Aerial insect responses to non-native Chinook salmon spawning in a Great Lakes tributary

We investigated whether spawning by non-native Chinook salmon influenced aerial insect abundance in the riparian zone of Thompson Creek, a tributary of Lake Michigan, located in Michigan, USA. Specifically, we evaluated whether decades of salmon disturbance affected patterns of aquatic insect emergence, and how both live salmon and salmon carcasses influenced the abundance of terrestrial carrion flies. Retaining wall timbers from a low-head dam on Thompson Creek were removed, providing a unique opportunity to compare stream reaches that were exposed to the immediate ecological impacts of salmon (i.e., disturbance, subsidy effects) with reaches experiencing decades of spawning activity. Using sticky traps to collect aerial insects, we observed fewer adult aquatic insects in downstream reaches conditioned to decades of salmon disturbance in comparison to naïve upstream reaches. Reduced abundance in downstream reaches was primarily driven by taxa more susceptible to disturbance in the larval life stage (e.g., Diptera: Simuliidae, Ephemeroptera). A greater abundance of adult Chironomidae midges were detected in upstream reaches with higher numbers of spawning salmon and carcasses. Though abundance of adults differed between upstream and downstream reaches, we observed no evidence of early emergence. In addition, carrion fly abundance was greatest at reaches with more live and dead salmon. Evidence from our study suggests that non-native salmon have the potential to influence patterns of aerial insect abundance in riparian zones. Our findings suggest that non-native Chinook salmon can affect aerial insect assemblages; however, the propagating effects of these changes through riparian food webs warrant further investigation.