The fish farming industry is becoming increasingly more intensive and the production of fish now occurs with the use of less water per fish produced. When the fish use the water, there is uptake of oxygen while the carbon dioxide and metabolites (waste products from metabolism) are excreted. This results in the water's oxygen level falling while levels of metabolites rise. When the fish converts proteins, it excretes ammonia, which is toxic for the fish.
Fish are in many ways closer to the surroundings in which they live as they have the same medium on the inside and outside of their body, separated only by biological membranes. As such, they are extremely exposed to influence from the environment in which they live. Consequently, the water quality can deteriorate a limited amount before a problem arises. When access to fresh water is limited, the alternative becomes regenerating the used water to a quality that ensures the fish have continued wellbeing and growth. This is often referred to as recirculation of the water, while purpose-built systems for use in aquaculture are known as recirculating aquaculture systems (RAS).
Recirculation provides effective utilisation of water
In brief, recirculating water involves using the water several times and restoring the quality of the water via treatment in several steps each time the water is used. In this way, only extremely limited amounts of the water are exchanged each time the water is reused. This is mainly to transport particles and waste from the fish tanks.
The water treatment covers the removal of particles, nitrogenous metabolites and carbon dioxide, as well as adding new oxygen. The particles are removed in a settling basin or through mechanical filtration, carbon dioxide is aired while the metabolites are normally removed by biological filtration. Numerous technological solutions are available for each of these processes. A well constructed RAS is characterised by good combinations of technology and production protocols for the different processes in the system.
Nofima's goal is through research to be able to contribute to knowledge-based use of recirculation in Norwegian aquaculture. Nofima is establishing a research facility at Sunndalsøra dedicated to recirculation of aquaculture. In addition, we have equipment designed to be used at commercial fish farms, e.g. that changes in the water quality may be documented over time.
Correct design and operation of RAS provide stable conditions for the fish
When central production parameters such as water quality and temperature exceed optimal conditions, the fish must adapt to the new conditions. The fish then often react by increasing the synthesis of special enzymes to handle the situation. This demands energy and as a result the fish have less energy available for growth and to tackle other challenges. Correct use of RAS can provide a stable temperature and water quality as the system is not influenced by external conditions to the same extent as a flow-through system. This can possibly lead to an enhanced quality of fish produced in RAS for transfer to cages, with respect to both survival and growth rate. Nofima and our collaborative partners are working actively to test this hypothesis.
Particle removal - feed adapted for use in RAS
A central component with RAS is the removal of organic material from the water. This material provides favourable growing conditions for bacteria and fungus and can also have an adverse effect on the biofilter/bioreactor. An important principle for RAS is to filter out particles as intact as possible. The removal of large particles is easier, cheaper and more effective than the removal of small ones.
Nofima has conducted research on adapting feed for use in RAS. This has focused on the development of feed with properties better suited to mechanical influence, which is stable in water and also retains the nutritional qualities for the fish. This reduces the need to filter substances containing minute particles and also contributes to reducing the amount of organic material in the water.
Carbon dioxide and metabolites
Reduced water quality leads to reduced appetite and growth and, in the worst case, an increased mortality rate. Nofima has via several projects studied the effect of different water environment parameters on production performance for fish and shell fish in both seawater and freshwater.
Environmental parameters such as oxygen, carbon dioxide, ammonia, nitrite and pH are relevant for RAS, and different levels of these are often included in Nofima's studies.
Biological filtration
Removing ammonia and nitrite is central in RAS. This normally occurs by biological filtering, where controlled bacteria cultures remove toxic ammonia from the water by converting it to nitrite then nitrate. Biological filtering functions in both saltwater and freshwater, but the effectiveness is different. The effectiveness is also closely linked to temperature, and Nofima has special focus on the operation and effectiveness of biofilters/bioreactors at low temperatures.
The example of Villmarksfisk
For more than 10 years, Nofima has collaborated with Villmarksfisk AS, which produces wild-captured charr using RAS technology at Bardujord in Inner Troms. During this period, the plant has been expanded from a small-scale pilot plant to a full-scale commercial plant with the capacity to produce around 75 tonnes of charr annually. The R & D work has covered the use and suitability of wild fish in aquaculture, product quality, different phases in operational optimisation in recirculating water at low temperatures, handling of fungus and water treatment via ozone treatment and protein skimmers, as well as the handling of waste and the use of waste from the land-based facility as plant fertiliser.
