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The fundamentals of successful RAS farming

March 19, 2020  Sponsored by by Alltech

Productivity in a recirculating aquaculture system (RAS) is far superior to traditional farming methods. The profitability of RAS depends on the productivity (kg/m3/year) of the system and, therefore, RAS requires a high stocking density and growth rate. A well-designed system, the right feed, good management practices and an optimal feeding strategy are fundamental to the success of both your fish and your farm

Importance of a great system
In terms of water quality in the system itself, a RAS is the most advanced. For optimal control, filters clean and recondition the water after the fish use it. First, the faeces and any spilt pellets are removed. This is done with a mechanical filter, like a sludge cone and/or a drum filter with a fine sieve. Both filters remove a large part of the organic matter. Then the ammonium and CO2 produced by the fish is removed.

This is done through a nitrification filter, in which bacteria like Nitrosomonas and Nitrobacter break down ammonium to nitrate, a molecule that is almost harmless to fish. Biofilters can be a moving bed, where bacteria grow on beads, or trickling filters using a fixed bio medium over which the water is trickled like a shower. After biological filtration, a degasser strips the CO2 from the water, and the oxygen is replaced. Usually, farmers use liquid oxygen in order to reach high levels of oxygen in the incoming water. Finally, in some cases, extra units and like UV or ozone, are used to reduce suspended solids and for hygiene reasons. By using this process, the essential water quality parameters are optimised for the fish, ensuring a consistent, high growth rate and good fish health.


When the provided water quality and, especially, oxygen levels are not limiting, the strategy of growth maximisation provides attractive financial benefits.

Management practices
In all types of fish farming, RAS especially, the water quality is the basis for healthy fish and optimal performance. Fish eat and grow well when the water quality is excellent and consistent. To a great extent, water quality determines the success of the RAS.

Fish performance is affected by several crucial parameters, such as dissolved oxygen, concentration of carbon dioxide, ammonia levels, nitrite, nitrate, water temperature and pH. All of them interact with one another and impact fish health.

The golden rule for consistent aquaculture farm results is to maintain a constant and optimal water quality. This applies to cage farming, raceway systems and also in RAS. The water quality affects feed uptake, digestion, growth and waste excretion. Keeping these parameters optimal at all times requires a great understanding of fish behaviour and the dynamics of the filters.

Feeds for RAS farming
Besides a state-of-the-art RAS, the farmer needs first-class RAS feeds and a proper feeding strategy to fulfil the farm’s full potential and achieve the best results. Being able to maintain a high daily feed rate is obviously what every fish farmer wants. Higher feed intake equals better growth. In Alltech Coppens, we developed a trout RAS guide that offers support to farmers in developing a successful production.

A RAS feed is characterised by high digestibility, leading to minimal amounts of faecal matter, and high protein retention that minimizes ammonia excretions so the filters can work more efficiently. A true RAS feed makes a big difference for the water quality and final productivity of the farm.

In a RAS, the feed quality and feeding regime always have an impact on the quality of the water your fish swim in. The best farm performance, in combination with the lowest impact on your water quality, can only be reached with the best feeds and feeding regime. Overfeeding pollutes the water, but using lower-quality feeds can also increase the pollution levels in your system. The lower digestibility of these feeds increases the levels of expelled waste, and that lowers the system conditions.

The basis of a good RAS feed starts with the selection of only high-quality ingredients. Critical selection criteria include protein level, amino acid profile, energy level, fatty acid profile, digestibility and palatability. Next to that, the composition must allow for maximum growth, which means a high-energy diet. The ultimate goal is to maximize nutrient utilization and minimize nutrient losses for optimal fish growth.

A RAS feed should not only satisfy the nutritional needs of fish, but the physical pellet properties should also minimize water contamination. Feed pellets and faeces must be firm so that they do not dissolve in the water and can be effectively removed by the mechanical filter. This can be checked relatively easily at the tank outflow.

Feed pellets need to be firm and durable so that they can withstand the friction they are exposed to in feeding and transportation systems. This is essential so that dust generation can be minimized. Fish cannot consume dust; it will pollute the water and possibly irritate the gills. Sinking speed is also important. The pellet should sink slowly so that all individual fish in the tank have enough time to eat the pellets. This helps to distribute the pellets to the whole batch of fish evenly and minimizes uneaten pellets.

The future of fish farming
Waste control and removal as well as water recycling in RAS are measures for the sustainable future of aquaculture. The amount of water now required to produce trout or other fish is much lower than in traditional systems. RAS can also be located close to the market, significantly reducing transportation costs and carbon footprint.

Although RAS farming has already undergone much intensification and professionalization, further development will result in an even higher level of sustainability. Proper design and management of RAS, optimum RAS feed quality and feeding management are essential for successful and sustainable fish production. In this way, we take proper care of our planet and contribute to a Planet of Plenty™.

Good knowledge of the management practices and feeding routines are fundamental to the success of both your fish and your farm. It is vital to have excellent day-to-day stock control and adequate feed rates. In general, a feed rate near satiation works best for most species, as it allows high performance without the risk of overfeeding. One should bear in mind that even small amounts of uneaten feed, usually of a high nutrient density, will pollute the water and lower the water quality. Only if optimal water quality is maintained can the fish and the system perform well.

To get a greater insight into the optimal operation and function of a RAS, it is crucial to understand the fundamental requirements. Find out more on www.alltechcoppens.com/ras-guide, where you can request a complete guide on trout farming in RAS.

Gijs Rutjes is passionate about fish, feed and fish farming. He studied aquaculture at Wageningen University in the Netherlands, with a Master of Science degree specializing in RAS, fish nutrition and immunity in fish. He did an internship with Marine Harvest (Mowi) in Scotland, where he was involved in selective breeding and supported the feed department with feeding tables and feeding strategies. He worked for a leading African catfish RAS hatchery in The Netherlands and a RAS catfish farm for nearly three years before entering the fish feed industry. In 2003, he joined Alltech Coppens, currently responsible for technical sales support within Coppens and travels extensively to provide on-farm support. Rutjes is strongly committed to the Alltech Coppens motto “dedicated to your performance”.

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