Movement and Microhabitat Associations of Guadalupe Bass in Two Texas Rivers

Abstract

The Guadalupe bass Micropterus treculii is endemic to Texas and is threatened by introgression with introduced smallmouth bass M. dolomieu as well as habitat degradation. This study described and quantified the movements and habitat associations of Guadalupe bass to assess the factors that may influence current populations. Radio-tagged adult Guadalupe bass were tracked in the Pedernales River (n = 12) and South Llano River (n = 12) from January through August 2008. Available microhabitats were measured and modeled in terms of depth, velocity, substrate, and cover for about 1.5 km in the Pedernales River and 1.2 km in the South Llano River. Rates of movement were greatest during the reproductive season, ranging from less than 1 to 9 m/d. Instream cover (such as undercut banks and woody debris) was preferred during daylight hours throughout the study period, although the distances from cover increased from January to August. Habitat shifts from cover to open water occurred at night and from woody structures to boulders and ledges during a large flood pulse. The habitats most suitable for adult Guadalupe bass had a depth of 1.0 m and a current velocity of 0.05 m/s, and habitat selection was strongest for eddy mesohabitats with smaller substrates. By July, the Guadalupe bass in the South Llano River were associated with runs with greater current velocities, whereas those in the Pedernales River were associated with pools with greater depths, largely owing to the low flows and reduced habitat availability in the Pedernales River. Environmental factors, including the availability and suitability of instream cover, are probably the strongest influences on the distribution and abundance of Guadalupe bass.