Effects of Substrate Size, Hyporheic Exchange, and Spawning Site Selection on Bull Trout Egg and Alevin Survival

Salmonid spawning success can be influenced by numerous environmental factors including dissolved oxygen concentration, substrate size, redd scour, and rates of interstitial water flow.  However, potential interactions between these factors, as well as the mechanisms that drive them, are commonly overlooked.  Our study assessed the collective impact of a suite of abiotic variables on egg and alevin survival of a threatened salmonid, bull trout Salvelinus confluentus.  We incubated a known number of eggs in man-made redds in three separate spawning streams with different geomorphic characteristics.  We assessed bull trout egg-to-fry survival in relation to variables measured within the egg pocket of each redd, including dissolved oxygen, pH, temperature, grain size distribution, and hydraulic conductivity.  We found a significant difference in survival rates among the three streams; survival was uniformly high in one stream ( 0.87 ± 0.09 [mean ± SD]), and variable in the other two (0.47 ± 0.34 and 0.27 ± 0.28).  We found consistent water quality among redds in all three streams and no evidence that dissolved oxygen was the limiting factor for egg and larval survival.  However, our results suggested that accumulation of fine sediment within redds negatively affected survival of eggs and alevins, but the effect of fine sediment varied depending upon additional hydrologic variables, including hydraulic conductivity (a measure of substrate permeability) and vertical hydraulic gradient.  These results suggest that multiple factors can concurrently impact spawning success, including complex interactions between hydrologic and geomorphic variables that create heterogeneous conditions within the hyporheic zone.  This research improves our understanding of factors affecting bull trout early life-stage survival, and demonstrates the importance of limiting fine sediment and maintaining hyporheic exchange in spawning habitat.