<p>Populationer av öring består av både migrerande och stationära individer, och det tycks vara både genetiska och miljömässiga faktorer som påverkar en individs beslut att vandra eller ej. Syftet med denna studie var att testa vilken effekt tillgång på föda har på tillväxt och smoltifieringsprocess hos öring (Salmo trutta). Min hypotes var att öring med hög tillförsel av mat växer bättre och smoltifieras därmed inte till samma grad som öring med låg tillförsel av mat. Som en konsekvens av skillnader i mattillgång förväntades att ATPas-aktiviteten, vilken indikerar smoltstatus, var lägre vid god tillförsel av mat än vid låg tillförsel. Experimentet utfördes från november 2003 till april 2004 vid fiskeodlingen i Brattfors, Värmland, där 2+ Klarälvsöringar utsattes för tre olika födonivåer, med fyra replikat av varje födonivå. Var 4-5 vecka fångades fisken för att vägas och mätas och slutligen togs biopsi på öringens gälar för en analys av Na+,K+ -ATPas aktivitet. I slutet av experimentet klassificerades fiskarna som parr eller smolt.</p><p>Parr hade en signifikant lägre Na+, K+ -ATPas aktivitet än smolt i de tre olika födonivåerna. Na+, K+ -ATPas aktiviteten visade dock ingen signifikant skillnad mellan födonivåerna. Parr hade ökat sin vikt och längd mest i samtliga födonivåer och störst skillnad var det i den lägsta födonivån, parr hade ökat i genomsnitt 17 g och 3,5 mm mer än smolt. Av de två högsta födonivåerna var andelen klassificerade som smolt 37 % och vid den lägsta födonivån var andelen smolt 76 %.</p> / <p>Populations of brown trout consist of both migratory and resident individuals, and it is likely that both genetic and environmental factors influence the decision to migrate. The purpose of this study was to test the effect of food availability on growth and the smolting process in brown trout (Salmo trutta). My hypothesis was that brown trout subjected to a high food supply grows better and smoltifies to a lesser degree than brown trout subjected to a low food supply. As a consequence of differences in food supply I expected that ATPase-activity, which indicates smoltification status, would be lower with a high food supply than with a low supply. The experiment was run from November 2003 until April 2004 in a fish hatchery in Brattfors, Värmland, where 2+ brown trout from the river Klarälven were exposed to three food levels, each replicated four times. Every 4-5 weeks the fish were weighed and measured and a biopsy was taken from the gills for analysis of Na+, K+ -ATPase activity at the end of the experiment. At the end of the experiment the fish were classified as parr or smolt.</p><p>Parr had a significantly lower Na+, K+ -ATPase activity than smolt for the three different food treatments. Na+, K+ -ATPase activity did not differ significantly between food levels. Parr grew faster (both weight and length) than smolt at all food levels, and the largest difference between parr and smolt was observed at the lowest food level, where parr increased on average 17 g and 3,5 mm more than smolt. At the two highest food levels the proportion classified as smolt was 37 % and at the lowest foodlevel the proportion of smolt was 76 %.</p>

Populationer av öring består av både migrerande och stationära individer, och det tycks vara både genetiska och miljömässiga faktorer som påverkar en individs beslut att vandra eller ej. Syftet med denna studie var att testa vilken effekt tillgång på föda har på tillväxt och smoltifieringsprocess hos öring (Salmo trutta). Min hypotes var att öring med hög tillförsel av mat växer bättre och smoltifieras därmed inte till samma grad som öring med låg tillförsel av mat. Som en konsekvens av skillnader i mattillgång förväntades att ATPas-aktiviteten, vilken indikerar smoltstatus, var lägre vid god tillförsel av mat än vid låg tillförsel. Experimentet utfördes från november 2003 till april 2004 vid fiskeodlingen i Brattfors, Värmland, där 2+ Klarälvsöringar utsattes för tre olika födonivåer, med fyra replikat av varje födonivå. Var 4-5 vecka fångades fisken för att vägas och mätas och slutligen togs biopsi på öringens gälar för en analys av Na+,K+ -ATPas aktivitet. I slutet av experimentet klassificerades fiskarna som parr eller smolt. Parr hade en signifikant lägre Na+, K+ -ATPas aktivitet än smolt i de tre olika födonivåerna. Na+, K+ -ATPas aktiviteten visade dock ingen signifikant skillnad mellan födonivåerna. Parr hade ökat sin vikt och längd mest i samtliga födonivåer och störst skillnad var det i den lägsta födonivån, parr hade ökat i genomsnitt 17 g och 3,5 mm mer än smolt. Av de två högsta födonivåerna var andelen klassificerade som smolt 37 % och vid den lägsta födonivån var andelen smolt 76 %. / Populations of brown trout consist of both migratory and resident individuals, and it is likely that both genetic and environmental factors influence the decision to migrate. The purpose of this study was to test the effect of food availability on growth and the smolting process in brown trout (Salmo trutta). My hypothesis was that brown trout subjected to a high food supply grows better and smoltifies to a lesser degree than brown trout subjected to a low food supply. As a consequence of differences in food supply I expected that ATPase-activity, which indicates smoltification status, would be lower with a high food supply than with a low supply. The experiment was run from November 2003 until April 2004 in a fish hatchery in Brattfors, Värmland, where 2+ brown trout from the river Klarälven were exposed to three food levels, each replicated four times. Every 4-5 weeks the fish were weighed and measured and a biopsy was taken from the gills for analysis of Na+, K+ -ATPase activity at the end of the experiment. At the end of the experiment the fish were classified as parr or smolt. Parr had a significantly lower Na+, K+ -ATPase activity than smolt for the three different food treatments. Na+, K+ -ATPase activity did not differ significantly between food levels. Parr grew faster (both weight and length) than smolt at all food levels, and the largest difference between parr and smolt was observed at the lowest food level, where parr increased on average 17 g and 3,5 mm more than smolt. At the two highest food levels the proportion classified as smolt was 37 % and at the lowest foodlevel the proportion of smolt was 76 %.

Les patrons de migration de saumonneaux sauvages du saumon Atlantique (Salmo salar L.) ont été déterminés dans un système fluvial / estuarien de la péninsule de Gaspé, Québec, Canada. Vingt-quatre saumonneaux en 2005 et trente saumonneaux en 2006 ont été munis d'émetteurs acoustiques codés et leur migration a été suivie en rivière et en estuaire grâce à un large déploiement de récepteurs fixes de type VR2. L'effet de la période du jour (jour vs. nuit), du débit et des caractéristiques des saumonneaux (poids et facteur de condition) sur le temps de transit d'une section fluviale a été déterminé par régression linéaire. La relation entre la vélocité observée des smolt (par rapport au sol) et les caractéristiques des saumonneaux et les conditions environnementales - mesurées et prédites par un modèle hydrodynamique validé - a été déterminée par modèle de régressions multiples. Un effet circadien important a été remarqué en rivière, les mouvements se produisant surtout durant la nuit. La migration était plus lente durant le jour. Le débit de la rivière avait un effet sur la vitesse de transit durant la nuit, suggérant une composante passive de la migration durant la nuit. Dans l'estuaire, la migration était aussi affectée par la vélocité des courants et les smolts se déplaçaient vers la mer aux marées baissantes et vers l'amont aux marées montantes. L'effet de la vélocité des courants était plus prononcé durant la nuit que durant le jour ce qui peut suggérer que les saumonneaux adoptaient un comportement d'alimentation ou d'évitement des prédateurs durant le jour comme en rivière. Suivant une augmentation de la salinité, les mouvements vers la mer étaient plus fréquents aux marées montantes et la vélocité observée des saumonneaux augmentait avec un biais vers la mer. Il est proposé que les saumonneaux subissant une augmentation de la salinité ambiante vont modifier leur comportement passant de déplacements plutôt passifs à une nage active orientée vers la mer.

La smoltification est un évènement métamorphique chez le saumon qui initie la migration de dévalaison et pré-adapte le juvénile à l’entrée en mer. Cette thèse a pour objectif d’étudier les régulations endocrines et environnementales de la smoltification chez le saumon Atlantique de la longue rivière Loire-Allier, population qui est en danger. Nous montrons la présence et la divergence fonctionnelle de deux paralogues de la sous-unité ß (tshßa & tshßb) de la thyrotropine (TSH) chez le saumon Atlantique et observons un pic d’expression de tshßb dans l’hypophyse à la smoltification, pic concomitant à l’initiation de la migration de dévalaison. Ce résultat est le premier à mettre en relation l’expression hypophysaire de TSH avec la smoltification et le comportement migratoire de dévalaison. L’exposition expérimentale à une photopériode constante de jours courts ou à une température augmentée n’affecte pas nettement le pic de tshßb ni l’initiation de la migration de dévalaison, ce qui met en évidence l’importance de contrôles endogènes. Cette étude apporte de nouvelles connaissances fondamentales sur le cycle de vie du saumon Atlantique avec la découverte de nouveaux acteurs dans le processus de smoltification et avec des implications dans le domaine de la conservation. / Smoltification is a metamorphic event in salmon, which initiates downstream migration and pre-adapts juvenile for seawater entry. The PhD aimed at investigating endocrine and environmental regulation of smoltification in the endangered long-river Loire-Allier Atlantic salmon. We report the presence and functional divergence of thyrotropin ß-subunit paralogs (tshßa & tshßb) in Atlantic salmon and showed a peak pituitary expression of tshßb at smoltification which was concomitant with the initiation of downstream migration. This is the first time pituitary TSH expression is related to smoltification and downstream migratory behavior. Experimental exposure to constant short-day photoperiod or to increased temperature did not markedly affect the peak of tshßb nor the initiation of downstream migration, highlighting the importance of endogenous controls. This study brings new insights to the life cycle of Atlantic salmon with the discovery of novel components of the smoltification process, and with implications for conservation.

<p>During the smoltification the Atlantic salmon (Salmo salar) develop different adaptations to survive in oceanic environment. One of the most important adaptations is the ability to excrete the surplus of salt through the gills. The excretion is controlled by an enzyme called Na+/K+-ATPase which is produced in an α-subunit by two gene isoforms called α1a and α1b. Enzyme activity is increasing during the smoltification process and is a strong indicator for that the process is taking place. The aim of this study was to investigate a landlocked strain of Atlantic salmon and see how the enzyme activity is developing in comparison with the gene expression of the mRNA that is coded for the enzyme. The study was carried out between March and May in the hatchery in Brattfors, Värmland. Fish were sampled at four occasions. The enzyme activity was compared between two groups of salmon where one group had full ration of food, 100% and the other group had a 15% food ration. The enzyme activity for the 100% group was then compared with the gene expression from the same group. The hypothesis was that food availability should effect smoltification and that the 15% group would have a faster increase in activity compared with the 100% group. There should also be some correlation between enzyme activity and gene expression. Na+/K+-ATPase enzyme activity showed no major differences between the groups except for a significant difference at the last sampling. Both groups had a large increase in activity from the second to the third sampling with a peak on 3.16 µmol ADP/mg/h at most. This was followed by a drop in activity at the last sampling date. The gene expression showed a fast increase of the α1b gene over the study with drop in the last sampling and the α1a gene had a constant increase from the first to the last sampling. The comparison with enzyme activity and gene expression showed a weak correlation. Compared with studies done on anadromous salmon and the land locked salmon in this study had a different development in gene expression. This could be explained that the different life strategies play an important role how the genes are expressed.</p> / <p>Under smoltifieringen utvecklar atlantlaxen (Salmo salar) olika anpassningar för att överleva i havsmiljö. En av de vikigaste anpassningarna är att utsöndra överskott av salt via gällarna. Exkretionen är kontrollerad av ett enzym som heter Na+/K+-ATPas som produceras i en α-subenhet av två isoformer av gener som heter α1a och α1b. Enzym aktiviteten ökar under smoltifieringen och är en stark indikator på att processen sker. Målet med denna studie var att undersöka en sjövandrande stam av atlantlax och se hur enzymaktiviteten utvecklas i jämförelse med gen expressionen av mRNA som kodar för produktionen av enzymet. Studien genomfördes vid fiskodlingen i Brattfors, Värmland där prover togs vid fyra tillfällen. Enzymaktiviteten jämfördes mellan två grupper av lax där en grupp fick full matranson 100 % och en grupp fick 15 % matranson. Senare jämfördes enzymaktiviteten för 100 % gruppen med gen expressionen inom samma grupp. Hypotesen var att tillgängligheten på mat skulle påverka smoltifieringen och att 15 % gruppen skulle ha en snabbare ökning i jämförelse med 100 % gruppen. Det skulle också vara en viss korrelation mellan enzymaktivitet och gen expression. Na+/K+-ATPas enzym aktiviteten visade inga större skillnader mellan grupperna förutom vid sista provtagningen. Båda grupperna hade en stor ökning från den andra till den tredje provtagningen med den högsta aktiviteten på 3.16 µmol ADP/mg/h. Detta följdes av ett fall i aktivitet vid sista provtagningen. Gen expressionen visade en snabb ökning av α1b genen över studien med en nedgång vid sista provtagningen och α1a hade en konstant men mindre ökning från första till sista provtagningen. Jämförelsen mellan enzymaktivitet och gen expression visade på en svag korrelation. Det fanns en skillnad i gen expression mellan studier gjorda på anadrom lax och sjövandrande lax i denna studie. Detta kan förklaras av att de olika livsstrategierna spelar en betydande roll i hur generna uttrycks.</p>

During the smoltification the Atlantic salmon (Salmo salar) develop different adaptations to survive in oceanic environment. One of the most important adaptations is the ability to excrete the surplus of salt through the gills. The excretion is controlled by an enzyme called Na+/K+-ATPase which is produced in an α-subunit by two gene isoforms called α1a and α1b. Enzyme activity is increasing during the smoltification process and is a strong indicator for that the process is taking place. The aim of this study was to investigate a landlocked strain of Atlantic salmon and see how the enzyme activity is developing in comparison with the gene expression of the mRNA that is coded for the enzyme. The study was carried out between March and May in the hatchery in Brattfors, Värmland. Fish were sampled at four occasions. The enzyme activity was compared between two groups of salmon where one group had full ration of food, 100% and the other group had a 15% food ration. The enzyme activity for the 100% group was then compared with the gene expression from the same group. The hypothesis was that food availability should effect smoltification and that the 15% group would have a faster increase in activity compared with the 100% group. There should also be some correlation between enzyme activity and gene expression. Na+/K+-ATPase enzyme activity showed no major differences between the groups except for a significant difference at the last sampling. Both groups had a large increase in activity from the second to the third sampling with a peak on 3.16 µmol ADP/mg/h at most. This was followed by a drop in activity at the last sampling date. The gene expression showed a fast increase of the α1b gene over the study with drop in the last sampling and the α1a gene had a constant increase from the first to the last sampling. The comparison with enzyme activity and gene expression showed a weak correlation. Compared with studies done on anadromous salmon and the land locked salmon in this study had a different development in gene expression. This could be explained that the different life strategies play an important role how the genes are expressed. / Under smoltifieringen utvecklar atlantlaxen (Salmo salar) olika anpassningar för att överleva i havsmiljö. En av de vikigaste anpassningarna är att utsöndra överskott av salt via gällarna. Exkretionen är kontrollerad av ett enzym som heter Na+/K+-ATPas som produceras i en α-subenhet av två isoformer av gener som heter α1a och α1b. Enzym aktiviteten ökar under smoltifieringen och är en stark indikator på att processen sker. Målet med denna studie var att undersöka en sjövandrande stam av atlantlax och se hur enzymaktiviteten utvecklas i jämförelse med gen expressionen av mRNA som kodar för produktionen av enzymet. Studien genomfördes vid fiskodlingen i Brattfors, Värmland där prover togs vid fyra tillfällen. Enzymaktiviteten jämfördes mellan två grupper av lax där en grupp fick full matranson 100 % och en grupp fick 15 % matranson. Senare jämfördes enzymaktiviteten för 100 % gruppen med gen expressionen inom samma grupp. Hypotesen var att tillgängligheten på mat skulle påverka smoltifieringen och att 15 % gruppen skulle ha en snabbare ökning i jämförelse med 100 % gruppen. Det skulle också vara en viss korrelation mellan enzymaktivitet och gen expression. Na+/K+-ATPas enzym aktiviteten visade inga större skillnader mellan grupperna förutom vid sista provtagningen. Båda grupperna hade en stor ökning från den andra till den tredje provtagningen med den högsta aktiviteten på 3.16 µmol ADP/mg/h. Detta följdes av ett fall i aktivitet vid sista provtagningen. Gen expressionen visade en snabb ökning av α1b genen över studien med en nedgång vid sista provtagningen och α1a hade en konstant men mindre ökning från första till sista provtagningen. Jämförelsen mellan enzymaktivitet och gen expression visade på en svag korrelation. Det fanns en skillnad i gen expression mellan studier gjorda på anadrom lax och sjövandrande lax i denna studie. Detta kan förklaras av att de olika livsstrategierna spelar en betydande roll i hur generna uttrycks.

The Atlantic salmon (Salmo salar) lifecycle is punctuated by distinct ontogenic stages which are routinely manipulated commercially by photoperiod regimes to enable year-round production. As such, light plays a critical role throughout the production cycle, however, it remains poorly characterised and light spectrum and intensity have not been defined optimally yet. This thesis was therefore set out to test the effects of narrow bandwidth light (Blue-&lambda;(max) 444 nm, Green-&lambda;(max) 523 nm, Red-&lambda;(max) 632 nm and White) and intensity in freshwater (FW). Fry-parr development, out-of-season smoltification and ocular and vertebral health were examined as was the long-term effects of FW light regimes on seawater (SW) growth and muscle structure. In addition, the impact of photoperiod regimes on out-of-season smolts following transfer to SW was investigated. &NewLine; Major findings from the trials conducted show that light spectrum and intensity influence parr development with lower intensities performing better than higher intensities. Both the initiation and duration of smoltification was impacted by spectrum. Importantly, this doctoral work showed that daily changes in light intensity, from low during the scotophase to high during the photophase applied for the duration of a standard out-of-season smoltification regime was capable of providing a sufficient cue for the induction of smoltification. Historic FW light exposure impacted SW performance and post-transfer SW photoperiod had significant impact upon growth and maturation development. Results based on changes to the gonadosomatic index provide important guidance for suitable post-transfer photoperiods for smolt transferred to SW around the winter solstice. Importantly, from the parameters tested, exposure to different spectrum or light intensities did not adversely affect vertebral or ocular health. &NewLine; This thesis did not only focus on the physiological effects of light but also aimed to characterise better the pathways involved in light perception and integration. To do so, the neural response to both broad spectrum white light, darkness and Red and Blue light was investigated through deep brain insitu-hybridisation and high throughput sequencing (NGS) of the pituitary gland. Results showed substantial spectral and light/dark changes in the both the deep brain and pituitary transcriptome. Overall, this research provides both scientifically interesting and commercially relevant guidance for the optimisation of lighting systems for use in captive salmon aquaculture. Major findings from the trials conducted show that light spectrum and intensity influence parr development with lower intensities performing better than higher intensities. Both the initiation and duration of smoltification was impacted by spectrum. Importantly, this doctoral work showed that daily changes in light intensity, from low during the scotophase to high during the photophase applied for the duration of a standard out-of-season smoltification regime was capable of providing a sufficient cue for the induction of smoltification. Historic FW light exposure impacted SW performance and post-transfer SW photoperiod had significant impact upon growth and maturation development. Results based on changes to the gonadosomatic index provide important guidance for suitable post-transfer photoperiods for smolt transferred to SW around the winter solstice. Importantly, from the parameters tested, exposure to different spectrum or light intensities did not adversely affect vertebral or ocular health. This thesis did not only focus on the physiological effects of light but also aimed to characterise better the pathways involved in light perception and integration. To do so, the neural response to both broad spectrum white light, darkness and Red and Blue light was investigated through deep brain insitu-hybridisation and high throughput sequencing (NGS) of the pituitary gland. Results showed substantial spectral and light/ dark changes in the both the deep brain and pituitary transcriptome. Overall, this research provides both scientifically interesting and commercially relevant guidance for the optimisation of lighting systems for use in captive salmon aquaculture.

Accurate smoltification and disease management in Atlantic salmon (Salmo salar) are key issues for the aquaculture industry. Due to their anadromous lifecycle the transfer of salmon from fresh water (FW) to seawater (SW) is crucial to their survival; too early can cause mortality, too late can cause desmoltification and long-term health problems. Both scenarios can increase susceptibility to four viral diseases: Salmon alphavirus (SAV), Infectious salmon anemia virus (ISAV), orthoreovirus (PRV), and Piscine myocarditis virus (PMCV). They all show similar clinical and histopathological symptoms and can easily spread throughout farms. Understanding the initial innate immune response to these viruses may provide biomarkers that could help identify and monitor infections. An in house and onsite Na+/K+ ATPase (NKA) qRT-PCR assay was developed for the salmon biomarker ATPase to test smoltification readiness in salmon smolts. Tested against NKA enzymatic assays it showed a similar success rate over 3 years: NKA qRT-PCR (57%), NKA activity assay (60%). Onsite tests confirmed that the ATPase mRNA transcript is a useful biomarker for smoltification detection. An in-lab and mobile multiplex qRT-PCR assay was developed for detection of SAV, PRV and PMCV. The analytical sensitivity of the SAV (86.5% SE 0.11), PRV (90.94%, SE 0.09) and PMCV (100.46%, SE 0.19) assays was 102 copies for PMCV and 103 for SAV and PRV. Initial results suggest individual assays could be run on site at farms. Addition of an internal control, probit analysis and viral positive tests are still required for multiplex assay integration. Salmon erythrocytes were infected with ISAV, SAV and Poly I:C to investigate whether they induce and up-regulate innate immune response genes. All genes were expressed at low levels in all parameters investigated including non-infected control erythrocytes. These findings suggest erythrocytes act as an initial buffer to viral infections and may help stimulate the innate immune response.

Pre-harvest sexual maturation in farmed Atlantic salmon, Salmo salar, remains a key biological bottleneck compromising biomass and financial output, production predictability, environmental respect, stock welfare and the overall sustainability of the on-growing industry. The management practices currently in place are not optimized and events of high maturation rate are still sporadically observed. From an ecological perspective, the escape of reproductively competent, domesticated Atlantic salmon constitutes a threat to the integrity of wild stocks. The forecasted expansion of the Scottish salmon industry compels the need for a comprehensive and more reliable control of sexual maturation. The general aim of this research project was to optimize the current management strategy (windows of light-manipulation and quality grading) and test alternative practices (lighting-technologies, selective harvest and triploidization) in the control of pre-harvest sexual maturation within the Atlantic salmon on-growing industry. In that end, a number of trials were performed using stock reared in sea-cages on a full commercial-scale or in tanks on an experimental scale. The results of this project are organized around three experimental chapters dealing consecutively with body-size dimorphism, grading and harvest quality; light manipulations and triploidy. In each chapter, two original manuscripts either published or in review are included. In addition to these experimental results, a literature review chapter composed of two review papers on the photoperiodic synchronization and developmental regulation of maturation in salmonids and on morphological skin colour changes in teleosts (published) are presented. In the first experimental chapters, we aimed at investigating the possibility of detecting and selectively harvesting a high proportion of sexually recruited fish before flesh quality deterioration. Results clearly showed that body-size dimorphisms between maturity cohorts at the end of the anabolic window of reproduction (June/July) are strong and standard predictors of maturation among related populations with the same freshwater history. Dimorphism can therefore be modelled to easily and accurately estimate maturation rate in a number of discrete rearing-units. If required, a high proportion of sexually recruited fish can be selectively harvested as superior quality product while leaving the immature fish for further on-growing. This provides an alternative to visual grilse grading that is not feasible in large-scale aquaculture systems, prevents downgrading and increases production predictability as compared to emergency harvests. Furthermore, our results showed immature males grow faster than immature females which should be further investigated to fully determine gender specific performances and nutritional requirements. Weight-grading performed earlier in the cycle affects the sex-ratio within individual pens and in turn apparent performance. This work also revealed that Atlantic salmon can exhibit significant variations in skin colouration resembling the onset of nuptial display but that are not related to sexual recruitment and do not correlate with reduced flesh quality. This originates from a lack of purine (silver) pigments which was also identified, to a larger extent, as characteristic of the nuptial display. This suggests a degree of desmoltification in these histologically immature fish. The instrumental colouration of the altered phenotype was shown to be improved towards a more silver-like appearance by direct ice-contact. This knowledge could facilitate post-harvest quality grading towards the most appropriate market channel and increase product acceptance and attractiveness. The second experimental chapter investigated the possibility of improving photoperiodic manipulation used to suppress early maturation, currently applied for 6-months during the second winter at sea using wide-spectrum, high-intensity lighting systems. Our results showed that the window of continuous artificial-light (LL) exposure can be reduced to 4-months following its onset in early January without compromising its efficiency in suppressing pre-harvest maturation. In addition, alternative lighting technologies were also highly potent at suppressing sexual maturation. The mean-irradiance (intensity) generated within a commercial sea-cage was inversely proportional to the suppression of nocturnal plasma melatonin (light perception hormone) and negatively correlates with the maturation rate within the commercial sea-pen. Threshold levels of light-intensity required to achieve optimal (total) suppression of sexual maturation are suggested. Alternative, narrow band-width lighting-technologies (cold cathode and light-emitting diodes) present an array of technical, practical, economic and welfare benefits comparing to the system currently in use. Clear improvements of the photoperiod-manipulation strategy were demonstrated and these would reduce economic and environmental costs but also potential impacts on animal welfare. The third experimental chapter showed the strong potential of sterile-triploid Atlantic salmon stocks both in freshwater and seawater. Triploid out-of-season smolts were produced for the first time using a classical accelerated "square-wave" photoperiod. Triploidization affected the smoltification pattern but had no detrimental effects on freshwater and early seawater performances under both a S0+ and S1 regime. This illustrates the need to adapt the timing of seawater transfer for successfully producing triploid Atlantic salmon post-smolts. Following one year of seawater rearing, the prevalence of external deformities was higher in triploids but remained within acceptable levels. Importantly, the incidence of vertebral deformities and ocular cataract was higher in triploids possibly due to their specific requirements. It is suggested that tailoring the diet to the nutritional requirements of triploids holds strong potential for remediation. This must be addressed if the use of sterile-triploid stock is to become a commercial reality. The present research project provides means to optimize the maturation management strategy within the Atlantic salmon on-growing industry through light-manipulation, maturation detection and selective harvest, and quality grading. Proposed improvements have the potential to increase biomass and financial output, production predictability, environmental respect and animal welfare and will allow standardization of the overall control of pre-harvest sexual maturation. Their implementation provides a comprehensive strategy likely to favour a sustainable expansion of the Atlantic salmon industry. From a longer term perspective, the rearing of sterile-triploid stocks is promising and should be actively investigated to isolate domesticated strains from their wild conspecifics. This would also eliminate the need for on-growers to deploy a maturation management strategy that that might still affect stock welfare and remains, despite the strong improvements demonstrated, not 100% reliable, costly, technical and protracted.