exchange rate gave a higher fraction of r-strategists in the water of the tank. Flowthrough and recirculation resulted in different microbiota in tanks. UV-treatment in the recirculating system made the microbiota resemble that of the flow through system more, whereas reuse made the microbiota resemble that of the recirculating system without disinfection. We developed a mathematical model describing the competition between r- and K-strategist bacteria in a larval rearing tank depending on system design, flow, disinfection and feeding/removal of organic matter. In the model competition is determined by the bacterial carrying capacity (CC, the number of bacteria) and the level (percent of CC occupied) above which K-selection is favoured. The model have given insights into the effects of system design and water treatment on the resulting composition of the microbial community (r- or K-dominated) over time (hours to weeks) in the culture tanks. The model shows that a gradual increase in CC (increased feeding) gives a faster dominance of K-strategists than fast changes. The model also shows that disinfection of the incoming water to the tanks do not have a large impact on the result and the competition between the K- and r-strategic bacteria. On the other hand, the water exchange rate in the tank and the number of bacteria entering the tank with the incoming water have a great impact on this competition and the resulting dominance.
The paradigm of this project is that a stable, elevated microbial abundance in the water phase of land based aquaculture systems can be beneficial for fish health and economically profitable. A common challenge in land based systems, and shown across species, is the loss of fish due to unfavourable conditions and disease outbreaks that may be linked to opportunistic bacteria. A popular approach to prevent this is to attempt to reduce the load of bacteria in the systems by the use of UV or ozone disinfection. This is however not possible or sufficient in the majority of systems, because disinfection has a non?lasting effect on the numbers and a destabilising effect on the composition of bacteria. In most systems, the water exchange rates and organic loading applied for biological reasons allow for microbial regrowth in the rearing tanks. Hence, alternative approaches to reduce the chances of disease outbreaks are needed. This project pursues the concept of establishing and maintaining stable microbial systems. Water treatment technology for promoting K-selection, which is a selective pressure disfavouring the r-selected opportunists, has shown very promising results for several marine species in small scale experiments, but the up?scaling and optimization for flow through systems (FTS) and recirculating aquaculture systems (RAS) remains. The paradigm favouring a stable and elevated bacterial abundance is foreseen to reduce fish mortality and also reduce water treatment costs. This project will investigated fish health and microbial carrying capacity correlations as well as identifying treatment requirements to achieve a certain microbial stability.