What is the best system for raising salmon smolts: RAS or FTS?

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By Milthon Lujan

Salmon smolt RAS. Courtesy: Nofima
Salmon smolt RAS. Courtesy: Nofima

The Norwegian salmon industry is a global powerhouse and consistently ranks number one in farmed salmon production. However, with growing concerns over environmental impact and sustainability, the industry is exploring innovative solutions such as Recirculating Aquaculture Systems (RAS).

Scientists from the University of Bergen, Lingalaks AS, Bluegrove As, and the Institute of Marine Research compared the growth performance, physiological traits, and environmental adaptation of Atlantic salmon (Salmo salar) after transferring them to a cage farming site from a Recirculating Aquaculture System (RAS) with a similar-sized group of the same genetic lineage and produced in the same facilities but under continuous flow-through system (FTS) conditions.

The research aimed to evaluate and compare the performance, physiological traits, and environmental adaptability of Atlantic salmon smolts raised in RAS and FTS, focusing on their adaptation to cage farming environments and their impact on performance. Both groups were monitored throughout the production cycle, from freshwater to harvest, under standardized management protocols in a Norwegian commercial setting.

RAS vs. Traditional Methods: Advantages and Challenges

While traditional Flow-Through Systems (FTS) rely on large volumes of freshwater, RAS offers a more controlled environment with benefits such as:

  • Reduction in water usage and environmental impact
  • Improved disease management and hygiene
  • Stable and controlled rearing environment

However, RAS also presents challenges such as:

  • High investment and operational costs
  • Greater technical complexity compared to FTS

Optimizing RAS for Salmon Production

Extensive research is being conducted to optimize RAS for salmon production, focusing on factors like:

  • Water salinity and flow rate
  • Levels of dissolved oxygen and carbon dioxide
  • Population densities and feeding rates
  • Temperature
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Current Applications and Challenges

Currently, RAS is primarily used to rear juvenile salmon (smolts) before transferring them to cage farms for grow-out. However, this transition can pose a challenge for fish and lead to high mortality rates. Researchers believe that the quality of smolts, influenced by early life experiences, plays a crucial role in their ability to adapt to cage farming environments.

Stable RAS vs. Variable FTS: A Comparative Study

This article describes a research project comparing the performance and adaptability of Atlantic salmon smolts raised in:

  • RAS: Controlled and stable environment with optimized temperature for growth.
  • FTS: Variable environment following natural seasonal fluctuations.

Both groups were monitored throughout the production cycle, from freshwater to harvest, under identical management protocols. Researchers evaluated growth performance, physiological traits, seawater adaptation, energy balance, and vertical distribution in cage farms.

The findings of the study provide valuable insights into the potential of RAS technology for sustainable and successful salmon farming.

Smolt Quality: The Key to Success

The research confirmed that smolt quality is crucial for successful salmon production. While RAS and FTS smolts appeared similar in size and development at the end of the freshwater phase, significant differences emerged after transfer to the cage farm environment.

RAS-reared smolts exhibited several physiological and molecular differences compared to FTS smolts, leading to:

  • Increased mortality: RAS fish experienced significantly higher mortality rates throughout the production cycle.
  • Slower growth: They grew more slowly and achieved a smaller harvest size compared to FTS fish.

According to the study, these findings can be attributed to several factors:

  • Lower energy reserves: RAS fish had lower levels of key indicators like HSI (hepatosomatic index), suggesting limited energy reserves to cope with the demanding cage farm environment.
  • Poor osmoregulatory and allostatic adaptation: RAS fish struggled to regulate their internal salt and water balance and adapt to the external environment, affecting their overall resilience.
  • Reduced stress response: RAS fish showed a weaker physiological response to unpredictable environmental changes such as water temperature and oxygen fluctuations, potentially impairing their ability to cope with these stressors.
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FTS Fish: a Smoother Transition

Conversely, smolts raised by FTS:

  • Adapted faster to the seawater environment: They demonstrated a greater ability to regulate their internal environment and adapt to changing external conditions.
  • Showed greater physiological robustness: They exhibited a stronger response to seasonal variations and unpredictable environmental changes, leading to better overall performance.

Importance for the Salmon Industry

The study findings highlight the fundamental role of freshwater rearing conditions in shaping a smolt’s ability to thrive in cage farm environments. FTS fish, exposed to natural temperature fluctuations and other factors during their freshwater phase, appeared better equipped to handle the complexities of the sea.

The research emphasizes the need to optimize freshwater rearing strategies within RAS systems to better prepare smolts for the transition to cage farms. By understanding the physiological differences and limitations of RAS-reared smolts, researchers and industry professionals can work to develop more effective rearing protocols and acclimatization procedures to ensure their success in the marine environment.


RAS technology holds promise for the future of sustainable salmon farming. However, addressing the challenges faced by RAS-reared smolts during their transition to cage farms is crucial to harnessing their full potential. By focusing on optimizing freshwater rearing conditions and developing strategies to improve smolt quality, we can pave the way for a more sustainable and successful future for the salmon industry.

The study was funded by the Research Council of Norway (RCN) NoFood2Waste.

Floriana Lai
University of Bergen, Department of Biological Sciences, Pb 7803, 5020 Bergen, Norway.
Email: floriana.lai@uib.no

Reference (open access)
Lai, F., Rønnestad, I., Budaev, S., Balseiro, P., Gelebart, V., Pedrosa, C., … & Handeland, S. (2024). Freshwater history influences farmed Atlantic salmon (Salmo salar) performance in seawater. Aquaculture, 740750.