meeting the high demands in feeds for Recirculating Aquaculture Systems


The PowerRAS concept meets the high demands towards feed for RAS in terms of feed efficiency, optimal water quality and subsequent fish growth. The concept is the result of continuous research and trial work at Aller Aqua Research in combination with field trials.

The development of feed for Recirculating Aquaculture Systems

At Aller Aqua we believe that constantly challenging the status quo is key to staying at the forefront of fish feed development. Together with our customers and research partners, we develop innovative feed concepts that address the demands from the market.

Farming fish in Recirculating Aquaculture Systems (RAS) has gained popularity across the globe. Whether RAS are built from scratch or existing fish farms are converted to RAS technology – fish produced in RAS represent a steadily growing volume of total aquaculture production. 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. Therefore, feeds for RAS need to fulfil the requirements of this highly sophisticated and complex production technology by taking the following aspects to a new level:

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

Aller Aqua feeds for testing
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 raw materials 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

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.

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. 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. 

In countless trials during the last years, Aller Aqua R&D has established a comprehensive data catalogue of raw material nutrient digestibility for fish at different life stages and under varying environmental conditions. This achievement led to the introduction of the successful POWER² concept in 2017 and is a key milestone in the development of optimal feed for fish husbandry in RAS.


Tools for 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. For example, low stability and weight of faeces dramatically reduces the removal efficiency by mechanical cleaning units, such as drum filters. 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 become substrate for undesired bacterial growth. The efficiency of the biofilter drops and potentially toxic nitrite levels in the water rise. Therefore, the RAS become biologically unstable which in turn negatively affects fish growth by unfavourable environmental conditions.

From trials, at Aller Aqua Research and partner institutes comprehensive knowledge has been obtained about feed composition and the impact of raw materials on faeces quality. Faeces quality is screened by different standardized methods including the determination of descent rate, harvest quantification and the faeces-driven impact on water turbidity and biofilter.

To develop the optimal feed for RAS we have created feed formulations as an integrated part of the POWER² concept considering a balanced digestibility of raw materials to ensure shaped and compact faeces particles. This ensures an effective removal and the lowest possible impact of suspended matter on fish environment and filtration technology.

Faeces samples at Aller Aqua Research

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 of the successful POWER² concept 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.

At Aller Aqua Research, life-stage-specific DP:DE ratios were identified in a series of trials which serve as essential cornerstones in formulation of a feed for RAS.


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 feed dust levels mean no feed particles affect 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 minimized 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 shall 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.

Realizing 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 most sophisticated demands.

Faeces sinking rates being tested at Aller Aqua Research

Feed functionality

Above it has been described which nutritional and physical characteristics are defined and implemented in a feed for RAS. All 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 optimized to reduce the contents of these potentially toxic 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.


All of the above findings are now commercialized and represent the latest addition of RAS-optimized feed technology by Aller Aqua.

For more information, please contact: 

Dr Robert Tillner

Product Manager

E. rt@aller-aqua.com
T. +49 159 018 419 89