Distribution and Amount of Hybridization Between Shoal Bass and the Invasive Spotted Bass in the Lower Flint River, GA

Shoal bass Micropterus cataractae are endemic to the Apalachicola drainage, including the Chattahoochee (Alabama & Georgia), Flint (Georgia), and Chipola (Florida) river systems. Habitat modification, including extensive dam construction, have reduced populations sizes in many areas of their native range, and in some cases have led to local population extinctions. The most abundant shoal bass populations reside in the Flint River, but even there they are suspected to be hybridizing with the invasive spotted bass M. punctulatus, which have become established in the system. To assess the threat that hybridization poses to the genetic integrity of shoal bass in the Flint River, presumptive shoal bass were collected from multiple locations above and below the Flint River Dam, and from Ichawaynochaway Creek, a southern tributary. These were genotyped using 17 polymorphic microsatellite loci and sequenced for the mitochondrial DNA 16S ribosomal RNA gene. Above the Flint River Dam, 12% of the specimens carried non-native alleles; however, genomic proportions of these alleles were low (1%). Below the Flint River Dam, the percentage of hybrid specimens ranged from 17-43% and sample genomic proportions increased to 6-7%. In Ichawaynochaway Creek, the percentage of hybrid specimens was 27% and the sample genomic proportion was 7%. Pure spotted bass were collected below the Flint River Dam; most F1 hybrids were collected in Ichawaynochaway Creek. The mtDNA of hybrids indicated that shoal bass males cross with spotted bass females more frequently than the converse. Hybridization was bidirectional; i.e., hybrids were found to backcross to both spotted bass and shoal bass. Because introgressive hybridization occurs, the flux of non-native alleles in this system should be monitored and all brood fish in stocking programs should be screened. Studies should be implemented to determine if natural genetic mechanisms (e.g., disruptive selection) will be sufficient to maintain the integrity of the species boundaries.