Importance and seasonal availability of terrestrial invertebrates as prey for juvenile salmonids in floodplain spring brooks of the Kol River (Kamchatka, Russian Federation)

Energy and resource fluxes between terrestrial and aquatic habitats of river flood plains can have reciprocal influences on food webs. Floodplain spring brooks may contain high densities (>1 m −2 ) of juvenile fish even when aquatic food resources appear to be low. A likely explanation is the allochthonous energy subsidy from riparian vegetation (e.g. leaves, invertebrates). To quantify this relationship in a rich salmon river ecosystem, we measured the temporal and spatial flux of terrestrial invertebrate and aquatic food resources in relation to diets of the two most abundant salmonid species in two spring brooks within early (<20 years) and late (>50 years) successional riparia on an expansive coastal flood plain of the Kol River, Kamchatka. Standing crops of benthic invertebrates and density of drifting invertebrates (dry mass) were fairly low (<4 g m −2 and <1 mg m −3 , respectively) at both sites, but annual average terrestrial invertebrate input among sites was 22.2 ±  0.1 mg m −2  day −1 . On average 68% of the coho salmon diet was of terrestrial origin, contrasting with 13% for Dolly Varden char at both sites, thus showing resource partitioning. Diet varied temporally and spatially with prey availability. Specifically, larvae of a weevil (Curculionidae) feeding on willow catkins were a dominant diet item for coho in the spring but only at the early successional site; Dolly Varden also ate these weevils but aquatic invertebrates continued to dominate their diets, further demonstrating behavioural segregation. The results show the importance of channel‐floodplain connectivity to management and conservation of salmon rivers. Copyright © 2009 John Wiley & Sons, Ltd.