The construction of salmon redds represents a localized but intense disturbance to benthic substrate and associated flora and fauna. Pink salmon (Oncorhynchus gorbuscha) have a fixed 2-y life cycle. Thus, their spawning run in Wolf Point Creek, a 1st-order coastal stream in southeast Alaska, alternates between high-density (odd years) and low-density (even years) returns. During July and August 1996 and 1998, counts of returning fish were low (<2000 fish) with little evidence of streambed disturbance by redd construction, whereas in 1997, counts of returning fish were >10,000 fish, and streambed disturbance was extensive. Coarse particulate organic matter and macroinvertebrate density and taxonomic richness were significantly lower in the main channel in 1997 than in 1996 or 1998. Macroinvertebrate densities and taxonomic richness were similar in all years in areas unsuitable for salmon redds (refugium habitat: margins, boulders, shallows). Macroinvertebrate drift density increased during the 1997 spawning run. Several taxa demonstrated circadian drift behavior, but the peak in redd construction activity coincided with an absence of nocturnal peaks for most macroinvertebrate taxa. Simulation of the abrasion and settling of material excavated during redd construction caused a modest but significant increase in total macroinvertebrate drift density compared with unmanipulated reference sites. Drift density declined after the cessation of experimental disturbance, a result suggesting that entombment and mortality associated with abrasion and settling was high and indicating that excavation, rather than abrasion and settling, was the most important process causing overall increase in drift density during redd construction. River habitat structure is a critical feature mediating the effects of disturbance associated with spawning salmon. Redd construction displaces macroinvertebrates and opens habitable space. Thus, it is an important determinant of community structure and function and could influence the course of community succession.
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Vol. 28 • No. 1