Factors associated with baseline mortality in Norwegian Atlantic salmon farming

Sampling time for different matrices in stress assessment of farmed Atlantic salmon post-smolt

The sustainability of commercial aquaculture production depends critically on prioritizing fish welfare management. Besides monitoring welfare parameters such as fish behaviour and water quality, fish stress level can also provide a reliable measure of the welfare status of farmed fish. Cortisol and 5 of its metabolites (5β-THF, cortisone, 5β-DHE, 5β-THE, β-cortolone) were previously identified by the authors as suitable stress biomarkers of farmed Atlantic salmon. Based on this knowledge, the present study aimed to investigate the time-related dynamics of these metabolites in plasma, skin mucus, bile and faeces over a 72 h- period. The objective was to determine the optimal sampling time for each matrix and to understand the clearance pathway of these metabolites following stress. An experiment was carried out using a total of 90 Atlantic salmon with an average weight of 438 (±132) g. The average sea temperature was 6.9 °C during the experimental period. A control group of 10 fish was first collected before the remaining 80 fish were submitted to a stress of netting and subsequent relocation into two separate cages. From each of these two stress groups, 10 fish were sampled at 1 h, 2 h, 4 h, 6 h and 12 h, 24 h, 48 h, 72 h after the stress event respectively. The concentrations of cortisol and its metabolites were measured at each of the sampling timepoint. The results demonstrated that plasma cortisol metabolites reached the highest concentration 4 h after stress and remained elevated despite the slight decrease for the remaining timepoints. The peak level was observed at 12 h post-stress in skin mucus and 24 h in bile and faeces. The findings suggest that these timepoints are the optimal for sampling Atlantic salmon post-smolt following stressful events in acute stress studies. Furthermore, the results reveal that analysing cortisol and its metabolites, both in free and conjugated forms, rather than free cortisol provides greater flexibility as their concentrations are less affected by sampling procedure. This study confirms the appropriateness of skin mucus and faeces as less-invasive sample matrices for fish stress evaluation and provides a basis for further developing low invasive tools for monitoring the welfare of farmed salmonid.

Impact of freshwater rearing history on Atlantic salmon gill response to viral stimulation post seawater transfer

Land-based recirculating aquaculture systems (RAS) have risen in prevalence in recent years for Atlantic salmon production, enabling intensive production which allows increased growth and environmental control, but also having the potential for reducing water use and eutrophication. The Atlantic salmon has an anadromous life history with juvenile stages in freshwater (FW) and on-growing in seawater (SW), enabled by a transformational process known as smoltification. The timing of smoltification and transfer of smolts from FW to SW is critical under commercial production with high mortalities during this period. The impact of FW rearing system on immune function following seawater transfer (SWT) is not well understood. In this study parr were raised in either RAS or a traditional open-LOCH system until smolting and then transferred to a common marine environment. Two-weeks post-SWT fish were immune stimulated with a viral mimic (poly I:C) for 24 h to assess the ability to mount an antiviral immune response, assessed by whole transcriptome analysis of gill tissue, an important immune organ in fish. We show that unstimulated smolts reared in the LOCH had higher immune gene expression than those reared in RAS as determined by functional analysis. However, following stimulation, smolts reared in the RAS mounted a greater magnitude of response with a suite of immune genes displaying higher fold induction of transcription compared to LOCH reared smolts. We suggest RAS smolts have a lower steady state immune-associated transcriptome likely due to an unvarying environment, in terms of environmental factors and lack of exposure to pathogens, which shows a compensatory mechanism following stimulation allowing immune 'catch-up' with those reared in the LOCH. Alternatively, the RAS fish are experiencing an excessive response to the immune stimulation.

Quantitative analysis of mass mortality events in salmon aquaculture shows increasing scale of fish loss events around the world

Globally, salmon aquaculture promises to contribute to sustainable sources of animal protein for a growing human population. However, the growth of the industry also includes increased reports of mass mortality events-disaster events where large numbers of fish die in short periods of time. As salmon production increases in scale and more technology is used to grow salmon in contexts otherwise not suited for them, there is a possibility for more frequent and more severe mortality events. Despite investigations into specific cases of mass mortality events-no global study has been conducted to see if large scale mortality is increasing in frequency and scale. Using a global dataset of publicly available and government-collated data on salmon mortality events including nations responsible for the majority of salmon aquaculture, we document trends in mortality events, showing that in some of the major salmon producing nations of the world (in particular Norway, Canada, and the UK), mass mortality events have increased in frequency from 2012 to 2022. We also show that the scope of mass mortality events has increased over time-that is, the upper bound of how many fish were killed in a specific mortality event has increased over time. Finally, the expected maximum size of a mass mortality event differs from country to country, but is likely much larger than site and jurisdictional thresholds of concern for animal welfare, early warning thresholds, and capacity to respond to mortality events. The consequences of the increased scale and scope of mass mortality events extend past aquaculture production to include severe consequences to aquaculture companies and to coastal communities who depend on aquaculture. Our results agree with predictions of the concept of "manufactured risk", which suggests that risk emerges from the aggressive use of technology to optimize production in variable environments, and we argue that there is a need for more fine-scale and standard data collection on salmon mortality events, and that future investigations into salmon aquaculture should increase focus on disaster potential and realization.

Consequences of reduced effectiveness of salmon lice treatments for lice control

The effective control of ectoparasitic salmon lice, Lepeophtheirus salmonis, in fish farms is challenged by the salmon lice having developed resistance towards several antiparasitic drugs and by the effectiveness of non-medicinal treatments being limited by considerations of fish welfare. When new antiparasitics are introduced to the market, these should be used sparingly to slow resistance development. Using a population model for salmon lice parameterised for salmonid fish farms in Norway, we quantified how reduced treatment effectiveness influences treatment frequency and lice abundance. Furthermore, we investigated when in the production cycle a highly effective lice treatment leads to the largest reduction in the total number of treatments, mean lice abundance and lice larvae production. Results showed that reductions in treatment effectiveness to lower than 50% led to the steepest increases in treatment frequency and mean lice abundance, as well as to increased risk that lice abundance increased beyond control. The timing of the most effective treatment had only moderate effects on the total treatment need and the mean number of adult female lice through the production cycle, but large effect on the production of lice larvae in spring. These findings imply that farmers can optimise the timing of the most effective treatment to reduce the release of lice larvae in the period of year when wild salmonids are in coastal waters, without compromising total treatment need or mean lice levels.

Agonistic effect of peptides derived from a truncated HMGB1 acidic tail sequence in TLR5 from Salmo salar

Based on the structural knowledge of TLR5 surface and using blind docking platforms, peptides derived from a truncated HMGB1 acidic tail from Salmo salar was designed as TLR5 agonistic. Additionally, a template peptide with the native N-terminal of the acidic tail sequence as a reference was included (SsOri). Peptide binding poses complexed on TLR5 ectodomain model from each algorithm were filtrated based on docking scoring functions and predicted theoretical binding affinity of the complex. The best peptides, termed 6WK and 5LWK, were selected for chemical synthesis and experimental functional assay. The agonist activity by immunoblotting and immunocytochemistry was determined following the NF-κBp65 phosphorylation (p-NF-κBp65) and the nuclear translocation of the NF-κBp65 subunit from the cytosol, respectively. HeLa cells stably expressing a S. salar TLR5 chimeric form (TLR5c7) showed increased p-NF-κBp65 levels regarding extracts from flagellin-treated cells. No statistically significant differences (p > 0.05) were found in the detected p-NF-κBp65 levels between cellular extracts treated with peptides or flagellin by one-way ANOVA. The image analysis of NF-κBp65 immunolabeled cells obtained by confocal microscopy showed increased nuclear NF-κBp65 co-localization in cells both 5LWK and flagellin stimulated, while 6WK and SsOri showed less effect on p65 nuclear translocation (p < 0.05). Also, an increased transcript expression profile of proinflammatory cytokines such as TNFα, IL-1β, and IL-8 in HKL cells isolated from Salmo salar was evidenced in 5LWK - stimulated by RT-PCR analysis. Overall, the result indicates the usefulness of novel peptides as a potential immunostimulant in S. salar.

The economic impact of decreased mortality and increased growth associated with preventing, replacing or improving current methods for delousing farmed Atlantic salmon in Norway

Impacts of salmon lice is a major concern for a sustainable production of farmed Atlantic salmon in Norway. Most treatment methods for removal of salmon lice have associated increased mortality and decreased growth in a period after delousing, which affects the profitability of the farmer, and causes poor welfare and sustainability. In addition, the variance in mortality and growth, especially after non-medicinal treatment methods, is high, which makes it hard for a farmer to decide which control measure to apply to keep lice levels below the legal limit. In this study, we have applied a stochastic partial budget approach to assess the economic impact of reducing mortality and increasing growth of farmed Atlantic salmon by preventing, replacing and improving current delousing methods in Norway. We have simulated a production cycle of two different smolt-groups to find the outcome (harvested biomass, average end weight of the salmon, number of dead fish and feed consumption) of production cycles without or with two, three or four delousing treatments in the on-growing phase at sea. The results suggest that accounting for the biological losses associated with lice treatments is important when making choices of delousing strategies. The biological costs of increased mortality and decreased growth associated with especially non-medicinal treatments are expected to be high, but varies substantially. Therefore, the economic benefit of preventing or improving can also be high. The calculations imply that salmon producers could invest a considerable amount in measures for prevention or improvement of thermal treatments before break-even. For example could a farmer use on average 535,313 €/cage/ 1-yearling production in measure to prevent four thermal treatments before it is no longer economical beneficial. Depending on the performance of the four thermal treatments a farmer could use from 319,196-737,934 €/cage/ 1-yearling production on measures of improvement. Replacing one thermal treatment with another immediate treatment method has a minor economic benefit. The results further shows that sales value and feed consumption constitutes the largest share of the change in profit between different treatment regimes. The results from this study also show that not taking into account the risk of mortality and reduced growth associated with the different treatment methods of delousing, could lead to underestimating the benefit of improving, preventing and replacing treatments.

Sex-dependent effects of mechanical delousing on the skin microbiome of broodstock Atlantic salmon (Salmo salar L.)

Delousing strategies, including mechanical delousing, are typically used to treat Atlantic salmon (Salmo salar) sea lice infestations. In this study, we evaluate the impact of mechanical delousing (Hydrolicer) on the skin bacterial microbiome of broodstock female and male Atlantic salmon. 16S rDNA sequencing of salmon skin microbial communities was performed immediately before delousing, right after delousing and 2 and 13 days post-delousing (dpd). The skin bacterial community of female salmon was more diverse than that of males at the start of the experiment. Overall, hydrolycer caused losses in alpha diversity in females and increases in alpha diversity in males. Hydrolicer also caused rapid shifts in the skin microbial community composition immediately after delicing in a sex-specific manner. There was a decrease in abundance of Proteobacteria and Bacteriodetes in both female and male salmon, whereas Firmicutes and Tenericutes abundances increased. Interestingly, the female community recovered faster, while the male community remained dysbiotic 13 dpd due to expansions in Bacteroidetes (Pseudomonadaceae) and Firmicutes. Our data suggest that female broodstock are more resilient to Hydrolicer treatment due to their more diverse skin microbiota community, and that sex influences the skin microbial community and therefore host health outcomes during common farming manipulations.

Chronic stress negatively impacts wound healing, welfare, and stress regulation in internally tagged Atlantic salmon (Salmo salar)

The desire to understand fish welfare better has led to the development of live monitoring sensor tags embedded within individuals for long periods. Improving and understanding welfare must not come at the cost of impaired welfare due to a tag’s presence and implantation process. When welfare is compromised, the individual will experience negative emotions such as fear, pain, and distress, impacting the stress response. In this study, Atlantic salmon (Salmo salar) underwent surgical implantation of a dummy tag. Additionally, half of this group was introduced to daily crowding stress. Both groups and an untagged group were followed for 8 weeks using triplicate tanks per group. Sampling took place once a week, and where stress was given, it was conducted 24 h before sampling. Stress-related measurements were taken to understand if tagging caused chronic stress and explore the chronic stress response and its impact on wound healing. Primary stress response hormones measured included CRH, dopamine, adrenocorticotropic hormone, and cortisol. Secondary stress response parameters measured included glucose, lactate, magnesium, calcium, chloride, and osmolality. Tertiary stress response parameters measured included weight, length, and five fins for fin erosion. Wound healing was calculated by taking the incision length and width, the inflammation length and width, and the inside wound length and width. The wound healing process showed that stressed fish have a larger and longer-lasting inflammation period and a slower wound healing process, as seen from the inside wound. The tagging of Atlantic salmon did not cause chronic stress. In contrast, daily stress led to an allostatic overload type two response. ACTH was elevated in the plasma after 4 weeks, and cortisol followed elevation after 6 weeks, highlighting a breakdown of the stress regulation. Fin erosion was elevated alongside cortisol increase in the stressed group. This data suggests that tagging previously unstressed fish in a controlled environment does not negatively affect welfare regarding stress responses. It also indicates that stress delays wound healing and increases the inflammatory response, highlighting how continued stress causes a breakdown in some stress responses. Ultimately, the tagging of Atlantic salmon can be successful under certain conditions where proper healing is observed, tag retention is high, and chronic stress is not present, which could allow for the possible measurement of welfare indicators via smart-tags.

Sustainable production of biofuels and bioderivatives from aquaculture and marine waste

The annual global fish production reached a record 178 million tonnes in 2020, which continues to increase. Today, 49% of the total fish is harvested from aquaculture, which is forecasted to reach 60% of the total fish produced by 2030. Considering that the wastes of fishing industries represent up to 75% of the whole organisms, the fish industry is generating a large amount of waste which is being neglected in most parts of the world. This negligence can be traced to the ridicule of the value of this resource as well as the many difficulties related to its valorisation. In addition, the massive expansion of the aquaculture industry is generating significant environmental consequences, including chemical and biological pollution, disease outbreaks that increase the fish mortality rate, unsustainable feeds, competition for coastal space, and an increase in the macroalgal blooms due to anthropogenic stressors, leading to a negative socio-economic and environmental impact. The establishment of integrated multi-trophic aquaculture (IMTA) has received increasing attention due to the environmental benefits of using waste products and transforming them into valuable products. There is a need to integrate and implement new technologies able to valorise the waste generated from the fish and aquaculture industry making the aquaculture sector and the fish industry more sustainable through the development of a circular economy scheme. This review wants to provide an overview of several approaches to valorise marine waste (e.g., dead fish, algae waste from marine and aquaculture, fish waste), by their transformation into biofuels (biomethane, biohydrogen, biodiesel, green diesel, bioethanol, or biomethanol) and recovering biomolecules such as proteins (collagen, fish hydrolysate protein), polysaccharides (chitosan, chitin, carrageenan, ulvan, alginate, fucoidan, and laminarin) and biosurfactants.

Towards better survival: Modeling drivers for daily mortality in Norwegian Atlantic salmon farming

Mortality in the production of farmed salmonids is a major constraint to the sustainability of this form of animal husbandry. We have developed a model for the daily mortality in salmon farming over a full production cycle from stocking to harvest, considering different environmental and production factors. These factors included sea temperature, salinity, day within year, fish weight at stocking, stocking day, four types of lice treatments and the possible occurrence of pancreas disease (PD). We considered a generalized additive model following full production cycles, allowing for non-linear descriptions of how relevant factors relate to the daily mortality. We saw a high overall mortality rate immediately after stocking, which decreased the first three months in the cycle and thereafter increased. We found that the total mortality could be reduced by 21% if avoiding all lice treatments, and similarly reduced by 20% if no PD infections occurred. If avoiding jointly PD and all lice treatments, the accumulated mortality could be reduced by 34%. A single thermal or hydrogen peroxide treatment was associated with a mortality of around 1.6% and 1.3%, respectively. This modeling approach gave a unique opportunity to model how different factors interact on the overall global mortality and can easily be extended by other factors, such as additional fish diseases.

Drug and pesticide usage for sea lice treatment in salmon aquaculture sites in a Canadian province from 2016 to 2019

We used 4 years of publicly available data (2016–2019) on chemical usage at salmon sites with information on production, stocking, locations and environmental conditions to explore patterns of anti-sea lice treatments in a Canadian province. Results show that sequential chemical treatments are prevalent, emamectin benzoate (EMB) with azamethiphos being the most used combination with a decrease in ivermectin usage. Relatively high rates of usage of EMB per fish biomass may point to potential lice resistance patterns with information needed on mechanisms and local populations. Lower or no chemical usage at some sites indicate less sea lice infestations likely influenced by localized site conditions (coves), and a lessened need for medication due to the usage of cleaner fish and possibly other non-chemical methods (not documented in this report). The year/climate influenced chemical input only in sites with higher treatment levels likely due to effects on sea lice growth and reproduction. Observed differences between years are warmer surface temperature in the fall, a higher freshwater input in spring, and stronger wind conditions for 2017 and 2018 with more medication usage for these two years. The lack of significant effect of site distances calculated in zones of influence based on 24 h potential connectivity patterns highlight the need to refine the resolution of hydrodynamic processes.

Combined effects of elevated rearing temperature and dietary energy level on heart morphology and growth performance of Tasmanian Atlantic salmon (Salmo salar L.)

Cardiac abnormalities may pose a threat to salmonid aquaculture due to their potential detrimental effect on fish health and welfare. The teleost heart is an extremely plastic organ with important morphological differences between wild and farmed fish that include ventricular shape, alignment of the bulbus arteriosus and epicardial fat deposition. However, little is known about how different factors and interactions among them may affect cardiac morphology of Atlantic salmon. To determine whether rearing temperature could induce cardiac malformations in large Tasmanian Atlantic salmon, we examined a range of cardiac morphology indicators and growth parameters in a population of 1-2 kg seawater salmon (n = 60 temperature^-1 diet^-1 ) exposed to control and elevated temperatures of 15 and 19°C, respectively, while fed one of two commercial feeds with different dietary energy levels. Most fish possessed conspicuous fat around the heart with a tendency towards a rounded ventricle and a more obtuse angle of the bulbus arteriosus. However, fish showed no significant differences in heart shape and bulbus alignment in relation to water temperature and dietary energy. These results suggest that cardiac morphology of large Atlantic salmon is unlikely to be affected by rearing temperature and dietary energy during the grow-out phase.

Mortality patterns during the freshwater production phase of salmonids in Norway

Mortality in Norwegian salmonid aquaculture has a major influence on fish welfare and represents economic losses for producers. We reviewed the estimated monthly mortality for freshwater farms with Atlantic salmon and rainbow trout between 2011 and 2019. We built a regression model for mortality which included the variables year, weight group, season, region and farm. Additionally, we distributed questionnaires to farmers to gather information regarding potential causes of mortalities. The analysis of data for Atlantic salmon showed that median monthly mortality increased from 0.15% in 2011 (interquartile range [IQR]: 0.06-0.39) to 0.25% (IQR: 0.1-0.67) in 2019. Mortality was highest in the North (0.27%, IQR: 0.11-0.72) and lowest in the Southwest region (0.16%, IQR: 0.07-0.4). The season with highest mortality was summer (0.24%, IQR: 0.1-0.64), while winter had the lowest (0.12%, IQR: 0.05-0.35). The smallest fish (3-12 g) showed highest mortality (0.31%, IQR: 0.14-0.69) compared to heavier fish. Results from the questionnaire showed that infectious or non-infectious diseases were the most commonly reported causes of mortality. The mortality patterns described in this study identifies several important risk factors. Highlighting causal links is an important step to reducing mortality and improving welfare in the freshwater production phase of salmonids in Norway.

Quantitative Genetics of Smoltification Status at the Time of Seawater Transfer in Atlantic Salmon (Salmo Salar)

High mortality during grow out in the sea is a challenge for farmed Atlantic salmon production in Norway and globally, which is partly attributed to suboptimal smolt quality. In this study, two groups of pre-smolts were put on a standard light smoltification regime with alternating 12L:12D per day for 6 weeks (Phase I), followed by 24L:0D per day for 6 weeks (Phase II); one group was 0 + smolt (EXP1) and the other 1 + smolt (EXP2). To monitor the smoltification status of the fish, 100 (EXP1) and 60 (EXP2) fish were randomly sampled per week during Phase II. The following phenotypes for smoltification status were studied: RT-qPCR relative mRNA expression of values of two alpha catalytic subunits of the variants of the Na⁺K⁺ATPase (NKA) expressed in the sampled gill tissues of each fish. The first variant, alpha1a with increased expression in freshwater (FW) and the second variant alpha1b with increased expression in seawater variant (SW), as well as their ratio SW/FW. At the optimal time for seawater transfer based on the SW/FW trait, 1,000 (at sixth sampling of EXP1) and 1,500 (at fifth sampling of EXP2) fish were sampled for genetic parameter estimation. The individual variation in FW, SW, and SW/FW was very large at each of the seven samplings indicating a large variation among individuals in the optimum time of transfer to seawater. SW/FW showed significant genetic variation in both 0+ and 1+ smolts, which indicates the possibility for selection for improved synchronization of smoltification status of Atlantic salmon at the time where the largest proportion of the fish is considered to be smolt. However, the genetic correlation between SW/FW of 0+ and 1+ was not significantly different from zero indicating very little shared genetic variation in SW/FW in 0+ and 1+ fish. Smoltification phenotypes showed temporal progression over the smoltification period, and this progression varied between 0+ and 1+ smolt highlighting the importance of correctly timing the major sampling point, and when cohorts are transferred to seawater. This also highlighted the need for further research into noninvasive methods of objectively measuring individual smoltification through time and subsequent smolt survival and growth rate at sea.