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

Abstract

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.

High mercury concentrations in steelhead/rainbow trout, sculpin, and terrestrial invertebrates in a stream-riparian food web in coastal California

“…Non-native species occur in many ecosystems and can have strong ecological effects, yet there has been very little research on their role in mercury dynamics in food webs or invertebrate subsidies to aquatic and terrestrial predators (Eagles-Smith et al 2018 ; Rundio and Lindley 2021 ). In Big Creek, non-native terrestrial isopods ( A. vulgare and P. scaber ) appear to be a potentially important source of mercury to trout due to the combination of their abundance in the diet (Rundio and Lindley 2008 , 2019 ) and high mercury concentration relative to most aquatic invertebrate taxa and many terrestrial taxa (Table 1 , Fig.…”

“…2 ). Non-native isopods have become established in temperate regions around the world and often reach very high densities (summarized in Rundio and Lindley 2021 ), and isopods are known to bioaccumulate heavy metals including mercury (Hopkin et al 1986 ; Dallinger et al 1992 ; Pedrini-Martha et al 2012 ), suggesting that they, and potentially other non-native species, may be important to mercury bioaccumulation and transfer in many areas.…”

“…Terrestrial invertebrates are a major prey source for stream-dwelling steelhead/rainbow trout ( Oncorhynchus mykiss ) in watersheds on the Big Sur coast of central California, providing 15–20% of the annual energy in the diet of age 0 (young-of-year) trout and up to 60% of the energy in age 1+ trout (Rundio and Lindley 2008 , 2019 , 2021 ). Steelhead populations in this region are listed as threatened under the U.S.…”

“…Our objectives were to determine the range of mercury concentrations among consumers within the food web and to test two basic hypotheses: (1) that concentrations would be higher in terrestrial invertebrates than aquatic invertebrates of the same trophic level; and (2) that, based on differences in diet, concentrations would be higher in age 1+ O. mykiss than in age 0 trout or sculpin, which consume primarily aquatic insects (Rundio and Lindley 2019 ; Moyle 2002 ). Among terrestrial invertebrates, we were particularly interested in mercury concentrations in non-native isopods, as they are a major prey item for trout in Big Creek (accounting for 20–30% of the annual energy in the diet, more than any other prey taxon; Rundio and Lindley 2008 ) and other Big Sur streams (Rundio and Lindley 2021 ) and are known to bioaccumulate heavy metals including mercury (Hopkin et al 1986 ; Dallinger et al 1992 Pedrini-Martha et al 2012 ).…”