Sophisticated feeds for RAS by Dr Robert Tillner, Product Manager, Aller Aqua Group
F
arming 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 nondigestible 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
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%).
18 | June 2019 - International Aquafeed