Abstract
As the world population continues to show exponential growth, wild fish stocks are becoming stagnant causing a shortfall in capture fisheries and increasing the price of fish meal as a source of protein. Thus, different combinations of raw materials, including plant-derived ingredients, need to be considered to fulfil the increasing demands of the aquaculture industry and maintain cost-effectiveness. Soybean meal became a preferred source of protein in aquafeeds, however, it has typically encountered a variety of challenges at high inclusion levels, especially in piscivorous and carnivorous species like rainbow trout (Oncorhynchus mykiss). Despite the adequate protein content and amino acid profile of SBM, the presence of antinutritional factors such as phytate, saponins, lectins, etc., has historically led to significant repression of growth performance. In addition, the inflammation of the distal intestine, referred to as soybean meal-induced enteritis (SBMIE), is a noteworthy sign of reduced tolerance to SBM, and it is related to poor nutrient absorption in the intestine, ultimately impacting other organs such as the liver. Nowadays, the use of feed additives in fish feed formulations has become a popular approach to enhance feed utilization while providing additional benefits, including affordability of the feed, and boosting immunity. Attenuation of SBMIE will make it possible to increase SBM inclusion levels in practical formulations while maintaining the health status of farmed fish and reduce risk for disease.
The overall goal of this thesis was to gain insight into the evolution of SBMIE in the long-term and attempted to identify biomarkers associated to SBMIE. In addition, this research evaluated the suitability of glutamine, bile acids and vitamin D as dietary supplements in mitigating said pathology in rainbow trout. Three feeding trials were carried out to study each additive over a period of thirty, eighteen, and ten weeks, respectively. This research further aimed to investigate the potential detrimental effects that either a prolonged exposure to dietary bile acids or extra high vitamin D doses (i.e., > 10 mg Kg-1) could have on this species.
Results from this work confirmed the occurrence of SBMIE when feeding SBM at 30% and 40% inclusion levels and that these effects are maintained throughout >18 weeks. In addition, the supplementation of high level SBM diets with either 2.0% glutamine or 1.5% bile acids had the potential to reduce intestinal inflammation. The research detailed herein also demonstrates the capacity of dietary bile acids in reverting hypocholesterolemia derived from 40% level SBM diets. Moreover, no apparent bile-associated cytotoxicity was detected over the long-term, further validating bile acids as an additive for use in aquafeeds. A significant increase in growth performance was observed from the supplementation with 2.0% glutamine in combination with vitamin D. The data obtained here are, however, insufficient to determine the potential benefits derived solely from dietary vitamin D.
Future research will be required to gain in-depth understanding of molecular markers that can shed light on underlying SBMIE mechanisms, which will allow continuing improvement of mitigation approaches as well as developing monitoring tools that can be used in farm operation.