Salmon carcass retention in recently formed stream habitat

Habitat manipulation for migratory fish is typically aimed at fulfilling particular life-cycle stages and rarely acknowledges the wider role of these fish in lotic ecosystems. Recent research, indicating the importance of salmon carcasses in stream ecosystems of the Pacific Northwest has highlighted their potential contribution in restoring degraded rivers. Previous research has emphasised the importance of in-stream habitat features such as pools and debris dams for carcass retention in mature river systems but carcass retention in other river habitats has not been well studied. This study documents the spawning runs of pink salmon (Oncorhynchus gorbuscha) and the retention of their carcasses in the early successional habitat of a recently formed stream. Eight years after initial colonisation, the estimated number of pink salmon spawners exceeded 10,000. Overall carcass retention was low and strongly related to discharge, with most carcasses transported out of the freshwater system to the estuary within a few days. The majority of carcasses that were retained accumulated in the slower moving waters in the stream margins, whilst the spatially scarce habitats of boulders, shallow areas and coarse woody debris were associated with comparatively high carcass densities. Mark and recapture of carcasses indicated that, with the exception of coarse wood debris, the retention of carcasses was an ephemeral and highly dynamic process with the structurally simple retentive features of this stream associated with a high turnover of carcasses.