Evidence for spawning habitat preference of eulachon, an endangered anadromous smelt in the Fraser River, British Columbia

The anadromous smelt, eulachon Thaleichthys pacificus , population was historically abundant in the Fraser River. Extensive anthropogenic impacts in the river have induced both short-term and long-term changes to the hydraulic and sediment transport characteristics of the river channel, potentially changing the availability of eulachon spawning habitat. Despite these extensive anthropogenic impacts to the riverbed, there is limited understanding of spawning habitat utilization by eulachon in the river. Our purpose was to characterize spawning habitat preference in Fraser River. Available habitat was characterized in 3 locations using a combination of multibeam sonar to describe bathymetry, side scan sonar to describe surficial bedforms, and sediment sampling to describe substrate composition. Artificial spawning substrate were deployed in available habitats to measure egg deposition rate throughout the migration period. Results show that of the 5 habitat types we examined, smooth bedform features composed of coarse sands and medium gravels were the most productive habitat type selected for spawning based on egg deposition. Un-eyed egg deposition followed a unimodal distribution pattern throughout the adult migration period. Drifting eyed egg deposition increased towards the end of the adult migration period, indicating downstream drift of eggs spawned further upstream. This study highlights the value of incorporating surficial bedform features and substrate composition to characterize and assess spawning habitat selection and preference and demonstrates the importance of incorporating egg developmental stage to distinguish spawning habitat use. River management decisions will need to consider both surficial bedforms and substrate composition when assessing and protecting critical spawning habitat.