ARTICLE Evaluating the influence of stocking history and barriers to movement on the spatial extent of hybridization between westslope cutthroat trout and rainbow trout Janet L. Loxterman, Ernest R. Keeley, and Zacharia M. Njoroge Abstract: In this study we examine how the stocking of rainbow trout (Oncorhynchus mykiss), as well as the presence of fish movement barriers, influences the probability of introgression with westslope cutthroat trout (Oncorhynchus clarkii lewisi). We measured the level of introgression in cutthroat trout sampled from 32 locations that occurred either above or below fish movement barriers along with the frequency and number of rainbow trout stocked in the watershed over a 43-year period. The occurrence and level of hybridization in cutthroat trout were not related to whether the site was above a movement barrier or not. In contrast, the level of introgression was related to the distance to the nearest stocking location, the number of stocking events within the nearest watershed, and the total number of rainbow trout stocked in that watershed. Our data indicate that westslope cutthroat trout located further from stocking have a lower risk of introgression with rainbow trout, but those isolated above movement barriers should not be considered free from introgression until a thorough genetic assessment of the location has been made. Résumé : Nous examinons l’influence de l’ensemencement de truites arc-en-ciel (Oncorhynchus mykiss) et de la présence de barrières au déplacement des poissons sur la probabilité d’introgression avec la truite fardée versant de l’ouest (Oncorhynchus clarkii lewisi). Nous avons mesuré le degré d’introgression chez des truites fardées prélevées de 32 sites en amont et en aval de barrières au déplacement des poissons, ainsi que la fréquence et le nombre de truites arc-en-ciel ensemencées dans le bassin versant sur une période de 43 ans. La présence de truites fardées et leur degré d’hybridation n’étaient pas reliés a ` l’emplacement du site par rapport a ` une barrière au déplacement (amont ou aval). En revanche, le degré d’introgression était relié a ` la distance par rapport au site d’ensemencement le plus proche, au nombre d’évènements d’ensemencement dans le bassin versant le plus proche et au nombre total de truites arc-en-ciel ensemencées dans ce bassin versant. Nos données indiquent que les truites fardées versant de l’ouest situées plus loin de sites d’ensemencement présentent un risque relativement faible d’introgression avec la truite arc-en-ciel, alors que celles qui sont isolées en amont de barrières au déplacement ne devraient pas être considérées comme étant exemptes d’introgression si l’emplacement où elles se trouvent n’a pas fait l’objet d’une évaluation génétique exhaustive. [Traduit par la Rédaction] Introduction The introduction of non-native and captive-bred species for rec- reational or conservation purposes has been common in species management programs (Naish et al. 2008; Laikre et al. 2010). While often intended to be beneficial, such introductions can have unintended interactions with native populations, including predation, competition, disease transmission, and hybridization with or without introgression (Sakai et al. 2001). Hybridization between introduced and native species poses a major threat to populations of plants and animals, reducing fitness through out- breeding depression, genetic swamping, and disruption of local adaptations. In some situations, especially with rare or threatened species, hybridization can lead to population or species extinction (Rhymer and Simberloff 1996). While hybridization poses a threat to a variety of plant and animal taxa, it is especially widespread in populations of freshwa- ter fishes, as there is a long history of population supplementa- tion through hatchery introductions, as well as the introduction of non-native species for use in aquaculture and for recreational purposes (Moyle et al. 1986; Scribner et al. 2000). For salmonid fishes, which are often artificially propagated for recreational fisheries, stocking in natural water bodies to support sport fish- eries is a common practice. Thus, hybridization commonly occurs in salmonids and poses a genetic risk to native species and popu- lations (Allendorf et al. 2001, 2004). The level of genetic difference between native and introduced individuals can affect the severity of hybridization. Examples range from hybridization without in- trogression, to the loss of local adaptations, to complete genetic mixing of populations whereby the genetic identity of native pop- ulations is completely lost (Kanda et al. 2002; Ruzzante et al. 2004; Rasmussen et al. 2012). In addition to genetic similarity, many other factors can affect the extent of hybridization in salmonids. For example, habitat conditions, water temperature, elevation, and life-history charac- teristics can influence the extent of invasion as well as the rate of hybridization (Rubidge and Taylor 2005; Gunnell et al. 2008; Muhlfeld et al. 2009b). Hybridization between native populations and non-native species also depends on the non-native introduc- tions themselves. Stocking history can be defined as the combina- tion of stocking events over time, the number of individuals stocked, and the distance between native populations and the locations of introductions. Although a few studies have noted a Received 6 August 2013. Accepted 17 March 2014. Paper handled by Associate Editor Michael Bradford. J.L. Loxterman, E.R. Keeley, and Z.M. Njoroge. Department of Biological Sciences, Stop 8007, Idaho State University, Pocatello, ID 83209, USA. Corresponding author: Janet L. Loxterman (e-mail: loxtjane@isu.edu). 1050 Can. J. Fish. Aquat. Sci. 71: 1050–1058 (2014) dx.doi.org/10.1139/cjfas-2013-0424 Published at www.nrcresearchpress.com/cjfas on 19 March 2014. Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Janet Loxterman on 08/01/14 For personal use only.