A feed is only as good as its ingredients ? a review of ingredient evaluation strategies for aquaculture feeds

The evaluation of feed ingredients is crucial to nutritional research and feed development for aquaculture species. In evaluating ingredients for use in aquaculture feeds, there are several important knowledge components that should be understood to enable the judicious use of a particular ingredient in feed formulation. This includes information on (1) ingredient digestibilities, (2) ingredient palatability and (3) nutrient utilization and interference. Diet design, feeding strategy, faecal collection method and method of calculation all have important implications on the determination of the digestible value of nutrients from any ingredient. There are several ways in which palatability of ingredients can be assessed, usually based on variable inclusion levels of the ingredient in question in a reference diet and feeding of those diets under an apparent satietal or self-regulating feeding regimes. However, the design of the diets, the parameters of assessment and the feeding regime can all be subject to variation depending on subtleties of the experimental design. Clearly, issues relating to feed intake are the key performance criteria in palatability assessments, and it is important that such experiments maintain sufficient stringency to allow some self-discrimination of the test feeds by the fish. The ability of fish to use nutrients from the test ingredient, or defining factors that interfere with that process, is perhaps the most complex and variable part of the ingredient evaluation process. It is crucial to discriminate effects on feed intake from effects on utilization of nutrients from ingredients (for growth and other metabolic processes). To allow an increased focus on nutrient utilization by the animals, there are several experimental strategies that can be adopted, which are based on variations in diet design and feeding regime used. Other issues such as ingredient functionality, influence on immune status and effects on organoleptic qualities are also important consideration in determining the value of ingredients in aquaculture feed formulations. A key aspect to note is the need to design all experiments with sufficient experimental capacity to detect significant effects.