Introduction
Issues affecting skin health have been a persistent challenge in Norwegian Atlantic salmon (Salmo salar) aquaculture. The skin is a multifunctional organ, playing a crucial role in protecting against environmental stressors, pathogens, and physical injury. Beyond serving as a physical barrier, it also performs essential immunological functions, housing complex mucosal layers that contribute to immune surveillance, physiological homeostasis, and protection against harmful agents. As such, skin health is central to the overall robustness and performance of salmon in aquaculture.
The surrounding production environment closely influences the integrity and functionality of the skin barrier. Often regarded as a living sensor, the skin reflects the quality of rearing conditions, making it a key indicator of health in response to environmental factors. Before Atlantic salmon are transferred to sea, they are reared in various land-based systems, with recirculating aquaculture systems (RAS) becoming increasingly common. This shift has largely replaced the previously widespread use of flow-through systems (FTS) and partial-reuse systems (PRS). While RAS offers enhanced control over water quality, they are frequently associated with challenges such as elevated levels of ammonia, nitrite, and organic load, among other factors. These environmental parameters have been linked to compromised skin health, although direct comparisons between these systems remain limited.
This study investigates the effects of different land-based production environments on the skin mucosal barrier of Atlantic salmon, with a focus on immune function, oxidative stress responses, and overall skin physiology.
Materials and Methods
A total of 1800 Atlantic salmon parr, with an initial weight of 30-40 g, were reared under three different production regimes: a flow-through system (FTS), a recirculating aquaculture system (RAS) with 95% water recirculation, and a partial-reuse system (PRS) with 40% water reuse. During the first five weeks (Phase I), all fish underwent smoltification, with each system operating in freshwater at 12 °C. In Phase II (post-smolt stage), which lasted three weeks, the salinity in the FTS and PRS systems was increased to 33 ppt, and the water temperature was maintained at 8-10 °C. The RAS group was kept at the same temperature, but the system operated with 17 ppt brackish water. In the final phase (Phase III), all groups were transferred to flow-through systems with a salinity of 33 ppt and a temperature of 8-10 °C to simulate sea transfer conditions.
Skin samples were collected at the end of each phase for molecular and histological evaluations. Mucus was also collected and subjected to proteomic and metabolomic analyses. To simulate exposure to a secondary stressor, scale cultures from fish in Phase III were established and exposed to hydrogen peroxide (H₂O₂). Molecular and biochemical assays were then performed on these samples.
Results and Discussion
Preliminary histopathological analysis indicated an increase in the severity of pathological changes in the skin in phase III across all groups. Skin morphometric parameters, including epidermis thickness and the number of mucus cells present in the epidermis, did not reveal any significant differences between groups or phases. However, a significant increase in dermis thickness was observed after phase I in the FTS group.
Gene expression analysis of skin samples from all production systems across the three phases showed a significant upregulation of immune- and stress-related genes in the PRS group during phase II. Mucin expression was significantly higher in the FTS group than in RAS and PRS groups, and other structural genes also showed a significant upregulation in the FTS and RAS groups in phase III. These findings suggest that production conditions influence skin functions related to barrier integrity and structure.
In the cell culture analysis, while the overall expression patterns of oxidative-stress genes were similar between control and stressed groups, there was a significant increase in the expression of some genes (e.g., gpx, gr, and mnsod) in the PRS group compared to the FTS group upon H2O2 exposure. In most of the oxidative-stress genes analysed, gene expression was lower in the FTS group than in the RAS and PRS groups, possibly indicating reduced cellular stress in salmon reared in flow-through conditions.
Oxidative stress biomarkers in the cell culture showed that in the FTS group, exposure to H₂O₂ led to an increase in total antioxidant capacity and a decrease in reactive oxygen species (ROS) and reactive nitrogen species (RNS), suggesting an efficient antioxidant system, characterized by a dynamic inverse relationship between antioxidant capacity and ROS/RNS levels. The opposite trend was observed in the PRS group.
Preliminary analyses of the mucus revealed differences in the protein and metabolite composition between the production systems, although data are still being analysed. These results will be presented during the meeting.
Conclusions
This study demonstrates that each land-based production system has a different impact on the skin barrier functions of Atlantic salmon, with a significant effect on immune and oxidative stress responses. Our findings suggest that the fish group reared in a partial-reuse system exhibited more changes in skin health conditions, exhibited by increased immune and stress responses, along with reduced antioxidant capacity, compared to fish from other rearing systems.
Acknowledgements
This research is part of the GillHealthAct project, funded by the Norwegian Seafood Research Fund (nº 901911). AL would like to thank ERASMUS+ Student Mobility Program and Centre for Interdisciplinary Research in Animal Health (CIISA) from Faculty of Veterinary Medicine, University of Lisbon, for financing her stay at Nofima.