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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Authors: | Focken Ulfert |
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Affiliation: | Department of Aquaculture Systems and Animal Nutrition in the Tropics and Subtropics, University of Hohenheim (480B), 70593 Stuttgart, Germany. Focken@uni-hohenheim.de |
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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. |
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