In the Texas Hill Country, drought and excessive groundwater withdrawals are expected to impact the economically important ecosystems of the spring fed North and South Llano Rivers (NLR, SLR). SLR flows are stronger, whereas NLR flows are weaker and become intermittent during summers. These rivers provide the favorable habitats of two major sport fishes of Texas: the endemic Guadalupe bass (GB, Micropterus treculii) and the largemouth bass (LB, Micropterus salmoides). These species segregate by habitat type and size (LB lentic, GB lotic) during normal flow regimes in both rivers. We examined patterns of habitat shift and availability under contrasting flow regimes during fish development on the feeding ecology of these fishes. We hypothesized greater diet overlap between GB and LB of larger sizes in the NLR (lower summer spring flows) compared to SLR. We hypothesized that LB would shift to piscivory earlier than GB due to reaching larger sizes earlier in development. Gut content analysis of fish caught in the summer of 2018 in both rivers were used to determine body sizes at which ontogenetic diet shifts occurred as well as calculate Schoener’s Index of Overlap throughout ontogeny.
Results/Conclusions
Standard length (SL) of GB and LB were used to form size classes (SC) for fish: 1= ≤ 50, 2=51-100, 3=101-150, and 4=151 ≥ mm SL. Through quantification of prey items found in gut contents, we found that all size classes of LB consumed fish, however increases in fish consumption start in SC 2 fish. GB also incorporated fish into their diets at smaller sizes, but did not increase their fish consumption until reaching SC 3. GB SC 4 consumed more decapods than fish. The NLR had the highest overlap (1.0) between GB SC 3 and LB SC 4. Lowest overlap (0.26) was between GB SC 4 and LB SC 4. In the SLR, SC 2 and SC 3 exhibited the most overlap (0.90). Lowest overlap (0.42) in the SLR occurred between GB SC 3 and LB SC 2, 3, and 4. Using the SLR as a “flow” control suggests summer low flow pooling of the NLR is likely associated with high competition between LB and GB. This serves as a prediction that GB may be impacted by LB if crucial SLR habitats decrease in abundance due to anthropogenic flow declines and climate change.