Over the last decade, fishing tackle has been documented in the digestive tracts of White Sturgeon Acipenser transmontanus in several fisheries in the Columbia River basin, raising concerns about the prevalence and types of tackle being consumed and the ability of these long-lived fish to shed such metal in a timely manner. We scanned 2,077 White Sturgeon with a metal detector and X-rayed (with a portable X-ray system) 443 fish in the Hells Canyon reach of the Snake River to characterize the incidence, quantity, and type of hooks and other metal fishing tackle that had been ingested. We also estimated the amount of time for fish to eliminate ingested metal and to ingest new metal. For the fish we captured, which averaged 118cm and ranged from 47 to 287cm (fork length), 21% contained metal in their digestive tract, with smaller fish (<100cm) less likely to contain metal (10%) than larger fish (>100cm; 36%). Much of the metal in the digestive systems of White Sturgeon was fishing tackle that was not gear targeting White Sturgeon and included large and small hooks, jigs, swivels, and pieces of broken metal. White Sturgeon with metal in their digestive systems, on average, weighed slightly less than fish without metal, suggesting that metal may slightly hinder food consumption or assimilation, though this affect was apparently short lived. White Sturgeon X-rayed at least twice over the course of the study were able to pass ingested metal on average in 492d, but one piece of metal was retained for at least 1,266d. White Sturgeon ingested new metal on average every 575d. In summary, our results suggest that White Sturgeon effectively process the metal they ingest, and that most if not all their metal passage is by oxidation through the digestive system.

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.

This paper describes a simulation study of reconnection options for white sturgeon Acipenser transmontanus subpopulations in adjacent river segments above and below CJ Strike Dam on the Snake River, Idaho, USA. In contrast to the downstream river segment, the upstream river segment is long and has areas that are suitable for spawning during normal and wet hydrologic conditions. We evaluated demographic and genetic consequences of upstream and downstream passage using different model assumptions about trashrack spacing and density-dependent effects on the spawning interval. Our genetic results predict that, although reconnection would introduce new alleles to the upstream subpopulation, it would also preserve alleles from the downstream subpopulation by propagating them in the larger subpopulation above the dam. Our demographic results predict that halving the space between trashracks would have large and unequivocal benefits, whereas the predicted effects of reconnection were smaller and more sensitive to model assumptions. Simulated upstream passage tended to benefit both subpopulations only in the absence of density-dependent limitation. In the presence of density dependence, the combination of halved trashrack spacing and upstream and downstream passage produced the best results. Narrower trashracks kept spawning adults in the upstream segment with spawning habitat, while allowing their progeny to migrate downstream. Screening appears to be the best option for such a species in this configuration of a long river segment acting as a demographic source above a short one acting as a demographic sink.

Five of the nine populations of white sturgeon Acipenser transmontanus, located between dams on the Middle Snake River, have declined from historical levels and are now at risk of extinction. One step towards more effectively protecting and managing these nine populations is ranking factors that influence recruitment in each of these river segments. We developed a model to suggest which of seven mechanistic factors contribute most to lost recruitment in each river segment: (1) temperature-related mortality during incubation, (2) flow-related mortality during incubation, (3) downstream export of larvae, (4) limitation of juvenile and adult habitat, (5) mortality of all ages during summer episodes of poor water quality in reservoirs, (6) entrainment mortality of juveniles and adults, and (7) angling mortality. We simulated recruitment with, and without, each of the seven factors, over a typical series of hydrologic years. We found a hierarchical pattern of limitation. In the first tier, river segments with severe water quality problems grouped together. Poor water quality during summer had a strong negative effect on recruitment in the river segments between Swan Falls Dam and Hell's Canyon Dam. In the second tier, river segments with better water quality divided into short river segments and longer river segments. Populations in short river segments were limited by larval export. Populations in longer river segments tended to be less strongly limited by any one factor. We also found that downstream effects could be important, suggesting that linked populations cannot be viewed in isolation. In two cases, the effects of a factor on an upstream population had a significant influence on its downstream neighbors.