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Rational feeding practices

the Fish Site Editor
30 December 2009, at 12:00am

Dominique P. Bureau, David J. Bevan, and C. Young Cho, from the Fish Nutrition Research Laboratory at the University og Guelph, look at how rationally predicting feed requirements could save money and prevent poor growth.

Feed cost is a major production cost in fish culture yet feeding fish is still mostly guesswork, each fish producer following different guidelines (feed charts) or adopting different practices. Feeding too much leads to feed wastage, a pure economic loss, and greater waste output. Feeding too little results in less growth and this also represents an economic loss. Feeding strategies minimiSing feed and total production costs as well as waste output per tonne of fish produced are key to the economic and environmental sustainability of fish production in Ontario.

Many fish producers rely, in part, on feed charts provided by feed manufacturers or found in the literature. However, these feed charts are based on a lot of ‘guessing’ and little or no real production data. The wide range of fish species, genetic stocks, feed composition, water temperature and growth rates encountered on Ontario fish farms makes it impossible for anybody to develop a single feed chart that would be correct for all individual farm situations. More rational site- or situation-specific approaches to determining feeding level should be used.

A spin-off of many years of research funded by the Ontario Ministry of Natural Resources (OMNR) at the Fish Nutrition Research Laboratory (University of Guelph) has been the development of an approach to rationally predict feed requirement of salmonids in a situation-specific manner. This approach is based on the principle that feed requirement is generally governed by how much protein, fat and minerals the animal deposit in its body and the biological cost of depositing these body components. Fish retain in their body a large proportion of the nutrients fed to them. Their feed and energy requirements are, therefore, very closely related to their rate of body components accretion.

Five steps to rational approach of calculating feed requirements

1. Diet selection

The amount of feed required by a fish depends firstly on the composition of the feed used. In general, a greater amount will be required of a lower nutrient density feed than a higher nutrient density feed to achieve the same performance level, if two feeds have similar protein and energy balance. Composition of the feed can also affect the composition of the fish produced which, in turn, affects the amount of feed required.

2. Growth prediction

The accurate prediction of the growth of the animal over the period for which the feed requirement is calculated is probably the most critical factor for the accurate prediction of feed requirements. Growth involves the deposition of nutrients (accretion of body components) which is the main factor determining feed requirement. Fish growing at different rates will, therefore, have different feed requirements. Production records are very valuable starting points when trying to predict the growth of the fish for which one wants to calculate ration allowance. A growth model, the thermal-unit growth coefficient TGC), has been developed to help predict growth of fish based on previous production records and current water temperature profile. It is important to note that the instantaneous growth rate model, better known as specific growth rate (SGR), is not up to the task and its use should be avoided.

Live weight gain is the result of deposition of water, protein, fat and minerals. The amount of these components deposited per unit of live weight gain is not constant but rather changes with fish species and size, feed used, etc. For this reason, knowledge of the composition of the fish reared is another key factor for the accurate determination of feed requirement. Research aiming at the development of models to predict rainbow trout composition at various sizes depending on the composition of the feed fed is underway. It might be faster and more reliable, however, for fish producers to invest in the determination of the chemical composition and energy content of their own fish. A number of private laboratories can perform, at very reasonable cost, the basic chemical analyses required (moisture, protein, fat, energy, phosphorus) on fish samples.

3. Waste estimation

The maintenance of life processes (integrity of the tissues of the animal, osmoregulation, respiration, circulation, swimming, etc.) and the deposition body components have costs in terms of nutrient and feed energy. Basic and practical research projects have allowed the development of simple, yet reliable, models (equations) to calculate these costs or wastes. Studies involving the rearing of fish under the variety of conditions (water temperature, feeding level, fish size, etc.) have shown that these biological costs are, surprisingly, fairly constant and, consequently, fairly easily predicted.

4. Ration allowance

Calculation of the ration can be done by simply to adding all the different components calculated above (body components deposited + waste produced) per unit of time (day, week) to calculate total cost. This cost is generally expressed as "digestible energy". Knowledge of the digestible energy content of the feed used allows calculation of the amount of feed to be served or the feeding level to be used. This calculated amount of feed generally represents the minimum amount of feed required to achieve the predicted growth of the fish.

5. Feeding strategies

Any model, as good as it is, cannot replace common sense when feeding fish and it is up to the producer to determine how much feed to serve and how to serve it depending on the prevailing conditions. Feed should be served in manner that allows adequate opportunity (time or space-wise) for the fish to consume the determined ration and achieve their growth potential while minimising feed wastage.

A detailed description of the approach can be found in the website of the Fish Nutrition Research Laboratory (http://www.uoguelph.ca/fishnutrition) and a number of scientific publications (available on request). A trial version of a programme called Fish-PrFEQ, based on this approach, can be downloaded from the same website. An "user-friendly", stand-alone, version of Fish-PrFEQ is currently under development by the OMNR.

This approach to rationally feed requirement has been used with much success by OMNR fish culture stations and for numerous studies at the Fish Nutrition Research Laboratory and the Alma Aquaculture Research Station. Used properly, it could become a valuable management tool for commercial fish culture operations, notably by providing yardsticks to compare current performance (example: feed conversion ratio, FCR) with what is estimated to be biologically achievable. The OMAFRA Aquaculture Research Programme is currently funding research looking at improving the models used in the calculation of feed requirement to make them more applicable to the various conditions found on commercial fish culture operations of Ontario. The fact that the approach presented above is in continuing development does not prevent fish producers from examining its potential for their particular operation. The development of site or situation-specific feed charts or standards may require some efforts by potential users, namely fish producers and feed manufacturers, but at current fish feed cost this may represent time and money well-spent. The Fish Nutrition Research Lab (Ph: 519-824-4120 ext. 6688) is always happy to provide assistance to those wanting to have a closer look at their feeding practices.

December 2009