Stable isotopes in animal ecology: the effect of ration size on the trophic shift of C and N isotopes between feed and carcass.

The feeding ecology of living or extinct animal species is now frequently studied by analysis of stable isotope ratios in small quantities of carcass or remains such as bones or teeth. Although there are numerous papers on these applications in natural systems, the theoretical and experimental basis of this method is weak. In order to evaluate the effect of different feeding levels on the carbon and nitrogen trophic shift, an experiment was carried out in which fish (Nile tilapia, Oreochromis niloticus, initial weight 40.8 g) were fed for 4 weeks at three levels from slightly above maintenance to almost satiation. For each treatment, three fish were reared individually. The isotopic ratios of carbon and nitrogen in feeds and fish carcasses were determined and in the case of carbon this was done separately for lipids and lipid-free matter. The trophic shift was calculated at each feeding level from the delta13C and delta15N ratios of feed and fish. There was a significant trend towards higher values for the trophic shift at higher feeding rates in all fractions analysed. Although further research is required, it can be concluded that the effect of feeding level cannot be ignored when the diet of an animal has to be calculated from the ratios of isotopes in its body.