Nutritional and immunological evaluation of soy-based diets for summer flounder (paralichthys dentatus) aquaculture

Summer flounder aquaculture has progressed significantly in the last twenty years, and diets have been optimized to provide optimal nutrition, ensuring a high quality final product. Recently, the focus of research has evolved toward finding a replacement for fish meal in diets for many species, as the demand for fish meal worldwide has caused the primary protein product to become increasingly economically volatile. Therefore, a series of experiments were designed to systematically evaluate feeds for summer flounder (Paralichthys dentatus) including soy-based products as the primary protein source. The first feed trial involved the comparison of three diets; a fish meal control, a 60% fish meal replacement diet with soybean meal (SBM), and a 60% fish meal replacement diet with soybean meal and soy protein concentrate (1:1 ratio, SBM/SPC), in order to investigate the impact of including soy products in diets for summer flounder on growth and survival following challenge (post-feeding trial) with a bacterial pathogen of summer flounder (Vibrio harveyi). The diet with the majority of the FM replaced with SBM/SPC produced significantly greater growth, and survival following bacterial challenge, suggesting that a combination of soybean meal and soy protein concentrate could provide a good alternative to fish meal-based replacement diets by providing equal growth to fish meal diets and enhanced resistance to bacterial challenge. A second trial was designed to systematically evaluate which combination of soybean meal and soy protein concentrate would result in the best growth and survival to bacterial challenge. A diet comprised of 40% fish meal, 48% soy protein concentrate, and 12% soybean meal resulted in significantly better growth and survival following bacterial challenge compared to a fish meal control diet, confirming the results from the first trial. Additional feed trials were designed to investigate the compounds responsible for both the reduced growth and the increased survival following bacterial challenge, in order to determine which compounds of soybean meal, but absent in soy protein concentrate, may be responsible for the effects on growth and survival to bacterial challenge. In the first feed trial in this series, ,in addition to a fish meal-based control, four additional diets were formulated to contain 60% soy protein concentrate and 40% fish meal as protein sources, with 0.53%, 1.23% or 1.94% soy molasses (w/v) added to the diet. Growth was reduced with soy molasses added to the diet, with the most decreased growth in the group fed the 1.94% soy molasses-supplemented diet, suggesting that antinutritional factors present within soy molasses are responsible for decreased growth observed in summer flounder fed soybean meal-based replacement diets. Therefore, a second feed trial was designed to further investigate the role of compounds present in soybean molasses on growth and survival. Soy molasses was fractionated using n-butanol to phase-separate the different compounds within the soy molasses into either a water (primarily enriched with oligosaccharides), interphase (containing a mix of saponins, oligosaccharides, ash and proteins) or butanol (including saponins) phase and 3 diets were prepared by adding each sub-fraction to a soy protein concentrate replacement (60%) diet at a level corresponding to a 12% soybean meal replacement level (best results from trial 2). Growth was significantly lower for all diets with soy molasses fractions added compared to groups fed either the fish meal or soy protein concentrate control diets. Survival following challenge was highest for the groups fed the diets containing either the water phase or butanol phase of soybean molasses, suggesting that oligosaccharides, alone or in combination with low levels or other antinutritional factors, may be responsible for increased survival to bacterial challenge compared to the fish meal diets. Finally, a feed trial was designed to test the effect of three different levels of oligosaccharide supplementation (0.2%, 0.4% or 0.6% supplementation with stachyose hydrate and raffinose pentahydrate combined at a 3.16:1 ratio (w/w)) to a 60% soy protein concentrate replacement on growth and survival to bacterial challenge. The diet which produced the greatest growth and significantly better survival than the soy protein concentrate-based control diet included an oligosaccharide supplementation level of 0.4% (w/w). These results demonstrate that soy protein concentrate can replace fish meal in summer flounder diets at a high level (60%) with no negative effects on growth. Furthermore, supplementation of the soy protein replacement diet with crystalline oligosaccharides or an oligosaccharide-rich fraction resulting from the phase-separation of soybean meal may decrease susceptibility to disease. These soy-based diets would provide a sustainable, economically viable alternative to fish meal diets for marine finfish aquaculture.