Article written by: Jason Danaher Ph.D.
Why Feeding Is So Important
In a recirculating aquaculture system (RAS), feeding is not just about growing fish. Feeding is the main factor controlling how the entire system operates. Every pound of feed added to a tank has the potential to become fish biomass (Figure 1). But, for every pound of feed added to the tank a portion will also become ammonium released through the fish gills and solid waste in the form of fish feces or uneaten feed. Therefore, feeding directly influences your RAS water quality, oxygen demand, and biofilter performance. Feeding also affects your production cost and is a significant expense for daily operation. Poor feeding practices can quickly lead to water quality problems, fish stress, and lost profits. Good feeding practices keep both the fish growth, fish health, and the system in balance.
Figure 1. Tilapia being fed a commercial, floating diet in a recirculating aquaculture system.
Two Common Feeding Methods
Percent Body Weight Feeding
Feeding fish in a RAS often utilizes a method based on a calculated ration, determined by the total estimated weight (biomass) of fish in each tank. This approach requires the operator to regularly sample the fish to accurately estimate the total biomass.Once the biomass is determined, the operator feeds a set percentage of this weight daily. Typically, grow-out tanks, containing fish from fingerlings to market size, are fed between 1% and 5% of the total tank biomass each day, though the exact percentage is species-dependent. The total daily feed ration is commonly split into multiple feedings. For example, a 30-pound total daily ration might be divided into three 10-pound feedings at 8 AM, 12 PM, and 4 PM.
Feeding fish based on percent body weight is a reliable RAS management strategy because it ensures a predictable feed input. This predictability is valuable for several reasons: it aids in managing consistent system loading, optimizing biofilter capacity, and effectively controlling feed costs.
Apparent Satiation Feeding
This feeding method involves observing the fish's behavior and continuing to feed them until they lose interest in the diet. A feeding event typically lasts 15 to 20 minutes and begins with a predetermined quantity of feed. During this time, the operator incrementally dispenses small amounts of food while closely monitoring the fish. Feeding stops when the operator observes a decrease in feeding activity and uneaten, floating feed remains on the water's surface.
To calculate the amount distributed, the operator weighs the remaining feed and subtracts this from the initial known amount. This method is highly effective for operators who wish to fine-tune feeding rates, maximize fish growth, and monitor fish health through daily observation.
Best Practices for Feeding in Recirculating Aquaculture Systems
The truth of the matter is that most successful operators of RAS employ a sophisticated, two-sided approach to fish feeding. This combined strategy creates a powerful feedback loop. The operator uses the quantitative percent body weight method to establish a new, accurate daily ration after each sampling event. They then rely on the qualitative apparent satiation method to adjust the feeding rates in real-time until the next scheduled sampling. This ensures that the feeding strategy is always both precisely calculated for long-term growth targets and dynamically responsive to the immediate health and environmental conditions of the RAS, striking the perfect balance between scientific control and operational flexibility.
In summary, an operator should feed the RAS based on the known fish biomass and average fish size. Large, single feeding events should be avoided while multiple, small feedings should be completed throughout the day. The operator should attempt to watch fish closely during each feeding event to monitor for uneaten feed. Floating diets are beneficial to observe fish feeding behavior. The feeding rates should be adjusted regularly based on fish feeding behavior. Also, water quality should be monitored consistently and feeding rates modified as needed because the biofilter may need time to adjust with increased feeding rates. Consistent, well-managed feeding helps prevent sudden changes in ammonium, oxygen, and solids concentration.
Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com for additional information on best practices in feeding your aquatic system.