UC Santa Cruz

Multiple human stressors including non-native species and habitat alterations can interact with complex consequences on native species. Human-modified habitats can change non-native predator functional and aggregative responses with additive impacts on native prey species. I assessed how the non-native predator, striped bass (Morone saxatilis), and habitat alterations (small diversion dam and other altered habitats) interact to influence mortality on native juvenile Chinook salmon (Oncorhynchus tshawytscha) migrating to sea on the lower Mokelumne River, CA (USA). Relative abundance and diet surveys across natural and human-altered habitats assessed functional and aggregative responses of striped bass. Striped bass showed elevated per capita consumption of juvenile salmon and behavioral aggregation (estimated as catch per unit effort - CPUE) at a small diversion dam site (Woodbridge Irrigation District Dam: per capita consumption= 3.54 juvenile salmon per striped bass and CPUE= 0.189) over other altered (0 juvenile salmon per striped bass; CPUE= 0.0024) and natural habitats (N/A; CPUE= 0.0003) creating a localized area of heightened predation. At this predation hotspot, experimental predator removals, diet energetic analysis, and before-after impact assessment estimated striped bass consumption of the population of out-migrating juvenile salmon to be between 10-29%. Striped bass per capita consumption rates among the three approaches were 0.92%, 1.01-1.11%, and 0.96-1.11% respectively. This study highlights how interactions between multiple stressors can exacerbate consequences for native species and are important to examine when predicting ecological impacts from stressors and planning local management strategies.