Rangewide Survey of the Introgressive Status of Guadalupe Bass: Implications for Conservation and Management

The stocking of fishes outside of their native range for the purpose of sport fisheries can lead to secondary contact and introgression between species that were historically allopatrically distributed. Smallmouth Bass Micropterus dolomieu were introduced within the range of Guadalupe Bass M. treculii in central Texas and introgressive hybridization subsequently occurred. One recent survey of temporal changes in introgression in the Blanco River found that introgression had increased and that Guadalupe Bass had been extirpated. Thus, a survey of changes in introgression across the range of the Guadalupe Bass was conducted in 12 subbasins in the Brazos, Colorado, Guadalupe–San Antonio, and Nueces drainages in Texas using 15 microsatellite loci. The results indicate that introgression is now occurring in four subbasins but no longer occurring in the Lampasas and San Gabriel rivers, where rates were previously 6% and 46%, respectively. Additionally, we found no evidence that stocking of hatchery-reared individuals in the Guadalupe and Nueces rivers has led to severely depressed genetic variation. The variable success of restoration efforts to prevent extirpation of the Guadalupe Bass suggests that protection of the remaining nonintrogressed populations should be a priority for the conservation of this species. Freshwater sport fishes are among the most widely introduced fish species in North America (Rahel 2000), and stocking of sport fish often results in previously allopatric species coming into secondary contact. As a result, sport fish introductions account for several known incidences of introgressive hybridization in fishes (e.g., Whitmore 1983; Koppelman 1994; Pipas and Bulow 1998; Johnson and Fulton 2004; Cordes et al. 2006). Introgressive hybridization is one mechanism by which extirpation and extinction of native species can occur (Rhymer and Simberloff 1996) and is a significant threat to many na*Corresponding author: [email protected] 1Present address: Llano River Field Station, Texas Tech University, 254 Red Raider Lane, Junction, Texas 76849, USA. Received April 24, 2012; accepted December 9, 2012 tive fishes in North America (Williams et al. 1989; Perry et al. 2002). The dynamics of introgression can result in multiple outcomes, including bimodal hybrid zones, hybrid swarms, and replacement of native parental species by hybrids. For example, hybridization between Dolly Varden Salvelinus malma and Bull Trout S. confluentus resulted in a bimodal hybrid zone in which both parental species co-occurred in greater abundance than the hybrids between the two species (Redenbach and Taylor 2003), whereas Childs et al. (1996) found that Pecos Pupfish Cyprinodon pecosensis severely declined and Pecos 681 D ow nl oa de d by [ T ex as T ec hn ol og y U ni ve rs ity ] at 1 1: 25 1 0 A pr il 20 13