Relations Between Shoal Bass and Sympatric Congeneric Basses in the Flint River, Georgia

Abstract

Shoal bass Micropterus cataractae is a fluvial specialist bass species endemic to the Apalachicola River drainage, which co-occurs with native largemouth bass M. salmoides throughout its range. Spotted bass M. punctulatus have been recently introduced to the upper Flint River in Georgia, causing concern among fisheries managers and anglers about the potential negative impacts the species may have on the population of shoal bass in the river. In this study, I investigated the potential for competitive interactions among the three species of basses in the Flint River at adult and juvenile life stages. In December 2008, 11 shoal bass, 10 largemouth bass, and 6 spotted bass were implanted with radio transmitters in the Flint River and were tracked for a period of one year. Basses exhibited habitat partitioning in this study; shoal bass were generally found near the fastest available water velocities in shoals with rocky substrates, largemouth bass were found almost exclusively near large woody debris in pools with sandy substrates and little to no flow, and spotted bass often showed habitat preferences intermediate to both species. Individuals of all three species were observed moving distances > 5 km to large shoal complexes during the spawning season; shoals may be important spawning and nursery areas for basses in the Flint River. Age-0 basses were collected from piedmont and coastal plain segments of the Flint River in early summer during 2008 and 2009 to see if relative hatch timing would provide spotted bass with a competitive advantage over the two native basses in growth or size during juvenile life stages. Basses generally hatched from early April to mid-June at water temperatures ranging from 20 to 25˚ C. Mean daily growth rates ranged from 0.822 to 1.07 mm/d across species and samples. Differences among species in mean hatch dates, mean growth rates, and mean total lengths were observed; however, these differences were not consistent across samples. Discharge varied widely between years and sampling sites and this may have contributed to the difficulty in detecting consistent patterns across samples. Relative hatch timing did not appear to convey any competitive advantage to spotted bass over the two native species. Spotted bass were collected from the Flint River in Georgia and native Alabama bass were collected from the Tallapoosa River in Alabama to compare life history strategies among native and introduced bass populations. Males began maturing at 1 year and 168 mm in the Flint River and 2 years and 173 mm in the Tallapoosa River. Females began maturing at 2 years and 186 mm in the Flint River and 2 years and 200 mm in the Tallapoosa River. No differences in age or size at 50% maturity were evident between the two populations. The invasive success of spotted bass in the Flint River does not appear to be related to a shift in life history characteristics. Competitive interactions between shoal bass and spotted bass do not appear to pose a large threat to the persistence of shoal bass in the upper Flint River. A potential for introgressive hybridization between the two species exists and warrants future investigation and monitoring. Fisheries managers must attempt to educate anglers about the dangers of moving species into new drainages, in order to prevent future invasions.