Aquaculture is an essential developing sector for world food production, however the attainment of sexual maturity during commercial on-growing is a major bottleneck to industry expansion. Sexual maturation brings a commercial loss due to reduced growth performance as well as reduced immune function. Furthermore, serious concerns exist over potential genetic interaction with native stocks through broadcast spawning or spawning interaction by escapees. In the north Atlantic region, the Atlantic cod (Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus) are key aquaculture species in which industry expansion is limited by pre-harvest sexual maturation. However, through a species specific combination of modern technologies and refinement in management practices it is possible that this sexual maturation can be controlled and on-growing potential enhanced. Thus the overall aim of this thesis was to conduct novel research that will improve our understanding of the underlying mechanisms that regulate sexual maturation, whilst also advancing the optimisation of technologies for the management of maturation in cod and halibut. In Atlantic cod, owing to the inconsistent inhibition of maturation in commercial conditions, ever increasing intensities of light and in some cases narrow spectrum technologies are being used to try to combat this problem. Firstly, this PhD project investigated the potential welfare impacts of high intensity artificial lighting which have not been studied to date (Chapter 2). The work specifically investigated the effect of traditional metal halide and novel green cathode lighting on the stress response, innate immunity, retina structure, feeding activity and light perception of Atlantic cod. Results indicated that although acute responses to light were observed, there were no clear significant long term effects of any of the lighting treatments on these parameters. Regarding light perception, interestingly even when subjected to high intensity constant lighting (metal halide mean tank intensity: 16.6 watts m-2), cod still demonstrated a day/night rhythm in melatonin release which suggests perception of the overlying ambient photoperiod. The second trial of this PhD project investigated the efficacy of shading of ambient photoperiod in addition to constant lighting to inhibit maturation of cod outdoors (Chapter 3). This aimed at improving the performance of artificial lighting regimes in the open cage system during commercial on-growing by reducing the relative difference between day/night light intensities. The trial was conducted over a one year period where a low and high shade treatment were tested in outdoor tanks. Shading increased the relative night time illumination to 6.6% and 31.3% of daytime levels respectively, compared to <2% in an unshaded set-up. Both shading treatments were effective at suppressing sexual development in cod as confirmed through measurements of gonadosomatic index, histological analysis of gonadal development, oocyte diameter measurements and sex steroid profiles as well as measurements of growth. In addition to research at the applied level in Atlantic cod, this thesis has also extended to the fundamental level and explored one of the potential mechanisms relaying photoperiod signal to the endogenous regulation of sexual maturation in cod, namely the kisspeptin system (Chapter 4). Partial sequences for the signal peptide Kiss2 and its receptor Kissr4 were isolated and described showing similarity to other teleost species such as the medaka, Oryzias latipes and stickleback, Danio rerio. Novel molecular qPCR assays were designed and developed to measure the expression of both genes in male and female cod over a maturation cycle and compared to cod under constant lighting which remained immature. Interestingly, expression patterns of kiss2 and kissr4 did not reveal any clear association with season or photoperiod treatment. However, pituitary expression of gonadotropins (FSH, follicle stimulating hormone; LH, luteinising hormone) did show a differential expression in relation to treatment from early winter approximately 4-6 months after the photoperiod change. These new results are in contradiction with the hypothesis that the kisspeptin system would be involved in the initiation of gametogenesis, as shown in mammals. However, the FSH/LH data defines a window during which time kisspeptin or another GnRH stimulating mechanism must be active, this compels the need further investigation. In Atlantic halibut farming, all-female production removes the concerns of production losses through sexual maturation. Accordingly, this thesis investigated the potential/feasibility of generating monosex populations by FACS (fluorescence activated cell sorting) semen sexing based on cellular DNA content, as proven in terrestrial agriculture. Results however did not show any clear differences between the DNA of sperm in a range of species tested (Atlantic halibut, cod, sea bass, perch) suggesting that this technique may not be applicable in such species. The project also focussed on the production of a population of sex reversed halibut broodstock (neomales) that will generate, in the long term, a basis for traditional monosex population generation in the UK. Two in feed MDHT (17α-methyldihydrotestosterone) treatments were tested with the aim to reduce the use of hormone. Results were very successful with a hormone treatment of 5ppm MDHT generating a 97% phenotypic male population thus suggesting the presence of sex-reversed halibut which can be used for future monosex production. Overall, this work aimed to develop and/or refine potential remediation techniques for sexual maturation in two key commercially important farmed marine fish species, cod and halibut, as well as further our understanding on the regulation of puberty. The knowledge gained from this work provides a means to optimise the techniques employed in the industry and has the potential to increase production and profitability without compromising farmed animal welfare, thus ultimately promoting the sustainable expansion of the Atlantic cod and halibut aquaculture.