Summer and Autumn Movement of Endemic Bartram's Bass, Invasive Alabama Bass and Hybrid Congeners in an Upper Savannah River Tributary

Abstract

Bartram's Bass Micropterus sp. cf. coosae is endemic to the Upper Savannah River Basin of the southeastern United States, and is threatened by hybridization with invasive Alabama Bass M. henshalli. Quantifying movement of these species and their hybrids will improve understanding of how nonnative alleles spread among riverine fish populations. We quantified summer/autumn movement of Bartram's Bass, Alabama Bass and hybrid bass in Eastatoee Creek—a tributary experiencing ongoing invasion from Keowee Reservoir. To do this, we first quantified factors associated with the longitudinal distribution and weekly movement rates of each species, then estimated probabilities of species transitioning among key habitats at the river-reservoir interface. We tagged 291 fish with passive integrated transponder tags, sampling the entire stream length of Eastatoee Creek four times in both 2021 and 2022. We radio-tagged an additional 52 fish and tracked them from early May through mid-October each year. We used mixed effect models and a Bayesian multistate model to quantify movement, river position, movement probability and the effects of abiotic factors thereon. Alabama Bass and hybrid bass moved more than Bartram's Bass and remained in the lower reaches of Eastatoee Creek, apparently restricted by a reach of high-gradient habitat that functioned as a natural barrier. Alabama Bass made greater upstream movements during cooler spring temperatures when higher reservoir levels inundated the creek mouth. Bartram's Bass were distributed throughout Eastatoee Creek, making shorter weekly upstream movements associated with increasing temperature in late spring. Movement of hybrid bass from lower portions of tributaries is likely a primary source of nonnative allele spread in this system.