Distinguishing between Natural and Hatchery Snake River Fall Chinook Salmon Subyearlings in the Field Using Body Morphology

Abstract

We used body morphology to distinguish between natural- and hatchery-origin subyearling fall Chinook salmon Oncorhynchus tshawytscha in rearing areas of the Snake River and at a downstream dam during seaward migration. Using subjective eye and body shape characteristics, field personnel correctly classified 88.9–100% of natural subyearlings (N = 626) and 90.0–100% of hatchery subyearlings (N = 867) in rearing areas from 2001 to 2008. The morphological characteristics used by these personnel proved to have a quantitative basis, as was shown by digital photography and principal components analysis. Natural subyearlings had smaller eyes and pupils, smaller heads, deeper bodies, and shorter caudal peduncles than their hatchery counterparts during rearing and at the dam. A discriminant function fitted from this set of morphological characteristics classified the origin of fish during rearing and at the dam with over 97% accuracy. We hypothesize that these morphological differences were primarily due to environmental influences during incubation and rearing because it is highly probable that a large portion of the natural juveniles we studied were the offspring of hatchery × hatchery mating in the wild. The findings in this paper might provide guidance for others seeking to differentiate between natural and hatchery fish.

Received April 15, 2010; accepted August 31, 2010

ACKNOWLEDGMENTS

We thank our colleagues at the U.S. Geological Survey's Western Fisheries Research Center and the U.S. Fish and Wildlife Service's Idaho Fisheries Resource Office, whose efforts contributed to the success of this study. We acknowledge the U.S. Army Corps of Engineers and the Washington Department of Fish and Wildlife for providing assistance at the Lower Granite Dam juvenile fish facility. Reviews by John Beeman, Chris Peery, Martin Unwin, and two anonymous reviewers greatly improved this manuscript. This study was funded by the Bonneville Power Administration and administered by Debbie Docherty. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the U.S. Fish and Wildlife Service.