Genetic Relationships of Florida Bass and Largemouth Bass Inhabiting Salt Marsh and Freshwater Habitats in Northwest Florida Coastal Streams

Coastal rivers provide important fishing opportunities throughout the southeastern United States. Black bass Micropterus spp. are major components of these fisheries and are highly valued by anglers, especially in northwest Florida, where few impoundments exist. In the Escambia and Yellow rivers, as in most other rivers on the northern Gulf Coast, salinity is low in the upper segments but may exceed 13 parts per thousand in the lower, mesohaline sections. Both rivers are located within the natural intergrade zone between Largemouth Bass M. salmoides and Florida Bass M. floridanus, and the age and size structure of fish inhabiting riverine reaches differ considerably from those in salt marsh habitats. We collected tissue samples from fish in both the riverine and salt marsh habitats in the Escambia and Yellow rivers and conducted genetic analyses to determine whether individuals from the two habitat types exist as a single randomly mating (panmictic) population in each system. Analysis of 15 microsatellite loci identified significant differences in the allele frequency distributions of samples collected from riverine versus salt marsh habitats in each river. Both rivers were found to be inhabited by intergrade populations that included pure Largemouth Bass and hybrids between Largemouth Bass and Florida Bass. Fish in the salt marsh habitats tended to have more Largemouth Bass alleles than fish in riverine habitats in both systems. Together, these findings indicate fish in the salt marsh and river habitats do not exist in panmixia and have different genetic compositions. Environmental differences between the riverine and estuarine habitats may have led to divergent selection and limited mating between subpopulations within the same river system. Largemouth Bass alleles may confer greater fitness than Florida Bass alleles in estuarine environments of the northern Gulf Coast, which could explain the greater frequencies of Largemouth Bass alleles in these unique habitats.