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.Received February 12, 2015; accepted November 8, 2015 Published online March 30, 2016
Extreme variability in abundance of California salmon populations is often ascribed to ocean conditions, yet relatively little is known about their marine life history. To investigate which ocean conditions influence their distribution and abundance, we surveyed juvenile Chinook salmon (Oncorhynchus tshawytscha) within the California Current (central California [37°30 0 N) to Newport, Oregon (44°00 0 N]) for a 2-week period over three summers (2010)(2011)(2012). At each station, we measured chlorophyll-a as an indicator of primary productivity, acoustic-based metrics of zooplankton density as an indicator of potential prey availability and physical characteristics such as bottom depth, temperature and salinity. We also measured fork lengths and collected genetic samples from each salmon that was caught. Genetic stock identification revealed that the majority of juvenile salmon were from the Central Valley and the Klamath Basin (91-98%). We constructed generalized logistic-linear negative binomial hurdle models and chose the best model(s) using Akaike's Information Criterion (AIC) to determine which covariates influenced the salmon presence and, at locations where salmon were present, determined the variables that influenced their abundance. The probability of salmon presence was highest in shallower waters with a high chlorophyll-a concentration and close to an individual's natal river. Catch abundance was primarily influenced by year, mean fork length and proximity to natal rivers. At the scale of sampling stations, presence and abundance were not related to acoustic indices of zooplankton density. In the weeks to months after ocean entry, California's juvenile Chinook salmon population appears to be primarily constrained to coastal waters near natal river outlets.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.