Trophic ecology and population attributes of two resident non-game fishes in riverine habitat engineered to enhance salmon spawning success

Habitat enhancement efforts meant to improve conditions for fish in degraded rivers have the potential to impact all resident fishes - not just the focal population. Yet post-project ecological monitoring is often inadequately conducted, and in cases where restoration targets a single species, the impact on non-target fishes may be neglected. We investigated the diet and population attributes of two non-focal, resident fish species, the Sacramento pikeminnow (Ptychocheilus grandis) and prickly sculpin (Coitus asper), upstream of and within engineered habitat (hereafter the reference and restored reaches, respectively) intended to improve Chinook salmon (Oncorhynchus tshawytscha) spawning success. Population density, body size, stomach fullness, and prey importance were compared between populations residing in the reference and restored reaches. We also determined if the primary carbon source that supported common pikeminnow and sculpin prey differed between reaches using stable isotope analysis. Pikeminnow residing in the restored reach were significantly larger but less densely populated, exhibited greater condition factor values, and consumed more large-bodied prey. In contrast, sculpin in the restored reach were significantly smaller and more densely populated relative to the reference reach. Analysis of isotopic signatures suggests that macroinvertebrates supporting fish populations as prey principally depend on diatomaceous algae in the restored habitat, while filamentous algae were most important in the reference reach. Results suggest that restored salmonid habitat may represent significantly different environmental settings for non-target fish species, with consequences for population structure and diet.