Effects on sediments and biota of cleaning a salmonid spawning channel

Prior to the cleaning of the Little Qualicum chum salmon channel, the sediments, of which 33% were greater than 64 μm, were distributed fairly evenly throughout the 56 cm gravel column and occupied 25% of the interstitial space. Filamentous algae, especially Didymosphenia, were abundant in summer and reduced the diversity of the zoobenthos relative to that of the parent stream. The mean number of salmonid food-organisms was 30 000 m–2 (sampled with a 200 μm net); 57% were Chironomidae at the upstream end of the channel, increasing to 93% at the downstream end. Numbers of coho salmon fry, Oncorhynchus kisutch, km–1 of channel exceeded those of productive natural streams. Scarification of the channel resulted in removal of 88% of the sediments, reduction of arthropods by 98%, and reduction of emerging insects by 88% across the centre of the channel and by 90% at the margins. Coho salmon fry showed no significant mortalities during cleaning and lived on marginal aquatic insects until the channel was re-colonized 8 weeks later.