Isotope Fractionation in Juvenile and Large Rainbow Trout (oncorhynchus Mykiss): Repeatability of Stable Isotope Measures and Their Relationship to Growth Rate

The study aimed to assess the repeatability of isotope ratios, delta 13C and delta 15N in four tissues (muscle, liver, adipose fin, and visceral adipose) and the relationship between growth rate and these measures in rainbow trout (Oncorhynchus mykiss). Tissue samples were from two experiments with, respectively, juvenile (n = 10; Exper-iment 1, one tank) and large (n = 120; Experiment 2, three tanks) rainbow trout, fed commercial diets. In Experiment 2, the estimated repeatability was high for both delta 13C and delta 15N in muscle and liver, making one measure suffice (r = 0.70-0.78 and r = 0.80-0.91, respectively). Using regression analysis, bioaccumulation of delta 13C in muscle and delta 15N in liver (over life), through fractionation (without feed enrichment or depletion stra-tegies) was found to be, respectively, negatively and positively associated with growth rate. Even though indi-vidual feed intake was not recorded, the significant relationship between GR (that correlates with feed efficiency) and, respectively, MC and LN, suggests isotopic fractionation as potential biomarkers for individual cumulative (over-life) feed efficiency in fish.