Gene Flow Among Modoc Sucker and Sacramento Sucker Populations in the Upper Pit River, California and Oregon

The Modoc sucker Catostomus microps received legal protection in the United States based partially on concerns that anthropogenic environmental changes had restricted migration among populations and catalyzed hybridization with a more abundant congener, the Sacramento sucker Catostomus occidentalis. We applied eight microsatellite markers to samples of both species collected from two tributaries to the Pit River, California (Ash Creek and Turner Creek), and one tributary of Goose Lake, Oregon (Thomas Creek). Modoc sucker populations in these three tributaries seemed to be largely isolated from one another: gene flow between Ash Creek and Turner Creek was no greater than that among these two creeks and Thomas Creek. In contrast, divergence estimates among collections of Sacramento suckers indicated greater gene flow between Ash Creek and Turner Creek than between either of these creeks and Thomas Creek. Samples collected at a single site (Ash Valley) were identified based on morphology as Modoc suckers, but genetic data suggested they were much more similar to Sacramento suckers. Interspecific hybrids were detected in all three tributaries. Collections of Modoc suckers yielded 0.0–3.9% hybrids, and collections of Sacramento suckers yielded 0.0–80.0% hybrids. The two collections with the greatest proportions of hybrids (54.5 and 80.0%) were both from tributaries to lower Thomas Creek, and neither of these tributaries is thought to have upstream populations of Modoc suckers. Based on 1) low levels of hybrid detection in all three tributaries, 2) the absence of hybrids from typical parental habitats (upstream habitats for Modoc suckers and Pit River mainstem for Sacramento suckers), and 3) highly significant RST (variance in allele size) values between the species, we conclude that hybridization is common but that significant introgression (i.e., loss of parental genotypes) has not occurred. We also note that hybridization, and subsequent introgression, may become a conservation concern in such cases when the habitat of one or both of these species is eliminated or modified.

Changes in Habitat and Microhabitat Partitioning within an Assemblage of Stream Fishes in Response to Predation by Sacramento Squawfish (Ptychocheilus grandis)

The Sacramento squawfish (Ptychocheilus grandis), a piscivorous cyprinid, was recently (ca. 1979) introduced into the Eel River, California, USA. We compared habitat and microhabitat use of resident fishes between areas where squawfish were present and absent at one location and between years before and after invasion by squawfish at a second location. The resident species showed a variety of responses to the presence of the predator. Juvenile rainbow trout (Oncorhynchus mykiss) and juvenile Sacramento suckers (Catostomus occidentalis) increased their use of fast-flowing riffles at both locations, in the presence of squawfish. Suckers also used significantly shallower depths within habitat types. Adult California roach (Lavinia symmetricus) decreased their use of run habitat and increased use of pools and riffles at the first site, but not at the second. When squawfish were present, juvenile roach and threespine stickleback (Casterosteus aculeatus) were found in shallower water closer to the stream edge in all habitats. Spatial overlaps tended to be lower in the presence of squawfish. The introduction of squawfish has resulted in changes in the habitat and microhabitat use of the resident fish assemblage but no loss of species.