Effect of Spawning Salmon on Seasonal Changes in Structure and Function of the Macroinvertebrate Community of Kennedy Creek

This study investigated the hypothesis that spawning salmon affect macroinvertebrates negatively and positively-the former due to the disturbance of redd excavation and the latter due to nutrients released during spawning and salmon carcass decomposition. To test this hypothesis, I monitored changes for five seasons in density, biomass, and salmon-derived carbon (C) and nitrogen (N) in the benthic macroinvertebrates of a small stream with a large run of chum salmon (Oncorhyncus keta). Stable isotope analysis showed that the macroinvertebrates contained salmon-derived C and N year around, as indicated by the results of pre-spawning samples: 20-41% salmon-derived C and 25-50% salmon-derived N, representing 22% of total macroinvertebrate biomass. Near the end of the spawning run, all macroinvertebrates sampled showed increases in salmon-derived C (41-68%) and N (51-87%) incorporated into their tissues; however, the total macroinvertebrate biomass decreased due to redd excavation. The percentage of salmon-derived C and N in macroinvertebrates remained high 3 months after spawning (49-88% and 60-97%, respectively). Because total macroinvertebrate biomass also increased, this period had the highest salmon-derived macroinvertebrate biomass (2.71 g m-2). Six months after spawning, there were no detectable differences in total macroinvertebrate biomass between reaches with and without salmon.