Michel, CJ, Henderson, MJ, Loomis, CM, Smith, JM, Demetras, NJ, Iglesias, IS, Lehman, BM, Huff, DD. Fish Predation on a Landscape Scale. Submitted to Ecological Applications.
Spatial patterns and environmental predictors of predator-prey dynamics are often investigated at discrete locations, limiting our ability to extrapolate to broader landscape scales. At these broader scales, landscapes often contain multiple complex and heterogeneous habitats, thus a spatially balanced sample throughout the different habitats is necessary to sufficiently represent them. This challenge is especially pronounced in California’s Sacramento-San Joaquin River Delta, where the landscape scale impact of fish predators on native prey fish is information needed by managers to protect multiple imperiled populations. During the spring of 2017, we quantified relative predation risk in the southern half of the Delta (South Delta) using floating baited tethers that record the exact time and location of predation events. We selected 20 study sites (1-km long) using a generalized random tessellation stratified survey design, which allowed us to infer relationships between the environment and predation across a broader spatial scale compared to previous studies. In addition to estimating relative predation risk at each study site, we also collected relevant environmental covariates including distance to nearest predators, water temperature, turbidity, depth, bottom slope, bottom roughness, water velocity, and distance to nearest riverbank and nearest submerged aquatic vegetation. We employed model selection to determine the environmental covariates that best predicted predation risk. Our most parsimonious model included water temperature, time of day, mean predator distance, and river bottom roughness. Using this model, we estimated predation risk for the entire South Delta at a 1-km resolution. This effort identified certain predation risk hotspots - the most important of which was corroborated by other studies - and allowed us to generate predicted survival rates for migrating fish transiting the South Delta. Because the 2017 water year was unusually wet, this methodology should be repeated across a suite of hydrologically distinct years to further develop the relationships.