The unimpounded lower Columbia River downstream from Bonneville Dam (LCR) supports the greatest abundance and density of white sturgeon Acipenser transmontanus reported in the species' range. The high productivity of the population resulted from growth that was as good as or better than reported for other populations, the highest mean relative weight reported for any white sturgeon population, and a relatively low median age (24 years) of first maturity for females (95% of females matured between 16 and 35 years of age). Estimated instantaneous natural mortality was 10%. During 1986-1990, the estimated instantaneous fishing mortality for age-12-17 fish averaged 36% in LCR fisheries. Over the same period, the average annual abundance estimate of LCR white sturgeon 54 cm fork length (FL) or longer was 895,500 fish, and the average density was 14.6 fish per ha. Population simulations, under the assumption of constant recruitment, predicted a maximum sustainable yield (MSY) of 3.0 kg/recruit at an 18% exploitation rate (of the 82-166-cm FL population). Reproductive potential was 93,400 eggs/recruit for an unexploited population and 4,800 eggs/recruit at the predicted MSY exploitation rate. The factors most responsible for the favorable production potential of the population were access to marine areas, abundant food resources, and consistently favorable hydrologic conditions during the spawning period.

Damming of large rivers in the U.S. Pacific Northwest and Canada has divided the historical population of white sturgeon Acipenser transmontanus into more than 36 fragmented populations, few of which are thriving. We now face the challenge of managing these populations to avoid extirpation. Two goals of this study were to identify extinction thresholds related to small size and inadequate habitat for this species. The minimum viable population size (MVP) is the threshold size above which populations support recruitment and grow and below which populations fail to support recruitment and decline. We estimated a single, cross-population MVP using data from multiple populations and quantile regression, which removed the effects of factors other than population size. Only two populations (those in the Bonneville and Dalles reservoirs on the Columbia River), both with significant increasing trends, were larger than our MVP estimate. We detected significant decreasing trends in two populations-those below Bonneville Dam and in the Kootenai River. To discover how site-specific differences in river habitat influence MVP, we used a population viability analysis (PVA) model that incorporated Allee mechanisms. The PVA model identified a river segment length below which extinction was certain regardless of initial population size. Above this threshold, simulated populations in river segments that were longer or that provided more frequent recruitment opportunities were able to persist with smaller initial sizes. Two priorities emerged for white sturgeon: monitoring age structure and understanding the circumstances preventing recruitment to age 1. Our results ultimately guided us toward thresholds in rearing habitat and age structure that promise to develop into more useful conservation tools than MVP for this and similar long-lived species.

Summary A sanctuary targeting the time and space occupied by reproductively active white sturgeon, Acipenser transmontanus, during spawning can be an effective method for reducing angling‐related stressors. A catch and release (C&R) fishery targeting over the legal limit (oversize; ≥137 cm fork length) white sturgeon had been intensifying since 1990 and grew in popularity through the 2000s in the lower Columbia River. A research project to describe the reproductive structure of the adult white sturgeon population below Bonneville Dam (rkm 220‐233) was initiated in 2000 and provided the first detailed observations of trauma linked to this C&R fishery. Fish captured from 2003 to 2011 were examined for evidence of in‐season angling damage (hook marks, leaders and lines in buccal cavity) from June through August each year. Carcass surveys were conducted weekly from June through August within a 24.8 km stretch downstream from Bonneville Dam. Catch rates were determined through angler interviews and aerial counts of boat and bank anglers. Four major regulation changes occurred during the course of the study affecting the seasonal spawning sanctuary in space and time. A total of 679 oversize white sturgeon were examined for evidence of in‐season angling damage. The proportion of individuals with evidence of angling, damage indicators per individual, number of carcasses found and the number of carcasses with observed evidence of angling or retained gear, and the total number of oversize fish handled by boat anglers declined after each fishing regulation change. The trends suggest that the sanctuary influenced the number of encounters between anglers and white sturgeon and that increases to the sanctuary in time and space reduced stress endured by oversize white sturgeon.

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