<em>Abstract</em>.—Shoal Bass <em>Micropterus cataractae</em> are native only to the Apalachicola–Chattahoochee–Flint River system of Georgia, Alabama, and Florida. Shoal Bass are vulnerable to extinction as a result of population fragmentation and introduction of nonnative <em>Micropterus</em> species. We assessed the genetic integrity of four isolated populations of Shoal Bass in the upper Chattahoochee River basin (above Lake Lanier, in Big Creek, below Morgan Falls Dam, and at Cochran Shoals) and sought to determine rates of hybridization with nonnative, illegally stocked Smallmouth Bass <em>M. dolomieu</em> and Alabama Bass <em>M. henshalli</em>. Collected specimens were evaluated using 10 highly polymorphic microsatellite markers and Bayesian population assignment software. Shoal Bass below Morgan Falls Dam exhibited high levels of hybridization with Smallmouth Bass and possibly Alabama Bass, coupled with a high incidence of the nonnative parental forms. Rates of hybridization among specimens from Cochran Shoals were similarly high, although fewer nonnative parental forms were observed. Among the 30 tested, three specimens morphologically identified as Shoal Bass in Big Creek were either pure Smallmouth Bass or hybrids of Shoal Bass and Smallmouth Bass. The only Shoal Bass population not significantly impacted by congeneric hybridization was found upstream of Lake Lanier. Conservation actions aimed at reducing or eliminating nonnative black bass species and their hybrids could maintain the genetic integrity of Shoal Bass populations in the upper Chattahoochee River basin and lessen their risk of extirpation.
Biology of black bass Micropterus salmoides (Lacepède, 1802) fifty years after the introduction in a small drainage of the Upper Paraná River basin, Brazil
