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Freshwater sport fisheries contribute substantially to the economies of England and Wales. However, many trout fisheries rely partly or entirely on stocking farmed trout to maintain catches within freshwater fisheries. Farmed trout often differ genetically from their wild counterparts and wild trout could be at risk of reduced fitness due to interbreeding or competition with farmed fish. Therefore, to protect remaining wild brown trout (Salmo trutta L) populations and as a conservation measure, stocking policy has changed. Legislation introduced by the Environment Agency (EA, 2009) will now only give consent to stocking of rivers and some stillwaters with sterile, all-female triploid brown trout. There are reliable triploidy induction protocols for some other commercially important salmonid species however; there is limited knowledge on triploid induction in brown trout. Previously, triploid brown trout have been produced by heat shocks although reduced survivals were obtained suggesting that an optimised heat shock had not been identified, or that heat shock gives less consistent success than hydrostatic pressure shock (HP), which is now recognised as a more reliable technique to produce triploid fish. Thus the overall aim of this thesis was to conduct novel research to support the aquaculture and freshwater fisheries sector within the United Kingdom by optimising the production and furthering the knowledge of triploid brown trout. Firstly, this PhD project investigated an optimised triploidy induction protocol using hydrostatic pressure (Chapter 2). In order to produce an optimised hydrostatic pressure induction protocol three experiments were conducted to (1) determine the optimal timing of HP shock application post-fertilisation, (2) define optimal pressure intensity and duration of the HP shock and (3) study the effect of temperature (6-12 °C) on triploid yields. Results indicated high survival to yolk sac absorption stage (69.2 - 93.6 %) and high triploid yields (82.5 - 100 %) from the range of treatments applied. Furthermore, no significant differences in triploid rates were shown when shock timings and durations were adjusted according to the temperature used. In all treatments deformity prevalence remained low during incubation (<1.8 %) up to yolk sac absorption (~550 degree days post hatch). Overall, this study indicated that the optimised pressure shock for the induction of triploidy in brown trout delivering high survival and 100 % triploid rate (a prerequisite to brown trout restocking) is a shock with a magnitude of 689 Bar applied at 300 Centigrade Temperature Minutes (CTM) for 50 CTM duration. Regarding the assessment of triploid status, the second experimental chapter tested the accuracy and efficacy of three ploidy verification techniques (Chapter 3). Techniques studied were erythrocyte nuclei measurements (Image analysis), flow cytometry (Becton Dickinson Facscalibur flow cytometer) and DNA profiling (22 polymorphic microsatellite loci) to assess the effectiveness of triploidy induction in brown trout. Results indicated the validity of using erythrocyte indices major nuclear axis measurements, flow cytometric DNA distributions expressed as relative fluorescence (FL2-Area), and polymorphic microsatellite loci (Ssa410UOS, SSa197, Str2 and SsaD48) for assessing ploidy status in brown trout. Accuracy of each technique was assessed and indicated that all techniques correctly identified ploidy level indicating 100 % triploid rate for that commercial batch of brown trout. These techniques may be utilised within aquaculture and freshwater fisheries to ensure compliance with the legislation introduced by the EA. As a result of the legislation introduced by the Environment Agency triploid brown trout will freely interact with diploid trout therefore there is a need to assess feeding response and behavioural differences between diploid and triploid trout prior to release. Therefore, in the third experimental chapter (Chapter 4) diploid and triploid brown trout were acclimated for six weeks on two feeding regimes (floating/sinking pellet). Thereafter, aggression and surface feeding response was compared between pairs of all diploid, diploid and triploid and all triploid brown trout in a semi natural stream (flume). In each pairwise matching, fish of similar size were placed in allopatry and rank determined by the total number of aggressive interactions initiated. Dominant individuals initiated more aggression than subordinates, spent more time defending a territory and positioned themselves closer to the food source (Gammarus pulex) whereas subordinates occupied the peripheries. When ploidy was considered, diploid trout were more aggressive than triploid, and dominated their siblings when placed in pairwise matchings. However, surface feeding did not differ statistically between ploidy irrespective of feeding regime. Triploids adopted a sneak feeding strategy while diploids expended more time defending a territory. In addition, an assessment of whether triploids exhibited a similar social dominance to diploids when placed in allopatry was conducted. Although aggression was lower in triploid pairs than in the diploid/triploid pairs, a dominance hierarchy was observed between individuals of the same ploidy. Dominant triploid fish were more aggressive and consumed more feed items than subordinate individuals. Subordinate fish displayed a darker colour index than dominant fish suggesting increased stress levels. However, dominant triploid fish seemed more tolerant of subordinate individuals and did not display the same degree of invasive aggression as observed in the diploid/diploid or diploid/triploid matchings. These novel findings suggest that sterile triploid brown trout feed similarly but are less aggressive than diploid trout and therefore may provide freshwater fishery managers an alternative to stocking diploid brown trout. In addition to research at the applied level in triploid brown trout, this thesis also examined the fundamental physiological effects of ploidy in response to temperature regime. Triploid salmonids have been shown to differ in their tolerance to environmental temperature. Therefore the fourth experimental chapter (Chapter 5) investigated whether temperature tolerance affected feed intake and exercise recovery. Diploid and triploid brown trout were exposed to an incremental temperature challenge (10 and 19 °C) and subsequent survival and feed intake rates were monitored. Triploids took longer to acclimate to the increase in temperature however feed intake were significantly greater in triploids at high temperature. In a follow on study, we investigated post-exercise recovery processes under each temperature regime (10 and 19 °C). Exhaustion was induced by 10 minutes of forced swimming, with subsequent haematological responses measured to determine the magnitude of recovery from exercise. Plasma parameters (alkaline phosphatase, aspartate aminotransferase, calcium, cholesterol, triglycerides, phosphorous, total protein, lactate, glucose, pH, magnesium, osmolality, potassium, sodium, chloride, lactate dehydrogenase) were measured for each ploidy. Basal samples were taken prior to exercise and then at: 1; 4, and 24 hours post-exercise. Contrary to previous studies, there was no triploid mortality during or after the exercise at either temperature. Although diploid and triploid brown trout responded metabolically to the exercise, the magnitude of the response was affected by ploidy and temperature. In particular, triploids had higher levels of plasma lactate, osmolality, and lower pH than diploids at 1 hour post exhaustive exercise. By 4 hours post-exercise plasma parameters analysed had returned to near basal levels. It was evident that the magnitude of the physiological disturbance post-exercise was greater in triploids than diploids at 19 °C. This may have implications where catch and release is practiced on freshwater fisheries. Overall, this work aimed to develop and/or refine current industry induction and assessment protocols while better understand the behaviour and physiology of diploid and triploid brown trout. The knowledge gained from this work provides aquaculture and freshwater fisheries with an optimised protocol, which delivers 100 % triploid rates and profitability without compromising farmed trout welfare, thus ultimately leading towards a more sustainable brown trout industry within the United Kingdom.