This WP looks at interactions between the microbial communities in the system and the culture animals in recirculating aquaculture systems (RAS). RAS were chosen, because they are from an environmental (Neori et al., 2007) and biosecurity (Pruder, 2004) point of view performing better than other aquaculture production systems. In modern RAS, the metabolic wastes are constantly removed and treated. In the long run, some of the RAS treatment technologies will also have to be applied to more open production systems, considering future needs to minimize environmental impacts from aquaculture.
Important goals of the RAS-farmer are to provide the culture animals with a constant and optimal water quality year round and to minimize discharge. The in-farm water treatment units effectively reduce the waste flows leaving the farm to < 5% of the inputs, with some farms operating for 1.5 – 2.5 years under zero-discharge, this with minimum accumulation of waste on-farm (Gelfand et al., 2003, Neori et al., 2007). The wastes accumulate mainly in the form of microbial flocs, while the reactors of the water treatment units rely on controlled microbial processes. Hence, each RAS harbours large and diverse microbial communities that influence the microbial composition of the water in the fish tanks. In contrast to open aquaculture production systems, a RAS-farmer has a higher degree of control of the microbial water quality, which he can use to the benefit of the culture organisms.
Sea water from which > 95% of bacteria were removed was matured in contact with a biofilter harbouring non-opportunistic bacteria. In this matured water, Atlantic halibut yolk sac larvae showed a 76% better survival than in non-matured filtered water (Skjermo et al., 1997). Growth of turbot larvae was better in matured water, an effect which was further enhanced after addition of algae (Salvesen et al., 1999). These methods are referred to as the matured water and green-water technique, respectively. Recirculation water is a type of matured water. Although many hatcheries apply these techniques, and with fish production in RAS growing fast, the effect of water-MC on gut-MC and vise versa is still insufficiently known.
In addition, the microbial flocs accumulating in RAS are rich reservoirs of minerals, fatty acids and vitamins (Avnimelech, 2007). In more extensive outdoor aquaculture systems flocs significantly contribute to the production of culture animals (e.g. Hari et al., 2006), but the potential contribution to nutrition of flocs in RAS has been insufficiently tested.

Care must be taken in developing this technology, as although most studies report beneficial effects from flocs, toxins might be produced that might be harmful to the culture organisms (Bender and Phillips, 2004).

The specific objectives are to:

• compare the microbial communities in the water in the different RAS components of a RAS, including the fish and the feed.
• evaluate the impact of water-MC on gut-MC in RAS.
• evaluate the impact of algae on the water-MC and its cascading effects on gut-MC.
• evaluate the nutritional qualities of microbial flocs grown aerobically, anoxically or anaerobically in RAS.
• determine limits and options to integrate microbial processes in RAS while safeguarding gut-MC integrity and hence well-being of cultured organisms.

1. Identification of principal drivers of water-MC (in fish tank water) in RAS, and spatial and temporal variation of water-MC in RAS
2. Determination of the effect of different sources of water-MC on larval development
3. Identification of the principal routes by which water-MC affects gut-MC in juvenile tilapias
4. Documenting the impact of algae on water-MC and its effect on larval development
5. Standardized production of 3 types of sludge in tilapia RAS: activated sludge, sludge of an USB+ denitrifying reactor and sludge from anaerobic digestion

1. Report on the temporal and spatial variability of MC in RAS
2. Scientific publication on the effect of water-MC source on larval development of cod
3. Report on the routes through which water-MC influences gut-MC in juvenile tilapias
4. Scientific publication on the effect of algae on water-MC and its impact on larval development of cod
5. Report on nutritional composition and palatability of different types of sludge to juvenile tilapias
6. Report on the growth of tilapia on diets containing different types of processed bioflocs
7. Report on limits and options to integrate microbial processes in RAS while safeguarding gut-MC integrity of cultured organisms

• WUR
  • Marc Verdegem

• NTNU
• SINTEF-FA