Morphological and swimming stamina differences between Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), rainbow trout (Oncorhynchus mykiss), and their hybrids Steven M. Seiler and Ernest R. Keeley Abstract: We hypothesized that body shape differences between Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), rainbow trout (Oncorhynchus mykiss), and their hybrids may influence swimming ability and thus play an important role in the invasion of nonnative rainbow trout and hybrid trout into native cutthroat trout populations. We reared Yellowstone cutthroat trout, rainbow trout, and reciprocal hybrid crosses in a common environment and con- ducted sustained swimming trials in order to test for genetically based morphological and swimming stamina differ- ences. Linear and geometric morphometric analyses identified differences in body shape, with cutthroat trout having slender bodies and small caudal peduncles and rainbow trout having deep bodies and long caudal peduncles. Hybrid crosses were morphologically intermediate to the parental genotypes, with a considerable maternal effect. Consistent with morphological differences, cutthroat trout had the lowest sustained swimming velocity and rainbow trout had the highest sustained swimming velocity. Sustained swimming ability of hybrid genotypes was not different from that of rainbow trout. Our results suggest that introduced rainbow trout and cutthroat–rainbow trout hybrids potentially out- compete native Yellowstone cutthroat trout through higher sustained swimming ability. Résumé : Nous avons émis l’hypothèse selon laquelle les différences de forme du corps entre la truite fardée de Yellowstone (Oncorhynchus clarkii bouvieri), la truite arc-en-ciel (Oncorhynchus mykiss) et leurs hybrides peuvent affecter la capacité de nage et ainsi jouer une rôle dans l’envahissement des populations indigènes de truites fardées par les truites arc-en-ciel non indigènes et les truites hybrides. Nous avons élevé des truites fardées de Yellowstone, des truites arc-en-ciel et de truites issues de croisements hybrides réciproques dans un environnement commun et nous avons mené des tests de nage soutenue afin de vérifier l’existence des différences d’origine génétique dans la morpho- logie et dans la vigueur de nage. Des analyses morphométriques linéaires et géométriques mettent en évidence des dif- férences dans la forme du corps: les truites fardées possèdent un corps élancé et un pédoncule caudal réduit, alors que les truites arc-en-ciel ont un corps profond et un long pédoncule caudal. Les truites des croisements hybrides ont une morphologie intermédiaire par rapport aux génotypes des parents avec un effet maternel marqué. En accord avec les différences morphologiques, les truites fardées ont la vitesse de nage soutenue la plus faible et les truites arc-en-ciel la vitesse de nage soutenue la plus élevée. La capacité de nage soutenue des génotypes hybrides ne diffère pas de celle des truites arc-en-ciel. Nos résultats indiquent que les truites arc-en-ciel introduites et les hybrides des truites fardées et arc-en-ciel peuvent potentiellement gagner la compétition avec les truites fardées indigènes à cause de leur capacité supérieure de nage soutenue. [Traduit par la Rédaction] Seiler and Keeley 135 Introduction Nonnative species are commonly thought to cause de- clines and extinction of native species through predation, competition, or habitat alteration; however, a more severe impact may occur by hybridization with native species (Rhymer and Simberloff 1996; Sakai et al. 2001). Intra- specific hybridization, occurring between distinct popula- tions, is thought to have played an important role in evolution of many plant taxa (Stebbins 1959) and some ver- tebrates (Dowling and DeMarais 1993). In contrast, hybrid- ization between species is often highly detrimental to native populations when closely related nonnative species are intro- duced into native species ranges and form interspecific hy- brids (Rhymer and Simberloff 1996). When hybrid offspring are sterile, genetic resources are wasted and small native populations may lose significant portions of important re- cruitment classes. When hybrids are fertile, reproduction between first generation hybrids and parental or advanced crosses of hybrids is termed introgression (Anderson 1949). Can. J. Fish. Aquat. Sci. 64: 127–135 (2007) doi:10.1139/F06-175 © 2007 NRC Canada 127 Received 13 February 2006. Accepted 7 December 2006. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 30 January 2007. J19172 S.M. Seiler 1 and E.R. Keeley. Department of Biological Sciences, Box 8007, Idaho State University, Pocatello, ID 83209, USA. 1 Corresponding author (e-mail: seilstev@isu.edu).