Effects of Stocking Catchable-Sized Hatchery Rainbow Trout on Wild Rainbow Trout Abundance, Survival, Growth, and Recruitment

Abstract

The Idaho Department of Fish and Game has proactively dealt with the potential adverse genetic effects of stocking catchable-sized hatchery trout in waters that support native salmonids by adopting a policy in 2001 whereby only sterile rainbow trout Oncorhynchus mykiss are stocked in flowing waters; however, concerns regarding the competitive effects of introducing hatchery trout into streams and rivers supporting wild trout have not been addressed. We stocked fish in the middle 3 years of a 5-year study to assess whether stocking hatchery rainbow trout of catchable size (hereafter, catchables) reduced the abundance, survival, growth, or recruitment of wild rainbow trout in streams. Catchables averaging 249 mm total length (TL) were stocked from 2006 to 2008 at an annual density of 4.2 fish/100 m² into 12 treatment reaches of stream that were paired with control reaches at least 3 km apart in the same stream in which no stocking occurred. Wild rainbow trout abundance (including all fish ≥75 mm TL), recruitment, survival, and growth were determined from population estimates and recaptures of fish tagged with passive integrated transponder (PIT) tags during mark–recapture electrofishing sampling. The abundance of wild rainbow trout averaged 13.2 fish/100 m² but varied substantially across sites and years, ranging from a low of 0.5 to a high of 131.3 fish/100 m²; similar variability was observed in recruitment to age 1. Estimates of total annual survival averaged 0.53 based on the population abundance estimates (which allowed for emigration and immigration) and 0.26 based on the PIT-tag recaptures (which allowed for emigration but not immigration). Our paired study design demonstrated that the abundance, survival, growth, and recruitment to age 1 of wild rainbow trout were all unaffected by stocking catchables. The lack of population-level effects from stocking catchables was not surprising considering the high short-term mortality and the socially and physiologically naive behavior typically exhibited by hatchery catchables stocked in lotic systems.

Received February 15, 2011; accepted June 14, 2011

ACKNOWLEDGMENTS

Funding for this study was provided by the Federal Sport Fish Aid and Restoration Act. Numerous colleagues assisted with fieldwork, most notably Nick Gastelecutto, Jeremiah Wood, Chris Sullivan, Pete Gardner, Greg High, and Brad Wright. Mary Conner and Kirk Steinhorst provided invaluable help in MARK modeling and statistical analyses, respectively. Charlie Petrosky, John Cassinelli, Greg Schoby, Bob Carline, and four anonymous reviewers provided numerous helpful comments on our work which greatly improved this manuscript. This study was initially inspired and subsequently improved by Dan Schill's encouragement and guidance.