Stayin' alive: Strategies to reduce predation on target species are context dependent

Abstract

Restoration and conservation efforts often aim to reduce predation, particularly by reducing interactions with nonnative predators in heavily disturbed and invaded ecosystems. For Chinook salmon Oncorhynchus tshawytscha , a highly valued but declining fishery, potentially useful freshwater rearing habitat restoration strategies may include elimination of deep‐water features, shallow off‐channel habitat enhancement, or provisioning cover such as large woody material features. However, reducing predation on target species requires a detailed understanding of existing variation in predation rates and context‐dependent effects of restoration techniques. In a series of stationary predation tethering assays, we found variation in relative predation rates on juvenile salmon across local and habitat level gradients of habitat suitability for juvenile salmon and nonnative fish predators in existing main channel habitats. Counter to expectations, relative predation rates were higher at locations characterized by small‐scale cover and near large woody material. Our results demonstrate that restoration efforts to shift habitat suitability to favor juvenile salmon would be expected to improve salmon survival during rearing and outmigration. However, not all restoration strategies are universally applicable, as large woody material and other forms of cover can produce counterproductive results under some conditions, possibly due to abundant nonnative predator species advantaged by heavily altered environments. Additional studies of spatiotemporal variation in predation risk and context‐dependent restoration impacts on predation rates are needed to inform restoration strategies and avoid unintentionally increasing risk to fragile populations.