Sand Seals in Coho Salmon Redds: Do They Improve Egg Survival?

Abstract

We investigated whether sand seals form in the upper egg pocket of salmonid redds and improve egg survival in a sediment-impacted coastal stream in northern California. Sand seals can potentially reduce infiltration of detrimental finer sand and silt into the lower egg pocket. We predicted sand seals would form when the redds were exposed to streamflows high enough to entrain coarse sand and form seals. Using artificial redds of coho salmon Oncorhynchus kisutch, we conducted sediment analyses of the upper and lower egg pockets and found that protective sand seals formed in redds when discharge was two or more times the flow that entrains the median particle size of the streambed. When the coarse sand in the upper egg pocket was incorporated into a two-stage model that predicted survival to hatching and emergence, it greatly improved predictions in years with higher flows in both natural and artificial redds. However, the sand seals provided little protection when suspended sediment flux was high from logging or road construction. We built the model used for these predictions with data from artificial redds and applied it to natural redds during 6 years of different flow regimes. Predictor variables included cumulative flow above the entrainment flow, peak discharge, coarse sand in the upper egg pocket, fine sediment in riffle gravel, coarse sand in the upper and entire egg pocket, suspended sediment flux, and presence of predaceous worms. The model explained 67% of the variance in egg survival to emergence in coho salmon natural redds. We suggest that managers consider the complex interactions of streamflow, sediment transport, formation of sand seals, and fine-sediment infiltration when estimating salmonid reproductive success in sediment-impacted streams.