Feed Efficiency Versus Feed Conversion Ratio -- Demonstrated on Feeding Experiments with Juvenile Cod (Gadus Morhua)

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Feed conversion ratio (FCR), which is used as a description of growth as a function of feed intake, depends mainly on the feeding rate but also on temperature. The estimation of the FCR is simple; however, the practical application of this ratio is limited to cases in which fish are fed beyond maintenance rate. In addition, FCRs are not comparable if they are measured at different temperatures. Ivlev (1939) proposed the more universal temperature independent metric K3 -- also known as feed efficiency. Even if the feed efficiency is a measure of the usable percentage of the food it has not been in common use in research and practice. This is most likely due to difficulties in determining this parameter in conjunction with temperature dependency of the maintenance rate and its temperature constant in practice. In this work we present an estimation method, which allows the determination of these parameters indirectly by the “system response” of fish. The parameter estimation is performed by least square fitting, comparing the food intake of the model with the food intake observed in feeding experiments with juvenile cod (Gadus morhua), carried out by Soofiani (1983) and Hawkins et al. (1985) over a wide range of feeding rates and temperatures. For individual fish, the model works not very accurately, while a comparison of the computed model results with the de facto consumed amounts (mean feed intake per fish and size group) demonstrates a high level of agreement. The relative estimation error is generally highest at the lowest consumption rates. The developed model is particularly suitable for the optimization of the feed ration at different temperatures in aquaculture systems.