Influence of Connectivity and Habitat Heterogeneity on Fishes in the Upper San Marcos River, Texas
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Fish distributions and habitat association models are useful for predicting fish community responses to anthropogenic modifications, such as reductions in water quantity and alterations of instream habitats. Among western gulf slope drainages, spring runs (i.e., from spring outflows to the confluence with larger streams) provide habitats for a large number of endemic fish fauna; however, current knowledge of fish distributions and habitat associations within spring runs is insufficient to accurately predict community changes or to assess community changes as a result of pre-existing anthropogenic modifications. Based on previous research, spring-associated fishes are distributed homogenously within spring runs and rarely in mainstem rivers, whereas riverine fishes of mainstem rivers rarely enter spring runs. Therefore, a primary prediction of this study was that spring-associated fishes of the upper San Marcos River would be homogenously distributed from spring origin (Spring Lake) to the confluence with the Blanco River. I quantified fish abundance, densities, and habitat associations during four seasons and among five reaches within the river, using multiple gear types to sample wadeable and non-wadeable habitats. Overall, spring-associated fishes were not homogenously distributed throughout the river, attributed to a lack of connectivity and likely habitat alterations. Also, riverine fishes occurred in high abundance throughout the river. Fish-habitat associations ranged from slack water specialists (i.e., endangered Fountain Darter, endemic Large Spring Gambusia) to swift water specialists (i.e., regional endemic Burrhead Chub and Guadalupe Darter). Results from this study and a companion study demonstrated that the upper San Marcos River fish community is highly persistent during a span of 100 years, have highly predictable habitat associations, and demonstrate the ecological function of habitat heterogeneity and constant water quantity .