Emergence of Atlantic Salmon Fry in Relation to Redd Sediment Infiltration and Dissolved Oxygen in Small Coastal Streams

Fine sediment infiltration is widely discussed as a stressor to wild salmonids’ spawning success, but its mechanisms and severity in wild salmonid redds are difficult to measure. This study examined wild Atlantic salmon (Salmo salar) redd survival using emergence traps in two small coastal streams with differing agricultural land-use intensities on Prince Edward Island, Canada. Measured environmental parameters included stream and hyporheic dissolved oxygen, water velocity, water level, redd temperature, redd substrate composition, and stream suspended solids. Wild Atlantic salmon redds were equipped with emergence traps during May to June in two study years to evaluate survival. No single environmental factor was strongly associated with the success of individual redd emergence. However, the West River exhibited approximately two-fold-higher portions of silt and clay in redd substrates. Despite this, only modest reductions in hyporheic dissolved oxygen were observed and appeared to be related to high discharge events rather than sediment accumulation. Mortality rates were highly variable across all study sites on both rivers, which may be attributed to fertilization success rather than environmental conditions, as natural mortality was at least 50%. Entombment of alevin from accumulated fine sediments was noted in several redds on the West River, suggesting this mechanism may contribute to mortality even when oxygen levels are adequate. Overall, the study highlights the resilience of salmon embryos with moderate hypoxic episodes and the challenges of linking sediment metrics to mortality in wild Atlantic salmon redds.