Assessing Short- and Long-Term Anthropogenic Threats to a Reintroduced Fish in a Restored Urban Riverscape

Abstract

1. Understanding which species can persist in human-modified environments is essential to biodiversity conservation in the Anthropocene. In urbanised environments, many stressors limit the persistence of imperilled native species via impacts on the abiotic and biotic environment. Habitat restoration followed by reintroduction of native species may be an effective strategy to maintain or even regenerate biodiversity in urbanised environments, but few studies have assessed these two conservation strategies concomitantly in urban freshwater ecosystems.
2. We assessed short-term population dynamics and habitat associations of an endemic species of conservation concern, Guadalupe bass (Micropterus treculii, ‘GB’), that was reintroduced in a restored urbanised riverscape already occupied by a generalist congener, largemouth bass (M. salmoides, ‘LMB’). We performed four seasonal surveys of habitat association and one capture-mark-recapture survey of the two species at eight sites distributed along a 17 km reach of the San Antonio River within San Antonio, Texas—the seventh largest city in the United States.
3. Detection of multiple size classes at all eight sites indicated that reintroduced GB have dispersed throughout this riverscape and are naturally recruiting. Juvenile GB were associated with restored riffles and transitioned to pools as adults—an ontogenetic habitat shift documented in rivers draining natural landscapes elsewhere in the GB native range. By contrast, juvenile and adult LMB were associated with pool habitats. Our results indicated that the construction of riffle habitats along this restored riverscape provide essential habitat for juvenile GB that was unavailable prior to restoration. Habitat overlap of adult bass indicated the potential for competition between the two species; however, GB body condition did not vary with LMB abundance across sites or seasons.
4. Next, we assessed long-term stressors by comparing dispersal barriers, hydrologic alteration, pollution proxies and fish kill frequencies in the urbanised restored riverscape to minimally-impacted riverscapes throughout the native GB range. The urbanised restored riverscape was subject to more barriers, flashier hydrology and more pollution; however, these stressors did not translate to more frequently-documented fish kills in our study.
5. We showed that restoration of instream habitat followed by reintroduction of native species enhanced urbanised biodiversity. Monitoring population responses to multiple urbanised stressors and mitigating those that threaten the long-term persistence of reintroduced species remains important.