The Effect of Coho Spawning on the Benthic Invertebrates of the Platte River, Benzie County, Michigan

Abstract

This study was undertaken to determine what effects a spawning run of coho salmon would have on the benthos of the Platte River, Benzie County, Michigan. It was hypothesized that the physical disruption of the substrate, by spawning salmon, would reduce the density of benthos by dislodging the organisms and causing their downstream displacement. Estimates of density of benthic invertebrates were made in two control sections (salmon excluded) and in one experimental section (salmon having free access) in May and August 1967 prior to coho spawning run, and in December 1967 and May 1968 after the run. Coho spawning activity in the fall of 1967 disturbed the bottom materials. The densities of 12 taxa, as well as the total number and total weight of organisms per square foot, decreased significantly in samples collected in December after disruption. The decrease in total number of organisms per square foot in the experimental section as compared to the controls was 270.5; the decrease in weight in the experimental section was 1.97 grams. This is a percent decrease in the experimental section relative to the controls of 66% for total number and 78% for total weight. In May 1968, the number of organisms per square foot was still significantly lower in the experimental section (57.4 per square foot), but weight of organisms per square foot was not. The percent decrease in the experimental section relative to the controls for total number was 39%. Three of twelve taxa, whose density was reduced in the fall of 1967, were still lower in May 1968. Complete recovery of benthos was not observed in the 5-month period from December to May.