The development of aquaculture in semi-intensive and intensive systems has imposed the development of increasingly efficient feed patterns, based on the nutritional requirements of each species under cultivation. For each animal to grow properly in the shortest time and at the lowest cost possible, the feed provided must include optimum nutrition. Therefore, in the fish feed industry, as in that for other animals, the ingredients and storage of processed feed must be subject to strict quality controls.
Feed quality is considered to be one of the major factors influencing the success of a harvest in both semi-intensive and intensive aquaculture: feed and feeding costs generally account for 50 to 70% of operational costs in this type of system. The quality of the feed will depend firstly on the quality of the ingredients used in preparing a balanced diet and secondly on the processing they undergo before and during production. In addition, nutritional qualities will be considerably affected by the storage conditions of both ingredients and finished feed.
Balanced feeds are prepared according to the nutritional needs of each species, but even when the diet is formulated for them it does not always contain the calculated level of nutrients once prepared. This is because the manufacturing process can alter the nutritional value. For example, heat can damage some nutrients and/or make them more available by eliminating certain heat labile toxic substances. At the same time, milling can affect the digestibility of proteins and carbohydrates (Harris, 1980). Feed quality also changes after a certain time in storage, and other alterations may occur, such as in colour, texture, taste and smell. In these conditions there is also a serious risk of contamination by rodent faeces, insects and micro-organisms that quickly lead to deterioration (Limborg, 1979; Chow et al., 1980; Smith and Moss, 1985).
When all these factors are considered they show the importance of quality control for feed ingredients and the finished product. This ensures that the diet contains the minimum required nutritional levels as it gives the exact composition of any material and the level of toxic substances normally present. This control begins with an analysis of the ingredients that will be used to make the feed and ends with certification of the nutrient levels of prepared feed and its proper storage (Frazer, 1967; Harris, 1980). Obviously it is only possible to formulate a complete diet if at least the proximate composition of ingredients isknown precise y. Tacon (1979) recommends that the nutritional value of any material used in the formulation of feed should be evaluated on the basis of:
Protein and non-protein nitrogen content.
Composition of oils and fatty acids.
Crude fibre and soluble carbohydrate content.
Mineral and vitamin content.
Presence of microbes.
Presence of toxic organic compounds.
Variability of chemical composition.
Ideally, all ingredients should be submitted to the above controls. If this is not possible due to budget restrictions or lack of equipment, then an effort should be made to perform at least the analyses that will guarantee minimum quality and and assurance that animals will not show adverse reactions to feed.
Although quality control is a subject that deserves to be discussed exhaustively, this manual is focused primarily on the basic methods of chemical analysis that are necessary for a proper evaluation of ingredients and manufactured diets. The aim is to make these techniques available to more and more aquaculturists and anyone with an interest in these evaluations who have a basic knowledge of chemistry. The methods suggested have been taken from several different sources, and most of them are considered standard.
For practical purposes the analyses have been grouped into three sections. The first includes proximate analysis methods for determining humidity content, crude protein, crude lipids, crude fibre, ash and nitrogen-free extract This analysis shows the percentages of each nutrient, but indicates nothing about their quality; further tests are required for this. A selection of these is included in the second section of this manual as specific analyses.
The third part includes the simplest methods for detecting the most common toxic and anti-nutrient factors whose presence can produce a negative effect on feed effectiveness and even cause mortality. We consider this section to be particularly important when feed include agro-industrial by-products such as soyabean meal, cottonseed meal, or other sources of protein, especially those of plant origin.
The following recommendations should be observed for any of the analysis techniques:
Make sure that the necessary reagents and equipment for the experiment are at hand because once an analysis has been started it cannot normally be suspended while missing elements are found.
Make each analysis in TRIPLICATE; mean values give more accurate results than a single reading. If results vary significantly, the entire analysis should be repeated.
Weigh samples as accurately as possible. Three or four decimal points will give greater precision in calculations. Always use precision scales.
After transferring the sample to the analyzing vessel, weigh the container in order to adjust the final weight. Weight accuracy is very important for good results, since most of the techniques are gravimetric.
Do not change sample weight, timing of the different stages or reagents unless prior tests have guaranteed accurate results. Remember that analyses are based on general standards accepted or demanded by regulatory authorities.
Always use proper safety equipment and precautions such as fume extraction hoods, face masks and lab coats.
You will be handling toxic or dangerous substances, so be extremely cautious in every step.