Morphometric and Meristic Differences among Bluehead Suckers, Flannelmouth Suckers, White Suckers, and Their Hybrids: Tools for the Management of Native Species in the Upper Colorado River Basin

—Hybridization with nonnative fishes is a major factor influencing the status of native catostomids in the Colorado River basin. In Wyoming, hybridization with nonnative white suckers Catostomus commersonii is a particular concern in the conservation of native bluehead suckers C. discobolus and flannelmouth suckers C. latipinnis. The purpose of this study was to describe the meristic and morphometric characteristics of bluehead suckers, flannelmouth suckers, white suckers, and their hybrids from Muddy Creek, Wyoming, for which genotypic data were available to verify the classification of individual fish as pure species or hybrids. We also sought to develop discriminant function models that can be used in the field. Bluehead suckers and flannelmouth suckers had more lateral line scales, shorter and narrower heads, and narrower caudal peduncles than white suckers. Bluehead suckers and bluehead sucker 3 white sucker hybrids had a cartilaginous biting ridge in their mouth. Important characteristics for discriminating among these taxa included the number of postdorsal lateral line scales and the exterior width of the mouth. The discriminant function correctly classified 100% of the bluehead suckers and bluehead sucker 3 white sucker hybrids. Flannelmouth suckers, white suckers, and flannelmouth sucker 3 white sucker hybrids lacked a cartilaginous biting ridge. The number of postdorsal scales, caudal peduncle depth, and ratio of mouth length to the interior width of the mouth discriminated among taxa without a ridge. The discriminant function correctly classified 100% of the flannelmouth suckers, 92% of the white suckers, and 91% of the flannelmouth sucker 3 white sucker hybrids. None of the white suckers or flannelmouth sucker 3 white sucker hybrids were misclassified as flannelmouth suckers. Our results suggest that native bluehead suckers and flannelmouth suckers, nonnative white suckers, and hybrids of these species can be accurately identified by a few, easily measured meristic and morphometric characteris-