The fate of carbon in growing fish: an experimental study of isotopic routing

The application of stable isotope analysis to ecology requires estimating the contribution of different isotopic sources to the isotopic signatures of an animal's tissues using mixing models. These models make the physiologically unrealistic assumption that assimilated nutrients are disassembled into their elemental components and that these atoms are then reassembled into biomolecules. We quantified the extent to which mixing models yield erroneous results with an experiment using Nile tilapia (Oreochromis niloticus). The tilapia were fed synthetic diets that varied in protein content and in which the carbon isotopic composition of protein differed widely from that of carbohydrates and lipids. We predicted that dietary protein would contribute disproportionately to the carbon in muscle, whereas the nonprotein components of diet would contribute disproportionately to lipids. Our experiment also allowed us to test the conjecture of a positive correlation between the (15)N enrichment in tissues and protein intake. As predicted, the contribution of protein carbon to muscle was higher than that expected by the assumptions of isotopic mixing in all treatments except that with the lowest dietary protein content. We hypothesized that the unexpectedly high contribution of nonprotein carbon to muscle was the result of assimilating both dispensable and indispensable amino acids synthesized by the fishes' gut microbiota. Although we expected the contribution of carbon in nonprotein dietary ingredients to be higher than expected from a mixing model, we found that protein contributed more than expected, probably as a result of differences in amino acid composition between diet and tissues, which led to excess carbon used for lipid synthesis. Finally, our results verified the positive relationship between dietary protein content and the enrichment in tissue (15)N. Assuming perfect mixing in field isotopic studies can lead to erroneous inferences about the relative contributions of different sources to an animal's diet.