Evaluation of Single-cell Protein in Trout Diets

Abstract

A yeast, bacteria, alga and two fungi were evaluated in pelleted trout feeds. Pellets were produced by dry and wet pelleting processes. The latter was more convenient and applicable to the production of small quantities of water stable pellets. An inexpensive feeding system, applicable for use in small-scale tank systems, was developed. This was adapted for controlled, satiation and demand feeding techniques. Satiation feeding appeared to be the most applicable to the experimental design. The five products were well tolerated in compound feeds at a level of 20% and 40% fed successively for 12 and 10 weeks, during which time no apparent pathological disturbances developed. Dietary performance was related to the protein content. Both fungi had low digestible protein contents, thus limiting their usefulness as a protein supplement in trout diets. The alga, bacterium and yeast had a higher digestible protein content and each was subsequently tested at four levels in semi-purified diets. The protein efficiency ratio varied in a manner similar to that shown for the rat, but maximum values were obtained at higher protein levels. The alga was palatable and the level of digestible protein was similar to that shown for several plant seed meals. It was efficiently utilized in one experiment and it could be used as a supplementary protein in trout feeds. Poor palatability of the bacterial protein was a consistent feature of the results and this may impose a limit to its usefulness in trout diets, despite the very high level of digestible protein found in this material. The yeast was extremely palatable and highly digestible. The material was efficiently utilized in the semi-purified diets, producing an acceptable growth and feed conversion efficiency and a relatively high protein efficiency ratio. This product could be used as a major component of balanced trout diets.