New Distributional Records for Etheostoma fonticola (Fountain Darter) in the San Marcos River, Texas

Etheostoma fonticola (Fountain Darter) is federally listed as endangered and endemic to a single river basin in Texas, occupying stenothermal headwaters (22–23°C) of the spring-influenced San Marcos River and Comal River. Reported distribution is limited to the upper 6.4 river kilometers (rkm) of the San Marcos River, but a collection of a Fountain Darters farther downstream (11.5 rkm from the headwaters) in August 2022, along with visual observations of other small percids in the immediate area, prompted a preliminary assessment to document extent of the Fountain Darter's downstream distribution. Using seines to target mesohabitats (i.e., riffle, run, pool, backwaters) likely to contain Fountain Darters, we resampled the same site as the 2022 collection in April 2023 to assess overwinter survival. In June 2023, we sampled 5 sites located 10.2–19.0 rkm downstream from the headwaters. Between both sampling dates, we collected 243 Fountain Darters among 13 mesohabitats at 3 sites located 10.2–16.0 rkm downstream from the headwaters, including a mesohabitat with water temperatures reaching 29 °C. The occurrence and abundance of Fountain Darters in a reach of the San Marcos River within a more eurythermal environment offer a unique opportunity to assess population viability of Fountain Darters outside of a stenothermal environment, which in the past was considered to be a primary limiting factor in wider distributions of Fountain Darters.