Effect of replacing fish oil and fish meal in aquaculture diets - on growth, feed utilisation and product quality.
By Gro-Ingunn Hemre, Institute of Nutrition, Directorate of Fisheries, PO Box 185, Sentrum, 5804 Bergen, Norway.

Abstract

For optimal growth fish require not only a minimum level of protein, but also that the essential amino acids are balanced to meet the requirement of each single one. This can easily be done by using fishmeal as the main protein source; however, the amount of fish meal on a world wide basis is not high enough to cover the need for the constantly growing aquaculture industry. Protein requirement varies between species, but is generally high (>30%) even for the most omnivorous species. For catfish (omnivorous) around 32% protein is required (Wilson R.P. 2001, pers.com.), and for this species all the protein (98%) can come from a mixture of plant protein - in commercial diets mainly soy-bean. Using this almost "all-veggie" feed still results in satisfactory growth and feed utilisation. In a study with Nile tilapia (Tilapia nilotica L.) (also omnivorous) indications of even better digestibility and feed utilisation were found when anchovy meal was fully replaced by soy-bean concentrate, again showing no need for fish meal in diets for this species. For the more strictly carnivorous species, such as Atlantic salmon, there is still a challenge to optimise dietary proteins focussing alternatives to fishmeal, and only special qualities of plant proteins, or low levels of these, can be used without any negative effects on production results. In intensive aquaculture in Norway (and EU) most species are strictly carnivorous, showing protein requirements from 38% and higher (dm), and giving the best production results when using high quality fish meals such as LT-meal with minor additions of other protein sources. In addition to well-balanced dietary amino acids, the protein digestibility and absorption rate of each amino acid are of utmost importance. To increase protein retention from the diet, the quality and mixture of different proteins and the inclusion of partly pre-digested proteins have shown good results.

Even more critical for the fast-growing aquaculture industry is the need for alternatives to fish oils. Several studies have shown that lipid digestibility, feed utilisation and product quality are highly influenced by the dietary lipid level and by the fatty acid pattern in the diet. Fish health, as well as human (consumers) health, will be influenced by intake of some of these fatty acids. In focus are both mono-unsaturated and poly-unsaturated fatty acids. There is a large EU-project going on where fish oils with high contents of PUFA are partly substituted by plant oils with higher levels of monoenes and where the contents of n-3 fatty acids are shorter (not PUFAs) (i.e. linoleic acid). Some results from this study will be presented and discussed in relation to beta-oxidation vs. retention of different fatty acids.