North American Journal of Fisheries Management © 2018 American Fisheries Society ISSN: 0275-5947 print / 1548-8675 online DOI: 10.1002/nafm.10177
MANAGEMENT BRIEF
Proactive Rainbow Trout Suppression Reduces Threat of Hybridization in the Upper Snake River Basin Ryan P. Kovach* U.S. Geological Survey, Northern Rocky Mountain Science Center, Missoula, Montana 59802, USA
Robert Al-Chokhachy U.S. Geological Survey, Northern Rocky Mountain Science Center, Bozeman, Montana 59715, USA
Tracy Stephens1 Wyoming Game and Fish Department, 420 North Cache Street, Jackson, Wyoming 83001, USA
Abstract Preserving remaining nonhybridized populations Cutthroat Trout Oncorhynchus clarkii is a conservation priority often requiring management action. Although proactive programs for Rainbow Trout O. mykiss and hybrid suppression offer a flexible tool, particularly in large interconnected river basins, this management approach is used less frequently than alternatives such as barriers and piscicides. We describe the results of a targeted Rainbow Trout hybrid suppression program spanning 15 years in the upper Snake River, Wyoming, a core stronghold for Yellowstone Cutthroat Trout O. clarkii bouvieri. Initially, Rainbow Trout hybrids were relatively common in the Gros Ventre River, a major tributary to the Snake River. Between 2002 and 2016, 926 individuals of Rainbow Trout ancestry were removed from the Gros Ventre River. Relative abundance of Rainbow Trout hybrids decreased over this time, while the Yellowstone Cutthroat Trout population increased. Temporal genetic data collected in 2007–2008 and again in 2014 demonstrate that the overall proportion Rainbow Trout admixture and the proportion of hybrids in a sample both significantly decreased in the Gros Ventre River and did not increase elsewhere in the Snake River basin. In conclusion, proactive Rainbow Trout suppression appears to have reduced the threat of Rainbow Trout hybridization in this river basin and helped protect an interconnected metapopulation that has a highly diverse life history and genetic variation important for long-term persistence.
Human-induced hybridization is problematic for the conservation of numerous fish species, especially the
various Cutthroat Trout Oncorhynchus clarkii subspecies distributed across western North America (Allendorf et al. 2001). Hybridization with nonnative Rainbow Trout O. mykiss—widely stocked for recreational purposes throughout the native ranges of all Cutthroat Trout subspecies—is considered a principal threat to the persistence of multiple subspecies of Cutthroat Trout (Peacock and Kirchoff 2004; Shepard et al. 2005; Gresswell 2011). Hybridization between Rainbow Trout and Cutthroat Trout has resulted in the genomic extinction (i.e., loss of native genotypic combinations through nonnative introgression) of innumerable populations, many lineages, and arguably, some subspecies (Allendorf and Leary 1988; Behnke 1992). Moreover, hybridization can reduce individual fitness and disrupt local adaptation (Muhlfeld et al. 2009; Kovach et al. 2015, 2016), which may ultimately influence persistence and population productivity. Throughout the western United States fisheries managers and conservationists are tasked with trying to preserve the remaining genetically pure (i.e., nonadmixed) Cutthroat Trout populations. An initial and warranted action is the cessation of stocking in watersheds occupied by genetically pure Cutthroat Trout. Unfortunately, terminating Rainbow Trout stocking does not address existing hybridization stemming from historical stocking practices (Bennett et al. 2010) and the fact that hybridization
*Corresponding author:
[email protected] 1 Present address: Arizona Game and Fish Department, 5000 West Carefree Highway, Phoenix, Arizona 850086, USA. Received December 8, 2017; accepted April 6, 2018
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appears to proceed from legacy stocking sources (Muhlfeld et al. 2017). Indeed, temporal genetic data from hybrid zones between Westslope Cutthroat Trout O. clarkii lewisi and Rainbow Trout suggest that Rainbow Trout admixture can increase rapidly in river basins where stocking has not occurred for several decades (Hitt et al. 2003; Muhlfeld et al. 2014, 2017). Thus, managers are often limited to one of three major options to address Rainbow Trout hybridization: (1) the use of fish barriers that prevent the invasion of Rainbow Trout or their hybrids; (2) chemical eradication (i.e., piscicide treatment) of genetically admixed populations; and (3) targeted suppression or removal programs. Although the use of fish barriers and chemical eradication are relatively widespread, targeted Rainbow Trout and hybrid suppression programs are rare in comparison. Recent evidence suggests that suppression programs may successfully limit the extent and magnitude of Rainbow Trout admixture in Westslope Cutthroat Trout (Al-Chokhachy et al. 2014) and Yellowstone Cutthroat Trout O. clarkii bouvieri (Meyer et al. 2017a). Nevertheless, the potential value of suppression as a management tool for minimizing hybridization is still poorly understood, in part because of limited implementation of such programs. Rainbow Trout and hybrid suppression programs may be particularly valuable in large river basins containing the last remaining interconnected metapopulations of Cutthroat Trout. Like all Cutthroat Trout, Yellowstone Cutthroat Trout populations are increasingly isolated due historical land management and more recently intentional isolation to prevent the spread of invasive species like Rainbow Trout (Gresswell 2011). Thus, protecting interconnected metapopulations of Yellowstone Cutthroat Trout is a management and conservation priority. Recent research provided evidence for ongoing Rainbow Trout hybridization in a major Yellowstone Cutthroat Trout stronghold, the upper Snake River basin in Wyoming (Kovach et al. 2011). Simultaneously, managers have been proactively suppressing a putative Rainbow Trout source population in this river basin. The primary purpose of this research was to quantify the effectiveness of this management program in terms of (1) reducing the numerical abundance of Rainbow Trout and their hybrids, (2) decreasing Rainbow Trout admixture within the source population, and (3) minimizing the spatial and temporal spread of Rainbow Trout admixture throughout the upper Snake River basin.
METHODS Study system.— The Snake River originates in Yellowstone National Park before it flows southerly into Grand Teton National Park, and enters Jackson Lake, Wyoming (2,067 m), a natural lake augmented by an earthen dam
to enhance water storage for agriculture. The dam at Jackson Lake is a complete upstream barrier to fish movement, and downstream from the dam the Snake River flows unimpeded for nearly 142 km into Palisades Reservoir (1,720 m), a second complete upstream barrier at the border with Idaho. The climate within the region is typical of the northern Rocky Mountains with cold winters where precipitation is dominated by snowfall and warm relatively dry summers. Accordingly, the hydrograph is primarily dictated by snowmelt with the peak hydrograph occurring during the late spring or early summer and streamflow tapering throughout autumn. The upper Snake River basin is largely (>90%) under public ownership, and the population of Yellowstone Cutthroat Trout within the basin is considered one of the largest strongholds across the species’ range (Endicott et al. 2016). Nevertheless, Rainbow Trout and Rainbow Trout– Cutthroat Trout hybrids are present within the system, particularly in a lower section of the Gros Ventre River, Wyoming (Figure 1). This section was previously known to contain Rainbow Trout and their hybrids, and subsequent radiotelemetry efforts in 2007 confirmed that Rainbow Trout and hybrids appeared to spawn in a small spring area within this section of the river (Gregory and Yates 2009). Genetic data also confirmed that this section of the river, and the lower Gros Ventre River more broadly, likely act as the primary source of Rainbow Trout genes to the upper Snake River basin (Kovach et al. 2011). The headwaters of the Gros Ventre River are protected as “Wild and Scenic,” but the lower river is subject to some human modifications and irrigation water withdrawal. Rainbow Trout suppression.— In an effort to maintain the Yellowstone Cutthroat Trout stronghold in the upper Snake River basin, the Wyoming Game and Fish Department initiated Rainbow Trout suppression efforts in the Gros Ventre River in 2002. Rainbow Trout were removed by means of boat electrofishing from a 5.1-km section of the Gros Ventre River immediately upstream from the boundary of Grand Teton National Park (Figure 1) in a section used for long-term population monitoring. Genetic and radiotelemetry data both confirmed that this section likely acts as a key source of and spawning area for Rainbow Trout and hybrids (Gregory and Yates 2009; Kovach et al. 2011). In 2002 and 2003 fish that were visually identified as hybrids or Rainbow Trout were collected and removed during a single-pass electrofishing effort over the entire suppression reach. Effort was increased from 2004 onward, and fish were collected and removed with three electrofishing passes conducted on consecutive days. No suppression effort was conducted in 2005 and 2006. Generally, visual identification of Yellowstone Cutthroat Trout from hybrids is accurate (Meyer et al. 2017b),
MANAGEMENT BRIEF
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FIGURE 1. The study area in Wyoming depicting the upper Snake River basin, Yellowstone and Grand Teton national parks (hatched), locations with temporal genetic data (gray circles; numbers refer to specific site information in Table 1), and area of Rainbow Trout suppression (black square).
particularly for individuals with Rainbow Trout admixture that exceeds 20% (Kovach et al. 2011). However, hybrid removal efforts were conservative in the sense that some individuals with low amounts of Rainbow Trout admixture (99% chance of detecting 1% Rainbow Trout admixture in a sample of 30 randomly mating individuals), the difference is marked when estimating individual admixture. For example, the probability of detecting Rainbow Trout alleles in an individual with 5% Rainbow Trout ancestry is 0.98 with 38 diagnostic loci, but only 0.76 with 14 diagnostic loci. All genotypic data (2007, 2008, and 2014) were produced at the University of Montana Conservation Genetics Laboratory. Prior to calculating summary statistics describing admixture, we examined the distribution of Rainbow Trout alleles across loci within populations. We looked to identify shared polymorphisms at some loci (i.e., loci that are potentially nondiagnostic in this study area), a problem commonly reported for genetic data at putatively species-diagnostic loci differentiating Cutthroat Trout from Rainbow Trout (Allendorf et al. 2004; Bingham and Buckskin 2016; Kovach et al. 2016). Specifically, we examined the data for evidence of Rainbow Trout alleles that were only present at one locus in otherwise nonadmixed (pure) populations. We removed four loci (OclYSD107607_Garza, OclYSD106367_Garza, OclYSD129870_Garza, and OclY SD123205_Garza) that satisfied this criterion among the 38 SNPs. Similarly, we reexamined the 2007 and 2008 genotypes and removed two loci (Occ42 and Omi0004) where there was evidence for shared polymorphisms among species. Additionally, there were a considerable number of missing genotypes (29.7%) for one SNP locus (OclYGD 106419_Garza) relative to other loci (
