The fat levels in the diet do not influence the smoltification and maturation of salmon

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By Milthon Lujan

Smolt salmon. Source: Nofima.
Smolt salmon. Source: Nofima.

The rapid expansion of Recirculating Aquaculture Systems (RAS) has revolutionized Atlantic salmon (Salmo salar) smolt production with impressive growth rates. But does this speed come with a hidden cost?

A study published by scientists from Nofima, the Norwegian University of Life Sciences, and Havbruksstasjonen i Tromsø AS delves into the intricate interplay between fat levels in the diet, water temperature, and key physiological processes such as smoltification and early sexual maturation in Atlantic salmon raised in RAS.

RAS and Smolt Production

Traditionally, salmon hatcheries raised juvenile fish (parrs) in freshwater until they transformed into smolts ready for seawater, then transferred them to oceanic cages for further growth. However, the rise of Recirculating Aquaculture Systems (RAS) has revolutionized this process, providing strictly controlled environments for optimal growth and resource efficiency.

However, continuous feeding, constant light, and relatively high temperatures applied in RAS facilities to produce smolts are known to interfere with smoltification and promote early sexual maturation.

Thus, RAS rearing presents a unique challenge: balancing rapid growth with crucial physiological processes like smoltification and maturation. Inadequate smoltification can hinder the fish’s ability to adapt to seawater, while early sexual maturation, especially in males, can affect fillet quality, growth, and even survival.

Fat Levels and Body Composition

Researchers studied the impact of fat levels in the diet (20%, 24%, and 28%) on young salmon (~19 g) in freshwater RAS at 12°C. As expected, high-fat diets led to fatter fish, with the low-fat group showing 23% less body fat. However, these differences disappeared once the salmon transitioned to seawater (continuous flow systems, 26% fat diet, 12 or 16°C).

Growth and Smoltification

Interestingly, despite different fat levels, all three diets supported comparable growth and smoltification success during the freshwater phase. Key indicators such as the k-factor (gill enzyme activity), blood electrolytes, and smolt index were unaffected. This suggests that within the tested range, fat levels in the diet do not hinder smoltification in RAS.

Temperature Adjustments: Larger Bodies, Early Maturation?

On the other hand, water temperature emerged as a key influencing factor. Salmon raised at 16°C grew more than those at 12°C, exhibiting a similar thermal growth coefficient (growth rate relative to temperature). However, the warmer environment also seemed to push some fish toward early sexual maturation. Elevated levels of the sex steroid androstenedione and early maturation stages observed in some testicles at 16°C suggest this possibility.

Implications for the Salmon Industry

The study’s findings highlight the following:

  • RAS can achieve optimal smoltification without compromising growth by carefully adjusting fat levels in the diet and water temperature.
  • Monitoring sex steroid levels and implementing temperature control strategies can help mitigate the risk of early male maturation in RAS-raised salmon.

Additionally, scientists suggest:

  • Researching the long-term impacts of early male maturation on salmon health and reproductive performance.
  • Exploring the potential of temperature manipulation as a tool to optimize growth and smoltification while minimizing the risks of maturation.

Conclusion: A Balancing Act

This study underscores the delicate balance between optimizing growth and potential physiological consequences in RAS-raised salmon smolts. While varying fat levels in the diet within the tested range (20-28%) do not seem to alter smoltification or early maturation, warmer rearing temperatures may trigger early maturation in some individuals.

The scientists emphasize the need for further research to fully understand the long-term implications of these findings and to optimize RAS conditions for healthy and sustainable salmon production.

The study was funded by The Research Council of Norway, Strategic Institute Research Nofima AS, project no. 12878 ‘Atlantic salmon biology in focus: for a robust fish in a shifting aquaculture industry.’

Reference (open access)
Mota, V. C., Verstege, G., Striberny, A., Lutfi, E., Dessen, J. E., Sveen, L., … & Bou, M. Smoltification, seawater performance, and maturation in Atlantic salmon (Salmo salar) fed different fat levels. Frontiers in Aquaculture, 3, 1323818.