The effect of salmon redd excavation on stream substrate and benthic community of two salmon spawning streams in Canterbury, New Zealand

Two neighbouring, stable spring-fed streams were studied during January-September 1982. The Glenariffe stream was 22 km long, with a mean flow of 6 m3 per s, and a mean salmon population of 883 spawning pairs per year. The Upper Double Hill stream was 1.5 km long, with a mean flow of 1.3 m3 per s, and an average annual salmon run of 140 spawning pairs. Redd excavation by female quinnat salmon (Oncorhynchus tshawytscha walbaum) modified the contours of the stream bed (parallel riffle lines were formed at right angles to the banks), removed fine substrate particles and detritus, and decreased benthic standing crop by an average 62-73 per cent. Invertebrate abundances were lowest in May-June, but recovery was rapid and recolonization was almost complete by September. Only minor changes in benthic community structure were observed.