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

Abstract

Salmonid 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

ACKNOWLEDGMENTS

We thank Erica Curles, Lindsay Hart, and Jon Wittouck for help with experimental design, fish husbandry, sample collection, and preparation; Jeff Duda, Peter Kiffney, and Sarah Morley for sharing stable isotope data; Cole Monnahan and Kiva Oken for helpful statistical input; and Rachel Hovel, Peter Kiffney, and two anonymous reviewers and editors for helpful comments on earlier versions of this manuscript. The work was funded in part by a grant from Washington Sea Grant, University of Washington, pursuant to National Oceanic and Atmospheric Administration Award Number NA10OAR4170075, Project R/LME-7, and by the H. Mason Keeler Endowment to the University of Washington. The conduct of this experiment was permitted by the Washington Department of Fish and Wildlife and the University of Washington's Institutional Animal Care and Use Committee.