There is uncertainty about the importance of various factors in explaining declines of chinook salmon (Oncorhynchus tshawytscha) populations in the Snake River basin of Oregon and Idaho. This uncertainty has prevented implementation of long-term recovery actions for these stocks. We used simulation models and decision analysis to evaluate three management actions for seven index stocks of Snake River spring and summer chinook salmon: (i) continue current operation of the Columbia River hydropower system, (ii) maximize transportation of smolts, and (iii) natural river drawdown (breaching) of four Snake River dams. Decision analysis provided a useful approach for including multiple hypotheses about population responses to environmental and anthropogenic factors, systematically assessing the importance of alternative hypotheses, and identifying risk-averse recovery strategies that meet survival and recovery goals over a wide range of uncertainties. We found that the most influential uncertainties were related to hypothesized causes of estuary and ocean mortality. Current monitoring provides limited information on survival in this life stage; carefully designed management experiments are more likely to generate useful information. Given that these uncertainties exist, drawdown was the most risk-averse action, meeting long-term survival and recovery goals over a wider range of assumptions than the other actions.Résumé : Il existe de l'incertitude au sujet de l'importance des divers facteurs explicatifs du déclin des populations de Saumons quinnat (Oncorhynchus tshawytscha) dans le bassin versant de la rivière Snake en Oregon et en Idaho. Cette incertitude a empêché la mise en oeuvre de stratégies de récupération à long terme de ces stocks. Des modèles de simulation et des analyses décisionnelles ont permis d'évaluer trois stratégies de gestion pour sept stocks indicatifs de saumons de printemps et d'été de la rivière Snake : (i) ou bien la poursuite de l'opération actuelle du système hydroélectrique du fleuve Columbia, (ii) ou alors la maximisation du transport des saumoneaux, (iii) ou enfin la baisse des eaux de la rivière à leur niveau naturel par ébrèchement de quatre barrages de la rivière Snake. L'analyse décisionnelle est une méthode utile pour incorporer des hypothèses multiples au sujet des réactions des populations aux facteurs environnementaux et anthropiques, pour évaluer systématiquement l'importance des hypothèses de rechange et pour identifier les stratégies de récupération qui évitent les risques tout en atteignant les objectifs de survie et de récupération sur une gamme étendue d'incertitudes. Les incertitudes les plus sérieuses concernent les causes présumées de la mortalité dans l'estuaire et l'océan. Le réseau de surveillance actuel ne fournit que peu de renseignements sur la survie à cette étape du cycle biologique; des expériences de gestion planifiées avec soin seraient plus susceptibles de générer des informations utiles. Étant donné l'existence de ces incertitudes, la baisse des niveaux des rés...

We used spawner-recruit data to estimate the instantaneous differential mortality (µ) experienced by seven Snake River spring and summer chinook (Oncorhynchus tshawytscha) stocks relative to six lower Columbia River stocks. We applied 37 Ricker stock-recruit models to these data, incorporating different assumptions about measurement error, transport survival, intrinsic productivity, methods of estimating µ, and common year-effects that affect the survival of all stocks. Estimates of mean µ for the 12 best models ranged from 0.55 to 1.90 (mean of 1.09), implying that passage from Lower Granite Dam to John Day Dam reduced recruitment of 1970-1990 Snake River broods by an average of 42-85% (mean of 66%). Differential mortality was cyclical and moderately high in the 1970s brood years, low for 1980-1983, near average in 1984-1989, and high in 1990. Our empirical estimates of µ showed low bias and were between those produced by two mechanistic passage models. The best empirical models included common yeareffects, which shifted from generally positive effects on 1952-1968 brood years to generally negative effects on 1970-1990 broods. Year-effects were not significantly correlated with two climate indices or with water travel time (the time that water takes to travel down the Columbia River).

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We developed an age-structured simulation model incorporating pH-dependent mortality to assess impacts of acidification on Atlantic salmon (Salmo salar) populations at the reach and river system level. Applied to the LaHave River, Nova Scotia, acidic reaches (mean annual pH 4.6–5.1) were predicted to have 0–15% of the annual smoit production and 0–31% of the maiden spawner production compared with circumneutral reaches (pH ≥ 5.6) and increased juvenile growth due to lower parr densities. The benefit of increased growth (earlier age of smoltification and increased early marine survival) was not sufficient to outweigh impacts of higher freshwater juvenile mortality. Simulated removal of acidic conditions resulted in a 41% increase in recruits, but only after several decades following complete chemical recovery. Circumneutral reaches were predicted to be 4–12 times more efficient at producing smolts and 3–7 times more efficient at producing adults from hatchery fingerlings compared with acidic reaches. To maintain current target egg deposition rates of 240 eggs/100 m2, circumneutral reaches could withstand grilse exploitation rates of up to 65%, while acidic reaches, even without harvesting mortality, could not maintain this level. Fisheries management strategies need to account for variability in freshwater production both within and between river systems.

ABSTRACT. The U.S. National Commission on Science for Sustainable Forestry recognized a need for effective adaptive management to support management for biological diversity. However, difficulties in implementing adaptive management in the U.S. Northwest Forest Plan led the Commission to wonder if comparisons across multiple adaptive management trials in the forest sector could provide insight into the factors that serve to enable or inhibit adaptive management. This comparison and the resulting discussions among a group of seasoned practitioners, with adaptive management experience at a variety of scales and levels of complexity, led to insights into a hierarchy of ten factors that can serve to either enable or inhibit implementation. Doing high quality adaptive management is about doing good science to enable learning from management experience. Enabling adaptive management though is about working with people to understand their concerns, to develop a common understanding and an environment of trust that allows adaptive management to proceed. Careful attention to enabling factors is critical to fulfilling the promise of adaptive management.