Diagnosing causes of white sturgeon (Acipenser transmontanus) recruitment failure and the importance of substrate condition to yolksac larvae survival

White sturgeon are a conservation concern throughout their range, and three populations in Canada are experiencing nearly zero recruitment. While the presence of large dams on the Nechako, upper Columbia and Kootenay rivers suggests a causal relationship between flow regulation and recruitment failure, persistent uncertainty regarding the species’ biology and its interaction with anthropogenic habitat change limits the understanding of the causal mechanisms of recruitment failure. Recruitment failure causation in the Nechako and upper Columbia rivers was evaluated by comparing hindcasted recruitment with historical environmental changes. For the Nechako River population, recruitment failure coincided with substrate changes at a known spawning site; these changes were identified based on air photos and changes in river bed elevation. Evaluation of the upper Columbia River population considered the effects of multiple dams and other anthropogenic impacts, as well as the effects of sturgeon aging error and population structure. Eleven recruitment failure hypotheses were evaluated based on fifteen criteria, including stock specific temporal and spatial patterns of recruitment decline. Estimated timing of stock-specific recruitment declines was also compared with reach and watershed scale environmental changes and site-specific habitat changes. For both analyses, substrate change at spawning sites was identified as the likely cause of recruitment failure. Embryos and yolksac larvae are expected to be the primary life stages affected in both cases. Laboratory and field enclosure studies assessed the effects of substrate on drift, hiding and survival of yolksac larvae. Contrary to many prior investigations, interstitial hiding was the predominant behaviour in laboratory studies in the presence of suitable interstitial habitat, and both drift and predation by sculpins (Cottus spp.) increased when interstitial habitat was unavailable. Rapid and sustained hiding by yolksac larvae and increased survival all suggest that hiding predominantly occurs in the vicinity of spawning habitat. Collectively my results indicate that recruitment failure results from increased early life history mortality in response to substrate changes at spawning sites. Increased survival of yolksac larvae in response to gravel augmentation in field studies suggests that larger-scale substrate restoration at spawning sites provides a habitat based approach for recruitment restoration.Science, Faculty ofZoology, Department ofGraduat