Experimental Determination of the Limits of Using Stable Isotopes to Distinguish Steelhead and Rainbow Trout Offspring

Abstract

AbstractSalmonid ecology and conservation projects often necessitate distinguishing the offspring from anadromous or nonanadromous mothers, but the forms are often physically and genetically similar. We reared Oncorhynchus mykiss fry produced by anadromous mothers on one of two diets: one rich in and the other depleted in marine nutrients. We then sampled and analyzed fin and muscle tissues from fish fed both diets for nitrogen stable isotopes over time. Tissue nitrogen values from fish on the marine‐nutrient‐depleted diet indicated the size and time cutoffs at which offspring of anadromous O. mykiss no longer reflected a marine origin (muscle tissue cutoff: 0.81 g, 50.8 mm, and 29.4 d postfeeding; fin tissue cutoff: 0.65 g, 45.9 mm, and 35.3 d). Salmonids smaller than 50 mm cannot provide a sufficiently large fin sample for isotopic analysis without significant risk of mortality. Therefore, our experiment did not reveal a period when fish were large enough to provide a nonlethal fin clip but still maintain their maternal nitrogen isotope signature. However, fish growing slower than those we examined would likely be distinguishable for a longer period of time than our data indicated.Received December 21, 2014; accepted May 2, 2015