Taurine Supplementation of All-plant Protein Diets for Rainbow Trout (Oncorhynchus mykiss)

—Taurine has been demonstrated to be conditionally indispensable for several carnivorous fish species. Current trends in trout production include decreasing levels of fish-meal content in feeds, along with faster growing strains of fish. Taurine may be a limiting nutrient in support of elevated planes of growth for rainbow trout. A 9-wk feeding trial was conducted using a factorial treatment design with protein source (fish meal or plant) and taurine supplementation (four levels) as the main effects. The fish-meal diet series included 23% herring meal and contained 1.76% total sulfur amino acids (TSAA). The plant diet series did not contain any animal proteins and substituted protein from soy protein concentrate in place of the herring-meal protein and contained 1.5% TSAA. Taurine was supplemented at 0, 5, 10, and 15 g/kg dry diet to each of the diets in the plant series and the fish-meal series of diets. All diets were formulated to contain 43.8% crude protein and 20% lipid with an estimated physiological fuel value of 4.2 kcal/g. Fifteen fish were stocked in each of 24 tanks with a mean initial weight of approximately 26.8 g per fish. The unsupplemented fish-meal diet contained 2 g/kg taurine, and the unsupplemented plant diet had taurine levels below the detection limit of 0.1 g/kg diet. Taurine supplementation improved growth, feed conversion ratios, protein retention efficiencies, and energy retention efficiencies of fish fed the plant protein diets. No effects of taurine supplementation were observed for these response factors in fish fed the fish-meal series diets. This study demonstrates that taurine supplementation may be necessary for rainbow trout fed plant-proteinbased feeds. The removal of fishmeal from the diet of rainbow trout and other carnivorous fish species has had wide interest globally. Current diet formulations often utilize a variety of protein sources to replace all but a small portion of fish meal. Substituting the last portion of fish-meal protein with plant-derived products often results in reduced growth rates. Animal renderings from industry, including meat and bone meals, poultry meals, feather meals, and blood products, have been widely evaluated as substitutes for fish meal, but these products also have limits based on nutrient profiles and nutrient digestibility for fish (Gaylord and Rawles 2005). A number of researchers have attempted to replace all the animal protein in trout feeds with plant proteins. Soybean meal has gained the most interest because of its abundance and relatively low cost per unit protein. Plant protein concentrates also have shown promise in trout feeds, including soy protein concentrate (Adelizi et al. 1998), canola protein concentrate (Thiessen et al. 2004), as well as corn gluten and wheat gluten. Although these concentrates have been shown to be effective in replacing some of the fish-meal component, cost considerations often limit their utilization in feed formulations. Other locally available products have also gained interest; however, each has specific limitations in fish feed formulations (Francis et al. 2001). Cheng et al. (2003) replaced up to 50% of the fish meal in the control diet with a mixture of plant proteins without a reduction in growth of rainbow trout. This equated to approximately 25% of the dietary protein coming from fish meal to support comparable growth to the control diet containing approximately 22% fishmeal protein. Further reduction of the fish-meal protein in the diets utilizing soybean meal and wheat gluten caused a reduction in growth and feed efficiency (Cheng et al. 2003). Additionally, Gomes et al. (1995) demonstrated that palatability may be reduced when feeding all-plant diets to rainbow trout. 1 Corresponding author. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 37, No. 4