Aquaculture, the cultivation of aquatic organisms in a controlled environment, offers both economic and nutritional benefits to human society. As there is an increased demand to feed a growing human population, many wild-caught fisheries have struggled due to the overexploitation of resources. Currently, production relies heavily on wild-caught fish to produce fishmeal to feed farm-raised fish. The demand for alternative materials in fish feeds has grown rapidly as fishmeal resources have become limited. Antibiotic resistance emergence in aquaculture systems is another area of concern. Reducing antibiotic use via alternate prophylactic measures to increase host health is an essential area of research; modulation of the host intestinal bacterial community via prebiotics is one possibility. Prebiotics refer to non-digestible food ingredients that are thought to stimulate the growth of beneficial bacteria, consequently benefiting host health by indirectly reducing the possibility of bacterial pathogen proliferation. This occurs through various measures such as competition for space and resources. The intestinal bacterial community has a significant impact on a variety of host factors that include host development, physiology, immunity, and nutrient acquisition. In turn, there are multiple factors impacting the bacterial community, including the presence of pathogens and/or antibiotics, environmental conditions, host genetics, and the diet consumed. To promote environmental sustainability and improve production and animal health in aquaculture, a collaboration was created with Anheuser-Busch of the brewing industry and Maltento, a functional ingredient company. With breweries around the globe, Anheuser-Busch produces consistent, food grade byproducts that are safe for human consumption. Two of the most prevalent brewery byproducts are brewer's spent yeast (BSY) and brewer's spent grain (BSG). BSY contains a variety of beneficial nutrients such as proteins, essential amino acids, and carbohydrates. BSG is high in fiber but low in protein; however, black soldier fly larvae can be cultured on BSG to convert the low-value product into insect biomass to be used in fish feed, as insects themselves are full of beneficial lipids and proteins. The objective of the work presented in this thesis was to evaluate the efficacy of using low-value brewery waste products, converted into high-value feed additives, for aquaculture practices. Specifically, the effects of dietary feed additives on the production, health, and intestinal bacterial community of aquaculture-raised rainbow trout were examined. Inadvertently, benefits of the feed additives on fish subjected to chronic and acute thermal stress were also assessed. Overall, the results of the study found that the feed additives did not significantly change the production efficiency of the rainbow trout, though some increase in growth was observed. When subjected to chronic thermal stress conditions, fish fed the experimental diets outperformed those fed the control diet regarding growth parameters. The intestinal bacterial community of the fish was significantly altered from the beginning of the trial compared to the end of the trial, though differences were not attributed to the feed additives. Instead, the resulting intestinal dysbiosis is believed to have stemmed from the physiological response of the fish to thermal stress conditions. When the fish underwent an acute thermal stress event, causing mortality, fish fed three of the five experimental diets were found to have higher survival rates compared to the control. Ultimately, results of this project suggest that the BSY and BSG-fed insect feed additives may have increased the health and robustness of the fish during a period of thermal stress. However, further research under controlled conditions is needed to evaluate if the observed host health benefits can directly be attributed to the feed additives. / Master of Science / Aquaculture refers to the method of rearing aquatic organism such as fish and shellfish under controlled conditions. Within the food industry, aquaculture is one of the fastest growing sectors, and provides important economic and nutritional benefits to humans. Additionally, aquaculture is an important alternative to fisheries that rely on catching fish from the natural environment. Wild-caught fisheries have struggled due to the overfishing, and unfortunately, many aquaculture practices still rely on wild-caught fisheries to produce fishmeal used in feed for carnivorous fish. Research into alternate protein sources to use in fish feed has been on the rise. Additionally, as the emergence of multi-drug resistant bacteria continues to increase, reducing antibiotic use has become a priority across all fields whether it be healthcare or the food industry. Within aquaculture, using alternative prophylactic measures such as prebiotics to increase animal health and disease resistance could lead to the overall reduction of antibiotic use. Prebiotics are non-digestible food ingredients believed to help the beneficial bacteria within the intestinal track to grow. In turn, the increased numbers of beneficial bacteria reduce the possibility of pathogenic bacteria invading and establishing a presence in the intestinal track. The intestinal microbiome refers to the various organisms, such as bacteria, viruses, and fungi, that live commensally within the host digestive tract. The bacterial community within the intestinal microbiome has many important roles, including effects on host development, physiology, immunity, and nutrient acquisition. Many factors also impact the bacterial community, including the presence of pathogens and/or antibiotics, environmental conditions, host genetics, and the diet consumed. To promote environmental sustainability and improve production and animal health in aquaculture, a collaboration was created with Anheuser-Busch of the brewing industry and Maltento, a functional ingredient company. With breweries around the globe, Anheuser-Busch produces consistent, food grade byproducts that are safe for human consumption. Two of the most prevalent brewery byproducts are brewer's spent yeast (BSY) and brewer's spent grain (BSG). These low-value waste products can consequently be converted into high-value feed additives for use in aquaculture. The objective of the work presented in this thesis was to evaluate the effects of BSY and BSG-fed insect dietary feed additives on the production, health, and intestinal bacterial community of aquaculture-raised rainbow trout. Unintentionally, benefits of the feed additives on fish subjected to chronic and acute high-temperature thermal stress were also explored. Overall, the results of the study found that while the feed additives did not significantly increase the growth of the rainbow trout, benefits were still observed. When subjected to chronically high-water temperatures, fish fed the experimental diets outperformed those fed the control diet regarding growth parameters. The intestinal bacterial community of the fish was significantly altered from the beginning of the trial compared to the end of the trial, though differences are not believed to be caused by the feed additives. Instead, the resulting shift in the bacterial community is believed to have stemmed from the stress-response of the fish triggered by high water temperature. When the fish underwent an acute thermal stress event, which caused mortality, fish fed three of the five experimental diets were found to have higher survival rates compared to the control. Ultimately, results of this project suggest that the feed additives may have increased the health and robustness of the fish while undergoing thermal stress. However, further research under controlled conditions is needed to evaluate if the observed host health benefits can be attributed directly to the feed additives.