River engineers: Evidence that spawning fish can alter small-scale stream hydrology

Background/Question/Methods Salmonid fishes (trout, salmon, and chars) can act as ecosystem engineers by physically altering stream habitat when they excavate nests in the gravel (redds) where they lay their eggs. Through this process, fish may impact the substrate composition and sediment transport within a river system, as well as induce greater rates of water flow through the gravel. Such physical changes can in turn impact the habitat quality and nutrient availability for other organisms. Our overall objective was to evaluate how redd formation modifies intra-gravel hydrology in ways that may potentially impact survival of organisms living within the stream substrate, specifically salmonid embryos and larvae. We conducted our study in two neighboring streams with different flow regimes in order to capture a range of hydrologic and habitat variability found in natural systems. We manually excavated river gravel to imitate the size, shape, and depth of a typical salmonid redd and compared rates of intra-gravel water flow, dissolved oxygen, and temperature at several depths both inside and adjacent to the redds. Results/Conclusions We observed heterogeneity in all response variables both inside and outside of redds; however, we also observed faster rates of water flow and higher rates of dissolved oxygen within the redds as compared to gravel adjacent to redds. Furthermore, in most sites, these physical habitat characteristics changed very little throughout the six month duration of egg development within the redds, indicating that fish-induced hydrologic alterations may persist for relatively long periods of time. While this study focused on small-scale habitat alterations created by spawning salmonids, in areas where mass spawning occurs, salmonids may have much larger scale (e.g., 10 - 20,000 m^2 in a single Alaskan river) impacts on physical and associated biotic processes within the gravel. As such, global declines in these salmonid ecosystem engineers may have unforeseen impacts on stream hydrology and other benthic organisms.