| In aquatic systems, stable isotopes as food-web energy and nutrient tracers have become standard over the past decade and now constitute the foundation for a rapidly growing sector of ecology. However, the putative trophic enrichment of delta15N and delta13C at rates of 3.4‰ and near 0‰ respectively in juvenile salmonids has not been rigorously examined. I tested the hypotheses that enrichment rates are not static but vary according to the health of the individual, quality of diet and tissue sampled. To quantify the isotopic variation attributable to fasting, tissue sampled, diet quality and diet change, controlled experiments using juvenile rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) was conducted. Fasting caused enrichment of delta15N in liver tissue only. High inter-tissue variation in both delta13C and delta15N signatures were observed in individuals raised on isotopically constant diets. Fish fed temporally variable diets had tissue signatures that closely tracked the change predicted by the growth dilution model. This study both, supports the utility of using stable isotopes to temporally track diet shifts in juvenile salmonids, and illustrates the importance of accounting for health condition and quantifying species and tissue specific enrichment rates when using stable isotopes for trophic and food-web analyses. |
