Abstract
A selected rainbow trout strain was used as a model to identify physiological parameters associated with improved performance when fed all plant-protein feed. Results from a digestibility trial showed that selection had no measurable effect on apparent digestibility coefficients of nutrients in the all plant-protein or a conventional fishmeal-based diet. While this result validated apparent digestibility as an evaluation tool for ingredient and diet quality, it demonstrated that other physiological mechanisms are responsible for the improved performance of the selected strain when fed an all plant-protein, high-soy diet. A subsequent experiment was conducted to evaluate the effects that alternate protein ingredients and plant protein mixtures with or without amino acid supplementation have on trout digestive physiology. Results from temporal plasma amino acids measurements at the absorption site (hepatic portal vein) and from the systemic blood (caudal vein) demonstrated that plasma amino acid level in the hepatic portal vein, measured at intervals following a single meal, are a valuable tool to evaluate the effects of candidate alternate proteins on fish digestive physiology. The findings showed that each ingredient affects digestive physiology of the fish in a singular manner when ingredients are fed individually. However, they do not have any predictable additive effect when fed together as a mixture. Moreover, the addition of crystalline amino acids to an all-plant protein mixture altered the plasma concentrations of all the amino acids as it did for uptake reflected in the hepatic portal vein. A major finding in the study was that the selected trout strain fed the plant protein mixture with amino acid supplemention showed a noteworthy difference compared to an unselected strain, specifically, a synchronous and homogenous decreasing pattern for all the essential amino acids over time in the hepatic portal vein. This indicates that homogeneous dietary amino acid uptake in the hepatic portal vein and rapid postprandial plasma amino acid disappearance are results of nutritional adaptation driven by selection for growth on and tolerance of all-plant protein diet. Results from gene expression of amino acid transporters, cholecystokinin and genes related with protein and amino acid metabolism supported the findings from the plasma amino acids. In conclusion, the results of the research described in this dissertation demonstrated that improved performance of the selected trout strain is associated with synchronous protein digestion of the plant protein mixture and synchronization of amino acid absorption leading to improved amino acid availability and utilization.