Introduction
Standard feeding strategies in Atlantic salmon farming are typically size-based, with high-protein diets (>40%) used for juveniles and high-fat diets (≥35%) during grow-out to promote protein sparing (Weihe et al., 2018). Ensuring good fish welfare and fillet quality is essential for sustainable production. Skin wounds are a major welfare concern, increasing vulnerability to infections and contributing to quality downgrading at harvest. Other relevant indicators include normal morphology and eye health (Moldal et al. 2025). Fillet quality is determined by factors such as nutritional composition, appearance, and texture. Visual attributes include the typical salmonid pink-red coloration and the absence of melanized spots, which influence consumer acceptance. Firmness is particularly important for both processing yield and sensory quality (Mørkøre, 2025).
This study evaluated three feeding strategies for autumn-transferred smolts farmed in Arctic conditions, assessing the potential benefits of high-protein diets with high marine protein share applied during winter alone or extended through both winter and spring, with respect to fish welfare and fillet quality.
Materials and methods
The trial was conducted at the Cermaq farming site Toknebuktneset, Finnmark (70.57°C, 24.07°E, Norway). Each of nine 120 m circumference open sea cages were stocked with 120 000 Atlantic salmon (Salmo salar L.) smolts averaging 75 g in June 2021. The trial started in October 2021, when the body weight averaged 570 g. The dietary regimes were:
Control: Standard feed during the production phase, October 2021-October 2022
HP-W: High protein feed during the winter period from October 2021-March 2022; thereafter the fish were fed the Control feed until harvest
HP-WS: High protein feed during winter and spring period from October 2021-June 2022; thereafter the fish were fed the Control feed until October 2022
Potential effects of the dietary treatments on growth performance, morphology, welfare indicators, nutritional status, and sensory quality were assessed at baseline in October 2021 and subsequently in March, July, and October 2022.
Results and discussion
Over the one-year feeding trial, average body weight increased from 0.57 kg to 3.17 kg. While no statistically significant differences in final body weight were observed among dietary treatments, the control group showed a tendency toward reduced growth, particularly in July and October. The thermal growth coefficient (TGC) averaged 3.23, which is above commonly reported values for salmon farmed under commercial conditions with optimal feeding and environmental parameters. TGC was significantly higher for the HP-W and HP-WS groups compared to the control between March and July. Biological feed conversion ratio (FCR 1.13) and mortality rates (6.9 %) remained acceptable and comparable across all treatments.
Operational welfare indicators (OWIs), including skin condition and eye lens opacity, indicated improved welfare in fish fed high-protein (HP) diets. In July, wound prevalence was significantly higher in the Control group (26.7%) compared to the HP-WS group (6.7%, P = 0.04), and scale loss was greater in the Control group than in the HP-W group (P = 0.05). In October, eye lens opacity affected 22.7% of fish in the Control group, versus 4% in both HP groups (P = 0.03), further supporting long-term welfare benefits of HP feeding.
Quality grading of whole fish showed a high proportion of superior share (91%), with no significant difference among the dietary regimes. Improvements were observed for fillet integrity and the occurrence of dark melanin spots. At harvesting in October, the share of fillets with severe gaping (score ≥ 3) and soft texture (breaking force ≤ 6 N) was halved for the high-protein (HP) groups compared with the Control (P ≤ 0.04). These results show that feeding the fish HP diets can prevent the development of gaping and soft texture, that are two commercially important traits. Fillet colour, measured using SalmoFan scores, showed that the HP-WS group achieved higher colour score in July (25.2 vs. 23.9; P < 0.0001), suggesting an interaction between diet composition and pigment retention. However, at harvesting four months later, visual colour was similar for all dietary groups.
Focal melanized spots are a major quality issue in farmed salmon (Mørkøre, 2025). In March, the prevalence of melanized spots in the cranio-ventral fillet averaged 11.1%, with no significant differences between the groups. By July, the prevalence of the Control group increased (30.7%), while the HP-W and HP-WS groups had lower prevalences (8.3% and 0%, respectively). At harvest, the Control group maintained a prevalence of 20%, while it was lower for the HP-W group (3.5%, P = 0.05). No significant difference was observed between the Control and HP-WS group (10%) (P = 0.22). From July onward, the number of dark-stained muscle segments was lower in the HP groups, with a significant difference observed between the Control and HP-WS groups in July (P = 0.05), and between the Control group and both HP groups in October (P = 0.02).
This study is the first to demonstrate that feeding high-protein diets with high marine protein share during the first winter (and following spring) can improve fish welfare by decreasing skin issues and effectively mitigate eye defects and quality issues such as gaping, soft flesh, and melanin deposition at slaughter.
References
1. Moldal, T., Wiik-Nielsen, J., Oliveira, V. H. S., Svendsen, J. C. og Sommerset, I.: Norwegian Fish health report 2024, Veterinærinstituttets rapportserie nr. 2025 1a/2025. N.V. Institute.
2. Mørkøre, T.: Feed to food: current status and perspective/Food quality. In V. Kumar (Ed.), Feed and Feeding for Fish and Shellfish: Nutritional management 2025, 1 ed., pp. 734.
3. Weihe, R., Dessen, J. E., Arge, R., Thomassen M. S., Hatlen, B., & Rørvik, K. A.: Improving production efficiency of farmed Atlantic salmon (Salmo salar L.) by isoenergetic diets with increased dietary protein-to-lipid ratio. Aquaculture Research 2018 49(4), 1441-1453.