Long-residence groundwater effects on incubating salmonid eggs: low hyporheic oxygen impairs embryo development

Atlantic salmon (Salmo salar) ova were incubated at a range of burial depths (50-300 mm) in a set of test arrays inserted at previously used redd sites in a near-pristine stream. Oxygen saturation and electrical conductivity of hyporheic water (obtained 0.5-1.0 m distant from each array from a depth of 200-300 mm below streambed level) were determined at similar to2-week intervals over the incubation period (November-March). The test arrays were recovered from the streambed in mid-March, and thereafter, the eyed ova were incubated in surface water until hatch (24 March - 17 April). Embryo survival rates, hatch date, and alevin size differed among and within arrays. Mean alevin body size varied markedly among incubation groups (length, 14.9-20.8 mm; dry mass, 3.9-7.7 mg). Within arrays, embryo size varied with burial depth on fine spatial scales (approximately 50 mm); greater burial depth tended to be associated with smaller size. Among arrays, embryo size at the 250-mm depth was correlated with oxygen saturation of hyporheic water. Across all sites, oxygen saturation of hyporheic water decreased with greater conductivity, indicative of the presence of groundwater. Variation in embryo size was therefore linked with variation in the local dominance of deoxygenated groundwater in the hyporheic zone.