Refining approaches to measuring, monitoring and appraising animal welfare in aquaculture research is key to (i) protecting and optimizing it, (ii) documenting the severity of how and when it deviates, and (iii) ensuring good scientific quality, reliable results and reproducibility, amongst other factors. However, different fish species and life stages can have varying welfare needs and assessing their welfare can be challenging. An array of welfare indicators (WIs) can be utilized when documenting fish welfare, and there is currently little consensus on which WIs are most applicable to the key fish species used in European aquaculture research. The aim of this review is to propose updated, fit for purpose and comprehensive WI toolboxes for aquaculture research involving Atlantic salmon ( Salmo salar ), rainbow trout ( Oncorhynchus mykiss ), European seabass ( Dicentrarchus labrax ), gilthead seabream ( Sparus aurata ), and the common carp ( Cyprinus carpio ). Where possible, these toolboxes will also include life-stage considerations. It also provides information on utilizing WIs in deciding humane end-points as well as information on how to sample different types of indicators. The review closes with information on how digitalization can affect the collection, collation and analysis of WI data in aquaculture research, including both practical and theoretical considerations. The toolboxes incorporate a range of WIs that go beyond those required for legally safeguarding fish welfare in both laboratory and operational experimental facilities in the current European 2010/63/EU Directive on the protection of animals used for scientific purposes and its amendment, the Commission Delegated Directive (EU) 2024/1262.

The welfare of non-human animals is central to ethical discussions on animal use, with increasing attention to fish welfare across research, aquaria, aquaculture, and fisheries. This paper reviews current theoretical approaches to animal welfare and recent advances in defining and assessing fish welfare since the seminal paper by Huntingford et al. (2006; J Fish Biol 68: 332-372), highlighting the growing role of cognitive and affective processes. It also includes the concept of positive welfare and some of the current research advances in this field. Methods for measuring, monitoring and assessing welfare via the utilisation of outcome- and input-based indicators are outlined, ranging from practical operational tools to laboratory-based measures. Welfare concerns in wild-capture fisheries are examined in relation to stress, flesh quality and sustainability, including the welfare of released fish. Recent advances in fish neurobiology, cognition and pain perception are summarised, together with technological innovations that enhance welfare monitoring and management. The paper also explores the relationship between fish welfare, sustainability, public concerns and consumer demand, and legal and moral recognition across contexts, situating fish welfare within the 'One Health' and 'One Welfare' frameworks that link animal welfare, environmental stewardship and human well-being. Ongoing challenges include climate change, cultural factors and the interpretation of fish sentience and cognition among others.

Harmful algal blooms (HABs) are a threat to fish welfare, occurring suddenly and unexpectedly causing significant consequences for fish and salmon farmers worldwide. Norwegian farmers have been facing this challenge at irregular intervals since the very beginning of the industry. This report describes the events on the first fish farm affected by the spring HAB of 2025 in northern Norway. Specifically at the production site Fornes, which at the time of the strike had more than 1 million Atlantic salmon (Salmo salar) with an average weight above 3 kg in its sea cages. First signs of the event were reduced fish appetite and cloudy water appearance, followed by changes in fish behaviour and acute mortality of 39.5%. Water samples showed dominance of the phytoplankton species Phaeocystis pouchetii, followed by Chrysochromulina leadbeateri. Necropsy of the fish and histopathological lesions on the gills and liver supported that the mortality was caused by the algae. The farm's contingency plan was enforced immediately when the acute high mortality was observed. Including, but not limited to, feed withdrawal followed by emergency harvest of the whole production site. The dramatic incidence at Fornes highlights the urgent need for monitoring and early-warning systems for HABs, as well as further development of suitable mitigation strategies and contingency plans to minimise the effect of HABs once they have been forecasted or detected.

Injuries from lice treatments are ranked as a major cause of mortality and poor welfare for Atlantic salmon in marine net pens. As a result, scoring schemes based on the external appearance of injuries and severity are now frequently used. One such recently developed scheme for farmed salmon is the LAKSVEL protocol, inspired by previous schemes such as the SWIM models and the FISHWELL scoring system. Here we review the injury-based welfare indicators (WIs) in the LAKSVEL protocol that can be associated with and induced by handling: Scale loss, Skin bleeding, Body wounds, Snout wounds, Eye injuries, Operculum damage, Gill status, and Fin damage. We used semantic modeling as a formalized procedure to attain information from the literature on how each of these injury-based WIs is likely to affect the injured fish in the days and weeks to come. Based on the reviews, we then followed the semantic modeling procedure to define weighting categories and score the welfare consequences of each WI and their levels. Results showed that injuries to eyes, gills, and snout are particularly harmful to the fish, followed by penetrating body wounds, operculum damage, and fin damage; but also injuries such as scale loss and skin bleeding can have severe consequences, especially if they indicate unseen internal injuries. It is our hope that the presented reviews and modeling of welfare consequences will be a valuable resource for fish farmers and fish health personnel in their decision-making processes and thereby contribute to improved fish welfare in the salmon aquaculture industry.

Vertebral deformities are a big challenge in Rainbow Trout seawater aquaculture. However, supportive scientific literature is missing. The present study used radiology to follow the development of vertebral deformities in a population of farmed Rainbow Trout as they grew from 36 g to 5.5 kg. In addition, separately collected deformity screening data from three other farmed populations were included for comparison. The fish developed deformities in different vertebral regions over time, eventuating to affect almost the entire vertebral column. At this point, the fish were similar to 5.5 kg and 93 % of the fish had one or more deformed vertebrae. A negative relationship between severity of deformity (number of deformed vertebrae per individual), and fish length and weight strongly suggest a negative impact on fish welfare. The most frequently affected area was the ural region of the vertebral column. This region is a part of the caudal fin complex which also suffered from degradation of the fin rays. Ural region deformities were also frequent in the separately investigated populations. The current results indicate that Rainbow Trout are not able to maintain normal bone development under current farming conditions, consequently, jeopardizing the welfare of the fish.

Crowding is a common aquacultural procedure and its monitoring and management is key to limiting its potential detrimental effects upon the fish. Crowding intensity in Atlantic salmon aquaculture is typically assessed manually using surface-based observations, such as changes in surface activity, or the numbers of dorsal fins or white sides that are visible during inspections of the crowd. However, surface observations, whilst having utility in many settings, do not always equate with the sub-surface behaviours of fish and the potential welfare risks that occur underwater. Here we suggest an underwater crowding risk scale for Atlantic salmon held in net pens based on underwater observations for crowd monitoring using a Remotely Operated Vehicle (ROV). The scale is based on a survey of fish farm professionals, fish welfare researchers and fish health students. A crowding event will typically fluctuate between different intensity levels over time and intensity and risk level can change in an instant. The scale is therefore based on what an ROV operator can see within a 10 s time frame, so that the ROV operator can help guide the staff managing the crowd in their corrective actions to lower the risk. This is a first attempt to make such an ROV-scale and to introduce stakeholders to the concept. It is therefore labelled v0.1. Given the importance of crowding for fish welfare, this study should be quickly followed up by work to improve and enhance the scale.

More Atlantic salmon (Salmo salar) post-smolt production is occurring at stable temperatures of 10-17 degrees C today, compared to the fluctuating ambient conditions used in the past. This increases growth, but also triggers unwanted sexual maturation. The latter can be avoided by using triploid females as they show no gonadal development during their life. However, triploids are believed to have a lower temperature optimum and suffer more from cataracts than diploids. Here, we assess the performance of mixed sex sibling circa. 260 g diploid and triploid post-smolts reared under one of eight temperatures from 3.0 to 20.5 degrees C (in increments of 2.5 degrees C) for 140 days in seawater and on continuous light. We assessed growth, sexual maturation, ocular cataracts, liver and heart size, and blood parameters associated with osmoregulation, anaerobic metabolism, and oxygen delivery. Cataract severity increased with temperature > 15.5 degrees C in diploids and > 5.5 degrees C in triploids, with the triploids at 20.5 degrees C being euthanised due to their severity. In male and female diploids, and male triploids, the prevalence of puberty increased with temperature. There were no pubertal diploids <= 5.5 degrees C, but all were pubertal >= 18 degrees C. In diploid females, the prevalence of puberty was 0 % <= 13 degrees C, but reached 76 % at 20.5 degrees C. Triploidy reduced the prevalence of male puberty to 75 % at 18 degrees C, whereas no triploid females entered puberty. In both ploidies, growth over the entire trial period was greatest at 10.5 degrees C, in which immature fish reached an average weight of circa. 1.2 kg. In general, triploids were significantly heavier than diploids >= 10.5 degrees C due to the higher levels of male maturation in diploids, but there was no ploidy effect on growth at any temperature in immature female fish. Triploids had larger livers (by 8 %) irrespective of temperature, whereas they tended to have smaller hearts <= 15 degrees C (1-7 %), but heavier hearts >= 18 degrees C (1-8 %), compared to diploids. Irrespective of temperature, plasma lactate was generally 17 % higher, but plasma chloride 1.5 % lower, in triploids. However, other ploidy effects, such as reduced haematocrit and haemoglobin (by up to circa. 30 %) in triploids, were time and temperature dependent. In conclusion, i) 10.5 degrees C was the optimal temperature for growth, ii) stable high temperatures induce puberty in post-smolts, and iii) triploids showed similar growth to diploids, lower levels of sexual maturation, but generally had a higher prevalence of cataracts.

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.

Reduced feed intake is a common stress response in Atlantic salmon (Salmo salar), yet the interaction between the stress-related hypothalamic–pituitary–interrenal (HPI) axis and the appetite-regulating hypothalamic melanocortin system remains poorly understood. This study explored the potential cross-regulatory interaction between these two systems by assessing feed intake, gastrointestinal content, and hypothalamic mRNA expression of crf1 paralogs, a key factor in stress handling, and appetite-related genes npy, agrp1, pomc, and cart in Atlantic salmon parr exposed to 21 days of unpredictable chronic stress, followed by a novel acute stressor. Our results indicated that regulation of the HPI axis and appetite-regulatory mechanisms appeared to depend on the type and duration of stress. Chronic stress reduced feed intake and gut content and increased feed conversion ratio, without changes in hypothalamic crf1 paralog expression but with reduced orexigenic npya1 transcript levels. Simultaneously, levels of the anorexigenic cart2b transcript variant were also reduced, possibly reflecting a compensatory response to prolonged appetite suppression. In contrast, exposure to the novel acute stressor induced a transient increase in crf1 paralogs and upregulated orexigenic neuropeptides npya1 and npya2, suggesting compensatory regulation to counteract stress-induced anorexia. In contrast, cart2b expression normalized to control levels, possibly reflecting an acute stress-induced compensatory response restoring appetite regulation. These results highlight a dynamic, stress-duration-dependent interaction between stress and appetite-regulatory systems in Atlantic salmon.

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.

Mechanical delousing methods are widely used in Atlantic salmon aquaculture, however, the effects of these treatments on fish welfare remain underexplored. This study followed a commercial production of Atlantic salmon that underwent three subsequent mechanical delousing treatments. The study focused on pathological changes in the gills, thymus, nasal cavity, skin, and hearts of the fish. The sampling was done every sixth week over a period of 8 months. Additionally, the study investigated changes in growth and mortality and the expression of specific immune and stress genes in the gills and thymus using qPCR analysis. Comparison of data from samplings before vs. after delousing showed reduced specific growth rate, increased mortality, more external injuries, cellular stress in the gills, and pathological changes in the exposed organs after all treatments. The immunohistochemistry showed that the number of PCNA-positive lymphocyte-like cells increased after each treatment, and especially after the third. At this sampling, we also observed degeneration and necrosis of pillar cells, and an increase in the number of mucus cells in the gills. However, the data does not allow us to determine whether these additional changes after the third treatment were due to a gradual weakening of the fish from repeated treatments or other co-correlating factors. An interesting observation near the end of the study was a significant increase in PCNA levels in the thymus and that the study of T cell markers showed a simultaneous increase in TCR delta and CD28.

We compared the performance and physiological response of diploid and triploid Atlantic salmon ( Salmo salar ) following 21 days of either unpredictable chronic stress (UCS) or no disturbance (control). Thereafter, all four groups were challenged with a novel acute stressor consisting of crowding, netting and transfer into another tank. Growth was monitored during the 21-day period, and stress response to the novel acute stress challenge was assessed by blood chemistry parameters and pituitary gene expression. Plasma parameters were analyzed before (0) and 15, 30, 45, 60, 90, 120, 240, and 300 min after acute stress induction, whereas gene expression was analyzed at 0, 30, 60, 120, and 240 min. The results showed: a) similar growth rate and condition factor reductions from UCS in triploids and diploids, b) higher post -stress plasma Acth and cortisol levels in triploids compared with diploids, c) wider post -stress fluctuations in most of the other plasma parameters for triploids compared with diploids, particularly for glucose and lactate, d) pomca1, pomcb, crf-r1a and crf-r1b mRNA levels were more upregulated in control diploids than control triploids, and e) only minor changes in pituitary gene expression from UCS in both diploids and triploids, but with a noticeable upregulation of pomca2 and crf-r1a and crf-r1b in the triploids. Our results suggest that triploid Atlantic salmon have a higher energy cost in response to stress than diploids salmon. This can make them more sensitive to suboptimal conditions and episodes of acute and chronic stress.

Periods of fasting occur for a multitude of reasons in Atlantic salmon aquaculture. Feed withdrawal is widely used prior to transport, parasite treatments, preslaughter and for depuration purposes in recirculating aquaculture systems. Voluntary fasting is a coping response when fish have poor health or are exposed to poor farm environments. Owing to increased attention to animal welfare in aquaculture, concerns have been raised regarding ethical issues when farmed fish are subjected to fasting. However, thorough science-based recommendations for fasting and feed-withdrawal regimes have been lacking. The purpose of this review is to provide a synthesis of the various causes for fasting in Atlantic salmon aquaculture and evaluate their associated welfare implications so that guidelines for appropriate practices can be formulated. To interpret impacts, we describe biological responses and tolerance limits to fasting in Atlantic salmon and consider adaptations in the wild. Fry and parr are highly sensitive to feed withdrawal. However, post-smolts and adults are well-adapted to endure prolonged fasting without experiencing compromised functionality or health. Here, short periods of feed withdrawal prior to operations should therefore not constitute significant welfare concerns. Serious concerns are instead associated with voluntary fasting that may continue for weeks. We emphasize that environmental extremes that exceed appetite impairing thresholds must be avoided. Additionally, farmed fish should not be subjected to practices that lead to chronic stress that induce cessation of appetite. Diseases or parasites that impair appetite should also be mitigated. Fasting is here a symptom rather than a cause for poor welfare.

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.

A 5-level crowding intensity scale for directing and auditing the crowding of Atlantic salmon in sea cages based on surface observations is currently included in standards, manuals, and guidelines for fish farmers. Here we test the feasibility of using this scale to create distinct crowding levels, the effects of these different levels upon fish welfare, and the suitability of a set of possible operational welfare indicators (OWIs) and laboratory-based welfare indicators (LABWIs) to be included in toolboxes for monitoring and assessing fish welfare in relation to the crowding of salmon in sea cages. Crowding level 1 was not included in this study since this is a very light level of crowding, and also not level 5 as this level clearly would harm the fish and lead to mortalities. We were able to use the scale to create three distinct crowding levels in two of three separate crowding events in 12×12m2 sea cages. Although the farm personnel were experienced, it soon became evident that underwater monitoring of fish behaviour and how the net was tightened around the fish was important to make sure that no pockets or irregularities that could harm the fish were formed during the crowding. Despite evidence of increased stress and epidermal damage with increased crowding intensity, there were no clear indications that this led to any long-term detrimental effects on fish welfare. In conclusion an OWI-toolbox for crowding should include both surface and underwater observations, monitoring of oxygen conditions, and morphological injury data to steer decisions to prevent welfare problems and mortalities. In addition, qualitative assessment of fish behaviour, plasma cortisol, and skin histology can be included in a LABWI-toolbox if more in-depth information on the effects from the crowding is wanted.

Dip nets are widely used to retrieve fish from the water but can cause injuries to the fish. The objective of this study was to document the potential effects of dip net mesh size with respect to external injuries, more specifically fin splits and scale losses, in Atlantic salmon (Salmo salar). The study included 273 post-smolt Atlantic salmon from two fish batches referred to as the Tank group (w- = 178 ± 36 g, n = 198) and the Cage group (w- = 1319 ± 334 g, n = 75). Four identical dip nets with mesh sizes of 5, 10, 15, and 20 mm, respectively, were used to net fish out of their enclosures before the external injuries were assessed by visual inspection. The results showed that the number and severity of fin splits increased with dip net mesh size, but no effect of mesh size on the percentage of scale losses was found. Dip-netting of five fish at a time instead of one with the 20 mm dip net, had no statistically significant effect on the examined injuries. It is concluded that the use of dip nets with smaller mesh sizes can be positive for fish welfare by reducing fin splitting.

Since salmon lice have developed resistance to pesticides, non-medicinal delousing treatments have become increasingly used to remove the parasite from fish on salmon farms in Norwegian waters. These novel techniques are an important means of controlling epidemics of lice at farms for maintaining the health of farmed fish and for preventing the spread of lice to wild salmonid populations. However, some treatments are associated with higher mortality rates and negative welfare impacts on the host fish. Furthermore, it is unclear how effective each treatment is in removing lice compared to alternatives. Here, in a controlled laboratory environment, we tested the efficacy of mechanical, warm water (28 °C and 34 °C), and freshwater treatments alone and in combination, and examined their welfare impact on host fish. Regardless of treatment group or control, the handling of fish through the experiment led to a reduction in lice load and decline in fish welfare. Among the treatments examined, the freshwater bath alone and in combination with other treatments had the greatest delousing efficacy. The 34 °C warm water baths also significantly reduced lice loads but led to worse welfare outcomes with fish having a higher prevalence of injuries and reduced growth and condition factor. Delousing treatments were however not associated with long-term effects on neuroendocrine parameters or stress coping ability, suggesting that immediate welfare impacts from these procedures are generally reversible. It was also found that sedating fish prior to treatment was shown to mitigate the welfare impact. These findings are useful for the needed optimization of delousing strategies for greater delousing efficacy and reduced welfare impact on fish.

Farmed Atlantic salmon ( Salmo salar ) welfare and performance can be strongly influenced by stress episodes caused by handling during farming practices. To better understand the changes occurring after an acute stress response, we exposed a group of Atlantic salmon parr to an acute stressor, which involved netting and transferring fish to several new holding tanks. We describe a time-course response to stress by sampling parr in groups before (T0) and 10, 20, 30, 45, 60, 120, 240, 300, and 330 min post-stress. A subgroup of fish was also subjected to the same stressor for a second time to assess their capacity to respond to the same challenge again within a short timeframe (ReStressed). Fish plasma was assessed for adrenocorticotropic hormone (ACTH), cortisol, and ions levels. Mucus cortisol levels were analyzed and compared with the plasma cortisol levels. At 5 selected time points (T0, 60, 90, 120, 240, and ReStressed), we compared the head kidney transcriptome profile of 10 fish per time point. The considerably delayed increase of ACTH in the plasma (60 min post-stress), and the earlier rise of cortisol levels (10 min post-stress), suggests that cortisol release could be triggered by more rapidly responding factors, such as the sympathetic system. This hypothesis may be supported by a high upregulation of several genes involved in synaptic triggering, observed both during the first and the second stress episodes. Furthermore, while the transcriptome profile showed few changes at 60 min post-stress, expression of genes in several immune-related pathways increased markedly with each successive time point, demonstrating the role of the immune system in fish coping capacity. Although many of the genes discussed in this paper are still poorly characterized, this study provides new insights regarding the mechanisms occurring during the stress response of salmon parr and may form the basis for a useful guideline on timing of sampling protocols.

Sterile triploid Atlantic salmon (Salmo salar) show inconsistent seawater grow-out, but the reason why remains unclear. The purpose of this study was to determine the salinity optima of triploid post-smolts. Diploids and triploids were assessed for smoltification status during an underyearling smolt regime before being transferred to one of four different salinities, 0, 11, 23 and 35 ppt at 12 degrees C and under 24 h continuous light for 83 days. During this period, fish growth, plasma biochemistry, and production traits (vertebral deformities, ocular cataracts, sexual maturation) were monitored. Molecular biomarkers in the gill (nka alpha 1a, nka alpha 1b, nkcc1a) suggested triploids reached peak smolt earlier than diploids and began the desmoltification process before the start of the salinity treatments, however this was not reflected in gill Na+/K+-ATPase enzyme activity. At the initiation of the salinity treatments triploids were significantly larger than diploids (mean weight g +/- SE: 71 +/- 0.7 and 87.2 +/- 0.8 for diploids and triploids, respectively) and there was a ploidy effect on post-smolt growth, with body weight showing a clearer positive trend with salinity in diploids (0 < 11 = 23 = 35 ppt) than in triploids (0 < 11 < 35 = 23 ppt) (final mean weight g +/- SE: 255.2 +/- 7.4, 303.9 +/- 9, 313.9 +/- 9 and 342.4 +/- 12 for diploids and 322.9 +/- 9.7, 361.7 +/- 10.7, 425.9 +/- 12.1, 415.2 +/- 12.2 for triploids at 0, 11, 23, and 35 ppt, respectively). Plasma Na+ and Cl- increased, but plasma pH decreased, with increasing salinity in both ploidy. However, ploidy only had transient effects on plasma biochemistry depending on the salinity treatment. There was no ploidy effect on vertebral deformities (21% of both ploidy had one or more deformed vertebra). In contrast, triploids had a significantly higher prevalence of ocular cataracts (84 vs 98% in diploids and triploids, respectively) with a higher mean cataract score (mean +/- SE: 1.93 +/- 0.1 and 2.78 +/- 0.1 for diploids and triploids, respectively), but a significantly lower prevalence of pubertal male post-smolts (15 vs 2% in diploids and triploids, respectively). Salinity treatment had no effect on vertebral deformities, cataracts, or post-smolt sexual maturation. In summary, there was a ploidy mismatch for smoltification biomarkers in the gill and salinity had a strong effect on postsmolt growth, but the effects were ploidy dependent.