Environmental drivers of salmonfly ecology in southwest Montana

Abstract

Aquatic insects have ecological, cultural, and economic value throughout the American West. They can control the processing of in-stream nutrients, are a vital component of both aquatic and terrestrial food webs, and support economically important species such as trout and the eco-tourism industries structured around these fisheries. Salmonflies (Pteronarcys californica) are one of the most well-known aquatic insects in the American West due to their large size and popularity among fly-fishers. However, mounting anecdotal evidence suggests that salmonfly populations could be in decline. We conducted surveys and compiled historical datasets that quantified salmonfly abundance, body size, and emergence timing along the Gallatin and Madison Rivers in southwest Montana to determine the status of salmonfly populations in these rivers, understand environmental drivers that are constraining their distribution and driving their development, and initiate long-term monitoring. Most notably, we found evidence for temperature-driven changes in salmonfly distribution and body size along the Madison River in the last four decades and observed marked differences in salmonfly emergence phenology and duration at multiple spatial scales between the Madison and Gallatin Rivers. Above-optimal summer water temperatures appear to be the major constraint on salmonfly populations in the Madison River, but only play a minimal role in dictating salmonfly distribution along the Gallatin River. This research provides rare empirical evidence of long-term biological change of an aquatic insect and highlights the importance of combining temporal and spatial datasets to explicitly address species' responses to environmental stressors across multiple spatial and temporal scales. Freshwater habitats are increasingly imperiled by climate change and human-induced habitat alteration, which will invariably continue to impact the ecology of aquatic insects like salmonflies. This work contributes to the understanding of how these ongoing changes will influence the structure of aquatic communities, the flow and transfer of energy and nutrients, consumer-resource dynamics, and stream--riparian food web linkages.