Investigating the effects of intermittent flow on the resilience and vulnerability of fish assemblage structure

Supra-seasonal droughts are natural occurrences that can have economic and environmental consequences that are exacerbated by climate change and anthropogenic water use. These long-term droughts have a direct impact on fish and fisheries. Our study aimed to assess the vulnerability and resilience of tributary fish assemblages within the Colorado River Basin (Texas) to drought-induced intermittent flows. Our objectives were to 1) document stream flow to determine the timing, duration, and severity of drying events, and 2) assess fish assemblage structure between intermittent and perennial stream reaches over differing seasonal flow conditions. Sites were sampled on four tributaries, focusing on logger deployment, habitat mapping, and fish assemblage surveys. Stream temperature intermittency and conductivity (STIC) loggers were used to document water presence. Drought severity varied among intermittent reaches from a minimum of 10 consecutive no-flow days on the San Saba River to a max of 60 days on Onion Creek. Drought severity did not follow expected longitudinal gradient patterns and more research should be done to determine if other factors such as local anthropogenic population size are driving forces of severity. The occurrence of fish was affected by habitat surface area (p = 0.04), sampling round (p <0.001), and temperature (p = 0.001). Species richness decreased by an average of 1 species per 10 days of no-flow across all sites (p = 0.007, R2 = 0.39). Fish assemblage composition differed among intermittent and perennial sites within tributaries as indicated by multivariate analysis. RLQ analysis showed that species that are elongated, long-lived, non-guarding, open substrate spawners or species with a primary diet of macrophytes and vascular plants are vulnerable to drought conditions. This information can be used to predict vulnerability of different species with similar traits that inhabit other basins.