Effects of prey and tissue type on δ13C and δ15N discrimination and turnover rates of rainbow trout

Stable isotope analysis is used often in ecological studies to examine dietary patterns of consumers and food web structure, yet we still know little regarding some underlying assumptions, including for well-studied species like rainbow trout Oncorhynchus mykiss. We assessed δ¹³C and δ¹⁵N discrimination (∆) and turnover rates in blood, liver and white muscle of rainbow trout fed natural prey items (sculpin and chironomids). Liver had the fastest turnover rates for both δ¹⁵N and δ¹³C (4–6 months), followed by blood (4–7 months) and white muscle (7–9 months). Metabolism accounted for more isotopic turnover (82–93%) than growth, with the exception of δ13C in the blood of rainbow trout fed chironomids (33% of isotopic turnover was metabolic). Discrimination factors differed by tissue and diet, with ∆δ¹⁵N showing highest enrichment in white muscle (3.8‰; 95% CI 3.3–4.3), followed by blood (2.9‰; 95% CI 2.4–3.4) and liver (2.5‰; 95% CI 1.9–3.1) compared to the hatchery diet. The ∆δ¹³C of liver showed higher enrichment (1.9‰; 95% CI 1.7–2.1) than white muscle (1.7‰; 95% CI 1.4–2.0) and blood (1.5‰; 95% CI 1.3–1.7). It is crucial that future models incorporate species-specific tissue turnover and discrimination factors to enable drawing strong inferences and improve accuracy of model predictions.