Intensive salmon farming is associated with high mortality rates, highlighting the need for new welfare indicators that can detect adverse conditions earlier and less invasively than many current approaches. Existing animal-based indicators used in the industry typically depend on subjective scoring and provide information mostly after welfare problems have already developed, thereby raising questions about their efficacy. Examples include emaciation, wounds, or scale loss, etc. Preliminary data and ongoing investigation suggest that melanin-based skin pigmentation may change dynamically with stress and condition in salmonid fishes. In this study, we present a semi-automated methodology for assessing changes in the grayscale intensity of melanin-based skin spots within the operculum region of adult Atlantic salmon (Salmo salar) kept in seawater. The pipeline combines computer vision models to detect the operculum, segment individual spots, and extract grayscale-based features for spot-level analysis over time. The method was applied to out-of-water images collected before and after exposure to a confinement episode. The results showed an overall shift in grayscale intensity from black to pigmentation fading after the challenge, although responses varied among individuals. These findings indicate that the proposed methodology can detect temporal changes in opercular melanin-based spots under applied experimental conditions. We therefore present this work as proof of principle for using computer vision to quantify changes in melanin-based skin spots as a potentially useful, non-invasive indicator of stress and welfare in Atlantic Salmon.

The salmon aquaculture industry is developing technologies to solve challenges with sustainability and poor animal welfare. A potential solution is to use semi-closed systems to mitigate lice infestations and provide more controlled rearing environments. Here, we tested Atlantic salmon in a novel semi-closed prototype cage and compared growth and welfare with a standard open cage to harvest sizes of similar to 5 kg. Additionally, data storage tags were used to record heart rate and activity in fish from both cage types. The semi-closed cage was subjected to a continuous photoperiod, constant moderate currents, and water supply from greater depths, while the open cage experienced a natural ambient surface environment. Growth trajectories differed throughout the trial, but final harvest sizes were similar in the two groups. Mortality was lower in the semi-closed cage, although welfare scores were worse, likely related to sub-optimal sampling methods. In both groups, heart rates were generally between 40 and 60 beats min(-1) and were influenced by seasonal temperatures. Owing to the continuous photoperiod, closed-cage fish showed weaker diurnal variations in heart rate and activity, while open-cage fish were calmer at night. Additionally, closed-cage fish had higher activity overall, indicative of higher swimming speeds. Finally, by using a refined tagging protocol, tag-related mortalities were prevented and tagged fish showed excellent growth following 7-months in sea cages. While data quality indexes did decline towards the end, this shows that bio-loggers can be used over long periods of fish production to record core welfare parameters for evaluating novel rearing systems.

Freshwater early rearing conditions can have long-lasting effects on feeding behaviour, physiology, and performance in Atlantic salmon during the seawater production phase. This study investigated how freshwater origin, recirculating aquaculture systems (RAS) versus flow-through systems (FTS), influences vertical behaviour, feeding dynamics, and thermal exposure throughout an entire seawater commercial production cycle.Across the seawater period, Atlantic salmon exhibited expected seasonal shifts in depth positioning and feeding behaviour. In summer, both groups avoided excessively warm surface waters, while in winter they avoided deeper cold layers, positioning themselves within thermally favourable zones supporting metabolic efficiency. Oxygen covaried with temperature but remained above known physiological thresholds for Atlantic salmon, suggesting that temperature was the primary driver of vertical behaviour.Clear behavioural differences emerged between production systems. FTS fish consistently occupied shallower depths than RAS fish indicating contrasting depth preferences. RAS fish exhibited more ordered and predictable vertical positioning over time, suggesting reduced behavioural flexibility. Feeding behaviour differed substantially between groups: RAS fish remained deeper before and during feeding events and were offered consistently shorter meals, reflecting reduced apparent appetite and coinciding with a marked decline in condition factor early in the seawater phase. In contrast, FTS stayed shallower during feeding and exhibited longer meal durations, particularly during summer and early autumn, suggesting higher feeding motivation coinciding with higher growth during early seawater phase.Overall, these results demonstrate persistent carry-over effects of freshwater rearing environment on behavioural plasticity, vertical positioning, and feeding performance in seawater, with implications for long-term production outcomes.

Efficient crowding of fish in sea cages for the purpose of transferring them to fasting, treatments or slaughter, is critical for insuring fish are moved quickly with minimal stress. With the development of larger offshore cages holding millions of fish and/or submerged cage salmon aquaculture, extraction of fish will potentially be more difficult. Particularly if farmers seek to remove portions of the biomass at a time without lifting cages to the surface. Using salmon innate swimming behaviours may aid to develop innovative, simple, welfare-friendly removal methods. Here we investigate whether the direction of crowding in a submerged cage influences the exit behaviour of salmon when extracted at depth for the purpose of pumping to a well-boat or otherwise. Using replicates of 46 large (similar to 4.3 kg) or 128 small (similar to 1.3 kg) Atlantic salmon and a prototype submerged cube cage (27 m(3)) fitted with a movable wall, we test to see if crowding salmon towards a 50 cm diameter circular opening in either the top, side, or bottom of the cage influences fish exit rate. Our results show that when crowding fish by incrementally reducing the cage volume by a factor of 12 over 25 mins, for both fish sizes similar to 80 % of fish exited the cage via sideways crowding, whereas only 20 to 50 % exited by top up or bottom down crowding directions. Furthermore, maximum relative fish densities reached during crowding tests were almost halved for sidewards crowding (37-43 kg m(-3)) compared to downwards or upwards crowding directions (59-73 kg m(-3)). We conclude that fish visualization of the exit hole and their natural circular swimming behaviour favoured sideways extraction. Hence, with a sea cage design that enables sideways crowding, it may be possible to extract fish quickly with minimal stress, and without needing to raise cages to the surface.

BackgroundChronic stress in aquaculture poses major challenges to fish welfare, health, and productivity. Detecting compromised welfare early requires validated biomarkers that link operational welfare indicators (OWIs) to underlying neuroendocrine states, including allostatic load and stress coping capacity. The central serotonergic system is a promising candidate for such validation, given its conserved role in stress regulation across vertebrates.MethodsWe sampled 40 Atlantic salmon (Salmo salar) from research-scale sea cages spanning a wide range of welfare states under production-relevant conditions. Fish were exposed to acute crowding during routine rearing operations immediately prior to sampling. Plasma cortisol was measured, along with serotonin (5-HT) concentration and turnover, proxied by 5-hydroxyindoleacetic acid (5-HIAA) and the 5-HIAA/5-HT ratio, in the brain stem and telencephalon. Associations among 5-HTergic variables, welfare indicators (OWI sum, scale loss, condition factor), and acute stress metrics (crowding time, cortisol) were analysed using principal component analysis (PCA), multivariate analysis of variance (MANOVA), and post hoc linear models.ResultsBrain stem 5-HT concentration and turnover were consistently associated with welfare-related indicators and loaded strongly with these variables in multivariate space, indicating sensitivity to cumulative stress and compromised welfare. In contrast, telencephalic 5-HT turnover aligned primarily with acute stress metrics rather than welfare state. MANOVA and post hoc analyses confirmed these region-specific associations. A significant interaction for telencephalic 5-HIAA revealed that fish in poorer welfare states exhibited a blunted 5-HTergic response to acute stress compared with individuals in better welfare condition, consistent with reduced stress responsiveness under chronic strain.ConclusionOur findings demonstrate a clear regional dissociation in central 5-HTergic stress processing in Atlantic salmon: brain stem 5-HT activity reflects chronic stress burden and welfare state, whereas telencephalic 5-HT turnover reflects acute stress reactivity conditional on baseline welfare. These results support the use of region-specific 5-HTergic measures as biologically grounded markers distinguishing chronic welfare compromise from functional acute stress responses in aquaculture, while emphasizing caution when interpreting acute stress reactivity in chronically stressed individuals.

A subpopulation of aquaculture salmon are characterized by abnormal swimming behavior, growth stunting and anorexia, as well as chronically elevated cortisol and brain serotonergic levels. This profile is associated with a depression-like state (DLS) and these fish are unable to respond to further stressors. Whereas the underlying causes behind this phenomenon remain elusive, the physiological profile strongly suggests that chronic stress plays a significant role in this phenomenon. We subjected Atlantic salmon to a chronic stress regime consisting of incremental increases in environmental CO2 concentrations during the freshwater phase for 68 days. Plasma corticosteroids, brain stem, hypothalamic and telencephalic serotonin concentrations and telencephalic whole transcriptome expression were then assessed under basal and acute stress conditions. We found that fish exposed to increased CO2 were characterized by a long-term increase in cortisol, cortisol+cortisone and serotonin (5-HT) signaling in the brain stem. Furthermore, in response to an acute confinement stressor, the CO2-treated fish increased their levels of cortisol and cortisol+cortisone, and decreased their cortisone/cortisol ratio. But unlike the control fish, they were unable to also respond to confinement by increased 5-HT signaling in the brain stem. In terms of their transcriptional response, post-stress gene regulation in CO2-treated fish was the opposite of that observed in control fish. We believe this profile is an example of allostatic overload, characterized by the inability to cope with stress. This profile is associated with DLS, suggesting that chronic stress may be an important factor leading to the development of the DLS phenotype in salmon.

Many species of fish, birds and mammals commonly live in human captivity; Atlantic salmon Salmo salar is one of them. The international legal status of the welfare of captive animals is slowly developing and still requires rigorous specification. For example, even though fish have complex cognition and elements of sentience, The United Nations’ animal welfare principles still take a functional health-centred perspective overlooking the cognitive-affective component. Wellbeing problems remain a major source of slow growth and high mortality in intensive aquaculture of Atlantic salmon. The value system for decision making in vertebrates is based on expectations of emotional wellbeing for the options available and is linked with the individual’s assessment of its future. We propose a new approach for monitoring and improving the welfare of salmon (or any other captive or wild vertebrate) based on modelling the salmon’s wellbeing system by digital twins, which are simulation models that implement major bodily mechanisms of the organism. Indeed, predictions on boredom, stress and wellbeing can all be captured by a computational evolutionary model of the factors underlying behaviour. We explain how such an agent-based model of salmon digital twins can be constructed by modelling a salmon’s subjective wellbeing experience along with prediction of its near future and allostasis (the bodily preparation for the expected near future). We attempt to identify the building blocks required in digital twin models to deliver early warnings about escalating issues that could eventually lead to negative effects on salmon health in aquaculture. These models would provide critical insights for optimizing production processes and could significantly reduce the reliance on animal experiments. Overall, reports of a population of digital twins could support the implementation of 3Rs - replacement, reduction, refinement - by offering actionable information to fish farmers as well as consumers, voters, politicians and regulators on relevant issues as well as guide experimental work on animal wellbeing across species.

Early rearing environment affects performance later in life. In Atlantic salmon (Salmo salar) aquaculture intensive smolt production has been linked to deviating cardiac morphology and increased mortality risks following stressful events during the marine production phase. To investigate the effects of early growth environment on later life-stages, two smolt groups were produced; a fast-growing group reared at 13 degrees C under continuous light and a slow-growing group reared at 6 degrees C under a natural photoperiod. The two groups were smoltified and transferred to 9 degrees C seawater at the same time and at similar sizes, although the slow smolts were approximate to 1000 day degrees older. Respirometry and swim tunnel experiments were performed to assess physiological performances along with morphological analyses of the hearts. We hypothesized that the slower growth trajectory would allow for the development of heart morphology more resembling that of wild salmon and that this should translate into improved physiological performance. Fast-growing smolt had more misaligned and enlarged bulbi as well as asymmetric ventricles compared to slow-growing smolt. However, contrary to our hypothesis, we did not find clear evidence for impaired physiological performance in fast-growing fish. That is, neither standard nor maximum metabolic rates, absolute critical swimming speed, stress recovery, or haematological parameters at fatigue differed between treatments. Mortality risks associated with deviating cardiac morphology first tend to occur in larger sized fish than investigated here. We therefore conclude that while early rearing environment clearly modulates cardiac morphology, recently seawater adapted Atlantic salmon do not yet show signs of compromised functionality associated with cardiac morphological differences at the whole- animal level. Future research should aim to incorporate larger sized fish in physiological experiments for a more appropriate representation of the latter production phase in Atlantic salmon aquaculture and its associated fish welfare problems.

Largely unpredictable, gelatinous plankton blooms are a growing concern for the ever-expanding mariculture industry. Recently, a bloom of a colonial hydrozoan known as Apolemia sp. was detected via a citizen-science initiative, Nye.dugnadforhavet.no, in mid-Norway in July 2023. By November 2023, the bloom spanned the entire 2500 km Norwegian coast, adversely impacting the aquaculture industry nationwide. This report describes the clinical presentation and pathology on farmed Atlantic salmon (Salmo salar) caused by an unprecedented bloom of Apolemia sp. at the Austevoll station of the Institute of Marine Research in Norway. Shortly after the Apolemia sp. arrived, many fish showed impaired swimming capacity and erratic behaviour. Over the following days, several fish displayed wounds on their eyes and skin, while daily mortality in some cages exceeded 17%. Histopathological analyses on moribund fish revealed acute gill damage and secondary complications, including necrosis and bacterial infections, in addition to ongoing amoebic gill disease (AGD). Blood biomarker profiles indicated severe physiological stress and organ dysfunction. Ultimately, all affected fish on site were euthanized due to welfare concerns, as was also the case for several other Norwegian salmon farms. This underscores the necessity for enhanced monitoring and mitigation strategies to protect farmed fish from such harmful events. The events of 2023 in Norway highlight the vulnerability of aquaculture to gelatinous plankton blooms and the necessity for research and innovation to develop effective monitoring and management practices.

Waste feed remains a major issue in open sea-cage Atlantic salmon aquaculture. "Echofeeding" is an appetite-led feeding method that stops meals based on fish biomass detected by an echo sounder. The method reduced waste feed and upheld fish growth in a relatively vertically unstratified coastal farming environment. Here, we tested echofeeding at a commercially relevant scale over an 8-month period in a fjord environment with seasonal vertical temperature and salinity gradients. We compared fish behaviour and growth between echofed fish, fed at high intensity and near surface, and control fish, with feeding regulated by pellet detection without surface feeding restriction (conventional practice). Growth (SGR>1.81) and FCR (<0.87) were excellent and similar for three months after sea-transfer in August. However, a strong halocline in late November (<5 degrees C surface water) led echofed fish to avoid surface feeding, resulting in underfeeding. Following the setting of a deeper depth interval for triggering feeding, the echofed fish fed more, and fed at similar levels to control fish when feeding intensity was reduced. Echofeeding underperformed in early spring as rising surface temperatures attracted salmon, making it difficult for the system to distinguish between feeding and routine behaviours. Both groups contracted salmonid alphavirus during winter, reducing appetite and promoting early harvest. Results highlight the need for echofeeding to take environmental changes into account. Further, as fish grew, a gradual decline in the echo signal measured during feeding suggests a method for refining meal termination threshold to minimize waste feed while maintaining good fish growth.

Feeding fish in excess is a common practice in the farming of Atlantic salmon to maximise fish growth and profitability. Since feed accounts for a large percentage of a farmer's operational expenses, periodic reduction of feed utilization that still enable to maintain production performance in the long run is a possible strategy to mitigate farmers economic costs and reduce feed waste. In this study we investigated how large (>3.3 kg) sea caged Atlantic salmon adapt to feeding being restricted from daily to every third day in an intermittent fasting regime for 6 weeks, followed by a 5-week refeeding period before harvest in June. Fish in control cages were fed daily, except for three days to compare feed intake with the intermittently fasted fish at the end of their fasting period. Both groups were fed based on visual observations and assessments using monitoring cameras. Fish growth, feed intake, gastrointestinal fullness, and endocrine signalling related to energy homeostasis and appetite regulation were analyzed. Salmon fed to satiation every third day gradually increased their feed intake over the first four weeks of intermittent fasting and showed a 3-fold increase in stomach content after feeding when analyzed at 3 and 6 weeks of intermittent fasting. Growth rate in intermittently fasted fish was reduced by similar to 50 % and feed conversion ratio increased by similar to 130 % compared to the control group. This was accompanied by a decrease in HSI and plasma levels of Ghrl as indicators of energy imbalance and an increase in hypothalamic mRNA expression of agrp1, a neuropeptide likely involved in glucose sensing and acting as feeding stimulant. At the end of the 6-week intermittent fasting regime, the fish had elevated levels of plasma cholesterol, glucose, and lactate, suggesting reallocation of energy sources and metabolism. The metabolic adjustments did not appear to be detrimental to the fish during the intermittent fasting regime applied in this study, as they continued to grow, albeit at lower rate than fully fed fish. Further, after returning to daily feeding regime, the intermittent fasted fish showed a tendency toward compensatory growth and differences in the endocrine signals related to appetite regulation and energy homeostasis were abolished. The present results provide novel insights into the physiological mechanisms occurring during intermittent fasting in Atlantic salmon and demonstrate a feeding regime that can be applied during periods of limited food availability, or when farmers aim to achieve strategic growth targets in commercial production.

The expansion of aquaculture production into more exposed harsh and remote ocean environments presents both new opportunities and challenges. To manage the complexities of exposed operations, research into fish welfare, personnel safety, and facilitating technology is thus of key importance. This paper reviews recent research advances in the areas of safety, fish welfare, and technology, while the focus is on the Norwegian salmon farming industry, the results could benefit exposed fish farming internationally. Regarding fish welfare, the study summarizes the current knowledge status of salmon coping abilities and welfare indicators in strong currents and waves. On the safety front, there has been significant progress in operational safety management, accident analysis, and emergency preparedness, all of which are crucial for human personnel in these demanding settings. Human safety and fish welfare also rely on structures and equipment, and recent research results include advances in environmental load analysis, vessel design, simulations of fish farms. Notably, the development of contact-free, autonomous lifting operations, and hole detection methods represents a significant leap in maintaining aquaculture infrastructure. This multidisciplinary study underscores the need for integrated research approaches to address exposed aquaculture, emphasizing that while recent innovations have enhanced safety and robustness, ongoing research and new strategies are critical for safety and fish welfare in exposed aquaculture operations.

A proportion of farmed salmon in seawater show a behaviorally inhibited, growth stunted profile known as a depression-like state (DLS). These DLS fish are characterized by chronically elevated serotonergic signaling and blood plasma cortisol levels and the inability to react further to acute stress, which is suggestive of chronic stress. In this study, we characterize the neuroendocrine profile of growth stunted freshwater parr and confirm that they show a DLS-like neuroendocrine profile with a blunted cortisol response and no serotonergic increase in response to acute stress. Furthermore, we attempted to reverse this DLS-like profile through pharmacological manipulation of the serotonin (5-HT) system with buspirone, an anxiolytic medication that acts as a serotonin receptor agonist (i.e., decreases serotonergic signaling). We found that while buspirone decreases anxiolytic-type behavior in healthy fish, no quantifiable behavioral change was found in DLS-like fish. However, there was a physiological effect of diminished basal serotonergic signaling. This suggests that at the physiological level, buspirone appears to reverse the neuroendocrine DLS profile. With a deeper understanding of what causes DLS profiles and growth stunting in juvenile fish, steps can be taken in terms of husbandry to prevent repeated stressors and the formation of the DLS profile, potentially reducing losses in aquaculture due to chronic stress.

The quality of smolts is one of the key factors for successful salmon production. We compared growth performance, physiological traits, and environmental adaptation in Atlantic salmon (Salmo salar) after transfer into a sea cages farm site from recirculating aquaculture system (RAS) to a similar sized group of the same genetic linage and produced in the same husbandry facility but under flow through system (FTS) conditions. Fish vertical distribution within the sea cages and water environment were continuously recorded, while biometry data and biological samples were collected monthly. No significant difference in size and smolt development between the FTS and RAS-produced fish were observed at the end of the freshwater phase. However, after transfer into seawater, the RAS fish showed an array of physiological and molecular differences that were maintained and resulted in significantly increased mortality and lower growth over the full production cycle. The RAS smolts were characterized by lower body weight, length, K factor, HSI (indicator of energy reserve), NKA activity, plasma levels of lactate, triacylglycerol, sodium, calcium, and phosphorus than FTS fish in seawater. This implicated osmoregulatory and allostatic maladaptation for the RAS fish during the first months in seawater. Both FTS and RAS fish showed physiological and behavioral adjustments during the seawater production linked with predictable (e.g., seasonal, diurnal) and short-term unpredictable variation in water temperature and oxygen. However, the RAS fish were characterized by lower physiological response to stressful effects of unpredictable environmental variability.Based on our data, the freshwater history set the basis for the performance success of the FTS and RAS smolts in seawater. FTS fish showed faster adaptation to the seawater environment, with higher osmoregulatory capacity, and higher physiological robustness to seasonal changes than RAS fish, which reflected in their difference in growth and harvest size. We emphasize the crucial role of the freshwater rearing conditions as the foundation for successful physiological adaptation to seawater, growth, and the success of salmon farming production.

Salmon lice (Lepeophtheirus salmonis) are the most severe health challenge facing Atlantic salmon (Salmo salar) aquaculture. Infestations threaten farmed and wild salmonids, drive up production costs, and limit industry growth. Submersible sea-cages can reduce infestation pressure by holding salmon below the surface layers where lice larvae are most abundant. However, submergence leads to loss of buoyancy over time, as salmonids must periodically refill their swim bladders by gulping air. Pockets of air held at depth within 'air domes' enable salmon to gulp air, but air domes are large and unwieldy structures. A more intuitive solution may be to deliver a stream of air bubbles at depth, as bubbling is commonly used in sea-cages to promote vertical mixing, and salmon interact with these bubbles. Here, we conducted a pilot study to test whether newly sea-transferred salmon would use bubbles for buoyancy regulation. We conducted 3 trials in sequence, each involving 15-19 days of continuous submergence, and recorded buoyancy-related behaviors in 1 cage with full surface access, 1 cage submerged without air, and 2 cages submerged with air bubbling. Bubbling was provided continuously in the first two trials, before being reduced to 90 min day(-1) in the third trial. Salmon submerged with bubbling maintained their swim bladder volume and swimming speed at similar levels to the cage with full surface access throughout all trials, indicating that they successfully refilled using bubbles. The same parameters deteriorated in salmon in the cage submerged without air. Acclimation to bubbles took similar to 3 days whether at the surface or after submergence, indicating that several days of bubbling before submergence may be beneficial. Some fish accumulated fluid in the swim bladder during submergence (more severe in fish submerged without bubbling). Together, the results suggest that bubbling can improve outcomes for salmon undergoing prolonged submergence. Moreover, bubbling offers an alternative to air domes that is less technically demanding, provides a greater spatial distribution of air, and supports welfare through behavioral enrichment.

The hearts of salmonids display remarkable plasticity, adapting to various environmental factors that influence cardiac function and demand. For instance, in response to cold temperature, the salmonid heart undergoes growth and remodeling to counterbalance the reduced contractile function associated with dropping temperatures. Alongside heart size, the distinct pyramidal shape of the wild salmonid heart is essential for optimal cardiac performance, yet the environmental drivers behind this optimal cardiac morphology remain to be fully understood. Intriguingly, farmed salmonids often have rounded, asymmetrical ventricles and misaligned bulbi from an early age. These deformities are noteworthy given that farmed salmon are often not exposed to natural cues, such as a gradual temperature increase and changing day lengths, during critical developmental stages. In this study, we investigated whether natural environmental conditions during early life stages are pivotal for proper cardiac morphology. Atlantic salmon were raised under simulated natural conditions (low temperature with a natural photoperiod; SIMNAT) and compared with those reared under simulated farming conditions (SIMFARM). Our findings reveal that the ventricle shape and bulbus alignment in SIMNAT fish closely resemble those of wild salmon, while functional analyses indicate significant differences between SIMNAT and SIMFARM hearts, suggesting diastolic dysfunction and higher cardiac workload in SIMFARM hearts. These findings highlight the profound influence of environmental factors such as water temperature and photoperiod on the structural development of the salmonid heart, underscoring the importance of early environmental conditions for cardiac health.

Submerged cage technology may enable placement of salmon farms at more exposed sites whilst avoiding the impacts of damaging weather on farm structures. Field trials have demonstrated that Atlantic salmon grow well in submerged cages for a full production cycle if an air dome is provided to enable salmon to refill their swim bladders at depth, but on occasion submerged fish have slower growth and poorer fish welfare. This could be due to submerged fish experiencing less optimal environmental conditions, the physiological effects of holding fish submerged at depth, restricted access to surface air, or an interaction between these factors. Here, we designed an experiment to remove possible environmental and depth-related effects, which enabled a direct test of the effect of restricted surface access to air due to submergence. Atlantic salmon were cultured for 11 months in either standard surface cages or submerged cages with air domes installed with a ceiling at 1-m depth to stop normal surface access. Environmental conditions were similar in surface and submerged cages. Restricted surface access had minimal effect on fish behaviour, growth, condition, welfare and harvest quality of submerged fish relative to fish held in standard surface cages. Backscattering from echo signals showed that the volume of air in the swim bladders of submerged and surface cage fish was similar, indicating submerged fish captured enough air via the air dome to maintain neutral buoyancy throughout the production period. These findings help progress commercial use of air domes and submergence, as they show the feasibility of maintaining optimal growth rates in Atlantic salmon when submerged for long periods if environmental conditions during submergence are equivalent or better than surface conditions.

Atlantic salmon will experience periods of fasting during its lifecycle. In nature, prolonged fasting periods occur owing to seasonal fluctuations in available feeds, migration or in relation to reproduction. In a culture setting, salmon is fasted mainly as part of planned operational handling prior to vaccination, delousing, transfer etc., and where fasting may last up to nine days. The mechanisms regulating the appetite during long-term fasting may vary among fish species. Here, we studied the impact of long-term fasting on neuro-endocrine regulation of appetite through the stomach-hypothalamic axis in Atlantic salmon post smolts (1.2 kg, similar to 46 cm), reared in two experimental conditions (Fed and Fasted; triplicated tanks), and sampled after 4 weeks and 6 weeks of fasting. Fasted fish showed lower condition factor and hepatosomatic index at both sampling points compared to Fed group. In qPCR analysis, hypothalamic relative mRNA expression of agouti-related protein 1 (agrp1) was upregulated in fasted group at both sampling points. Among neuropeptide Y (npy) paralogs, only npya1 at 4 weeks was upregulated by fasting. As for cocaine- and amphetamine-regulated transcripts (cart), cart2a was elevated at 4 weeks, and cart2b at both 4 and 6 weeks in fasted group, while cart3a and cart4 showed no response to fasting. The pro-opiomelanocortin (pomc) a1, a2 and melanocortin-4 receptor (mc4r) a2 increased only after 6 weeks of fasting, while mc4rb1 did not respond to fasting. In stomach, 6 weeks of fasting resulted in a decrease of ghrelin1 (ghrl1), while expression of mboat4 was unaffected. The elevated levels of hypothalamic agrp1 and npya1 in fasted group support orexigenic roles for these neuropeptides. In addition, upregulation of cart2a, cart2b, pomca1 and pomca2 indicate that these play vital roles in appetite regulation and that fasting may halt and/or counteract hunger signals (agrp1 and npya1) to save energy from foraging search activities during catabolic conditions. Another possibility is that these neuropeptides play a role in fasting-induced stress. Based on the drop in mRNA expression of ghrl under catabolic conditions, we hypothesize that Ghrl might return as hunger signal once feed becomes available. We also propose that agrp1 is a potential appetite biomarker gene under feed deprived conditions.