Salmon spawning reduces benthic macroinvertebrate density and diversity in Great Lakes tributaries

Pacific salmon (Oncorhynchus spp.) were introduced to the Great Lakes in the 1960s to control invasive alewife and for sportfishing.  In Alaska, Pacific salmon provide nutrients during decomposition and disturb benthic communities during spawning, whereas in the Great Lakes these effects on stream ecosystems are poorly understood.  We tested the hypothesis that salmon redd construction decreases benthic macroinvertebrate density and diversity in two Upper Great Lakes tributaries: the Garden River, Ontario, and Thompson Creek, Michigan.  In each stream we sampled macroinvertebrates in a downstream reach receiving salmon and in a reach above a barrier to salmon migration.  Samples were taken from three subreaches within each reach before and immediately after the salmon run.  Results showed declines in macroinvertebrate biomass, density, and taxa richness after spawning, indicating that salmon have at least a short-term disturbance effect on benthic macroinvertebrate communities.  Although these changes may have a direct impact only at relatively small spatial and temporal scales, alterations in the availability and composition of invertebrate food items may result in altered feeding behavior among higher trophic levels, such as invertivorous fish and predatory and filter-feeding insects.