Feeding fish with diets of different ratios of C3- and C4-plant-derived ingredients: a laboratory analysis with implications for the back-calculation of diet from stable isotope data

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Abstract
Back‐calculation of the diet is one of the most frequent applications of stable isotope techniques in animal ecology. These calculations are often based on two assumptions: a constant trophic shift for all dietary items and a linear response of the isotopic ratios to different mixtures of two isotopically distinct feeds. In a laboratory experiment, fish (Nile tilapia, Oreochromis niloticus) were fed semi‐synthetic diets prepared either from wheat or corn ingredients, or from three blended diets (25, 50, 75% wheat components). Isotopic analysis of the lipid‐free and lipid fraction of the fish after the experiment revealed that the trophic shift was not constant for wheat‐ and corn‐based diets. The isotopic response to the mixed diets was not linear, leading to a statistically significant over‐estimation of the corn component in the back‐calculation. Both effects are in agreement with published data on the isotopic effects of C3‐ and C4‐plant materials in the diet. Copyright © 2004 John Wiley & Sons, Ltd.

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