As aquaculture continues to contribute high quality protein to a greater proportion of the worlds growing population, fish producers have been pressured to increase overall production. However, associated with elevated production is greater stress due to crowding, reduced water quality, and other factors. These stressors impact the health and welfare of the farmed animal which has become of increasing concern to a more environmentally aware and health conscious consumer. New strategies must therefore be developed and adopted by the aquaculture industry to counteract negative consumer perceptions of industrial fish production while also stabilizing the industry. Better nutrition may enhance disease resistance of farmed fish, while fillet accumulations of specific health-related nutrients may simultaneously add value to the final product. This thesis summarizes research undertaken in an effort to enhance the nutritional value of fish by increasing fillet levels of selenium (Se). In addition, various biomarkers of fish health (lysozyme, ceruloplasmin and glutathione peroxidase (GSH-Px) activities), were examined to determine whether dietary Se supplementation had a positive impact upon fish immunocompetence. Moreover, the effect of vaccination was also examined using lysozyme and growth as indicators of fish performance. Hybrid striped bass (HSB), the fourth most valuable farmed fish and fifth in tonnage produced in the United States, were employed as a model animal. Se, an essential component of the antioxidant enzyme, glutathione peroxidase with many established health benefits was supplemented to HSB diets at various concentrations but was found to be without effect upon serum immune proteins or GSH-Px activity. This finding likely reflected the use of fishmeal within the dietary formulation, which possessed relatively high Se levels, together with sufficient storage of tissue Se within the experimental animals. Nevertheless, these studies determined that organic sources of Se were more efficiently accumulated in HSB muscle than traditional inorganic sources. A linear response occurred up to the highest dose used (3.2 mg kg⁻¹) over a 6 week study. Fillet Se accumulation (r²=0.95) proved to be a better indicator than the liver (r²=0.87).Se enhanced fish therefor appear to offer a route of entry for fish producers into the lucrative designer food market - especially since many hundreds of millions of people worldwide are believed to be Se-deficient. Studies undertaken with Se-deficient HSB confirmed findings from the aforementioned research and also indicate that Se-enhanced fillets might be produced using a finishing feed containing 1.5 mg Se kg⁻¹ 6-8 weeks prior to harvest. Accumulation of Se using this strategy resulted in a 100g portion of HSB fillets containing between 33-109 µg Se, amounting to a dietary intake of between 25-80 µg Se; a level that would satisfy present daily intake recommendations. Vaccination of HSB with a Streptococcus iniae oil-in-water vaccine was examined for its potential negative impacts upon HSB production performance. Vaccinated fish did not exhibit any significant reductions in growth but microarray studies revealed that together with many hundreds of genes, four immune-related genes were impacted by this procedure. This thesis discusses the results obtained with regard to their practical implications to the industry and welfare of cultured fish. / Master of Science