Conservation Status of Shoal Bass in the Chipola River, Florida: The Threat of Hybridization with Native and Nonnative Congeners
Michael D. Tringali, Patrick A. Strickland, Richard A. Krause, Seifu Seyoum, Brandon L. Barthel, Alicia C. Alvarez, and Cecilia Puchulutegui
Abstract.—Following reports of invasive Spotted Bass Micropterus punctulatus in the Chipola River, Florida, we initiated a microsatellite DNA survey of black basses in that system to assess genetic risks to the native population of Shoal Bass M. cataractae. Nonnative Spotted Bass and naturally occurring Largemouth Bass M. salmoides × Florida Bass M. floridanus intergrades were sampled from the Apalachicola and lower Chipola rivers to serve as reference specimens. In 2007, we identified five Shoal Bass hybrids among 45 specimens of Micropterus. Two of the hybrid specimens had introgressant alleles from Spotted Bass and another two had introgressant alleles from Largemouth Bass × Florida Bass intergrades. The introgressing taxon for the remaining hybrid was not immediately identifiable and was later determined to be an undescribed species of Micropterus (provisionally named Choctaw Bass), which inhabits the coastal plain rivers of the eastern Gulf of Mexico. From 2008 to 2010, an additional 217 specimens from the system were genotyped, resulting in the identification of 36 hybrids. Overall, 15.9% of the unknown specimens were heterospecific; the overall genomic proportion of introgressant alleles was 4.0%. The largest fraction of introgressant alleles (3.0%) originated from Choctaw Bass, most likely carried by emigrants from the nearby Choctawhatchee River system. Largemouth Bass × Florida Bass intergrades contributed the next highest introgressant proportion (0.6%), followed by Spotted Bass (0.4%). One genetically pure Spotted Bass was detected among the upper Chipola River specimens. Point estimates of genetic effective size for the Chipola River Shoal Bass population ranged from 110.4 to 131.4 (95% confidence interval [CI] = 79.1 to 195.9); values of this magnitude could signal an increased susceptibility to hybrid-swarm formation and reduced efficacy of purifying selection. While this population harbors a nontrivial amount of introgressant alleles, the majority of interbreeding appears to originate via natural processes. Nonetheless, if nonnative Spotted Bass become further established in the Chipola River and increase their interactions with Shoal Bass, the genetic integrity of the endemic population could deteriorate rapidly.