Autocorrelation in fish recruitment and environmental data can complicate statistical inference in correlation analyses. To address this problem, researchers often either adjust hypothesis testing procedures (e.g., adjust degrees of freedom) to account for autocorrelation or remove the autocorrelation using prewhitening or first-differencing before analysis. However, the effectiveness of methods that adjust hypothesis testing procedures has not yet been fully explored quantitatively. We therefore compared several adjustment methods via Monte Carlo simulation and found that a modified version of these methods kept Type I error rates near . In contrast, methods that remove autocorrelation control Type I error rates well but may in some circumstances increase Type II error rates (probability of failing to detect some environmental effect) and hence reduce statistical power, in comparison with adjusting the test procedure. Specifically, our Monte Carlo simulations show that prewhitening and especially first-differencing decrease power in the common situations where low-frequency (slowly changing) processes are important sources of covariation in fish recruitment or in environmental variables. Conversely, removing autocorrelation can increase power when low-frequency processes account for only some of the covariation. We therefore recommend that researchers carefully consider the importance of different time scales of variability when analyzing autocorrelated data.

To improve the understanding of linkages between ocean conditions and salmon productivity, we estimated effects of ocean temperature on survival rates of three species of Pacific salmon (Oncorhynchus spp.) across 120 stocks. This multistock approach permitted more precise estimates of effects than standard single-stock analyses. The estimated effects were opposite in sign between northern and southern stocks and were quite consistent across stocks within species and areas. Warm anomalies in coastal temperatures were associated with increased survival rates for stocks in Alaska and decreased survival rates in Washington and British Columbia, suggesting that different mechanisms determine survival rates in the two areas. Regional-scale sea surface temperatures (SST, within several hundred kilometres of a stock's ocean entry point) were a much better predictor of survival rates than large-scale climate anomalies associated with the Pacific Decadal Oscillation (PDO), suggesting that survival rates are primarily linked to environmental conditions at regional spatial scales. With appropriate cautions, these results may be used to predict the potential effects of climatic changes on salmon productivity in different areas of the Northeast Pacific.Résumé : Dans le but d'améliorer notre compréhension des liens entre les conditions océaniques et la productivité du saumon, nous avons estimé les effets de la température de l'océan et les taux de survie chez 120 stocks de 3 espèces de saumons du Pacifique (Oncorhynchus spp.). Cette approche impliquant de nombreux stocks a fourni des estimations plus précises que l'analyse habituelle de stocks individuels. Les effets estimés sont de signe contraire dans les stocks du nord et du sud et sont uniformes d'un stock à l'autre pour une même espèce dans une même région. Des réchauffements anormaux des températures côtières sont associés à des augmentations des taux de survie des stocks d'Alaska et des diminutions des taux de survie au Washington et en Colombie-Britannique, ce qui laisse croire que des mécanismes différents régissent la survie dans ces deux régions. Les températures de surface de la mer (SST) à l'échelle régionale (sur une distance de plusieurs centaines de km du point d'entrée d'un stock dans l'océan) sont de meilleures variables prédictives du taux de survie que les anomalies climatiques à grande échelle associées à l'Oscillation décennale du Pacifique (PDO), ce qui indique que les taux de survie sont liés principalement aux conditions environnementales à l'échelle spatiale régionale. Avec les précautions appropriées, ces résultats pourraient servir à prédire les effets potentiels des changements climatiques sur la productivité des saumons de différentes régions du nord-est du Pacifique.[Traduit par la Rédaction] Mueter et al. 463

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We used a multi-stock comparison to identify spatial and temporal characteristics of environmentally driven sources of variability across four decades in the productivity of 29 sockeye salmon (Oncorhynchus nerka) stocks from British Columbia (B.C.) and Alaska. We examined patterns of covariation among indices of survival rate (residuals from the best-fit stock-recruitment curve) and found positive covariation among Fraser River sockeye stocks (southern B.C.) and, to a greater extent, among Bristol Bay stocks (western Alaska) but no evidence of covariation between these two regions or with stocks of other regions in B.C. and Alaska. This indicates that important environmental processes affecting variation in sockeye survival rate from spawners to recruits operate at regional spatial scales, rather than at the larger, ocean-basin scale. The observed covariation in survival rates of Bristol Bay stocks appears to be due to a combination of both freshwater and, to a greater degree, marine processes. Bristol Bay sockeye stocks showed a dramatic and persistent increase in survival rates coinciding with the abrupt changes in the North Pacific environment in the mid-1970s; however, there was little evidence of a similar response for Fraser River stocks.

We examined spatial patterns of covariation in indices of survival rate (residuals from the best-fit stock- recruitment curve) across four decades among 43 wild pink salmon (Oncorhynchus gorbuscha) stocks from 14 geographical regions in Washington, British Columbia, and Alaska. We found strong evidence of positive covariation among stocks within each region and between certain adjacent regions (e.g., correlations from 0.3 to 0.7) but no evidence of covariation between stocks of distant regions (e.g., separated by 1000 km or more). This suggests that important environmental processes affecting temporal variation in survival rates of pink salmon from spawners to recruits operate at regional spatial scales rather than at the larger ocean basin scale. Based on limited fry abundance data, we found that this covariation in spawner-to-recruit survival rates may be strongly influenced by marine processes.