Habitat Alterations and a Nonnative Predator, the Striped Bass, Increase Native Chinook Salmon Mortality in the Central Valley, California

Anthropogenic stressors are the leading causes of species and biodiversity declines, driving wide-scale ecosystem changes. Additionally, synergistic effects of multiple anthropogenic modifications, including species introductions and habitat alterations, can have complex outcomes for native species. We assessed how a nonnative predator (the Striped Bass Morone saxatilis) and habitat alterations (a small diversion dam and other altered habitats) interacted to influence mortality of native juvenile Chinook Salmon Oncorhynchus tshawytscha during their emigration from the lower Mokelumne River, California. Relative abundance and diet surveys across natural and human-altered habitats were used to assess Striped Bass functional and aggregative responses. Per capita consumption (PCC) of juvenile salmon and behavioral aggregation (CPUE) by Striped Bass at a small diversion dam (Woodbridge Irrigation District Dam [WIDD]) were elevated in comparison with those at other altered and natural habitats (WIDD: PCC = 3.54 juvenile salmon, CPUE = 0.189 Striped Bass/s of electrofishing; other altered habitats: PCC = 0 juvenile salmon, CPUE = 0.0024 Striped Bass/s; natural habitats: PCC = not estimable, CPUE = 0.0003 Striped Bass/s). Increased aggregative and functional predator responses created a localized area of heightened predation at WIDD. At this predation hot spot, we used three approaches (experimental Striped Bass removals, diet energetic analysis, and before-after impact assessment) to estimate Striped Bass consumption at 8-29% of the emigrating juvenile salmon population. Striped Bass PCC rates for juvenile salmon as determined by the three approaches were 0.92% (predator removals), 0.71-1.20% (diet energetic analysis), and 0.96-1.11% (before-after impact assessment). Our results (1) illustrate how the synergistic effect of habitat modification and a nonnative predator can exacerbate the mortality of native juvenile salmon during their emigration and (2) highlight the importance of considering interactions among stressors when planning local management strategies and assessing population-level impacts on salmon.