Patterns of Hybridization of Nonnative Cutthroat Trout and Hatchery Rainbow Trout with Native Redband Trout in the Boise River, Idaho

Abstract

Hybridization is one of the greatest threats to native fishes. Threats from hybridization are particularly important for native trout species as stocking of nonnative trout has been widespread within the ranges of native species, thus increasing the potential for hybridization. While many studies have documented hybridization between native cutthroat trout Oncorhynchus clarkii and nonnative rainbow trout O. mykiss, fewer have focused on this issue in native rainbow trout despite widespread threats from introductions of both nonnative cutthroat trout and hatchery rainbow trout. Here, we describe the current genetic (i.e., hybridization) status of native redband trout O. mykiss gairdneri populations in the upper Boise River, Idaho. Interspecific hybridization was widespread (detected at 14 of the 41 sampled locations), but high levels of hybridization between nonnative cutthroat trout and redband trout were detected in only a few streams. Intraspecific hybridization was considerably more widespread (almost 40% of sampled locations), and several local populations of native redband trout have been almost completely replaced with hatchery coastal rainbow trout O. mykiss irideus; other populations exist as hybrid swarms, some are in the process of being actively invaded, and some are maintaining genetic characteristics of native populations. The persistence of some redband trout populations with high genetic integrity provides some opportunity to conserve native genomes, but our findings also highlight the complex decisions facing managers today. Effective management strategies in this system may include analysis of the specific attributes of each site and population to evaluate the relative risks posed by isolation versus maintaining connectivity, identifying potential sites for control or eradication of nonnative trout, and long-term monitoring of the genetic integrity of remaining redband trout populations to track changes in their status.

Received February 24, 2011; accepted August 4, 2011

ACKNOWLEDGMENTS

This study was funded by Trout Unlimited (TU) through salary support to H.M.N. and an Embrace-A-Stream grant to the Ted Trueblood Chapter of TU. We thank the many members of the Ted Trueblood Chapter and the Boise Valley Fly Fishermen; Mike Kellet (Boise National Forest); and Jeff Dillon, Lance Hebdon, Sharon Clark, Liz Mamer, Chris Kozfkay, and Stacey Dauwalter (IDFG) for their assistance in sample collection and providing stocking information. We are grateful to Veronica Kirchoff and Mary Peacock (University of Nevada) for performing the laboratory work and to Matt Mayfield and Sabrina Beus (TU) for help with figures. This manuscript was greatly improved by comments from Chris Kozfkay, Jeff Dillon, Dan Dauwalter, Jack Williams, Stephen Bennett, and four anonymous reviewers. The previous study was funded by the National Fire Plan, U.S. Forest Service Rocky Mountain Research Station, U.S. Forest Service regions 1 and 4, the U.S. Geological Survey's Forest and Rangeland Ecosystem Science Center, and the Boise National Forest. For acknowledgment of the many additional people who helped with this study, please see Neville et al. (2009). Reference to trade names does not imply endorsement by the U.S. Government.