Variation and plasticity and their interaction with urbanization in Guadalupe Bass populations on and off the Edwards Plateau

Abstract

The Colorado River Basin in Texas has experienced major alterations to its hydrologic regime due to changing land and water use patterns. These anthropogenic influences on hydrologic variability have had major implications for riparian and aquatic ecosystems and the species dependent upon them. However, these impacts are usually assessed at limited temporal and spatial scales, i.e., research tends to be focused on relatively short and discrete periods or portions of the river basin. It is not clear how basin-wide alterations occurring across decades affect species that maintain populations throughout a basin. Guadalupe Bass Micropterus treculii are endemic to central Texas and are typically associated with shallow runs and riffles in small streams. However, Guadalupe Bass are found throughout the Colorado River Basin, including the mainstem portion of the river downstream of Austin where they support a thriving trophy fishery. Because Guadalupe Bass exist across a wide range of stream orders within the Colorado Basin, it is unclear whether populations from different parts of the basin respond similarly to anthropogenic disturbances or to conservation and restoration activities. Therefore, my objectives were to 1) quantify the response of Guadalupe Bass to flow alteration and changing land use patterns throughout the Colorado River Basin, and 2) document differences between populations. To accomplish this, I used a combination of morphometric analyses, age and growth estimation, and radio telemetry. Watershed variables were evaluated for percent difference in urban and agricultural land use as well as flow alteration using Indicators of Hydrologic Alteration (IHA). I evaluated the influence of flow alteration on morphology, growth, recruitment, and movement of Guadalupe Bass. Relationships between hydrologic alteration and landscape changes were compared to shape variation determined using geometric morphometric for Guadalupe Bass in the late 1970s and present. I identified significant ecomorphological variation, as well as variation in standardized growth and recruitment, of Guadalupe Bass in response to flow alteration. Standardized growth was determined from Von Bertalanffy growth curves fit to the back-calculated length-at-age data for otoliths of contemporary Guadalupe Bass throughout the Colorado River Basin. In addition to life history traits, I evaluated the movement, habitat use, and behavior of Guadalupe Bass in the lower Colorado River. I used both radio telemetry and electrofishing surveys to describe the similarities and differences between tributary Guadalupe Bass populations and the unique mainstem population in the lower Colorado River. Radio telemetry allowed me to evaluate movement rates, home range size and the influence of habitat on activity levels. This research revealed differences in Guadalupe Bass habitat associations and movements, contrasts in age and growth, and morphological variation across a gradient of disturbance throughout the Colorado River Basin. There was a longitudinal gradient in growth throughout the basin with individuals in the lower Colorado River growing faster than in tributaries. The highest growth rates occurred under moderate flow conditions throughout the basin and weaker year classes in the lower Colorado River were related to high and low flow conditions. Spatial differences in growth rates between mainstem and tributary populations were accompanied by temporal and spatial morphological differences which were associated with altered flow regimes and landcover transitions. In addition to growth rate and morphological variability throughout the basin, there were differences in the movement and habitat associations of lower Colorado River in comparison to tributary individuals. The ability of the Guadalupe Bass population to not only persist but flourish downstream of a heavily populated urban area presented a unique opportunity to investigate the response of a native species to anthropogenic disturbance. Results of this work provide information on the potential effects of population growth and increased water withdrawals on Guadalupe Bass populations. Additionally, this work adds to an understanding of the unique Guadalupe Bass population found in the lower Colorado River and how it differs from upstream tributary populations. Providing more population-level information facilitates conservation actions critical to preserving preferred habitat and promoting growth rates for Guadalupe Bass in streams of different sizes and flow conditions while highlighting interpopulation differences that may warrant consideration for stocking programs and other management strategies.