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.

Vaccination is a critical prophylactic measure to prevent disease outbreaks in Atlantic salmon (Salmo salar L.) and multivalent oil-adjuvanted vaccines administered intraperitoneally (IP) are widely used for this purpose. The Atlantic salmon in this observational study were sampled during commercial production in a salmon farm in Northern Norway. Fish were sampled in the hatchery prior to vaccination, and further every 6 weeks after sea transfer up until slaughter. The key findings revealed that Atlantic salmon vaccinated IP with a commercial multivalent oil-based vaccine developed specific and long-lasting antibody responses against A. salmonicida, V. anguillarum, Y. ruckeri, and IPNV for up to 20 months post-vaccination. However, there was variation in the measured specific antibody response to the different antigens. Notably, the antibody responses against A. salmonicida and V. anguillarum antigens were higher and more prolonged compared to the antibody responses against Y. ruckeri and IPNV antigens. Histological examination showed presence of vaccine-induced granulomas in the pancreas at the injection site for up to 50 weeks post-vaccination and V. anguillarum antigens were found in the granulomas up to 44 weeks post-vaccination. There were also individual IgM-positive cells present in these vaccine-induced granulomas, and qPCR confirmed increased gene expression of membrane-bound and secretory IgM up to 37 weeks post-vaccination. Altogether the qPCR data of membrane and secretory IgM suggests local production of IgM in the vaccine-induced granulomas, while the head kidney might be responsible for the longterm secretion of IgM. Neither the low temperature the fish experienced for two winter seasons (below 5 degrees C) nor the seasonal changes in photoperiod affected the vaccine-induced specific antibody responses.

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.

Understanding the biomechanics of fish scales is crucial for their survival and adaptation. Ultrasonic C-scan measurements offer a promising tool for non-invasive characterization, however, existing literature lacks uncertainty analysis while evaluating acoustic impedance. This article presents an innovative integration of uncertainty into the analytical framework for estimating stochastic specific acoustic impedance of salmon fish scale through ultrasonic C-scans. In this study, the various types of uncertainties arising due to variation in biological structures and aging, measurement errors, and analytical noises are combined together in the form of uncertain reflectance. This uncertain reflectance possesses a distribution which is derived using a theory of waves by assuming suitable stochasticity in wavenumber. This distribution helps in development of a stochastic-specific acoustic impedance map of the scales which demonstrates the possible deviations of impedance from mean value depending on uncertainties. Furthermore, maximal overlap discrete wavelet transform is employed for efficient time-frequency deconvolution and Kriging for spatial data interpolation to enhance the robustness of the impedance map, especially in scenarios with limited data. The framework is validated by accurately estimating the specific acoustic impedance of well-known materials like a pair of target medium (polyvinylidene fluoride) and reference medium (polyimide), achieving over 90% accuracy. Moreover, the accuracy of the framework is found superior when compared with an established approach in the literature. Applying the framework to salmon fish scales, we obtain an average specific acoustic impedance of 3.1 MRayl along with a stochastic map visualizing the potential variations arising from uncertainties. Overall, this work paves the way for more accurate and robust studies in fish scale biomechanics by incorporating a comprehensive uncertainty analysis framework.

Studying inflammatory responses induced by vaccination can contribute to a more detailed understanding of underlying immune mechanisms in lumpfish (Cyclopterus lumpus). Tissue samples from lumpfish intraperitoneally immunized with a divalent oil-adjuvanted vaccine (Aeromonas salmonicida and Vibrio salmonicida) at water temperatures of 5, 10, and 15°C were collected at 630 day degrees and 18 weeks post injection. The relative amount of secretory and membrane-bound immunoglobulin M (IgM) gene transcripts in the head kidney was determined by qPCR. Vaccine-induced inflammatory lesions were assessed on histological sections of abdominal pancreatic/intestinal tissue from vaccinated fish in all three temperature groups. Inflammatory cells forming dense aggregations in lesions showed proliferative activity, many of which were identified as eosinophilic-granulocyte-like cells. IgM+ cells were scattered in inflammatory tissue dominated by connective tissue, showing no difference in numbers between lesions from fish vaccinated at 5, 10, and 15°C. Relative gene expression analysis of secretory and membrane-bound IgM revealed low overall expression in the head kidney of vaccinated fish at both 630 day-degrees and 18 weeks post injection. The results of this study indicate that the vaccine stimulated prolonged local inflammatory responses at the injection site, which were not influenced by temperature.

The scavenger endothelial cells (SECs) of vertebrates are an important class of endocytic cells responsible for clearance of foreign and physiological waste macromolecules, partitioning in the immune system, functioning as a cellular powerplant by producing high energy metabolites like lactate and acetate. All animal phyla possess SECs, but the tissue localization of SECs has only been investigated in a limited number of species. By using a specific ligand for scavenger receptors (formalin treated bovine serum albumin), the study revealed that in all tetrapod species (amphibia, reptiles, birds and mammals) the SECs were found lining the sinusoids of the liver. No SECs were found in the liver of any of the bony fishes (Osteichthyes) investigated. Interestingly, we found the SECs not only to be located in the heart of marine species but also in some freshwater species such as Lota lota, Percichthys trucha and Perca fluviatilis. In some fish species, the SECs were found both in the heart and/or kidney in a number of marine and freshwater fishes, whereas in some marine, diadromous and freshwater fishes the SECs were confined only to the kidney tissue. However, from these results it can be suggested that there is neither a clear phylogenetic trend when it came to anatomical localization of SECs nor any pattern in terms of habitat (salinity preferences).

ABSTRACT Scanning Acoustic Microscopy (SAM) emerges as a versatile label-free imaging technology with broad applications in biomedical imaging, non-destructive testing, and material research. This article presents a framework for the estimation of stochastic impedance through SAM, with a particular focus on its application to the salmon fish scale. The framework leverages uncertain reflectance, marking its pioneering application to uncertainty quantification in the acoustic impedance of fish scales through acoustic responses. The study uses maximal overlap discrete wavelet transform, to decompose acoustic responses effectively and is further processed to predict the acoustic impedance. To establish the effectiveness of the proposed framework, well-known materials like a pair of target medium (polyvinylidene fluoride) and reference medium (polyimide) are employed for impedance characterization. Results demonstrate over 90%accuracy in PVDF impedance estimation, validating the framework. A stochastic impedance map, using Kriging with a Gaussian variogram, offers insights into the complex biomechanics of a fish’s scale.

The aim of this study was to evaluate histologic post-mortem autolytic changes in farmed Atlantic salmon. The fish were either stored at room temperature (RT, 21°C), refrigerated (4°C) or frozen (-20°C), while fish necropsy was performed at 0, 1, 4, 24 and 48 h post-storage (hps). In addition, gills were sampled at 0, 5, 10, 15, 30 and 45 min post-storage (mps) at room temperature (RT). The haematoxylin and eosin-stained tissue slides were evaluated and scored by using a semi-quantitative scoring system. Our findings demonstrated gills and pyloric caeca/pancreas as the most severely autolysed organs while heart and skeletal musculature were least affected. Generally, moderate to severe autolysis appeared first at 4 hps, while severe changes were seen at 24 hps. Gills demonstrated autolytic changes as early as 10 mps and pyloric caeca/pancreas at 1 hps. Freezing did not prevent the autolysis and even contributed to freezing artefacts, which may lead to misdiagnosis. Keeping organs refrigerated slowed the autolytic progress within the first 4 hps marginally. This study recommends gills and pyloric caeca/pancreas should be sampled as early as possible, at least within 10 min post-necropsy.

Warm water treatment has in recent years become widely used for ridding salmonids of sea lice in aquaculture although the consequences of the treatment for fish welfare are not adequately investigated. The objective of this study was to document potential long-term welfare effects of repeated warm water treatments on Atlantic salmon (Salmo salar). Five weeks after a baseline welfare indicator scoring, non-anaesthetised Atlantic salmon (w = 1379 +/- 313 g, n = 332) were treated individually in a chamber with seawater at a temperature of 34 degrees C (warm water treatment) or 9 degrees C (control treatment) for 30 s. The treatment was repeated after 23-24 days. During the second treatment, a subset of the fish was video recorded for behavioural analysis. Seventeen to eighteen days after the second treatment, welfare indicators were again scored, and organ samples were taken for histopathological examination. The repeated warm water treatments resulted in a significantly increased prevalence and/ or severity of scale losses, snout wounds, various eye problems, and active fin injuries as well as a significantly reduced specific growth rate. The fish displayed an immediate, strong behavioural reaction when exposed to warm water, which was probably the main cause of the detected injuries.

Intact skin is of uttermost importance for fish welfare. The fish skin provides an environmental barrier and protects against invading pathogens. However, both pathogens and physical insults cause skin wounds that are of major concern in modern fish farming. The behavior and interactions between keratocyte cells and sheets of cells are not well understood. The collective migration of keratocytes (skin epithelial cells) is of central importance for wound healing in fish. In this study, we aimed to elucidate the complex wound healing process in fish skin by studying in vitro cultures of these highly motile cells. Using explant cultures from farmed Atlantic salmon ( Salmo salar ) and differential interference contrast microscopy (DIC), we have captured the dynamics of sheets of cells from harvested fish scales and of individual cells interacting in the cell sheet vicinity. In addition to direct contact, the cells were observed to interact through long membrane tubes, turn, rotate, merge, and/or detach. Additionally, stationary cells and cells moving on top of the cell sheets were observed. Cell sheets approaching one another from different scales did not merge but dispersed.

The definition of scavenger endothelial cells (SEC) is exclusively based on functional and structural characteristics. The following characteristics are common hallmarks for the vertebrate SEC: (a) All vertebrates examined are furnished with a population of special SEC that plays a role in the catabolism of physiologic and non-physiologic soluble waste macromolecules. (b) From the ligands that are endocytosed, SEC in all seven vertebrate classes appear to express the collagen α-chain receptor and the scavenger receptors. In addition, the hyaluronan and the mannose receptors are present on SEC of mammalia (several species) and osteichthyes (e.g., salmon and cod). It is likely that all four receptor types are present in all vertebrate classes. (c) Like liver endothelial cells (LEC) in mammals, SEC in all vertebrate classes are geared to endocytosis of soluble macromolecules, but phagocytic uptake of particles is taken care of mainly by macrophages. (d) The most primitive vertebrates (hagfish, lamprey and ray) carry their SEC in gill vessels, whereas phylogenetically younger fishes (salmon, carp, cod and plaice) carry their SEC in either kidney or heart and in all terrestrial vertebrates-SEC are found exclusively in the liver. (e) SEC of all vertebrates are localized in blood sinusoids or trabeculae that carry large amounts of slowly flowing and O2 poor blood. (f) SEC differs functionally and structurally from what is normally associated with "conventional vascular endothelium."

Photonic chip-based total internal reflection fluorescence microscopy (c-TIRFM) is an emerging technology enabling a large TIRF excitation area decoupled from the detection objective. Additionally, due to the inherent multimodal nature of wide waveguides, it is a convenient platform for introducing temporal fluctuations in the illumination pattern. The fluorescence fluctuation-based nanoscopy technique multiple signal classification algorithm (MUSICAL) does not assume stochastic independence of the emitter emission and can therefore exploit fluctuations arising from other sources, as such multimodal illumination patterns. In this work, we demonstrate and verify the utilization of fluctuations in the illumination for super-resolution imaging using MUSICAL on actin in salmon keratocytes. The resolution improvement was measured to be 2.2-3.6-fold compared to the corresponding conventional images.

Quantifying small, rapidly evolving forces generated by cells is a major challenge for the understanding of biomechanics and mechanobiology in health and disease. Traction force microscopy remains one of the most broadly applied force probing technologies but typically restricts itself to slow events over seconds and micron-scale displacements. Here, we improve >2-fold spatially and >10-fold temporally the resolution of planar cellular force probing compared to its related conventional modalities by combining fast two-dimensional total internal reflection fluorescence super-resolution structured illumination microscopy and traction force microscopy. This live-cell 2D TIRF-SIM-TFM methodology offers a combination of spatio-temporal resolution enhancement relevant to forces on the nano- and sub-second scales, opening up new aspects of mechanobiology to analysis.

The present study investigated how the concentration of PUFA in blood and muscle of Atlantic salmon (Salmo salar L.) changed when fed 4 diets containing very different amounts of LC-PUFA in fresh water for 84 days. The n-3 LC-PUFA constituted 4.4, 24.2, 18.4, and 51.5% of the total fatty acids in the diets made with rapeseed oil, anchoveta oil, a combination of rapeseed oil (60%) and anchoveta oil (40%), and EPA/DHA concentrate, respectively. No significant differences in the growth were observed in the groups during the feeding experiment. The changes in n-3 LC-PUFA in blood and muscle were however quite different in the 4 feeding groups. In the blood, DHA was mobilised to or retained at high level (30-36%) even with a very low amount of DHA in the feed. The concentration of EPA in the blood reflected to a large degree the concentration in the feed, with a rapid increase of EPA in the blood during the first 21 days of feeding. The level of EPA in the blood remained at this level for the rest of the study. However, when only small amounts (1.8%) were present in the feed the fatty acid appeared to be selectively mobilised to or retained in the blood. In the muscle, the percentage of EPA, DHA and other fatty acids mirrored the amounts in the feed suggesting increased TAG (triacylglycerides) during the feeding period.

Combating and controlling sea lice causes large economic costs for the farmers, with estimated values of more than 305 million euros (euro) per year. Increased resistance against traditional chemotherapeutants due to evolutionary drivers in the sea lice combined with the lack of an effective vaccine and few other chemical treatments available are expected to cause these costs to increase. Several possible methods for managing sea lice infestations have been investigated, but only cleaner fish has proven to have an effect on lice levels. Cleaning activity is well known in marine fish and has been observed in the wild as a form of symbiosis between two species: one species, the 'client' fish, seek out the other species, the 'cleaner' fish, to have ectoparasites and dead tissue cleared from its body. The Atlantic lumpfish is a relatively new aquaculture species, and wild-caught mature fish are used as brood stock for farmed production. This poses a biosecurity risk, as wild fish can carry pathogens, and the use of quarantine and health screening is recommended. Vaccine development is unfortunately lagging far behind relatively to the wide spread and high utilisation of the fish. This review contains description of the main pathogens and diseases that affect cleaner fish.

In today's aquaculture of Atlantic salmon (Salmo salar L.), a majority of viral disease outbreaks occur after seawater transfer. A relevant question is how the parr-smolt transformation influences the efficacy of viral vaccines and the innate resistance against viral diseases. In this study, vaccinated and unvaccinated A. salmon parr were exposed to different photoperiodic regimens (1-, 3- or 6-week continuous light-WCL). Fish groups at different stages in the smoltification process were induced, as demonstrated by differences in morphological and physiological smolt parameters. At the time of seawater transfer, the 6-WCL group had reached a more pronounced stage in the smoltification process than the 1-WCL group. In unvaccinated fish, the subsequent cohabitation challenge with infectious pancreatic necrosis virus (IPNV) gave a significantly higher accumulated mortality in the 6-WCL group (87%) compared to the 1-WCL group (39%). In the vaccinated groups, this effect was not apparent and there were no differences in accumulated mortality between the 1 WCL, 3 WCL and 6-WCL groups. These data suggest that the resistance to IPN in A. salmon was negatively influenced by smoltification, while vaccine-mediated protection to IPN was maintained equally well irrespective of smolt status.

Atlantic lumpfish were vaccinated by intramuscular (im) or intraperitoneal (ip) injection with a multivalent oil-based vaccine, while control fish were injected with phosphate-buffered saline. Four lumpfish per group were sampled for skin/muscle and head kidney tissue at 0, 2, 7, 21 and 42 days post-immunization (dpi) for histopathology and immunohistochemistry (IHC). Gene expressions of secretory IgM, membrane-bound IgM, IgD, TCRα, CD3ε and MHC class IIβ were studied in tissues by using qPCR. Im. vaccinated fish showed vaccine-induced inflammation with formation of granulomas and increasing number of eosinophilic granulocyte-like cells over time. On IHC sections, we observed diffuse intercellular staining of secretory IgM at the injection site at 2 dpi, while IgM + cells appeared in small numbers at 21 and 42 dpi. Skin/muscle samples from im. vaccinated fish demonstrated an increase in gene expression of IgM mRNA (secretory and membrane-bound) at 21 and 42 dpi and small changes for other genes. Our results indicated that im. vaccination of lumpfish induced local IgM production at the vaccine injection site, with no apparent proliferation of IgM + cells. Eosinophilic granulocyte-like cells appeared shortly after im. injection and increased in numbers as the inflammation progressed.

We present a study on the effect of water temperature on immunization of Atlantic lumpfish. In total, 360 fish were vaccinated with either 50 μl of an oil-based injection vaccine (VAX), with Aeromonas salmonicida and Vibrio salmonicida antigens, or PBS. Fish were vaccinated at three different water temperatures, 5°C, 10°C and 15°C, and sorted into six groups (N = 60). Lumpfish were weighed every 3 weeks after vaccination, sampled at 3, 6, 9 and 18 weeks post-immunization (wpi) and evaluated by modified Speilberg score, ELISA and immunoblotting. Vaccinated fish showed low antibody response against V. salmonicida. Fish vaccinated at 5°C showed significantly lower antibody response against A. salmonicida throughout the study. At higher temperatures, vaccinated fish showed significantly increased antibody responses, at 18 wpi for 10°C and at 6 and 18 wpi for 15°C. Immunoblotting demonstrated specific response against the LPS antigen of A. salmonicida in the 10°C and 15°C VAX groups. Mean body weight increased in all groups throughout the study. Vaccinated fish had low Speilberg scores with no melanization of abdominal tissue. Our results show that vaccinating lumpfish at a lower water temperature may lead to a low antibody response against A. salmonicida.

Blocking infective salmon lice larvae, that typically aggregate near the surface, from entering sea cages has become a widely-adopted preventive strategy in salmon aquaculture. The most prevalent shielding technique is fitting a skirt around the upper part of the cage. However, limited scientific data is available on the effects skirts have on lice levels, water quality and fish welfare. The present study compares lice infestation levels on farmed Atlantic salmon, oxygen saturation levels and fish welfare scores between cages with and without 10 m deep skirts from May to September at a commercial farm. The effectiveness of skirts at reducing lice infestations varied during the observation period, but at the last sampling 80% less lice were found on the fish in skirt cages compared to in standard cages. From early July and onwards, oxygen levels inside the skirts were reduced by 5 to 35 percentage points saturation compared to standard cages. However, the lowest oxygen value registered was still above 70% oxygen saturation. No differences in fish welfare scores and mortality between treatments were observed. Under the presented conditions, we therefore conclude that the skirts effectively reduced lice levels while preserving oxygen levels and fish welfare. More studies are, however, needed to reveal how the effects from skirts change with season and site.