Beyond Correlation in the Detection of Climate Change Impacts: Testing a Mechanistic Hypothesis for Climatic Influence on Sockeye Salmon (Oncorhynchus nerka) Productivity.

Detecting the biological impacts of climate change is a current focus of ecological research and has important applications in conservation and resource management. Owing to a lack of suitable control systems, measuring correlations between time series of biological attributes and hypothesized environmental covariates is a common method for detecting such impacts. These correlative approaches are particularly common in studies of exploited fish species because rich biological time-series data are often available. However, the utility of species-environment relationships for identifying or predicting biological responses to climate change has been questioned because strong correlations often deteriorate as new data are collected. Specifically stating and critically evaluating the mechanistic relationship(s) linking an environmental driver to a biological response may help to address this problem. Using nearly 60 years of data on sockeye salmon from the Kvichak River, Alaska we tested a mechanistic hypothesis linking water temperatures experienced during freshwater rearing to population productivity by modeling a series of intermediate, deterministic relationships and evaluating temporal trends in biological and environmental time-series. We found that warming waters during freshwater rearing have profoundly altered patterns of growth and life history in this population complex yet there has been no significant correlation between water temperature and metrics of productivity commonly used in fisheries management. These findings demonstrate that pairing correlative approaches with careful consideration of the mechanistic links between populations and their environments can help to both avoid spurious correlations and identify biologically important, but not statistically significant relationships, and ultimately producing more robust conclusions about the biological impacts of climate change

Measuring the net biological impact of fisheries enhancement: Pink salmon hatcheries can increase yield, but with apparent costs to wild populations

Hatchery production of juvenile fish for release into the wild has been practiced for well over a century in an effort to increase the number of salmon available to harvest. In this study we evaluate the net impact of the largest such program in North America, the hatchery program for pink salmon in Prince William Sound (PWS) Alaska. At the same time the hatchery program was increasing in output, there was a major change in productivity in the North Pacific so that throughout Alaska pink salmon increased dramatically in abundance between the 1970s and the 2000s. Using other regions of Alaska as reference sites, we estimate that the PWS hatchery program has increased the total catch by an average of 17 million fish, of which 8 million have been allocated to pay hatchery operating expenses. We estimate that the maximum sustainable yield (MSY) of wild spawning fish in PWS has increased slightly (28%), while in regions of Alaska without pink salmon hatchery programs the MSY has tripled. Our results support the use of a precautionary approach to future large-scale stock enhancement efforts.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Kvichak sockeye salmon life-history and main life-history pathways.

<p>Percentages give long-term average age composition. Dotted lines indicate variable life history pathways and are labeled with the long-term average proportions of individuals following each pathway. Fry length frequency histogram shows relationship between freshwater growth and smolt age.</p

Results from AIC selected smolt age composition model.

<p>Partial plots showing the effects of a) fry length, b) spring air temperature in year of migration and c) year of migration on proportion of a brood year migrating at age-1.d) Observed (points and lines) and predicted (shaded area: 95% confidence interval) proportions.</p

Results from AIC selected smolt survival model.

<p>Expected marine survival is shown in relation to length for age-1 (blue) and age-2 (red) smolts. Light colored lines show predicted survival as a function of length for each age in each year (random intercepts from model SAS-1R). Bold lines indicate mean values across all years. Histograms show frequency distributions of smolt length (x-axis) and expected survival at mean length (y-axis) for age-1 and age-2 smolts; dashed lines indicate medians of each frequency distribution, for each smolt age.</p

Summary of trend analysis results for freshwater environmental variables.

<p>Summary of trend analysis results for freshwater environmental variables.</p

Summary of intermediate relationship model results.

<p>Summary of intermediate relationship model results.</p

Kvichak River drainage, Alaska showing biological and environmental data collection sites.

<p>Spatial data retrieved from the USGS National Hydrography Dataset (nhd.usgs.gov).</p

Kvichak River annual sockeye run size and 5-year moving average, 1960–2014.

<p>Kvichak River annual sockeye run size and 5-year moving average, 1960–2014.</p

Summary of trend analysis results biological variables.

<p>Summary of trend analysis results biological variables.</p