Importance of non‐native isopods and other terrestrial prey resources to steelhead/rainbow trout Oncorhynchus mykiss in coastal streams in Big Sur, California Ecology of Freshwater Fish

Terrestrial invertebrates provide important prey subsidies to many stream fishes. Non‐native invertebrates are widespread in many ecosystems, yet they have received little attention in studies of subsidies to stream food webs. We sampled coastal basins in Big Sur, California, to determine the importance of non‐native isopods and other terrestrial invertebrates in the summer diet of steelhead/rainbow trout (Oncorhynchus mykiss) and to estimate the density of isopods along streambanks. Terrestrial invertebrates contributed more than 40% of the energetic value of O. mykiss stomach contents at most sites and up to 75% at some locations. Non‐native isopods Armadillidium vulgare or Porcellio scaber occurred at all sites, reaching mean densities of up to 13 individuals/m2 on streambanks, and accounted for up to 20% of the energy in the diet. The proportion of non‐native isopods in the diet was positively correlated with their density at a site, and the frequency and energetic value of terrestrial invertebrates and non‐native isopods in O. mykiss diets increased with fish size. In terms of the broader diet, the energetic value and taxonomic composition of stomach contents varied among sites but there was no major geographic pattern or trend to the variation, and overall O. mykiss diets were generally similar across the region. Our results highlight that non‐native terrestrial invertebrates may provide a considerable but overlooked subsidy to stream fishes.