by Dr Robert Tillner, Product Manager, Aller Aqua Group

Farming fish in increasingly sophisticated Recirculating Aquaculture Systems (RAS) has gained popularity across the globe. Whether RAS are built from scratch or existing fish farms have converted to RAS technology – fish produced in RAS represent a steadily growing volume of the total aquaculture production volume.

One of the reasons is that the control measures in RAS technology allow for fish production under constant environmental conditions. This accurate optimisation between water parameters and fish biomass requires external factors to seamlessly integrate into this equilibrium. In this respect, feed is the most influential external factor in RAS and needs to provide the following benefits from a farmers' point of view:

  • High feed efficiency
  • Optimal water quality
  • High fish growth

Consequently, feeds for RAS need to target the specific requirements of this highly sophisticated and complex production technology by taking the following features to a new level:

  • Nutrient digestibility and palatability
  • Faeces quality
  • Fish metabolism and DP:DE ratio
  • Technical quality of the feed
  • Feed functionality

Nutrient digestibility and palatability

In RAS, feeds with high nutrient density can unleash their full potential if the nutrients are readily available for fish metabolism and growth. The gross energy level is therefore not a useful indicator as nutrients get lost via faecal excretion.

In contrast, the digestible nutrient content of raw materials indicates the nutrients retained in the fish body after non-digestible energy has been lost in the faeces. Thus, assessing nutrient digestibility of each raw material is the groundwork for any feed of stable quality. Nevertheless, for accurate determination of nutrient digestibility the following obstacles need to be considered:

  • Raw materials differ in nutrient composition: obviously, fish meal and wheat differ in nutrient composition. But fish meal also varies in composition between different types of fish meal as well as within the same type. These differences in nutrient composition within a raw material need to be accounted for in the same way as between different raw materials like fish meal and wheat. This is required to provide the necessary accuracy when determining the nutrient digestibility of each raw material.
  • Raw materials differ in nutrient digestibility: the nutrients from raw materials vary in digestibility. This is due to the nature of each raw material as well as the processing conditions and processing grade. Consequently, raw materials need to be evaluated in accordance to exact origin and season. This means that determining the nutrient digestibility of each raw material just once is not enough because of seasonal variations of raw materials as well as changes in processing conditions.
  • Fish life stage and environmental factors affect feed nutrient digestibility: the digestive system of fish fry is not fully developed in many species when they start feeding. As fish mature so does their digestive system in terms of morphology and enzymatic capacity to digest feed. Therefore, fish show different digestive capacity to digest raw materials according to their life stage. In addition, environmental conditions determine the capacity of fish to digest different raw materials. Especially water temperature largely influences raw material digestibility in fish and needs to be taken into consideration in feed formulation to obtain evenly digestible feeds at varying water temperatures.

Taking these obstacles into account, the determination of nutrient digestibility of raw materials for different life stages under varying environmental conditions and subsequent feed formulation results in high and constant performance.


Faeces quality

Nutrient digestibility and palatability play an essential role but are not enough to develop an optimal feed for RAS. The physical quality of fish faeces largely influences the water quality in RAS and is mainly dependent on the feed the fish has eaten. Consequently, low stability and weight of faeces dramatically reduces the efficient removal by mechanical cleaning units, such as drum filters in RAS.

As small faeces particles become smaller and more and more suspended, they are not removed by the drum filter but accumulate in the water and are carried into the biofilter. Here, the faeces particles become substrate for undesired bacterial growth. Eventually, the efficiency of the biofilter drops and potentially toxic nitrite levels in the water rise. Consequently, the RAS becomes biologically unstable which in turn negatively affects fish growth by unfavourable environmental conditions.

Balancing nutrient digestibility of raw materials and faeces quality to ensure shaped and compact faeces particles will allow effective removal and the lowest possible impact of suspended matter on fish environment and filtration technology.


Fish metabolism and DP:DE ratio

Knowledge about nutrient digestibility of raw materials and their effects on faeces quality are necessary and powerful tools to formulate stable quality feeds. These tools are applied to determine the exact amount of nutrients required by fish of different life stages for optimal growth and the optimal ratio of digestible protein to digestible energy (DP:DE).

The optimal ratio between digestible protein and digestible energy allows for minimal waste of feed protein, mainly in the form of ammonia and urea, and at the same time allows the optimal usage of precious feed protein for fish growth. Consequently, lowest possible excretion of nitrogen into the water positively affects water quality and the dimensioning and loading of the biofilter. The equation is simple: better water quality equals better fish growth and health.


Structure for stability

The ideal feed formulation must allow the production of a feed with physical characteristics matching the high standards of RAS:

  • Low dust: Lowest possible dust levels in the feed increase water quality and biofilter efficiency
  • Low leaching: While leaching of nutrients from the pellets may serve as attraction for the fish, this effect needs to be optimised in order to ensure the delivery of precious nutrients into the fish
  • High stability: The feed needs to be robust enough to withstand transport, storage and automatic feeding systems with air pressure. At the same time the feed needs a certain level of plasticity to absorb impacts from handling
  • Optimal density and smooth surface: A smooth feed surface reduces the amount of breaking off fines to a minimum, but also impacts feed sinking speed. However, the feed should not sink too fast, because most fish tend to eat in the upper part of the water body.

The demands towards nutritional and physical feed quality are unquestionably high. Realising these demands in feed production is a result of dedicated trials, but also decades of experience on how to reach the desired target with big scale machinery. Upgrades in factory equipment parallel the development of RAS and allow for production of physical feed quality matching the most sophisticated demands.


Feed functionality

The previous passages described the nutritional and physical characteristics that are defined and implemented in a feed for RAS. Both are important in the development of a feed for RAS, but the highest demands go beyond nutrition and physical quality.

Feed functionality targets ways of supporting RAS by direct effects of the feed that are neither of nutritional nor of physical nature:

  • From laboratory experiments direct effects of special feed ingredients on water ammonia and nitrite levels have been investigated and the feeds have been optimised to reduce the contents of these potentially detrimental compounds and to relieve the biofilter
  • Bioactive components have been evaluated to shift the intestinal microbiome towards an improved gut flora and metabolic processes for enhanced fish health and sustainable performance.

Introducing Aller Aqua's POWERRAS concept

The abovementioned features have been optimised through continuous research and trials, and now match the sophisticated demands for feed for RAS. Ultimately, the features create benefits for the farmer in terms of feed efficiency, water quality and fish growth.

These findings jointly represent the latest addition of RAS-optimised feed technology by Aller Aqua's new POWERRAS concept, which is the culmination of efforts and sophistication in feed development over the last couple of years. Aller Aqua Research has closely followed the increasing sophistication of RAS in order to be able to match the latest requirements by RAS farmers.

Left: Difference in declared versus digestible protein content between batches in conventional feed formulation. The feed is formulated based on a fixed protein level which results in variable digestible protein contents between batches and variable performance on the farm.

Right: Feed formulation based on a fixed digestible protein content between batches which results in variable declared protein content, but constant performance on the farm.



Relative water turbidity in four decoupled tanks with four different feed test batches eight hours after the water circulation has been turned off to fine-tune feeds towards lower water turbidity (values are in % compared to respective control at time 0, control=100%).

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