Spatial and Temporal Trends in Habitat Associations of the Fish Community in Spring Lake and the Headwaters of the San Marcos River

As part of the San Marcos River Observation Systems, we assessed spatial and temporal trends of the fish community in Spring Lake and the upper San Marcos River.  The upper San Marcos River is critical habitat for four, federally-listed taxa and supports a number of indigenous fauna and flora.  Spring outflows from the Edwards Aquifer are attributed to support a stable and diverse biological community.  The San Marcos spring-fed system, while environmentally stable, is still susceptible to flooding, changing stream morphology, sediment transport and species expansions and contractions .  Trends in occurrence, abundance and habitat associations were assessed for each geomorphic unit by multiple line transects, seasonally for one year using three sampling techniques.  Eight sites within the headwaters were seined and electrofished, and six additional sites were quantified with underwater observations.  Species were segregated along substrate and vegetation gradients and their relative abundances were estimated.  The long term goal of the study is to relate changes is occurrence, abundance and habitat associations to natural and anthropogenic influences.