Environmental Controls on Salmonid In-Redd Survival

Salmonids burry their eggs in open gravel structures (redds), beneath the streambed in the hyporheic zone. Survival between spawning and hatch depends on a range of environmental controls, but critically the delivery of adequate dissolved oxygen to meet the needs of developing embryos. Historically, studies of in-redd survival have focussed almost exclusively on the role of fine sediment intrusion in controlling spawning success, the assumption being that this reduces interstitial velocity and consequently oxygen delivery. However, a number of studies have also suggested that this relationship should not be accepted uncritically. More recently a greater consideration and understanding of hyporheic zone processes has revealed that a previously unrealised range of processes and controls influence embryo survival. The hyporheic zone is a spatially and temporally variable dynamic environment where groundwater (GW) and surface water (SW) mix and water quality depends on the relative contribution of source waters, their physico-chemical characteristics and in-situ biogeochemical processes. Importantly for embryo survival, long-residence groundwater can often be characterised by low dissolved oxygen (DO) concentrations.

We investigated the processes controlling hyporheic water quality and salmonid embryo survival at nested spatial and temporal scales in an upland Scottish catchment with a documented history of Atlantic salmon spawning. Spatial scales of investigation ranged from catchment (ca. 30km²) to reach (ca. 300 m²) to redd (<1m²) to the scale associated with individual egg pockets (ca. 5cm). Temporal scales ranged from inter-annual to individual hydrological events. At the catchment scale hydrochemical investigations revealed a wide range of hyporheic DO conditions which mirrored local GW contributions. At the reach scale, chemical tracers combined with temperature and hydraulic head data facilitated more detailed process-based understanding of hyporheic processes which revealed the importance of local sedimentary structure and channel geomorphology in controlling local hydrological exchange. At the finest spatial scales (<1m), small volume hyporheic sampling and in-situ continuous DO measurement techniques revealed compressed hydrochemical gradients suggesting a temporally shifting GW-SW boundary with associated changes in DO. At all spatial scales embryo survival and performance scaled directly with observed dissolved oxygen and consequently also with local GW-SW interactions. Investigation of the relationships between DO, velocity and survival at the finest spatial scales suggested that DO is a relatively good predictor of embryo survival, but that velocity was a relatively poor predictor indicating the dangers of focussing on single processes in environments that are spatially and temporally heterogeneous and complex.