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Increased knowledge of heritable traits in Atlantic Salmon (Salmo salar) is important to overcome bottlenecks in salmonid aquaculture. Atlantic salmonid populations, both landlocked and anadromous, represent an interesting model to gain insight into anadromy related traits, most notably, the probability to smoltify. While a previous study has identified several genomic regions diverging between anadromous and landlocked populations across the species range, the present study explores these data further with the aim to uncover if some of these genomic regions are linked to beneficial genetic traits associated with smoltification. In this study 17 of these loci were monitored in 669 anadromous salmon originating from 36 full-sibling families that had been reared under common garden conditions. The Smolt Index was calculated, using multiple visual markers, and provided a means of assessing smoltification stage. One SNP, located in Ssa04, showed a significant association with probability to smoltify, where individuals homozygous for the landlocked variant (LL) displayed a decrease in probability of smoltifying after one winter when compared with the homozygous for the anadromous variant (AA). This effect was independent of individual fish size. A separate common garden study comprising 200 individuals from either anadromous or landlocked strains showed that expression levels of ncor1, a thyroid mediator hormone located on the same chromosomal region (Ssa04), were significantly reduced in landlocked individuals post smoltification but remained constant in their anadromous counterparts. This study therefore suggests that while size is still the most important trigger for the induction of smoltification, there may also be an additional genetic component or trigger that has been 'lost' during the years deprived of SW transfer. In conclusion, the LL genotype identified here could potentially be used by the industry to delay smoltification and may also represent one of the first clues to the genetic regulation of smoltification in Atlantic salmon. [ABSTRACT FROM AUTHOR]

Introduction: The use of recirculating aquaculture systems (RAS) to produce Atlantic salmon smolts has resulted in exceptionally high fish growth rates. However, there are potential negative trade-offs between fast growth and key physiological processes, such as inadequate smoltification and early sexual maturation, which can both be linked to body energy reserves. Methods: This study determined the effect of i) dietary fat levels on Atlantic salmon whole-body fat and fatty acids composition, growth performance and smoltification and ii) a previous dietary regime on seawater growth performance and male early sexual maturation. In freshwater RAS, salmon parr (~19g) were fed 3 fat levels (20, 24, and 28%) over a 14-week period at 12℃. Subsequently, in seawater flow-through systems, smolt (~96g) were fed a control diet (26%) for 12 weeks at 2 temperatures (12 and 16℃). Results: Dietary fat levels resulted in differences in k-factor, fish whole-body fat and fatty acids composition; the low fat diet resulted in fish with a 23% lower whole-body fat when compared to the other two dietary groups at the end of the freshwater phase. These differences in whole-body fat faded at the end of the seawater phase. During the freshwater phase, all three feeds resulted in a comparable growth performance and smoltification indicator values, including k-factor, gill Na+, K+-ATPase, blood serum chloride and smolt index score. In contrast, water temperature resulted in fish body weight differences, where groups reared at 16℃ were larger than those reared at 12℃. Nevertheless, both temperature regimes supported a similar thermal growth coefficient. Smolts grown at 16℃ showed a higher level of the sex steroids androstenedione compared to those at 12℃. However, 11-ketotestosterone and testosterone levels did not differ despite a trend for higher levels at 16℃. Furthermore, testis histology at the final sampling indicated that some individuals showed initial signs of maturation (stage 3). Discussion: In conclusion, varying dietary fat levels (20 - 28%) during the freshwater phase did not influence smoltification or male early sexual maturation during the subsequent grow-out phase. However, a temperature increase from 12 to 16℃ resulted in larger fish and appeared to stimulate early male maturation in some fish individuals. [ABSTRACT FROM AUTHOR]

Introduction: Maturation in seawater-adapted (smoltified) male Atlantic salmon is undesirable yet frequently reported in fish weighing up to 1 kg reared under intensive conditions in land-based facilities. Although the photoperiod regulates both smoltification and maturation, the optimal post-smoltification regimen for the prevention of maturation is unclear. Methods: In this study, mixed-sex Atlantic salmon weighing 95 g were exposed to three different photoperiod regimes: continuous daylength, gradually decreasing daylength, and short-to-long daylength after the induction of smoltification by the "winter signal" (12 h:12 h light:dark). Fish were maintained under these conditions in triplicate for 18 weeks in a brackish water flow-through system at 12℃ and with a salinity of 20 ppt. This initial phase was followed by a grow-out period (1 kg-2.5 kg) in seawater (32-ppt salinity) at 13℃ for 11 weeks, with exposure to decreasing daylength. Throughout the 29-week study, body condition, gonad development, and morphological traits were assessed. Additionally, during exposure to brackish water, serum 11-ketotestosterone (11-KT) levels were quantified, and the testes were examined histologically. Upon termination of the study, the maturation rate was determined. Results: This study exclusively detected maturation among male salmon. Under continuous light exposure, the number of fish initiating maturity steadily increased from week 8 and continued to increase in the subsequent samplings. In the short-to-long daylength group, a subset of male fish showed elevated 11-KT levels at week 14, although their gonadal development and morphological changes were not evident until week 18. Male fish exposed to decreasing daylength showed no substantial increase in 11-KT levels, although a few fish exhibited advanced gonad development by week 18. At the end of the trial, markedly lower percentages of maturing and fully mature male fish were observed among those fish previously exposed to decreasing daylength. The maturation rates at study termination were 44.4% for the short-to-long daylength group, 29.4% for the continuous daylength group, and 4.3% for the decreasing daylength group. Discussion: These results show that photoperiod can be utilized to regulate maturation in male Atlantic salmon. In addition, decreasing the photoperiod after smoltification seems to limit early sexual maturation without compromising body size development. Thus, decreasing the photoperiod could be utilized to limit maturation in the production of post-smolt salmon or salmon in landbased facilities. [ABSTRACT FROM AUTHOR]

The anadromous Atlantic salmon undergo a preparatory physiological transformation before seawater entry, referred to as smoltification. Key molecular developmental processes involved in this life stage transition, such as remodeling of gill functions, are known to be synchronized and modulated by environmental cues like photoperiod. However, little is known about the photoperiod influence and genome regulatory processes driving other canonical aspects of smoltification such as the large-scale changes in lipid metabolism and energy homeostasis in the developing smolt liver. Here we generate transcriptome, DNA methylation, and chromatin accessibility data from salmon livers across smoltification under different photoperiod regimes. We find a systematic reduction of expression levels of genes with a metabolic function, such as lipid metabolism, and increased expression of energy related genes such as oxidative phosphorylation, during smolt development in freshwater. However, in contrast to similar studies of the gill, smolt liver gene expression prior to seawater transfer was not impacted by photoperiodic history. Integrated analyses of gene expression, chromatin accessibility, and transcription factor (TF) binding signatures highlight chromatin remodeling and TF dynamics underlying smolt gene regulatory changes. Differential peak accessibility patterns largely matched differential gene expression patterns during smoltification and we infer that ZNF682, KLFs, and NFY TFs are important in driving a liver metabolic shift from synthesis to break down of organic compounds in freshwater. Overall, chromatin accessibility and TFBS occupancy were highly correlated to changes in gene expression. On the other hand, we identified numerous differential methylation patterns across the genome, but associated genes were not functionally enriched or correlated to observed gene expression changes across smolt development. Taken together, this work highlights the relative importance of chromatin remodeling during smoltification and demonstrates that metabolic remodeling occurs as a preadaptation to life at sea that is not to a large extent driven by photoperiod history. [ABSTRACT FROM AUTHOR]

Conflicts can arise in developmental pathways that prevent an individual entering different developmental life stages that result in the expression of different phenotypes within a specific time period. In salmonids, theory suggests that sexual maturation may inhibit subsequent smolting within the same 12‐month period and that this is partly the result of the time and the apparently conflicting physiological changes for these processes to occur, and partly because of the energy requirements for these physiologically taxing processes. This study tested whether sexually mature male Atlantic salmon (Salmo salar L.) parr, caught in the autumn, would subsequently smolt the following spring. Through individual identification using PIT telemetry, minimum estimates of 3.0% (n = 6/203) and 5.9% (n = 1/17) of Atlantic salmon parr that were sexually mature in two river catchments during the autumn were subsequently identified as smolts in the following spring. We therefore suggest that, in line with previous studies on domesticated Atlantic salmon and laboratory‐based experiments, there is no developmental conflict but that life‐history expression is mediated by environmental and genetic processes. [ABSTRACT FROM AUTHOR]

A cohort of Chinook salmon juveniles was vaccinated, with an autogenous bivalent vaccine against New Zealand RLOs (NZ‐RLO1) and Tenacibaculum maritimum. A proportion of the cohort was not vaccinated to act as controls. At smoltification, the fish were challenged with NZ‐RLO1, NZ‐RLO2, or T. maritimum. We found that challenge with T. maritimum by immersion in (7.5 × 105 cfu/mL of water) did not yield any pathology. Challenge with RLOs produced clinical signs that were more or less severe depending on the challenge route, dose or vaccination status. Survival was significantly higher for vaccinated fish within the groups challenged with NZ‐RLO1 by intraperitoneal injection with a relative percent survival (RPS) of 48.84%. Survival was not significantly different between vaccinated and non‐vaccinated fish for groups challenged with NZ‐RLO2 by intraperitoneal injection or by NZ‐RLO1 by immersion. Yet, anecdotally the clinical disease presentation (manifesting as haemorrhagic, ulcerative skin lesions) was more severe for the non‐vaccinated fish. This study demonstrates that autogenous vaccine against NZ‐RLO is protective against severe disease and death by NZ‐RLO1 challenge which warrants implementation and further evaluation under field conditions. Yet, this study also highlights the importance of the route of administration and dose when evaluating pathogenicity and vaccine efficacy. [ABSTRACT FROM AUTHOR]

The biochemical mechanisms of the effect of additional lighting on the growth and early development of salmonids in aquaculture allowing identification of patterns of slow or accelerated growth, as well as the onset of the smoltification, are studied. The results of changes in lipid status in parr and smolts of Atlantic salmon after the "winter window" (December–January) are presented. These results were obtained from three experimental groups exposed to different lighting and feeding regimes at the fingerling stage. The changes in lipid composition of the studied young salmon indicate the onset of smoltification in all experimental groups. However, this effect is the most pronounced in the fish previously reared under 24-hour lighting during the summer–autumn season. [ABSTRACT FROM AUTHOR]

Anadromous salmonids exhibit partial migration, where some individuals within a population migrate down to the ocean through complex interactions between body size and photoperiod. This study aimed to integrate the ontogenetic and seasonal patterns of smoltification, a series of changes for future marine life, in a strain of masu salmon (Oncorhynchus masou). Spring smoltification, as evidenced by the activation of gill Na+,K+-ATPase (NKA), was induced during winter under an advanced photoperiod. In addition, juveniles showed an additional peak in gill NKA activity in August regardless of the photoperiod. When juvenile masu salmon were subjected to feeding manipulations during the first spring/summer, only fish exceeding a fork length of 12 cm exhibited an increased gill NKA activity. We tested whether size-driven smoltification required a long-day period by exposing juveniles to a constant short-day length (9-h light and 15-h dark) from January to November. Juveniles under short-day conditions exceeded 12 cm in June but showed no signs of smoltification. Thus, masu salmon undergo photoperiod-limited, size-driven smoltification during the first summer and size-limited, photoperiod-driven smoltification the following spring. The findings of the present study provide a framework for further elucidation of the physiological mechanisms underlying partial migration in salmonids. [ABSTRACT FROM AUTHOR]

Nephrocalcinosis is a widespread challenge in intensive production of salmon smolt. There is however no consensus on its aetiology, which makes it problematic to implement proper measures to limit its development. We performed a survey of nephrocalcinosis prevalence and environmental factors in 11 different hatcheries in Mid‐Norway as well as a 6‐month monitoring in one of the hatcheries. A multivariate analysis indicated that the most influencing factor for the prevalence of nephrocalcinosis was the supplementation of sea water during smolt production. In the 6‐month monitoring, the hatchery introduced salinity in the production water prior to the change in day length. Mismatch in those environmental signals may increase the risk for developing nephrocalcinosis. Salinity fluctuations prior to smoltification can cause osmotic stress and result in unbalanced levels of ions in fish blood. This was clearly illustrated in our study, as the fish experienced chronic hypercalcaemia and hypermagnesaemia. Both magnesium and calcium are excreted over the kidneys and it is possible that their prolonged, elevated levels in plasma resulted in an oversaturation of the urine when finally excreted. This again could have led to the aggregation of calcium deposits within the kidney. This study indicates a relationship between osmotic stress induced by salinity changes in juvenile Atlantic salmon and the development of nephrocalcinosis. Other factors that may affect the severity of nephrocalcinosis are currently subjects for discussion. [ABSTRACT FROM AUTHOR]

Within-clutch variation in the size or the amount of yolk in the egg has been widely studied because of its importance for both maternal and offspring fitness. In particular, egg size has been used as a proxy for survival in first-feeding salmonids alevins. We tested the hypothesis that egg size has implications for the phenotypes (development, physiology, and behavior) of juvenile steelhead trout (Oncorhynchus mykiss) up to 1 year post fertilization in both hatchery and wild fish. Egg diameter was measured for 38 families; the smallest and largest eggs were selected and reared individually to study embryonic development, and then collectively as "small" and "large" egg groups. The development and growth of individual embryos were followed up to first feeding. We showed that hatchery fish have smaller eggs than wild fish, offspring from small eggs grow better during their first year than those from larger eggs, offspring from large eggs hatch later and at a more advanced stage than those from small eggs, and the degree of smolting and saltwater preference differed between hatchery and wild offspring. By looking at not only mean egg size but also individual mediated–egg size phenotypic traits, we show the significant contribution of egg size in promoting phenotypic variation within a population and its potential for among-population variation. [ABSTRACT FROM AUTHOR]

A comparative study of the lipid and fatty acid composition of Atlantic salmon underyearlings reared under natural and continuous light in commercial aquaculture in the summer–autumn period in North Ossetia–Alania was carried out. The change in the main indicators of lipid metabolism in salmon underyearlings from September to November was found to have a stable trend aimed at preparing juveniles for smoltification. A change in key fatty acid indicators was detected: an increase in the content of PUFAs due to (n-3)PUFAs, and the "marine" type of PUFA due to 22:6(n-3), high values of the ratios (n-3)/(n-6) PUFA, 18:3(n-3)/18:2(n-6), 22:6(n-3)/18:3(n-3). This study has shown the stimulating effect of the combined and stable action of the continuous photoperiod and feeding in the summer–autumn period in the preparation for smoltification of Atlantic salmon fingerlings grown under aquaculture conditions. [ABSTRACT FROM AUTHOR]

Photoperiod, the portion of 24-h cycle during which an organism is exposed to illumination, is an important phenological cue in many animals. However, despite its influence on critical biological processes, there remain many unknowns regarding how variations in light intensity translate into perceived photoperiod. This experiment examined how light intensity variations affect perceived photoperiod in Atlantic salmon (Salmo salar) to determine whether photoperiod interpretation is, a) fixed such that anything above a minimum detection threshold is regarded as 'illumination', or b) adaptive and varies with recent light exposure. To do this we compared the frequency of smoltification and sexual maturation between groups of male parr which were exposed to one of eight light regimes on a 12:12 cycling regime (12-hour day/12-hour night). The eight regimes were divided into two treatments, four with 'High' daytime light intensity and four with 'Low' daytime light intensity. The 'High' and 'Low' intensity treatments were each sub-divided into four groups for which the subjective 'night' light intensity was 100%, 10%, 1% and 0% of the daytime light intensity, with four replicate tanks of each treatment. The results show that above a minimum detection threshold, Atlantic salmon have adaptive photoperiod interpretation which varies with recent light exposure, and that adaptive photoperiod interpretation modulates the timing of the parr-smolt transformation and sexual maturation. Further, we show that photoperiod interpretation varies between closely related families. Given the influence of phenological timing on species survival, our results reveal a critical role for integration of photoperiod interpretation in attempts to understand how geographically shifting thermal niches due to climate change will affect future populations. [ABSTRACT FROM AUTHOR]

The life history of Atlantic salmon (Salmo salar) includes an initial freshwater phase (parr) that precedes a springtime migration to marine environments as smolts. The development of osmoregulatory systems that will ultimately support the survival of juveniles upon entry into marine habitats is a key aspect of smoltification. While the acquisition of seawater tolerance in all euryhaline species demands the concerted activity of specific ion pumps, transporters, and channels, the contributions of Na+/HCO3− cotransporter 1 (Nbce1) to salinity acclimation remain unresolved. Here, we investigated the branchial and intestinal expression of three Na+/HCO3− cotransporter 1 isoforms, denoted nbce1.1, -1.2a, and -1.2b. Given the proposed role of Nbce1 in supporting the absorption of environmental Na+ by ionocytes, we first hypothesized that expression of a branchial nbce1 transcript (nbce1.2a) would be attenuated in salmon undergoing smoltification and following seawater exposure. In two separate years, we observed spring increases in branchial Na+/K+-ATPase activity, Na+/K+/2Cl− cotransporter 1, and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no attendant changes in nbce1.2a expression. Nonetheless, branchial nbce1.2a levels were reduced in parr and smolts within 2 days of seawater exposure. In the intestine, gene transcript abundance for nbce1.1 increased from spring to summer in the anterior intestine, but not in the posterior intestine or pyloric caeca, and nbce1.1 and -1.2b expression in the intestine showed season-dependent transcriptional regulation by seawater exposure. Collectively, our data indicate that tissue-specific modulation of all three nbce1 isoforms underlies adaptive responses to seawater. [ABSTRACT FROM AUTHOR]

Simple Summary: Smoltification is a developmental process that preadapts Atlantic salmon for a life in seawater. Suboptimal smoltification and poor timing of transfer to seawater is associated with increased mortality. MicroRNAs (miRNAs) are small non-coding genes. They regulate gene expression post-transcriptionally as part of the miRNA induce silencing complex (miRISC) where they guide miRISC to particular mRNAs (target genes). The aim of this study was to identify Atlantic salmon miRNAs expressed in liver that are associated with smoltification and adaptation to seawater as well as to predict their target genes. In total, 62 guide miRNAs were identified, and by their expression patterns they were clustered into three groups. Target gene predictions followed by gene enrichment analysis of the predicted targets indicated that the guide miRNAs were involved in post-transcriptional regulation of important smoltification associated biological processes. Some of these were energy metabolism, protein metabolism and transport, circadian rhythm, stress and immune response. Together, the results indicate that certain miRNAs are involved in the regulation of many of the important changes occurring in the liver during this developmental transition. Optimal smoltification is crucial for normal development, growth, and health of farmed Atlantic salmon in seawater. Here, we characterize miRNA expression in liver to reveal whether miRNAs regulate gene expression during this developmental transition. Expression changes of miRNAs and mRNAs was studied by small-RNA sequencing and microarray analysis, respectively. This revealed 62 differentially expressed guide miRNAs (gDE-miRNAs) that could be divided into three groups with characteristic dynamic expression patterns. Three of miRNA families are known as highly expressed in liver. A rare arm shift was observed during smoltification in the Atlantic salmon-specific novel-ssa-miR-16. The gDE-miRNAs were predicted to target 2804 of the genes revealing expression changes in the microarray analysis. Enrichment analysis revealed that targets were significantly enriched in smoltification-associated biological process groups. These included lipid and cholesterol synthesis, carbohydrate metabolism, protein metabolism and protein transport, immune system genes, circadian rhythm and stress response. The results indicate that gDE-miRNAs may regulate many of the changes associated with this developmental transition in liver. The results pave the way for validation of the predicted target genes and further study of gDE-miRNA and their targets by functional assays. [ABSTRACT FROM AUTHOR]

The process of smolting is a critical phase in the life cycle of anadromous salmonids, and it has been associated with substantial rates of mortality. Survival during freshwater and marine migration is known to have population‐level effects; thus, an understanding of the patterns of mortality has the potential to yield important insights into population bottlenecks. Despite important advancements in tracking techniques, the specifics of mortality events in anadromous salmonids during their initial migration to sea remain somewhat elusive. Here, we develop a framework combining spatial and temporal detections of smolt riverine migration from two tracking techniques, which enable inferences to be made about mortality locations, causes, and rates. In this study, we demonstrate that during their initial riverine transitional phase, smolts were particularly vulnerable to predators. Specifically, avian predation appeared to be the main cause of mortality (42%), although piscine predation events were not trivial (14%). Our results suggested some direct and indirect tagging‐induced mortality (e.g., through increased predation vulnerability), which highlights the importance of determining tagging mortality in a telemetry study to ensure adequate interpretation of migration success. Overall, by estimating migration loss and its variability, our study framework should help to guide management actions to mitigate the widespread population declines these species are currently facing. [ABSTRACT FROM AUTHOR]

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue—adrenocorticotropic hormone (ACTH)—as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts. [ABSTRACT FROM AUTHOR]

Rainbow trout aquaculture has large potential for growth, but requires better understanding of aspects like smoltification and its effect on trout performance after seawater transfer. The present study examined the effect of three different salinities (fresh water, 3–5 ‰ and 15–16 ‰) on smoltification and growth of rainbow trout (70 ± 10 g) reared under constant light between March–June 2018; it also documented hypoosmoregulatory performance and growth of trout from all salinity treatments transferred to full seawater at three different times (Transfer 1: 26 April; Transfer 2: 24 May; Transfer 3: 21 June). Smolt development was assessed with markers commonly used for Atlantic salmon like gill Na+, K+-ATPase activity, gill mRNA transcription of key genes (nkaα1a, nkaα1b and nkcc1a) and plasma concentration of Cl− and Na+. Results evidenced that trout in brackish water experienced an earlier onset of smoltification, displayed by rapid changes in mRNA transcription of the ATPase α-isoforms nkaα1a (freshwater isoform) and nkaα1b (seawater isoform), which, in turn, induced an earlier peak of the Na+, K+-ATPase (NKA) activity. However, smoltification also commenced and progressed in fresh water most likely as a result of reaching a certain size. The salinity treatment did not influence growth during the smoltification period. After seawater transfer, all treatments in Transfer 1 (26 April) displayed the best capacity to acclimatize to full seawater (with clear upregulations of nkaα1b and nkcc1a after seawater entry) and the highest growth, compared to all treatments in Transfers 2 and 3. In contrast, few differences in growth and smoltification markers occurred between salinity treatments within each of the seawater transfers. This suggests that trout in the first transfer had the best smolt condition (were at the peak of smolt window), which allowed them to adapt and grow better in seawater. Overall, such results indicate that timing of seawater transfer can be more important for trout performance in the grow-out phase in the sea than the water salinity during the early rearing period. As such, although brackish water can be used to induce rainbow trout smoltification, transferring the fish at the peak of the smolt window can increase chances for best seawater performance. • Brackish water (3–5‰ and 15–16‰) induced earlier onset of smoltification in trout. • This was evidenced by rapid changes in gill transcription of nkaα1a and nkaα1b. • However, smoltification also progressed in fresh water induced by size. • Groups transferred at the peak of the smolt window performed better in seawater. • Time of transfer seemed more important than prior salinity for seawater performance. [ABSTRACT FROM AUTHOR]

The concentrations of Cl–, Ca2+, Na+, and K+ in the blood of the brown trout Salmo trutta from two rivers of the Lake Ladoga basin (Alatsoya and Ulmasenjoki) are compared. The concentrations of Cl– and Ca2+ + in the fish blood differ and correlate with the concentrations of chlorides and bicarbonates in the river water. The level of Na+ (in fish blood from both rivers) and Cl– (in fish blood from the Ulmasenjoki River) increases until the age of 2+ in specimens, and then (at the age of ≥3+) remains almost unchanged. Sexual dimorphism is found by the concentration of K+ in the blood: in females its concentration is 13% higher than in males. The concentration of K+ is much higher in females at the age of 1+ from the Ulmasenjoki River than in females of the same age in the Alatsoya River. It is assumed that, along with other factors, changes in the ionic composition of brown trout six months before smoltification can determine the degree of their preparation for downstream migration. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Background: In species showing partial migration, as is the case for many salmonid fishes, it is important to assess how anthropogenic pressure experienced by migrating individuals affects the total population. We focused on brown trout (Salmo trutta) from the Guddal River in the Norwegian Hardanger Fjord system, which encompasses both resident and anadromous individuals. Aquaculture has led to increased anthropogenic pressure on brown trout during the marine phase in this region. Fish traps in the Guddal River allow for sampling all ascending anadromous spawners and descending smolts. We analyzed microsatellite DNA markers from all individuals ascending in 2006-2016, along with all emigrating smolts in 2017. We investigated (1) if there was evidence for declines in census numbers and effective population size during that period, (2) if there was association between kinship and migration timing in smolts and anadromous adults, and (3) to what extent resident trout were parents of outmigrating smolts. Results: Census counts of anadromous spawners showed no evidence for a decline from 2006 to 2016, but were lower than in 2000-2005. Estimates of effective population size also showed no trends of declines during the study period. Sibship reconstruction of the 2017 smolt run showed significant association between kinship and migration timing, and a similar association was indicated in anadromous spawners. Parentage assignment of 2017 smolts with ascending anadromous trout as candidate parents, and assuming that unknown parents represented resident trout, showed that 70% of smolts had at least one resident parent and 24% had two resident parents. Conclusions: The results bear evidence of a population that after an initial decline has stabilized at a lower number of anadromous spawners. The significant association between kinship and migration timing in smolts suggests that specific episodes of elevated mortality in the sea could disproportionally affect some families and reduce overall effective population size. Finally, the results based on parentage assignment demonstrate a strong buffering effect of resident trout in case of elevated marine mortality affecting anadromous trout, but also highlight that increased mortality of anadromous trout, most of which are females, may lower overall production in the system. [ABSTRACT FROM AUTHOR]

Populations of northern Dolly Varden (Salvelinus malma malma) exhibit partial seaward migration, yet little is known about this phenomenon in Dolly Varden populations. Our study analyzed data from three different Dolly Varden populations in the western Canadian Arctic in order to determine if: (1) differences in size-at-first seaward migration exist between fish that migrate at early and late ages among populations inhabiting different river systems, and (2) annual growth influences anadromous or resident life history choice. Otolith strontium analysis and back-calculation were used to determine age- and size-at-first seaward migration, respectively. Differences in age- and size-at-first seaward migration were determined across river system and migration age. Back-calculated fish lengths were compared using a mixed effect model to determine how early growth influences migratory tactics (early or late aged smolt, or resident). Our results indicate that fish exhibiting faster early growth migrated in earlier years and at smaller sizes than slower growing fish, however size- and age-at first seaward migration varied by river system. Faster growing Dolly Varden tended to become either residents or early smolts, while slower growth was associated with smolting later in life. This is contrary to life history theory where the fastest growing fish in a population should mature as a resident. Our results indicate factors other than growth may be influencing life history 'decisions' in Dolly Varden. Future work on growth efficiencies and metabolic rates is needed to assess how they affect migratory behaviours. [ABSTRACT FROM AUTHOR]

The effect of photoperiod manipulation on growth, smoltification and maturation was assessed in Atlantic salmon through an observational study performed in a commercial RAS facility from June (25.0 ± 11.0 g) to October 2018 (151.1 ± 25.5 g). Half of the commercial cohort 721 was raised in continuous light LD24:0 (LL) while the other received a 6‐week LD12:12 winter signal (WS) for smoltification from 12 August to 21 September. Parameters related to growth (body weight, gene expression of pituitary gh1 and gh2, and liver ghr1, igf1 and igfbp1a), smoltification (condition factor, plasma sodium and cortisol, gill NKA activity and nka1a, nka1b and nkcc1a expression) and maturation (GSI and pituitary fshb and lhb expression) were analysed. Afterwards, a multivariate analysis was performed on production data from five commercial cohorts raised in the facility (including 721) to identify variables potentially linked to early maturation in RAS. Results from the observational study indicated weak compensatory growth and slightly better smoltification in WS, although signs of size‐induced smoltification were present in LL. Smoltification indicators were poor in both treatments, suggesting that smolts may not be yet ready for seawater. No maturation was observed in any photoperiod treatment; however, the multivariate assessment suggested that such lack of maturation might be rather linked to the low mean temperature and SGR experienced by our cohort. [ABSTRACT FROM AUTHOR]

Migration behaviour, route selection and mortality of Atlantic salmon, Salmo salar L., smolts were studied at four different hydroelectric facilities in the River Mustionjoki, Finland, in May 2017. Radio‐tagged smolts were released upstream of the power stations and tracked by stationary antenna‐receiver systems and hand‐held receivers. Tracking revealed a general tendency of smolts to move downstream with the main flow, but also variable behaviour and mortality related to diverse conditions characteristic of each power station. Average migration delay at the power stations ranged between 13.8 and 101.1 h (median: 1.7–61.5 h). Estimated mortality ranges were 0%–50% in the forebays, 4%–64% in the power stations and 2–30%/km during river migration after passage of the dam. This study provided essential information on behaviour and mortality in relation to local conditions at each power station required for successful application of fish bypass systems in a salmon restoration project. [ABSTRACT FROM AUTHOR]

Anadromous salmonids begin life adapted to the freshwater environments of their natal streams before a developmental transition, known as smoltification, transforms them into marine-adapted fish. In the wild, smoltification is a photoperiod-regulated process, involving radical remodeling of gill function to cope with the profound osmotic and immunological challenges of seawater (SW) migration. While prior work has highlighted the role of specialized "mitochondrion-rich" cells (MRCs) and accessory cells (ACs) in delivering this phenotype, recent RNA profiling experiments suggest that remodeling is far more extensive than previously appreciated. Here, we use single-nuclei RNAseq to characterize the extent of cytological changes in the gill of Atlantic salmon during smoltification and SW transfer. We identify 20 distinct cell clusters, including known, but also novel gill cell types. These data allow us to isolate cluster-specific, smoltification-associated changes in gene expression and to describe how the cellular make-up of the gill changes through smoltification. As expected, we noted an increase in the proportion of seawater mitochondrion-rich cells, however, we also identify previously unknown reduction of several immune-related cell types. Overall, our results provide fresh detail of the cellular complexity in the gill and suggest that smoltification triggers unexpected immune reprogramming. [ABSTRACT FROM AUTHOR]

The developmental transition of juvenile salmon from a freshwater resident morph (parr) to a seawater (SW) migratory morph (smolt), known as smoltification, entails a reorganization of gill function to cope with the altered water environment. Recently, we used RNAseq to characterize the breadth of transcriptional change which takes place in the gill in the FW phase of smoltification. This highlighted the importance of extended exposure to short, winter-like photoperiods (SP) followed by a subsequent increase in photoperiod for completion of transcriptional reprogramming in FW and efficient growth following transfer to SW. Here, we extend this analysis to examine the consequences of this photoperiodic history-dependent reprogramming for subsequent gill responses upon exposure to SW. We use RNAseq to analyze gill samples taken from fish raised on the photoperiod regimes we used previously and then challenged by SW exposure for 24 hours. While fish held on constant light (LL) throughout were able to hypo-osmoregulate during a 24 hours SW challenge, the associated gill transcriptional response was highly distinctive from that in fish which had experienced a 7-week period of exposure to SP followed by a return to LL (SPLL) and had consequently acquired the characteristics of fully developed smolts. Fish transferred from LL to SP, and then held on SP for the remainder of the study was unable to hypo-osmoregulate, and the associated gill transcriptional response to SW exposure featured many transcripts apparently regulated by the glucocorticoid stress axis and by the osmo-sensing transcription factor NFAT5. The importance of these pathways for the gill transcriptional response to SW exposure appears to diminish as a consequence of photoperiod mediated induction of the smolt phenotype, presumably reflecting preparatory developmental changes taking place during this process. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The elasmoid scales of anadromous sea trout Salmo trutta L. represent a significant internal reservoir of Ca2+. Although more is known about long‐term remodelling of scales in response to calciotropic challenges encountered during smoltification and migration, very little is known about the contribution made by scales to the short‐term, minute‐to‐minute regulation of Ca2+ homeostasis in the extracellular fluid (ECF) during these phases of the life cycle. This gap in the knowledge is partly due to the technical challenges involved in measuring small Ca2+ fluxes around the scales of live fish in real time. Here, this study describes exfoliating, mounting and culturing scales and their resident cells from parr, smolt and adult sea trout from a freshwater environment, as well as from adult sea trout caught in sea or brackish water. All the scales were then examined using an extracellular, non‐invasive, surface‐scanning Ca2+‐sensitive microelectrode. The authors quantified the Ca2+ fluxes, in the absence of any systemic or local regulators, into and out of scales on both the episquamal and hyposquamal sides under different extracellular calcemic challenges set to mimic a variety of ECF‐Ca2+ concentrations. Scales from the life‐cycle stages as well as from adult fish taken from sea, brackish or fresh water all showed a consistent efflux or influx of Ca2+ under hypo‐ or hypercalcemic conditions, respectively. What were considered to be isocalcemic conditions resulted in minimal flux of Ca2+ in either direction, or in the case of adult scales, a consistent but small influx. Indeed, adult scales appeared to display the largest flux densities in either direction. These new data extend the current understanding of the role played by fish scales in the short‐term, minute‐to‐minute homeostatic regulation of ECF‐Ca2+ concentration, and are similar to those recently reported from zebrafish Danio rerio scales. This suggests that this short‐term regulatory response might be a common feature of teleost scales. [ABSTRACT FROM AUTHOR]

Pink salmon hatch in fresh water, but their highly anadromous life history requires them to migrate into the ocean immediately after gravel-emergence, at a very small size. During their down-river migration these larvae undergo rapid smoltification that completely remodels their osmoregulatory physiology. At this time, the larvae reportedly have high whole-body Na+ contents and we hypothesised that the active accumulation of internal Na+ occurs in preparation for ocean entry. Using a comparative approach, the present study characterised the ontogeny of Na+ regulation in larvae of the anadromous pink salmon and the fresh-water rainbow trout. Our results indicate that larvae from both species actively accumulated Na+; however, whole-body Na+ content was higher in rainbow trout larvae compared to pink salmon. The time-course of this response was similar in the two species, with highest Na+-uptake rates ( J Na + in ) shortly after yolk sac absorption, but the mechanism of Na+ accumulation differed between the species. Rainbow trout larvae greatly increased J Na + in to overcompensate for a large simultaneous increase in Na+-efflux rate ( J Na + out ), whereas pink salmon mounted a smaller increase in J Na + in while maintaining tight control over J Na + out , which is supported by a significantly lower paracellular permeability. Our results indicate that the transient accumulation of internal Na+ is not a unique feature of the highly anadromous life history in pink salmon and may be a common ontogenetic pattern during larval development in salmonids; and perhaps it is associated with the development of the cardiovascular system during the larvae's transition to a more active lifestyle. [ABSTRACT FROM AUTHOR]

Despite the preponderance of exorheic lakes in rivers home to anadromous salmonids, little research has focused on how salmon, trout, and char use lakes as part of their anadromous life histories. The literature on this subject has so far revealed that some parr move into lakes to feed and grow before smoltification but that smolts moving through lakes tend to have high mortality in disproportion to what is observed in other habitats they migrate in or through. Adults have been observed using lakes for behavioural thermoregulation prior to spawning, and kelts of iteroparous species often exploit lakes to overwinter before returning to sea to recondition. We summarized knowledge on lakes as salmonid habitat and identified knowledge gaps about the use of lakes by anadromous salmonids related to whether lakes are barriers that structure genetics of populations, whether mortality in lakes is compensatory or additive, and whether systems with lakes have higher rates of repeat spawning among iteroparous salmonids. Human activities that alter lakes require further study to understand how changes in temperature, oxygen, ice, or circulation affect navigation and fate. [ABSTRACT FROM AUTHOR]

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na+/K+-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day–night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered the MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence. [ABSTRACT FROM AUTHOR]

Knowledge of the relative importance of genetic versus environmental determinants of major developmental transitions is pertinent to understanding phenotypic evolution. In salmonid fishes, a major developmental transition enables a risky seaward migration that provides access to feed resources. In Atlantic salmon, initiation of the migrant phenotype, and thus age of migrants, is presumably controlled via thresholds of a quantitative liability, approximated by body size expressed long before the migration. However, how well size approximates liability, both genetically and environmentally, remains uncertain. We studied 32 Atlantic salmon families in two temperatures and feeding regimes (fully fed, temporarily restricted) to completion of migration status at age 1 year. We detected a lower migrant probability in the cold (0.42) than the warm environment (0.76), but no effects of male maturation status or feed restriction. By contrast, body length in late summer predicted migrant probability and its control reduced migrant probability heritability by 50–70%. Furthermore, migrant probability and length showed high heritabilities and between-environment genetic correlations, and were phenotypically highly correlated with stronger genetic than environmental contributions. Altogether, quantitative estimates for the genetic and environmental effects predicting the migrant phenotype indicate, for a given temperature, a larger importance of genetic than environmental size effects. [ABSTRACT FROM AUTHOR]

Atlantic salmon migrate to sea following completion of a developmental process known as smolting, which establishes a seawater (SW) tolerant phenotype. Smolting is stimulated by exposure to long photoperiod or continuous light (LL) following a period of exposure to short photoperiod (SP), and this leads to major changes in gill ion exchange and osmoregulatory function. Here, we performed an RNAseq experiment to discover novel genes involved in photoperiod-dependent remodeling of the gill. This revealed a novel cohort of genes whose expression rises dramatically in fish transferred to LL following SP exposure, but not in control fish maintained continuously on LL or on SP. A follow-up experiment revealed that the SP-history dependence of LL induction of gene expression varies considerably between genes. Some genes were inducible by LL exposure after only 2 weeks exposure to SP, while others required 8 weeks prior SP exposure for maximum responsiveness to LL. Since subsequent SW growth performance is also markedly improved following 8 weeks SP exposure, these photoperiodic history-dependent genes may be useful predictive markers for full smolt development. [ABSTRACT FROM AUTHOR]

Pacific salmon stocks are in decline with climate change named as a contributing factor. The North Pacific coast of British Columbia is characterized by strong temporal and spatial heterogeneity in ocean conditions with upwelling events elevating CO2 levels up to 10-fold those of pre-industrial global averages. Early life stages of pink salmon have been shown to be affected by these CO2 levels, and juveniles naturally migrate through regions of high CO2 during the energetically costly phase of smoltification. To investigate the physiological response of out-migrating wild juvenile pink salmon to these naturally occurring elevated CO2 levels, we captured fish in Georgia Strait, British Columbia and transported them to a marine lab (Hakai Institute, Quadra Island) where fish were exposed to one of three CO2 levels (850, 1500 and 2000 μatm CO2) for 2 weeks. At ½, 1 and 2 weeks of exposure, we measured their weight and length to calculate condition factor (Fulton's K), as well as haematocrit and plasma [Cl−]. At each of these times, two additional stressors were imposed (hypoxia and temperature) to provide further insight into their physiological condition. Juvenile pink salmon were largely robust to elevated CO2 concentrations up to 2000 μatm CO2, with no mortality or change in condition factor over the 2-week exposure duration. After 1 week of exposure, temperature and hypoxia tolerance were significantly reduced in high CO2, an effect that did not persist to 2 weeks of exposure. Haematocrit was increased by 20% after 2 weeks in the CO2 treatments relative to the initial measurements, while plasma [Cl−] was not significantly different. Taken together, these data indicate that juvenile pink salmon are quite resilient to naturally occurring high CO2 levels during their ocean outmigration. [ABSTRACT FROM AUTHOR]

The anadromous salmon life cycle includes two migratory events, downstream smolt migration and adult homing migration, during which they must navigate with high precision. During homing migration, olfactory cues are used for navigation in coastal and freshwater areas, and studies have suggested that the parr–smolt transformation has a sensitive period for imprinting. Accordingly, we hypothesized that there would be significant changes in gene expression in the olfactory epithelium specifically related to smoltification and sampled olfactory rosettes from hatchery‐reared upper growth modal juvenile Atlantic salmon at 3‐week intervals from January to June, using lower growth modal nonsmolting siblings as controls. A suite of olfactory receptors and receptor‐specific proteins involved in functional aspects of olfaction and peripheral odor memorization was analyzed by qPCR. Gene expression in juveniles was compared with mature adult salmon of the same genetic strain caught in the river Gudenaa. All mRNAs displayed significant variation over time in both modal groups. Furthermore, five receptor genes (olfc13.1, olfc15.1, sorb, ora2, and asor1) and four olfactory‐specific genes (soig, ependymin, gst, and omp2) were differentially regulated between modal groups, suggesting altered olfactory function during smoltification. Several genes were differentially regulated in mature salmon compared with juveniles, suggesting that homing and odor recollection involve a different set of genes than during imprinting. Thyroid hormone receptors thrα and thrβ mRNAs were elevated during smolting, suggesting increased sensitivity to thyroid hormones. Treatment of presmolts with triiodothyronine in vivo and ex vivo had, however, only subtle effects on the investigated olfactory targets, questioning the hypothesis that thyroid hormones directly regulate gene expression in the olfactory epithelium. [ABSTRACT FROM AUTHOR]

We assessed the fish length – otolith length relationship (FL–OL) in Dolly Varden (Salvelinus malma malma) to verify proportional growth. A decoupling was detected during first ocean migration where fish growth was occurring at a greater rate than otolith growth. Because of this decoupling, the application of traditional back-calculation models overestimated the size-at-age in premigratory char. We developed modified back-calculation equations from existing traditional models to account for this decoupling based on discontinuous piecewise regressions. The new biological intercept breakpoint method (BI–BP) provided the most accurate representation of fish size-at-age throughout all life history stages when compared with known size-at-capture values in fish. The decoupling indicates that factors other than somatic growth are important for otolith accretion. Physiological changes during smoltification likely alter calcium uptake and thereby affect calcium deposition rates on otoliths during this short but biologically critical time period of life history. It is probable that species exhibiting similar complex ontogenetic shifts in life history will likely exhibit decoupling to some extent in the FL–OL relationship. [ABSTRACT FROM AUTHOR]

To study seawater tolerance and make a morphological assessment of yearling sea trout, their maturation and smoltification signs were estimated in two different fish rearing systems in Latvia,—in recirculating and flow-through system. For yearling sea trout (Salmo trutta L.), fish hypo-osmoregulatory ability was evaluated using seawater tolerance test, also fish morphological parameters were analyzed from January to May. April and May are months when smoltification occurs for wild sea trout in nature. Sea trout from recirculating system initially showed better growth, higher survival rate, and hypo-osmoregulatory ability, due to the elevated rearing water temperature. However, the situation completely changed in May when natural smoltification peaked and wild sea trout migration to the sea occurred—survival rate for yearling sea trout reared in recirculating system dropped to zero. Nevertheless, survival of fish from flow-through system increased, reaching 33%. Furthermore, in flow-through systems, sea trout that survived had significantly lower condition factor, also silvering level was higher compared those who did not survive. Weight was not a useful factor for determination of sea trout smoltification. The most part of yearling sea trout did not smoltify at the age of 1 year and should be reared for one more year before release in natural watercourses foreseen for migration to the sea. [ABSTRACT FROM AUTHOR]

The age and growth of Vistula River migratory sea trout, Salmo trutta L., caught in commercial catches in the Vistula in 2017-2018 were analyzed. The mean length of the sea trout caught was 61.4 cm in 2017 and 62.8 cm in 2018. It was confirmed that the fish caught reached an average of 56.5 cm following the first year at sea, 69 cm after the second year, and 82 cm after the third. The dominant age class among the fish in the catches was a sea age 1+. No older age classes than 3+ were noted. The age at smoltification of the sea trout examined in most instances was 2+. The results obtained were compared and discussed in light of historical results. The condition coefficient calculated was lower than that of the 1960s. [ABSTRACT FROM AUTHOR]

Photoperiod is thought to be the main zeitgeber that induces smoltification in salmonids. However, its effects on the smoltification of rainbow trout (Oncorhynchus mykiss) are not fully understood and no published data documents the effects of the photoperiod regime currently used commercially, continuous light (LL). The present study compared the effect of four different photoperiod regimes (i.e. advanced phase photoperiod (APP), delayed phase photoperiod (DPP), LL and simulated natural photoperiod (SNP)) on the smoltification and growth of juvenile rainbow trout during their freshwater phase of winter-spring and the following summer post smolt phase. Smoltification was evaluated by monitoring gill Na+,K+–ATPase (NKA) activity and transcription of NKA α-subunit isoforms 1a and 1b , and Na + ,K + , 2Cl ‐ cotransporter 1a. Growth was measured as specific growth rate of both length and weight, and through molecular growth proxies such as the levels of circulating insulin-like growth factor 1 (IGF-I) in plasma and transcription of igf-I , igf binding protein 1b (igfbp1b), growth hormone receptor 1 (ghr1) and cathepsin L (ctsl) in the liver. Results indicate that APP induces a longer smolt window and higher levels of plasma IGF-I in both freshwater and seawater (two months post transfer), while DPP led to a shorter smolt window, lower plasma IGF-I levels in freshwater and seawater, an earlier decrease in liver igf-I and ctsl transcription in freshwater (as seen by modelling over time) and lower specific growth rate in freshwater. The transcription analysis of osmoregulatory genes complemented NKA activity and allowed for the detection of a transient response to light and of differences between the osmoregulatory capacity of parr and desmolted fish. Furthermore, an upregulation of the liver transcription of igf-I , ghr1 and ctsl was found in all treatments during the smolt window, which corresponded to the periods with highest growth. Finally, both plasma IGF-I and liver igf-I in seawater were found to be significantly correlated to fish growth in seawater. However, our data did not show that plasma IGF-I prior to seawater transfer could be used as a reliable predictor of growth in seawater. Overall, and especially when compared with other salmonid species, photoperiod seems to be a weaker inducer of smoltification in rainbow trout, according to the parameters that were tested, suggesting that other environmental cues might be more important drivers of this process. • nkaα1a , nkaα1b and nkcc1a provide relevant information that the NKA activity does not reflect. • The liver transcription of igf-I , ghr1 and ctsl is directly related to fish SGRL. • While the best photoperiod in terms of duration of the smolt window and abundance of plasma IGF-I seemed to be APP, DPP was clearly the worst one. • While plasma IGF-I and liver igf-I in seawater was correlated to growth, plasma IGF-I prior seawater transfer was not a predictor of growth. [ABSTRACT FROM AUTHOR]

Abstract Variations in the mRNA expression of hepatic and muscle genes that are related to calcium signaling were analyzed by real-time qPCR in farmed Atlantic salmon (Salmo salar L. 1758) to determine changes in expression between parr and smolt stages. These organs were selected due to their close relationship with calcium signaling and metabolism (e.g., glycolysis, oxidative phosphorylation, muscle contraction). Differential expression between smolt and parr specimens and between organs was observed. Compared to parr specimens, smolts exhibited upregulated expression of the calcitonin receptor precursor, calcitonin receptor, calcitonin isoform, parathyroid hormone, and calmodulin in the liver. This pattern was inverse in muscle, with the exception of calmodulin, which was significantly upregulated in smolts compared to parr. Additionally, plasma calcium was decreased in the smolt condition. This study is the first to characterize the expression pattern of calcium signaling-related genes in the liver and muscle of parr and smolt S. salar. However, further functional studies are required to obtain a wider understanding about the physiological changes that accompany the productive conditions during smoltification. Graphical abstract The results suggest a transcriptional activation of genes involved in oxidative stress in both liver and muscle. The present study evaluated the mRNA expression of a group of genes related to calcium signaling in the liver and muscle during S. salar smoltification under farming conditions. This careful selection could reduce existing high mortality rates during farm-based smoltification, thus promoting the sustainable development of salmon farming. Unlabelled Image Highlights • Atlantic salmon (Salmo salar L. 1758) is the most widely farmed Salmonidae species worldwide • The aim was to screen changes in the mRNA expression of different genes related to calcium signaling in smoltification • Differential expression between smolt and parr specimens and between organs was observed • Plasma calcium was decreased in the smolt condition • It is first study to characterize the expression of calcium signaling-related genes in liver and muscle during smoltification [ABSTRACT FROM AUTHOR]

The article focuses on Danish studies about smolt predation to examine the impact of cormorant predation. Information about the breeding population of great cormorant, Phalacrocorax carbo sinensis (L.) as well as the results of the influence of cormorant predation on Atlantic salmon and Sea trout smolt survival is presented.

We used over 154 000 scale samples collected from salmon fisheries in the large River Teno system over a 40-year period to quantify life history diversity and long-term trends. We identified 120 different life history strategies, including combinations of smolt (2–8) and sea ages (1–5) and previous spawning events. Most strategies were rare; 60% of individuals matured after 1 year at sea following 3–5 years in fresh water. Age at maturity changed with an increase in two-sea-winter salmon and previous spawners and a decline in three-sea-winter fish. Smolt age distribution showed a decreasing proportion of age-3 smolts, while that of age-5 smolts increased. Fishing gear and fishing season times selected for fish differing in life history strategies. Temporal variation in life histories reflected changes in both fisheries and the changing environment. There was an inverse relationship between years spent in fresh water and sea age. Biocomplexity was manifested by the multiple year classes (6–11) present in annual runs, which increased with years, reflecting an increase both in previous spawners and sampling effort. The high number of cohorts spawning simultaneously each year indicates strong generational overlap, which has been suggested to maintain genetic diversity and thereby resilience via the portfolio effect. [ABSTRACT FROM AUTHOR]

Otolith strontium and multi-year mark-recapture information were used to characterize associations between migration patterns and spawning frequencies in an anadromous Dolly Varden (Salvelinus malma) population (Rat River, Northwest Territories, Canada) that undertakes a long migration between freshwater spawning/overwintering (Fish Creek; a tributary to Rat River) and marine feeding habitats (Beaufort Sea) (~800 km round trip). Reconstructions of lifetime annual migration histories among otolith annuli was matched to information on reproductive status (current-year ‘spawner’ or ‘non-spawner’) that was known in two different, sometimes successive, years for each fish. Two migratory life histories were observed: fish either migrated annually after smoltification or periodically skipped an annual ocean migration to remain in freshwater and spawn. Different spawning frequencies were detected where fish not migrating annually tended to spawn in alternate years (84.6%) more often than those migrating annually (50%). Additionally, annually migrating fish had lower longevity (≤9 years vs. ≤13 years). The evaluation of differences in spawning frequency between sexes, independent of migration tactic, revealed males (84.6%) skipped spawning more often than females (51.2%) suggesting fitness trade-offs between life histories differ between sexes. Further, some fish returned from the sea considerably earlier than the majority of other current-year migrants. Our findings demonstrate intrapopulation diversity in migration behaviour and reproductive frequency. [ABSTRACT FROM AUTHOR]

In this article, we present the first use of genetic assignment to determine the origins of farmed Atlantic salmon (Salmo salar L.) escaping directly into freshwater. Juvenile fish (n = 220) sampled from rotary screw traps operating on tributaries of Loch Shin in Scotland had characteristics suggesting they were of farm origin (scale losses, fin damage, and vaccination marks). To investigate their origins, baseline samples (n = 1200) were collected from the two freshwater smolt rearing facilities on the Loch. Baseline analysis with 186 single-nucleotide polymorphic (SNP) markers identified three assignment units associated with the aquaculture strains in production, Mowi and Aquagen at Site A and Fanad at Site B. Exclusion analysis identified at least 37% of the suspected escapees as being of farmed origin, 95% from Site A and 5% from B. The non-random nature of fish collection and trapping locations prevented determination of either absolute proportions of escapees in the entire Shin system or escape proportions from each site. However, it was clear that fish from both sources had escaped. The study demonstrates that genetic assignment to farm origin is possible in a novel situation in which fish have escaped at the smolt stage directly into freshwater. [ABSTRACT FROM AUTHOR]

Mouthrot, caused by Tenacibaculum maritimum is a significant disease of farmed Atlantic salmon, Salmo salar on the West Coast of North America. Smolts recently transferred into saltwater are the most susceptible and affected fish die with little internal or external clinical signs other than the characteristic small (usually < 5 mm) yellow plaques that are present inside the mouth. The mechanism by which these smolts die is unknown. This study investigated the microscopic pathology (histology and scanning electron microscopy) of bath infected smolts with Western Canadian T. maritimum isolates TmarCan15-1, TmarCan16-1 and TmarCan16-5 and compared the findings to what is seen in a natural outbreak of mouthrot. A real-time RT-PCR assay based on the outer membrane protein A specific for T. maritimum was designed and used to investigate the tissue tropism of the bacteria. The results from this showed that T. maritimum is detectable internally by real-time RT-PCR. This combined with the fact that the bacteria can be isolated from the kidney suggests that T. maritimum becomes systemic. The pathology in the infected smolts is primarily mouth lesions, including damaged tissues surrounding the teeth; the disease is similar to periodontal disease in mammals. The pathological changes are focal, severe, and occur very rapidly with little associated inflammation. Skin lesions are more common in experimentally infected smolts than in natural outbreaks, but this could be an artefact of the challenge dose, handling and tank used during the experiments. [ABSTRACT FROM AUTHOR]

For anadromous Pacific salmonid (Oncorhynchus spp.) smolts, the physiological state of individuals can influence migration fate. This critical life stage is typically associated with poor survival and influences population productivity, highlighting the need to identify intrinsic factors associated with outmigration fate. To better understand and identify such factors, we combined acoustic telemetry with nonlethal gill biopsies and used high-throughput real-time quantitative polymerase chain reaction to assess how infectious agents and host gene expression profiles influence migration fate for hatchery steelhead smolts (Oncorhynchus mykiss). Redundancy analyses of gene expression, infectious agent loads, and body condition highlighted gene expression profiles indicative of migratory fate. Smolts never detected after release in the river had significantly elevated expression of the immune genes Il-17D and RPL6, and lower expression of the osmoregulatory gene NKA α1b relative to other individuals. Flavobacterium psychrophilum and "Candidatus Branchiomonas cysticola" were detected in gill samples, but neither influenced survival. We demonstrate rare evidence of gene expression profiles relating to migration fate in juvenile salmonids and highlight potential mechanisms influencing fate for hatchery steelhead smolts. [ABSTRACT FROM AUTHOR]

Dams and turbines associated with hydroelectric power plants (HEP) disrupt connectivity by affecting fish movement and survival. There has been an increasing focus on measures to facilitate downstream migration at HEPs. The fish guidance efficacy (FGE) of downstream mitigation measures largely remains suboptimal and calls for development of knowledge on factors influencing FGE. In this study, we analyse 6 years of wild Atlantic salmon smolt passive integrated transponder (PIT)‐telemetry data (N = 1,498) from a neighbouring small‐ and a large‐scale HEP. Timing of the smolt migration period was significantly different between the two rivers. Thus, river‐specific smolt‐run timing is imperative for proper measures management in regulated rivers aiming at maximizing smolt‐descent survival. A generalized linear model including additive effects of relative bypass discharge and scaled river discharge on the FGE for descending smolts received highest Akaike's information criterion support in the data and explained 74.2% of the FGE variation. This model, including no river effect, predicted high FGE (up to 90%) at low river flow (≤30% of HEP maximum capacity) when 7% of the water is allocated through the bypass. Many run‐of‐the‐river HEPs have highly variable river flow during the smolt‐run period. Our model suggests that these HEPs could utilize their manoeuvre flexibility to obtain water allocation routines between bypass and turbines that optimize both FGE and hydroelectric production. [ABSTRACT FROM AUTHOR]