By Ingunn Stubhaug and Paulo Mira Fernandes
Feed is the primary external input to recirculating aquaculture systems. It is therefore essential to design the feed correctly, as in RAS, we are not only feeding the fish, but also feeding the system.
By Ingunn Stubhaug and Paulo Mira Fernandes
Although the fundamental nutritional requirements don’t change in RAS compared to flow-through systems, the growth performance can be affected by water quality if the feed is not optimised for the systems.
RAS feeds need to be high performing in terms of physical and nutritional quality to support fast growing and healthy fish, while maintaining good water quality in the systems. Good water quality will also reduce downstream waste treatment and handling.
In order to promote fish and system health, RAS feeds should meet several criteria. They should be optimised, balanced with essential nutrients and be highly digestible, be of excellent physical quality, and improve faeces quality for easy removal from the system. This article will mainly focus on faecal quality derived from dedicated RAS diets.
In any RAS, the first step of waste removal should be the separation of the faecal solids and uneaten feed from the rearing water. This is easily achieved with high efficiency by mechanical filtration and sedimentation. However, both are dependent on faecal structure, integrity and size. Sedimentation is also dependent on the buoyancy and settling properties of the faeces. To ensure proper maintenance of solids removal efficiency, the faeces must be as robust and as large as possible.
If the faeces are not effectively removed from the system, because they are too small or disintegrate too easily, it can have a snowball effect on the rest of the system and on the fish (Table 1). Residual organic solids can greatly impact biofiltration, degassing and aeration, efficiency of microparticle treatment (such as protein skimmers and contact filters) and microbial control. An excess of nutrients in the water will represent an additional load to the biofilter and can accumulate in and mechanically damage the gills, resulting in poor oxygen uptake by the fish. This can, in turn, lead to a fish that is less robust and more susceptible to disease.
Happy system, happy fish
The key to faeces management is to develop diets that promote good quality faces for removal and good growth. One way to achieve this is by utilising indigestible ingredients that bind the faecal matter. The use of faecal binders is not new in the nutritional world, and ensuring low dosage in the formulation of diets is key to their effectiveness.
Binding of faeces must occur in a way that nutrient digestibility and uptake is not compromised, while also ensuring that the indigestible fraction of the diet leaves the fish in good integrity. This way, leaching of nutrients from the faeces in the system is minimised, while ensuring the faeces are taken out of the system in a quick and gentle way. As water quality is enhanced, fish grow better and healthier, and the production capacity of the system may be increased.
Taking care of the effluent
Another consequence of being able to remove faecal components more easily is that the effluent treatment is simplified. In most of the places where RAS are located, strict regulations about release of nutrients (N, P, organic matter) into the environment have been implemented. The task of treating the solids is easier with good faecal physical quality. Good faecal physical quality will also assist in harvesting nutrients for further valorisation as manure, energy, etc. For instance, more nitrogen and more phosphorus will be faeces-bound when adding faecal binders, thus improving the potential recovery of these nutrients for other applications (Table 2). Likewise, less of these nutrients will be leached into the system, which will have a retroactive effect in fish and system performance.
Faeces that are easier to handle will greatly reduce costs associated with effluent treatment. When such faeces are produced, the waste stream flow is reduced, and the waste treatment reactors and processes will become more efficient.
Skretting introduced dedicated RAS diets into the market back in 2009 – an industry first. Since then, we have introduced a specialised concept dedicated to RAS, RecircReady, which takes into account growth and waste predictions, nutrient recycling solutions and health monitoring in addition to tailored feed solutions using patented faecal binders. Skretting has been investing research resources into RAS over many years, with three dedicated RAS-based research facilities across the world, each with a unique focus to support the growth of the land-based industry.
Dr. Ingunn Stubhaug and Dr. Paulo Mira Fernandes are researchers at the Skretting Aquaculture Research Centre (ARC). Ingunn specialises in fish nutrition, and Paulo specialises in RAS.