Effects of Prey and Tissue Type on δ13C and δ15N Fractionation and Turnover Rates, and Assimilation Efficiency of Rainbow Trout

Stable isotope analysis is increasingly used in ecological studies to examine dietary patterns of consumers, yet little attention has been given to some of the underlying assumptions.  Critical assumptions of stable isotope mixing models include knowledge of δ¹³C and δ¹⁵N fractionation (D) and turnover rates in each tissue examined along with assimilation efficiencies.  We conducted laboratory experiments to examine effects of prey (sculpin and chironomid) and tissue type (blood, liver and white muscle) on δ¹³C and δ¹⁵N fractionation and turnover rates in rainbow trout, along with determining assimilation efficiencies.  Liver showed the most rapid turnover times for both δ¹⁵N and δ¹³C (T₉₅ = 4-6 months), followed by blood (T₉₅ = 4-7 months) and then white muscle tissue (T₉₅ = 7-9 months).  Turnover rates were metabolically dominated (82-93% of turnover), with the exception of δ¹³C of blood in rainbow trout fed a chironomid diet (33%).  Fractionation rates differed by tissue and diet.   Based on the hatchery diet, Dδ¹⁵N was 3.8‰ (95% CI 3.3-4.3) for white muscle, 2.9‰ (2.4-3.4) for blood and 2.5‰ (1.9-3.1) for liver, whereas Dδ¹³C was 1.9‰ (1.7-2.1) for liver, 1.7‰ (1.4-2.0) for white muscle, and 1.5‰ (1.3-1.7) for blood.  Assimilation efficiency averaged 55.8% (SE + 0.90) and 64.5% (SE + 1.98) at the 10% and 25% ration level, respectively.  Based on the turnover rates we observed many food web studies using stable isotope analysis are likely to violate the assumption that δ¹⁵N and δ¹³C values of tissues are in equilibrium with a given diet.  Additionally, fractionation rates of Dδ¹⁵N, and to a lesser extent Dδ¹³C, need to be considered in the context of inter-tissue variability.  Knowledge of fractionation rates, tissue turnover rates and assimilation efficiency can be crucial to effectively using stable isotope mixing models to assess dietary source contributions.