Atlantic salmon farming in Recirculating Aquaculture Systems (RAS) offers significant advantages, such as reduced environmental impact and improved disease control. However, optimizing fish growth and maintaining water quality within these closed systems requires specialized approaches.
A study published in the journal Fishes by scientists from Cargill and the Norwegian University of Life Sciences (NBMU) evaluated the effectiveness of a diet adapted to RAS (ERAS) compared to a conventional flow-through system (FTS) diet for Atlantic salmon raised in RAS.
The Problem with Traditional Diets
Historically, diets for open pens and FTS systems aimed to break down fecal matter to prevent local sedimentation. While convenient, this approach increases the release of nutrients into the surrounding water.
The Role of Diets in RAS
Diet plays a crucial role in nutrient management. Unlike traditional diets, RAS diets promote the fixation and sedimentation of fecal particles, facilitating their removal through mechanical water treatment. This approach offers several benefits:
- Reduced Biofilter Load: The load on the biofiltration system is reduced, simplifying operations.
- Improved Water Quality: Creates a healthier environment for the salmon.
- Reduced Nitrogen Discharge: Less nitrogen is released into the surrounding water.
The Study: Comparing Diets in RAS
The research team conducted a four-week trial using six identical RAS units, each stocked with post-smolt Atlantic salmon. Key aspects of the study included:
- RAS Design: Each RAS unit had a total volume of 1,500 liters with 1,000-liter rearing tanks.
- Fish Stocking: Salmon with an average weight of 199.7 grams were stocked at a density of 30 kg/m³, reaching approximately 49 kg/m³ by the end of the trial.
- Dietary Adaptations:
- ERAS Diet: Formulated for greater stability in RAS environments, with higher fecal stability, reduced protein (-12%), and increased fat content (+8%) to maintain energy levels comparable to the FTS diet.
- FTS Diet: A standard flow-through system diet used as a control.
- Monitoring: Water quality parameters, system performance, and fish health were closely monitored throughout the trial.
Better Water Quality with a RAS-Adapted Diet
The study found no significant differences in fish growth, health, or survival between the two diets. However, RAS units using the ERAS diet showed significant improvements in water quality, including:
- Higher pH (+1.2%) and alkalinity (+17%)
- Lower total ammonia nitrogen (TAN) (-18%) and nitrite (NO2-) (-46%)
- Fewer drum filter activations (-13%)
These benefits suggest that the ERAS diet led to lower waste production and better waste management within the RAS. This was further supported by the finding that RAS units using the ERAS diet were more effective in removing particles from the drum filter, requiring fewer daily activations.
Reduced Nutrient Accumulation
Perhaps the most surprising finding was the significantly lower phosphorus accumulation in RAS units using the ERAS diet. Total phosphorus (TP) was reduced by an impressive 90%, and dissolved phosphorus (DP) by 92% compared to FTS units. This indicates that the RAS-adapted diet minimized nutrient pollution within the system, contributing to a more sustainable production process.
Benefits of RAS-Adapted Diets
This study demonstrates the potential of using RAS-adapted diets to optimize Atlantic salmon production in recirculating systems. These diets offer several advantages:
- Better Water Quality: By reducing waste production, maintaining lower ammonia levels, and promoting efficient particle removal, RAS-adapted diets can create a healthier environment for fish.
- Lower System Maintenance: Fewer drum filter activations translate to lower operational costs and increased efficiency.
- Environmental Sustainability: Reduced nutrient accumulation helps minimize environmental impact and contributes to more sustainable aquaculture practices.
Conclusion
These findings suggest that diets adapted to RAS systems can be a valuable tool for optimizing Atlantic salmon production in RAS. By tailoring diets to the specific needs of a closed system, fish farmers can achieve optimal fish health, maintain water quality, and ensure efficient system operation.
The study highlights the potential of RAS-adapted diets for RAS systems. Future research may explore how these diets can be further optimized for different fish species and RAS configurations.
The study was funded by the Research Council of Norway.
Reference (open access)
Flo, Vegard Øvstetun, Thomas Cavrois-Rogacki, Jon Øvrum Hansen, Jannicke Vigen, Thomas Gitlesen, and Odd-Ivar Lekang. 2024. “RAS-Designed Diets Result in Lower Accumulation of Nitrogen, Phosphorus, and Zinc in Recirculating Aquaculture System Compared with Traditional Flow-Through Designed Diets” Fishes 9, no. 8: 300. https://doi.org/10.3390/fishes9080300
