In 2019, Norway exported 1.4 million tons of farmed Atlantic salmon and the government has the vision to significantly increase the production in the years to come in a sustainable manner (Ministry of Trade, I.a.F., 2021). This ambitious production target is dependent on how the industry battles major challenges including sea lice (Lepeophtheirus salmonis) and escapees from the farms, both of which have serious economic and societal consequences. In recent years, many salmon aquaculture stakeholders have argued that the increased production must be achieved through the use of additional sustainable production technologies as alternatives to the traditional net pens that are mainly used today. New production technologies must be proven to support optimal conditions for fish health and welfare, and environmental functionality including stringent protection from lice and possibilities for escapees. Floating semi-closed containment systems (S-CCS) are an example of such sustainable systems, as they are proven to be efficient against lice and escapees (Nilsen et al., 2017; Nilsen et al., 2020). Floating S-CCS are aquaculture production systems in which a physical barrier keeps the lice out of the system and prevents the fish from escaping. Water is pumped into the system through pipes, with the inlet located below the level where sea lice normally thrive. Sea lice are normally found in the surface waters, between 1-6 m (Nelson et al., 2018; Fernandez-Gonzalez et al., 2022). However, they may show diel cycles and have been found at 10-17 m during daytime (Nelson et al., 2018). The vertical distribution of lice is also highly dependent on water current and may vary a lot (Asplin et al., 2014). So far, these SCC systems have been used mainly to produce post-smolt up to one kilogram and have thus far served as a stopover before the fish are transferred to net pens, where they are reared until harvest. This means of rearing allows the fish to stay away from the open sea for an extended period and, thus, the chances of sea lice infestation and escapees are lessened. Despite having a significant potential to address biological challenges, the semi-closed systems entail high investment costs and are today regulated similarly to traditional net pens—that is, with license costs and limitations in the allowed maximum biomass. The high costs and regulatory restrictions mean that many producers and farmers of the systems hesitate to proceed and adopt these technologies because profitability is often not assured. To increase the likelihood of improving the regulatory conditions for the semi-closed systems and to further develop them into more sustainable and feasible alternatives, there is a need for scientific documentation of fish performance, health, and welfare in these systems, in addition to holistic research on environmental functionality.

Because the semi-closed systems are relatively new production technologies, the scientific peer-reviewed papers documenting their environmental functionality and biological performance are scarce. Most of the available literature describes and documents the engineering aspects, particularly the hydrodynamic properties and construction (Gorle et al., 2018; Klebert et al., 2018; Kristiansen et al., 2018; Chu et al., 2020; Su et al., 2021). Further, there are only a handful of publications on production capacity (Nilsen et al., 2017) and health, welfare, and performance (Rud et al., 2017; Balseiro et al., 2018; Nilsen et al., 2019). Recently, it was reported that post-smolts from two types of semi-closed facilities had better growth, lower FCR (feed conversion ratio), and lower levels of cortisol compared to fish that had been raised in open net pens (Øvrebø, 2020). In general, these publications describe promising performance, health, and welfare in S-CCS compared to the traditional open net pens. However, most systems are still in a pilot/prototype stage, and further documentation of environmental functionality and biological performance is needed, as the systems are still developing. The scientific challenges with the prototypes are that most of them exist in one version, thus, without replicates, and reliable controls or reference cages. It is also important to note that various S-CCS have different system requirements, which are major contributory factors to how the fish behave and cope when reared in these artificial environments. Some of the technological developments being carried out recently include water and waste treatment, which play an important role in ensuring that the conditions of the rearing environment are optimal. It is being explored how to develop systems and processes at lower costs, as one of the major drawbacks for S-CCS and other closed systems are that the investment costs are much higher than those for traditional cages (Flimlin et al., 2008; Calabrese et al., 2017; Moe, 2021). Additionally, the Norwegian Food Safety Authority is requiring welfare documentation from all new technologies; hence, technology suppliers are obliged to document the welfare conditions of fish in these new environments. For the most optimal welfare monitoring and evaluation, the indicators for monitoring may be system- and procedure-specific (Noble et al., 2018).

One of the S-CCSs available at present is the FishGLOBE V5 ( Figure 1A ). It is a 3500 m³ S-CCS where water is pumped in through pipes from approximately 14 m deep. The facility is located in Lysefjorden, Rogaland County, Western Norway and is situated on a pier approximately 100 m from land. Each of the three pumps has a capacity of approximately 1000 L/s. The water retention time is 20 min. The system water is oxygenated and maintains dissolved oxygen at 90% saturation. Sludge is collected in the storage tanks. The production of fish starts with the delivery of smolt from the well boat and they are stocked through a hatch in the roof of the system. Feed is provided through perforated pipes that distribute feed from the storage compartment centred 1.5 m above the water level and horizontally to each side of the tank, thus securing even distribution of feed ( Figure 1B ). Fish appetite is observed with a submerged video camera or with a video camera placed at the filter. From the video, feed spill is evaluated, and feeding may be adjusted according to the fish appetite. When the fish reach the post-smolt size of over 800 g, they are transferred to the well boat via a pipe at the bottom of the globe.

The main aim of this study was to document the fish performance, health, and welfare in FishGLOBE V5 to support the further development of this semi-closed system. Extensive commercial use of FishGLOBE and other semi closed systems demand the security of fish health and welfare, in addition to their functionalities such as being free from sea lice and escapees. A monitoring plan was developed in which a group of production fish was followed during a production cycle from the pre-smolt RAS tanks on land to the FishGLOBE V5 tank in the fjord, and to the first months after transfer to net pens.

The study documented the health and welfare of a group of Atlantic salmon from a land-based RAS farm until three months after transfer to a net pen at sea, where, in between these productions, fish were reared in FishGLOBE V5, a novel semi-closed containment system. The use of semi-closed systems in the industry is still at a pilot stage, and most of the systems available still exist as one unit (N = 1) with no reference cage nearby. This has been considered a major challenge in several studies documenting the biological consequences of rearing fish in S-CCS prototypes (Balseiro et al., 2018; Karlsen et al., 2018), since we at present lack the possibility of controlled experiments with reliable replicates and controls. Also, most farmers are prohibited by regulations to place a reference cage so close to the semi-closed system. We are aware of these limitations, but still, the present study is needed to further develop the systems that are believed to be of importance for future salmon aquaculture since they have shown to be very promising regarding lice and escapees. One alternative in future study would be to replicate the study with new populations in the same system, but this also entails a certain limitation since populations differ in so many ways. This may require advanced mathematical models, so that monitoring data can be compared.

The water quality inside the FishGLOBE V5 was within the recommended levels for salmon post-smolts. Optimum hydrodynamics is important for even distribution of good water quality in all areas of the system to secure fish health and welfare. In addition, optimal hydrodynamics helps the self-cleaning of the system. Though not provided in this paper, the hydrodynamics inside the FishGLOBE system were monitored and found to be sufficient in ensuring that the parameters are within the limits supporting the biological requirements of post-smolts (Espmark et al., 2020).

The mortality inside the FishGLOBE V5 was low, at 1.4%, but increased to 4.9% during the first three months following transfer to net pens (S3). The mortality reported by the production staff was mainly attributed to weaker fish that did not grow. The weight from S0 to S3 was steadily increasing, with the best growth, in terms of SGR and TGC, during the 3.5 months that the fish stayed inside the FishGLOBE V5. The growth in FishGLOBE V5, in terms of SGR and TGC, is also acceptable with reference to commercial growth tables and reference studies (Espmark et al., 2017; Ytrestøyl et al., 2020). The drop in SGR and TGC from S2 to S3 was likely not associated with a drop in condition, but rather that the growth inside the FishGLOBE V5 was much better than during three months after transfer to cage, since SGR and TGC in the cages were not low compared to the mentioned references. Decreased growth right after transfer to new systems is expected, but in this case, the transfer from S0 to S1 did not cause a drop in growth, while transfer from FishGLOBE V5 to cage showed the expected drop. Also, the condition factor (CF) indicates that the fish at all sampling points were in good condition. The drop in CF from S0 to S1 may be due to sea water transfer (Alne et al., 2011). The second drop in CF from S2 to S3 may also be due to transfer. However, it is worth mentioning that the condition factor during S3 was approximately 1.2 and not low compared to benchmark studies (e.g., Ytrestøyl et al., 2020).

The cardio somatic index was higher inside FishGLOBE V5 (S1 – S2) compared to both phases before (S0) and after (S3). Larger hearts may be a consequence of training (Ytrestøyl et al., 2020) and can be seen as an indicator of good condition in fish. However, the benefit of increased CSI is not conclusive (Timmerhaus et al., 2021). Also, the hepatic somatic index showed differences between the phases, with significantly larger HSI during S1, and significantly lower HSI during S3. Smaller HSI during the sea phase (S3) was likely a direct effect of the fact that the fish was bigger in size. Also, larger HSI during the first period of the FishGLOBE V5 phase (S1) in relation to S2 and S3 can be fish-size-related. We would like to emphasise that the HSI was calculated from un-gutted fish. HSI is also considered to be an indicator of feeding status, where low feeding rate may result in smaller livers (Pandit and Gupta, 2019). Since in this study we did not systematically control and determine feed intake, we do not have reliable data to verify the difference in feed intake between the sampling points.

The general external welfare status was not sub-optimal and that rearing fish in FishGLOBE V5 did not have substantial negative consequences for the fish. Nonetheless, some indicators presented concerns for future consideration. Over 50% of the evaluated fish during the FishGLOBE V5 phase, regardless of whether it was during the early or late period of rearing, exhibited varying degrees of eye damages. Cataracts have many different causal factors, such as genetic conditions, nutritional deficiency, and water temperature (Hargis, 1991; Ersdal et al., 2001). Cataracts in fish are often considered irreversible, but osmotic cataracts have been reported to be reversible (Hargis, 1991). We could not establish the cause of the increased cases of cataracts, however, the moderate osmotic imbalance after sea transfer (S0 to S1) could be linked to the reported eye lesions. Around 25% of the evaluated fish demonstrated opercular shortening and allowing the fish to stay in FishGLOBE V5 prior to the open net pen did not aggravate the condition. Operculum deformities seem to have genetic causes, in addition to problems that occurred in early larval stages (Galeotti et al., 2000). This deformity can be an influencing factor for infection because gills are more exposed to the environment in malformed operculum (Lazado et al., 2020). This reflects some non-specific pathologies inferred from gill histology. The fish in this study showed increased snout damage from 6% to 17% from S1 to S3. This may originate from transport out of the FishGLOBE V5 and exemplify the issue of increased mechanical damage during transport in salmon (Sveen et al., 2020).

Fin erosion or damage is considered an important visible measurement of the welfare of the animals (Noble et al., 2007). These damages can be caused by several factors during production including handling, pumping, water velocity, density, pathogenic infection, and/or aggression (Timmerhaus et al., 2021). Salmon commonly exhibit fin damage at farms (Noble et al., 2007; Noble et al., 2018), and, usually, “mild” forms of damage (small area affected) are not potentially harmful at first, though when fish are crowded or stressed, and if water quality deteriorates, the condition rapidly increases in severity and causes serious damage to the affected animals. Except for dorsal fin damage, the FishGLOBE V5 phase did not worsen the cases of fin damage. The prevalence of dorsal fin damage with a score of 2 increased at the end of the FishGLOBE V5 phase but a considerable recovery was noted during the net pen phase.

Skin damage by visual gross scoring did not correspond well with the histology results, and the methods of evaluation could account for this difference. The gross scoring of skin evaluates the overall appearance of the skin surface while histological evaluation covers only a small portion. Nonetheless, both offer insights into the skin health status. The increased cases of higher scores (>2) for skin damage in S2 and S3 could be attributed to multiple factors, though very likely they were caused by the increased density (from S1 to S2) and handling and transport-related stress during transfer from FishGLOBE V5 to the net pen. Mechanical-related skin wounds are often an issue in salmon reared in high densities, and could be predisposing factors for infections (e.g. winter ulcers) (Sveen et al., 2020). In the future, it would be interesting to follow, at several time points during the FishGLOBE production, the skin gross appearance to establish whether rearing in this system has a pronounced impact on skin quality and the development of the damage. Nonetheless, most of the skin damage accounted for minor scale loss and haemorrhaging.

Nephrocalcinosis has been identified as a growing concern in salmon farming, especially in the last few years and it appears to be as common in flow-through as in RAS systems (Klykken et al., 2021). Around 20% of the fish evaluated during the RAS phase exhibited mild nephrocalcinosis (scores 1 and 2). Though there is an ongoing debate over the main causes of such a condition, water quality, such as the increased use of seawater during the hatchery stage, and CO₂ in RAS, have been implicated as potential causal factors (Fivelstad et al., 2018). It is important to highlight that the CO₂ levels were very low both during the RAS and FishGLOBE V5 phase and far below the threshold that has been shown to influence fish welfare (Skov, 2019). It is interesting to highlight that not a single case of nephrocalcinosis was identified during the FishGLOBE V5 and net pen phases. Despite the limited sampled fish, this suggests the possibility that the conditions in FishGLOBE V5 did not trigger nephrocalcinosis, or it could be that the condition in the system allowed the fish to recover. This supports the recent survey that mild nephrocalcinosis appears to be reversible after transfer to sea (Klykken et al., 2021).

Cortisol, the commonly measured primary stress hormone, was elevated in S1 to S2, compared to S3. In particular, the cortisol level at the beginning of the FishGLOBE V5 phase (S1) was lower than at the end period in the system (S2). In conditions that are less controlled, as in the present study, numerous factors might have influenced this profile, which could be difficult to single out. One example is the fact that the fish were sampled in different ways, because different systems required different strategies. While scoop-net was used in tanks and cage, the inserted net was used in FishGLOBE V5 to obtain a better representative sample in this huge system. However, the values were very similar to those of previous studies, demonstrating the cortisol response after stress induction in salmon. The values at S3 were close to the baseline resting cortisol levels in salmon. There were small but significant differences in plasma glucose level among sampling points, where fish sampled during the last part in FishGLOBE V5 (S2) and in sea cage (S3) phase had slightly higher glucose levels compared to the two first stages in RAS (S0) and early phase in FishGLOBE V5 (S1). The glucose ranged from 6 – 6.7 mmol/L and indicated mild stress, which partly indicates that the increased cortisol level was not chronic but an instantaneous response, likely due to handling-related stress during sampling. Despite the calming effect of anaesthesia, it is known that different chemicals, including Finquel, as used in this study can cause an immediate increase in physiological stress indicators such as cortisol. However, the increase is more severe if the fish are repeatedly exposed over time, and it is not expected to be large compared to the increase in cortisol because of handling (Zahl et al., 2012). In this study all fish were treated similarly, and Finquel was used for euthanasia.

Plasma magnesium is normally between 1 – 2 mmol/L. Levels deviating from this often indicate stress and a poor ability to regulate osmoregulation (Iversen and Eliassen, 2009). Here plasma Mg increased to 2.6 mmol/L in S2, indicating a slightly weakened seawater adaptation. The shift in salinity from 5 ppt in S0 (RAS) to 32 ppt in S1 and S2 (FishGLOBE V5) may have caused the significant increase in Mg (Bakke et al., 1991).

The skin and gills are multifunctional mucosal organs that represent the first line of defence in fish (Cabillon and Lazado, 2019). Because of their direct contact with the environment, their responses are often regarded as a reflection of the quality of the environment. Histological evaluation revealed that gill health status did not improve during the stay in FishGLOBE V5. Lifting, clubbing, and hyperplasia were the 3 pathologies that constituted the major deviations in the analyzed gill samples, accounting for 25 – 30% of the recorded cases. On the other hand, we can also state that rearing in FishGLOBE V5 did not worsen these structural alterations. As for the skin, the surface quality was significantly influenced during the early period in FishGLOBE V5. At the beginning of the period in the system, the skin surface exhibited roughness and, in many cases, the epidermal layer was missing. The skin significantly recovered at the latter phase of rearing in FishGLOBE V5, with an average score less than 2. This may indicate that the rearing environment provided a condition for the skin to partially recover. It may be that the elevated score in S1 is due to handling during transfer. Skin health, characterised by lesions and wounds, is a known issue for salmon during transportation from one production unit to another (Noble et al., 2018; Lazado et al., 2020; Sveen et al., 2020).

To further elucidate the observations in the skin, we performed gene expression analysis of key genes for tissue structure and integrity, stress, mucus, and antibacterial defence. The biological function of EMP3 is not fully known, but current evidence linked it to cell-cell interactions, control of cell proliferation, and apoptosis (Taylor and Suter, 1996). Both of the emp genes were significantly influenced by time in the current study, where a significant increase in expression from S1 to S2 was observed. It was earlier reported that these genes have been implicated in the skin development of salmon in S-CCS (Karlsen et al., 2018). Their pronounced regulation at the end of the FishGLOBE V5 phase likely contributed to the observed improvement in surface quality, as they may have facilitated the regeneration process. Two other genes, krt20 and thbs2, have key functions in the extracellular matrix (ECM) of skin (Agah et al., 2004; Rakers et al., 2010; Karlsen et al., 2018) and their increasing expression as the production progressed could be attributed to the changes relative to growth and further development of the skin in seawater (Karlsen et al., 2018). In a study by Sveen et al., (Sveen et al., 2019), ECM proteins were shown have key involvement in the early of wound healing mechanisms in the skin of salmon. The viscous mucus is an impervious gel, which acts as a lubricant, a physical barrier, and a trap for microbes (Linden et al., 2008; Osório et al., 2022). Here, we identified a significant temporal variability in the expression of mucin, the major glycopolymeric component of mucus, in the skin during the production cycle. We also observed that following transfer to FishGLOBE V5, mucin expression decreased, then was elevated again at the end of the rearing phase in the system. The elevated expression of mucins at S2 relative to S1 was perhaps a form of recovery response in the skin provided by the FishGLOBE V5 environment.

In summary, the monitoring programme employed in the current study documented the health and welfare of salmon from a RAS facility until transfer to a net pen. FishGLOBE V5, a S-CCS, was employed as a transition rearing phase between RAS and net pen. The adoption of S-CCS in salmon farming can shorten the time for the fish in on-growing net pens where they are more vulnerable to infection and erratic environmental conditions. Overall, we have shown that allowing salmon to stay for some time in FishGLOBE V5 did not significantly affect fish health, welfare and performance. Water quality in FishGLOBE V5 was within the threshold that supports a good rearing environment for salmon. This was reflected in the desirable growth, moderate mortality, and a potential recovery from nephrocalcinosis. The prevalence of eye, fin, and skin lesions following transfer to FishGLOBE V5 should be considered in the risk assessment with regard to the use of this S-CCS. Skin health may have benefited from the transition phase in FishGLOBE V5 based on the skin quality immediately after transfer compared to the end of the FishGLOBE V5 phase. Indications of stress induction including elevated plasma cortisol and magnesium levels should also be taken into consideration in the further development of this prototype. Future verification trials for FishGLOBE system and similar semi-closed systems should aim for including replicated systems and comparing reference net pens situated in locations with almost similar environmental conditions, including documentation of the temporal variability of different factors.

The raw data supporting the conclusions of this article will be made available by the authors on request and after the approval from all authors.

In accordance with the Norwegian Food Safety Authority, ethical review and approval was not required for this study.

Conceptualisation: AB and ÅE; Planning: ÅE, KTS and AB; Sampling: KTS, KS, BR, YM, JG, FS, and BE; Laboratory analysis: KTS and CL; Data analysis and curation: ÅE, KTS, and CL; Interpretation; ÅE, KTS, and CL; Visualisation and presentation: ÅE, KTS, and CL; Writing: ÅE, KTS, and CL. All authors contributed to the article and approved the submitted version.

The study was funded by Regionale forskningsfond RFF (ref 259057). The project is associated with the Centre of Research based Innovation project CtrlAQUA (RCN project #237856/O30).

AB was affiliated with FishGLOBE AS.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.