Instream habitat use and management of invasive armored catfishes in the upper San Marcos River

The San Marcos River is a spring-fed aquatic system located in central Texas fed by the San Marcos Springs. This spring system is characterized by high water quality and relatively constant temperatures, pH, discharge and dissolved ion concentrations. The San Marcos River complex provides habitat for eight endemic species that are federally-listed as threatened or endangered. Environmental stability and high concentration, and associated high density, of aquatic macrophytes and macroinvertebrates has made the San Marcos River an easily invaded aquatic system, especially by organisms at lower trophic positions. Two established invasive fish species in the San Marcos River are members of family Loricariidae- the suckermouth catfish (Hypostomus plecostomus) and the vermiculated high-fin catfish (Pterygoplichthys disjunctivus). In order to better characterize how these invasive species utilize the San Marcos River, a total of 15 snorkel surveys were conducted between December 2016 and May 2017. During these surveys, a total of 115 sightings of Loricariid spp. were recorded. Of these sightings, 26 were identified to species. During these surveys, water quality (surface temperature, dissolved oxygen, specific conductance, pH, flow) and physical (percent vegetation cover, substrate type, and distance from the Spring Lake Dam) were recorded. Negative binomial regression indicated the most influential factor associated with armored catfish occurrence was distance downstream from the Spring Lake Dam (P<0.0001). Data collected during this study indicate that armored catfish are more frequently observed in the lower reaches of the upper San Marcos River. These areas are characterized by possessing deeper pools created by dams. These pools contained dense stands of submerged vegetation and slow moving water. Data from armored catfish removal efforts conducted from 2013 to 2016 was also analyzed to assess the effectiveness of contracted removal efforts. Pairwise annual comparisons showed significant reductions in biomass over the removal period. Loricariid burrowing habits may displace sediment which can lead to altered water quality as well as uprooting of aquatic macrophytes during the burrowing and foraging. Furthermore, given the degree in which these invasive species are present in the San Marcos River, nutrient cycling within the river may be altered. It appears that ongoing suppression efforts are effectively reducing the biomass of armored catfish populations in the San Marcos River.