Water temperature at the time of the catch-and-release event is a better predictor of survival in Atlantic salmon (Salmo salar) than acute water temperature changes before and after.

Despite studies on the effect of catch-and-release on the survival of Atlantic salmon (Salmo salar) being widespread in the literature, studies to date have failed to evaluate the potential role of thermal history. Herein, we show that despite cooler conditions post-release, 4/18 (22%) salmon died when caught-and-released at water temperatures ≥20°C, whereas 1/13 (8%) salmon caught-and-released at water temperatures ≤20°C, but who encountered mean water temperatures ≥20°C post-release, died. We conclude water temperature at time of the catch-and-release event remains the most suitable predictor of post-release survival.

The effects of environmental enrichment on hatchery-performance, smolt migration and capture rates in landlocked Atlantic salmon.

Enrichment of rearing environment with natural elements has been suggested to improve the welfare and post-release survival of cultured fish. We studied the combined effects of shelter structures, periodical water flow and water level changes on pre- and post-release performance of critically endangered landlocked Atlantic salmon (Salmo salar m. sebago). Relative to standard (plain) rearing tanks, provision of enrichment improved fish condition factor and survival during the first year of rearing when most mortality was attributable to parasitic and bacterial infections. The consequent higher density in enriched tanks probably induced greater growth variation and more dorsal fin damages than found in fish of standard tanks. Possibly this was partly due to the applied changes in water level. Experimentally determined smolt migration tendency at age 3 did not differ, on average, between the rearing groups, but enriched-reared fish showed clearly less variation in total movement activity than standard-reared fish. Experimental angling in earthen ponds did not suggest divergent vulnerability between the differentially reared fish at age 3, but decreased condition during the preceding growth season increased vulnerability to fishing. Based on long-term post-stocking tag returns in large-lake fisheries, fish length at release but not rearing method affected the capture rates of fish released at age 2. When released at age 3 the fish grown in enriched environment had a higher risk to be captured with stationary gears and earlier by hook and line gears compared to standard-reared conspecifics. Earlier time of maximal smolt migration activity was associated with an increased risk of being captured. We suggest that environmental enrichment may modulate growth- and behavior-related qualities that indirectly increased the vulnerability to fishing in natural conditions but not in experimental setting. The favorable effects of enrichment on early survival encourages adopting enriched rearing practices in supportive breeding of landlocked salmon.

Phenotypic stress response does not influence the upper thermal tolerance of male Atlantic salmon (Salmo salar).

Fish can be identified as either low responders (LR) or high responders (HR) based on post-stress cortisol levels and whether they exhibit a proactive or reactive stress coping style, respectively. In this study, male Atlantic salmon (Salmo salar) from 17 families reared at 9 °C were repeatedly exposed to an acute handling stress over a period of four months, with plasma cortisol levels measured at 1 h post-stress. Fish were identified as either LR or HR if the total Z-score calculated from their cortisol responses fell into the lower or upper quartile ranges, respectively; with intermediate responders (IR) classified as the remainder. Salmon characterized as LR, IR or HR were then subjected to an incremental thermal challenge, where temperature was raised at 0.2 °C day-1 from their acclimation temperature (12 °C) to mimic natural sea-cage farming conditions during the summer in Newfoundland. Interestingly, feed intake remained high up to 22 °C, while previous studies have shown a decrease in salmon appetite after ∼16-18 °C. After the first three mortalities were recorded at elevated temperature, a subset of LR and HR salmon were exposed to another acute handling stress event at 23.6 °C. Basal and post-stress measurements of plasma cortisol, glucose and lactate did not differ between stress response phenotypes at this temperature. In the end, the average incremental thermal maximum (ITMax) of LR and HR fish was not different (25.1 °C). In comparison, the critical thermal maximum (CTMax; temperature increased at 2 °C h-1) of the remaining IR fish that had been held at 12 °C was 28.5 °C. Collectively, these results: 1) show that this population of Atlantic salmon is very thermally tolerant, and further question the relevance of CTMax in assessing responses to real-world temperature changes; and 2) indicate that characterization of stress phenotype at 9 °C is not predictive of their stress response or survival at high temperatures. Therefore, selection of fish based on phenotypic stress response at low temperatures may not be beneficial to incorporate into Atlantic salmon breeding programs, especially if the goal is to improve growth performance and survival at high temperatures in sea-cages.

Viral Infection Drives the Regulation of Feeding Behavior Related Genes in Salmo salar.

The feeding behavior in fish is a complex activity that relies on the ability of the brain to integrate multiple signals to produce appropriate responses in terms of food intake, energy expenditure, and metabolic activity. Upon stress cues including viral infection or mediators such as the proinflammatory cytokines, prostaglandins, and cortisol, both Pomc and Npy/Agrp neurons from the hypothalamus are stimulated, thus triggering a response that controls both energy storage and expenditure. However, how appetite modulators or neuro-immune cues link pathogenesis and energy homeostasis in fish remains poorly understood. Here, we provide the first evidence of a molecular linkage between inflammation and food intake in Salmon salar. We show that in vivo viral challenge with infectious pancreatic necrosis virus (IPNV) impacts food consumption by activating anorexic genes such as mc4r, crf, and pomcb and 5-HT in the brain of S. salar. At the molecular level, viral infection induces an overall reduction in lipid content in the liver, favoring the production of AA and EPA associated with the increment of elovl2 gene. In addition, infection upregulates leptin signaling and inhibits insulin signaling. These changes are accompanied by a robust inflammatory response represented by the increment of Il-1b, Il-6, Tnfa, and Pge2 as well as an increased cortisol level in vivo. Thus, we propose a model in which hypothalamic neurons respond to inflammatory cytokines and stress-related molecules and interact with appetite induction/inhibition. These findings provide evidence of crosstalk between pathogenesis-driven inflammation and hypothalamic-pituitary-adrenocortical axes in stress-induced food intake behavior in fish.

Early life stress causes persistent impacts on the microbiome of Atlantic salmon.

Farmed fish are commonly exposed to stress in intensive aquaculture systems, often leading to immune impairment and increased susceptibility to disease. As microbial communities associated with the gut and skin are vital to host health and disease resilience, disruption of microbiome integrity could contribute to the adverse consequences of stress exposure. Little is known about how stress affects the fish microbiome, especially during sensitive early life stages when initial colonisation and proliferation of host-associated microbial communities take place. Therefore, we compared the effects of two aquaculture-relevant early-life stressors on the gut and skin microbiome of Atlantic salmon fry (four months post hatching) using 16S rRNA amplicon sequencing. Acute cold stress applied during late embryogenesis had a pronounced, lasting effect on the structure of the skin microbiome, as well as a less consistent effect on the gut microbiome. Follow-up targeted qPCR assays suggested that this is likely due to disruption of the egg shell microbial communities at the initial stages of microbiome colonisation, with persistent effects on community structure. In contrast, chronic post hatching stress altered the structure of the gut microbiome, but not that of the skin. Both types of stress promoted similar Gammaproteobacteria ASVs, particularly within the genera Acinetobacter and Aeromonas, which include several important opportunistic fish pathogens. Our results demonstrate the sensitivity of the salmon microbiome to environmental stressors during early life, with potential associated health impacts on the host. We also identified common signatures of stress in the salmon microbiome, which may represent useful microbial stress biomarkers.

Hydropower-induced selection of behavioural traits in Atlantic salmon (Salmo salar).

Renewable energy projects such as hydropower facilities contribute towards meeting the world`s growing energy demands and urgent need for mitigating climate change. However, such infrastructure has the potential to substantially alter the environment which, in turn, can induce new challenges related to for instance fish migration conditions. As a consequence, local adaptations related to pre-development migration conditions may be affected for influenced populations. To explore selection regimes operating at a river hydropower plant, we monitored Atlantic salmon smolt individuals during their seaward migration. When passing the hydropower plant, the smolts chose between a surface fish passage or a submerged turbine intake. Smolts were scored for behavioural type (basal locomotor activity, net restrain (a measure of escape responses) and willingness to leave a familiar environment) prior to their migration choice, and we found that smolts with high basal activity had higher probability of using the fish passage than the turbine intake. In addition, migration route choice was a partly consistent trait in that fish that had previously passed a hydroelectric facility by using a fish passage rather than the turbine intake were significantly more likely to use it again when faced with the same choice. Higher mortality among turbine migrants could potentially reduce or eliminate particular behaviour types within populations- and the corresponding population genetic diversity that is essential to cope with future environmental challenges.

Identification of spectral signature for in situ real-time monitoring of smoltification.

We describe the use of an optical hyperspectral sensing technique to identify the smoltification status of Atlantic salmon (Salmo salar) based on spectral signatures, thus potentially providing smolt producers with an additional tool to verify the osmoregulatory state of salmon. By identifying whether a juvenile salmon is in the biological freshwater stage (parr) or has adapted to the seawater stage (smolt) before transfer to sea, negative welfare impacts and subsequent mortality associated with failed or incorrect identification may be reduced. A hyperspectral imager has been used to collect data in two water flow-through and one recirculating production site in parallel with the standard smoltification evaluations applied at these sites. The results from the latter have been used as baseline for a machine-learning algorithm trained to identify whether a fish was parr or smolt based on its spectral signature. The developed method correctly classified fish in 86% to 100% of the cases for individual sites, and had an overall average classification accuracy of 90%, thus indicating that analysis of spectral signatures may constitute a useful tool for smoltification monitoring.

Large inter-stock differences in catch size-at-age of mature Atlantic salmon observed by using genetic individual origin assignment from catch data.

Genetic individual assignment of river stock of origin of mixed stock catch fish offers a tool to analyze size differences among river stocks. Data on the genetically identified river stock of origin of individual fish from commercial mixed stock catches were used to compare the catch size-at-age of mature Atlantic salmon catch fish (Salmo salar) from different rivers in the Baltic Sea. In this application of genetic mixed stock modeling, individual assignments of the river stock of origin were analyzed together with length- and weight-at-age data for individual catch fish. The use of four genetic stock identification based methods was compared for defining the length distributions of caught mature salmon in different river stocks. The catch data included information on maturing salmon in the northern Baltic Sea over the years 2000-2013. DNA microsatellite data on 17 loci and information on the smoltification age were used to assign spawners to their stock of origin. All of the compared methods for using probabilistic stock of origin data in our case yielded very similar estimates of the final mean length distributions of the stocks. The Bayesian mixture model yielded slightly more conservative estimates than the direct probability method, threshold method, or the modified probability method. The catch size between spawners of a same sex and age from river stocks differed significantly and the differences were large. The mean catch weight of 1-sea-winter old mature males in different rivers varied from 1.9 kg to 2.9 kg, from 5.1 kg to 7.5 kg for 2-sea-winter old males, from 5.0 kg to 7.2 kg for 2-sea-winter old females, and from 8.2 kg to 10.8 kg for 3-sea-winter-old females. The mean size of caught wild salmon spawners in each year-class was on average smaller than that of the hatchery-reared and sea ranched stocks.

Melatonin and osmoregulation in fish: A focus on Atlantic salmon Salmo salar smoltification.

Part of the life cycle of several fish species includes important salinity changes, as is the case for the sea bass (Dicentrarchus labrax) or the Atlantic salmon (Salmo salar). Salmo salar juveniles migrate downstream from their spawning sites to reach seawater, where they grow and become sexually mature. The process of preparation enabling juveniles to migrate downstream and physiologically adapt to seawater is called smoltification. Daily and seasonal variations of photoperiod and temperature play a role in defining the timing of smoltification, which may take weeks to months, depending on the river length and latitude. Smoltification is characterised by a series of biochemical, physiological and behavioural changes within the neuroendocrine axis. This review discusses the current knowledge and gaps related to the neuroendocrine mechanisms that mediate the effects of light and temperature on smoltification. Studies performed in S. salar and other salmonids, as well as in other species undergoing important salinity changes, are reviewed, and a particular emphasis is given to the pineal hormone melatonin and its possible role in osmoregulation. The daily and annual variations of plasma melatonin levels reflect corresponding changes in external photoperiod and temperature, which suggests that the hormonal time-keeper melatonin might contribute to controlling smoltification. Here, we review studies on (i) the impact of pinealectomy and/or melatonin administration on smoltification; (ii) melatonin interactions with hormones involved in osmoregulation (e.g., prolactin, growth hormone and cortisol); (iii) the presence of melatonin receptors in tissues involved in osmoregulation; and (iv) the impacts of salinity changes on melatonin receptors and circulating melatonin levels. Altogether, these studies show evidence indicating that melatonin interacts with the neuroendocrine pathways controlling smoltification, although more information is needed to clearly decipher its mechanisms of action.

Synchrony and multimodality in the timing of Atlantic salmon smolt migration in two Norwegian fjords.

The timing of the smolt migration of Atlantic salmon (Salmo salar) is a phenological trait increasingly important to the fitness of this species. Understanding when and how smolts migrate to the sea is crucial to understanding how salmon populations will be affected by both climate change and the elevated salmon lice concentrations produced by salmon farms. Here, acoustic telemetry was used to monitor the fjord migration of wild post-smolts from four rivers across two fjord systems in western Norway. Smolts began their migration throughout the month of May in all populations. Within-population, the timing of migration was multimodal with peaks in migration determined by the timing of spring floods. As a result, migrations were synchronized across populations with similar hydrology. There was little indication that the timing of migration had an impact on survival from the river mouth to the outer fjord. However, populations with longer fjord migrations experienced lower survival rates and had higher variance in fjord residency times. Explicit consideration of the multimodality inherent to the timing of smolt migration in these populations may help predict when smolts are in the fjord, as these modes seem predictable from available environmental data.

Impact of down-stream processing on functional properties of yeasts and the implications on gut health of Atlantic salmon (Salmo salar).

Yeasts are becoming popular as novel ingredients in fish feeds because of their potential to support better growth and concomitantly ensure good fish health. Here, three species of yeasts (Cyberlindnera jadinii, Blastobotrys adeninivorans and Wickerhamomyces anomalus), grown on wood sugars and hydrolysates of chicken were subjected to two down-stream processes, either direct heat-inactivation or autolysis, and the feed potential of the resulting yeast preparations was assessed through a feeding trial with Atlantic salmon fry. Histological examination of distal intestine based on widening of lamina propria, showed that autolyzed W. anomalus was effective in alleviating mild intestinal enteritis, while only limited effects were observed for other yeasts. Our results showed that the functionality of yeast in counteracting intestinal enteritis in Atlantic salmon was dependent on both the type of yeast and the down-stream processing method, and demonstrated that C. jadinii and W. anomalus have promising effects on gut health of Atlantic salmon.

The effect of fasting period on swimming performance, blood parameters and stress recovery in Atlantic salmon post smolts.

In this study, Atlantic salmon post smolts (~250 g, ~29 cm) were fasted for four weeks at 12 °C in full strength seawater. During this period, the critical swimming speed (Ucrit) was measured after 1, 2 and 4 weeks of fasting, as well as in a fed control group. Furthermore, blood samples were taken in each treatment group prior to the swim test, at fatigue, and following 3 h and 24 h of subsequent recovery. Four weeks of fasting gradually reduced the condition factor from 1.03 to 0.89. However, the Ucrit remained statistically unaffected at 3.5 body lengths s-1. Exhaustive exercise stress caused large increases in plasma osmolality, [Cl-], [Na+], [Ca2+], [lactate] and [cortisol], while haematocrit and [haemoglobin] also increased. Plasma ions and lactate had increased further after 3 h recovery, and osmolality, [Cl-] and [Na+] were still elevated above control levels after 24 h while other blood parameters were fully recovered. Osmotic disturbances may therefore be considered the most challenging stressor during strenuous exercise in seawater. Only minor effects of fasting period on blood parameters in response to exhaustive exercise were detected, which included slightly higher osmotic disturbances and a repressed response in red blood cell recruitment at fatigue in fasted fish. Furthermore, the 4-week fasting group had a reduced cortisol response following fatigue compared to the other treatment groups. In conclusion, these results show that Atlantic salmon maintain their full swimming capacity as well as their ability to respond and recover from acute stress during an extended period of food deprivation.

Comparison of Circulating Markers and Mucosal Immune Parameters from Skin and Distal Intestine of Atlantic Salmon in Two Models of Acute Stress.

Ensuring salmon health and welfare is crucial to maximize production in recirculation aquaculture systems. Healthy and robust mucosal surfaces of the skin and intestine are essential to achieve this goal because they are the first immunological defenses and are constantly exposed to multistressor conditions, such as infectious diseases, suboptimal nutrition, and environmental and handling stress. In this work, Atlantic salmon, split from a single cohort, were subjected to acute hypoxia stress or 15-min crowding stress and observed over a 24-h recovery period. Samples were collected from fish at 0, 1, 3, 6, 12 and 24 h post-stress to analyze plasma-circulating markers of endocrine function (cortisol), oxidative stress (glutathione peroxidase) and immune function (interleukin 10 (IL-10), annexin A1). In addition, mucosal barrier function parameters were measured in the skin mucus (Muc-like protein and lysozyme) and distal intestine (simple folds, goblet cell size and goblet cell area). The results showed that both acute stress models induced increases of circulating cortisol in plasma (1 h post-stress), which then returned to baseline values (initial control) at 24 h post-stress. Moreover, the hypoxia stress was mostly related to increased oxidative stress and IL-10 production, whereas the crowding stress was associated with a higher production of Muc-like protein and lysozyme in the skin mucus. Interestingly, in the distal intestine, smaller goblet cells were detected immediately and one hour after post-hypoxia stress, which could be related to rapid release of the cellular content to protect this organ. Finally, the correlation of different markers in the hypoxic stress model showed that the circulating levels of cortisol and IL-10 were directly proportional, while the availability of Muc-like proteins was inversely proportional to the size of the goblet cells. On the other hand, in the crowding stress model, a proportional relationship was established between plasma cortisol levels and skin mucus lysozyme. Our results suggest key differences in energy partitioning between the two acute stress models and support the need for further investigation into the interplay of multistressor conditions and strategies to modulate immunological aspects of mucosal surfaces.

Climate change and anadromous fish: How does thermal acclimation affect the mechanics of the myotomal muscle of the Atlantic salmon, Salmo salar?

In response to accelerated temperature shifts due to climate change, the survival of many species will require forms of thermal acclimation to their changing environment. We were interested in how climate change will impact a commercially and recreationally important species of fish, Atlantic salmon (Salmo salar). As climate change alters the thermal environment of their natal streams, we asked how their muscle function will be altered by extended exposure to both warm and cold temperatures. We performed a thermal acclimation study of S. salar muscle mechanics of both fast-twitch, or white, and slow-twitch, or red, myotomal muscle bundles to investigate how temperature acclimated Atlantic salmon would respond across a range of different temperatures. Isometric contraction properties, maximum shortening velocity, and oscillatory power output were measured and compared amongst three groups of salmon-warm acclimated (20°C), cold-acclimated (2°C), and those at their rearing temperature (12°C). The Atlantic salmon showed limited thermal acclimation in their contraction kinetics, and some of the shifts in contractile properties that were observed would not be predicted to mitigate the impact of a warming environment. For instance, the maximum shortening velocity at a common test temperature was higher in the warm acclimated group and lower in the cold-acclimated group. In addition, critical swimming speed did not vary with temperature of acclimation when tested at a common temperature (12°C). Our results suggest that Atlantic salmon populations will continue to struggle in response to a warming environment.

Frequency of vateritic otoliths and potential consequences for marine survival in hatchery-reared Atlantic salmon.

Otoliths are inner-ear structures of all teleost fish with functional importance for hearing and balance. The otoliths usually consist of aragonite, a polymorph of calcium carbonate, but may also take the form partly or entirely of vaterite, a different polymorph of calcium carbonate. Vateritic otoliths occur sporadically in wild fish, but with a higher frequency in hatchery-reared fish. Abnormal otoliths have direct consequences for the inner-ear functions of fish and may be a symptom of environmental stress. In this study, the authors assess the differences in the frequency of abnormal otoliths and degree of abnormality (% vaterite) for different groups of hatchery-reared Atlantic salmon (Salmo salar) smolt and adults. The groups differed in parental broodstock origin (number of generations in hatchery) and treatment temperature. Smolt from the same groups were also released to complete their ocean migration. The otoliths of the returning and recaptured adults were subsequently extracted to assess the difference in frequency and degree of abnormality between the adults and the smolt from corresponding groups. Return rate varied among groups (0.2%-2.6%). The frequency of vateritic otoliths was high (11.4%-64.4%) and differed among smolt groups. The lowest return rates corresponded with the highest frequency of abnormal otoliths for the groups, suggesting that abnormal otoliths may have negative consequences for marine survival. Furthermore, indications of an effect of fast growth on the formation of abnormal otoliths were found for only one of the experimental groups, and for none of the groups after correcting for Type 1 error. This contradicts previous reports, suggesting rapid growth as the main cause of abnormal otoliths. Adult return rates were generally low, but abnormal otoliths were common, with high coverage (% vaterite).

Heart rates of Atlantic salmon Salmo salar during a critical swim speed test and subsequent recovery.

In this study, heart rate (HR) bio-loggers were implanted in the abdominal cavity of 12 post-smolt Atlantic salmon Salmo salar weighing 1024 ± 31 g and acclimated to 12°C sea water. One week after the surgical procedure, a critical swim speed (Ucrit ) test was performed on tagged and untagged conspecifics, whereafter tagged fish were maintained in their holding tanks for another week. The Ucrit was statistically similar between tagged and untagged fish (2.67 ± 0.04 and 2.74 ± 0.05 body lengths s-1 , respectively) showing that the bio-logger did not compromise the swimming performance. In the pre-swim week, a diurnal cycle was apparent with HR peaking at 65 beats min-1 during the day and approaching 40 beats min-1 at night. In the Ucrit test, HR increased approximately exponentially with swimming speed until a plateau was reached at the final speed before fatigue with a maximum of 85.2 ± 0.7 beats min-1 . During subsequent recovery tagged fish could be divided into a surviving group (N = 8) and a moribund group (N = 4). In surviving fish HR had fully recovered to pre-swim levels after 24 h, including reestablishment of a diurnal HR cycle. In moribund fish HR never recovered and remained elevated at c. 80 beats min-1 for 4 days, whereafter they started dying. We did not identify a proximal cause of death in moribund fish, but possible explanations are discussed. Tail beat frequency (TBF) was also measured and showed a more consistent response to increased swimming speeds. As such, when exploring correlations between HR, TBF and metabolic rates at different swimming speeds, TBF provides better predictions. On the contrary, HR measurements in free swimming fish over extended periods of time are useful for other purposes such as assessing the accumulative burden of various stressors and recovery trajectories from exhaustive exercise.

Photoperiodic regulation of pituitary thyroid-stimulating hormone and brain deiodinase in Atlantic salmon.

Seasonal timing is important for many critical life history events of vertebrates, and photoperiod is often used as a reliable seasonal cue. In mammals and birds, it has been established that a photoperiod-driven seasonal clock resides in the brain and pituitary, and is driven by increased levels of pituitary thyroid stimulating hormone (TSH) and brain type 2 iodothyronine deiodinase (DIO2), which leads to local increases in triiodothyronine (T3). In order to determine if a similar mechanism occurs in fish, we conducted photoperiod manipulations in anadromous (migratory) Atlantic salmon (Salmo salar) that use photoperiod to time the preparatory development of salinity tolerance which accompanies downstream migration in spring. Changing daylength from short days (light:dark (LD) 10:14) to long days (LD 16:8) for 20 days increased gill Na+/K+-ATPase (NKA) activity, gill NKAα1b abundance and plasma growth hormone (GH) levels that normally accompany increased salinity tolerance of salmon in spring. Long-day exposure resulted in five-fold increases in pituitary tshßb mRNA levels after 10 days and were sustained for at least 20 days. tshßb mRNA levels in the saccus vasculosus were low and not influenced by photoperiod. Increased daylength resulted in significant increases in dio2b mRNA levels in the hypothalamus and midbrain/optic tectum regions of the brain. The results are consistent with the presence of a photoperiod-driven seasonal clock in fish which involves pituitary TSH, brain DIO2 and the subsequent production of T3, supporting the hypothesis that this is a common feature of photoperiodic regulation of seasonality in vertebrates.

Improved mitochondrial function in salmon (Salmo salar) following high temperature acclimation suggests that there are cracks in the proverbial 'ceiling'.

Mitochondrial function can provide key insights into how fish will respond to climate change, due to its important role in heart performance, energy metabolism and oxidative stress. However, whether warm acclimation can maintain or improve the energetic status of the fish heart when exposed to short-term heat stress is not well understood. We acclimated Atlantic salmon, a highly aerobic eurythermal species, to 12 and 20 °C, then measured cardiac mitochondrial functionality and integrity at 20 °C and at 24, 26 and 28 °C (this species' critical thermal maximum ± 2 °C). Acclimation to 20 °C vs. 12 °C enhanced many aspects of mitochondrial respiratory capacity and efficiency up to 24 °C, and preserved outer mitochondrial membrane integrity up to 26 °C. Further, reactive oxygen species (ROS) production was dramatically decreased at all temperatures. These data suggest that salmon acclimated to 'normal' maximum summer temperatures are capable of surviving all but the most extreme ocean heat waves, and that there is no 'tradeoff' in heart mitochondrial function when Atlantic salmon are acclimated to high temperatures (i.e., increased oxidative phosphorylation does not result in heightened ROS production). This study suggests that fish species may show quite different acclimatory responses when exposed to prolonged high temperatures, and thus, susceptibility to climate warming.

Development of supermale and all-male Atlantic salmon to research the vgll3 allele - puberty link.

BACKGROUND: Farmed Atlantic salmon are one of the most economically significant global aquaculture products. Early sexual maturation of farmed males represents a significant challenge to this industry and has been linked with the vgll3 genotype. However, tools to aid research of this topic, such as all-male and clonal fish, are still lacking. The present 6-year study examined if all-male production is possible in Atlantic salmon, a species with heteromorphic sex chromosomes (males being XY, females XX), and if all-male fish can be applied to further explore the vgll3 contribution on the likelihood of early maturation. RESULTS: Estrogen treatment of mixed sex yolk sac larvae gave rise to one sexually mature hermaphrodite with a male genotype (XY) that was used to produce both self-fertilized offspring and androgenetic double haploid (dh) offspring following egg activation with UV treated sperm and pressure shock to block the first mitotic division. There were YY supermales among both offspring types, which were crossed with dh females. Between 1 and 8% of the putative all-male offspring from the eight crosses with self-fertilized supermales were found to have ovaries, and 95% of these phenotypic females were also genetically female. None of the offspring from the one dh supermale cross had ovaries. When assessing the general contribution of the vgll3 locus on the likelihood of early post-smolt sexual maturation (jacking) in the all-male populations we found individuals that were homozygous for the early maturing genotype (97%) were more likely to enter puberty than individuals that were homozygous for the late maturing genotype (26%). However, the likelihood of jacking within individuals with an early/late heterozygous genotype was higher when the early allele came from the dam (94%) compared to the sire (45%). CONCLUSIONS: The present results show that supermale Atlantic salmon are viable and fertile and can be used as a research tool to study important aspects of sexual maturation, such as to further explore the sex dependent parental genetic contribution to age at puberty in Atlantic salmon. In addition, we report the production of viable double haploid supermale fish.

Transcriptome sequencing and histology reveal dosage compensation in the liver of triploid pre-smolt Atlantic salmon.

Triploid Atlantic salmon (Salmo salar L.) is seen as one of the best solutions to solve key issues in the salmon farming industry, such as the impact of escapees on wild stocks and pre-harvest sexual maturation. However, the effects of triploidy on salmon smoltification are poorly understood at the molecular level, even though smoltification is a very sensitive period that has a major influence on survival rate and performance of farmed salmon. In this study, we have compared the liver transcriptomes of diploid and triploid Atlantic salmon at three ontogeny stages: fry, parr and smolt. In diploid fish, a total of 2,655 genes were differentially expressed between fry and parr, whereas 506 genes had significantly different transcript levels between parr and smolts. In triploids, 1,507 and 974 genes were differentially expressed between fry and parr, and between parr and smolts, respectively. Most of these genes were down-regulated and 34 genes were differentially expressed between ploidies at the same stage. In both ploidy groups, the top differentially expressed genes with ontogeny stage belonged to common functional categories that can be related to smoltification. Nucleotide and energy metabolism were significantly down-regulated in fry when compared to parr, while immune system processes were significantly down-regulated in parr when compared to smolts. The close resemblance of enriched biological processes and pathways between ploidy groups suggests that triploidy is regulated by genome dosage compensation in Atlantic salmon. Histological analysis revealed that areas of vacuolization (steatosis) were present only in fry and parr stages, in contrast to a compact cellular histology with glycogen granules after smoltification. There was no significant difference in vacuolization between ploidy groups at the fry stage but the liver of diploid parr had a 33.5% higher vacuolization area compared to their triploid counterparts. Taken together, our data provide novel insights into the changes that occur at the molecular and histological level in the liver of both diploid and triploid Atlantic salmon prior to and during smoltification.