Declining long-term trends in Atlantic salmon abundance: inferring mechanisms using catch data for Norway and Scotland

Long-term time-series of Atlantic salmon catches for c. 250 Norwegian rivers (some spanning the period 1876 to present) and 48 Scottish rivers (1952 to present) will be subject to statistical analyses in order to deduce what regulates the long-term dynamics of spawning populations. Preliminary analyses confirm that the catch data contain biologically interpretable signals. The temporal correlation between time series appears to vary over time, with trends persisting over several decades, while the spatial correlation appears relatively short range. We aim to consolidate these preliminary analyses and extend them to a consideration of mechanisms by pinpointing the factors generating differences in spatial trends for production and spatial variations in the trends. We aim at identifying what types of climatic (local, regional, large-scale, river-based or marine-based) factors most strongly influence catches in various rivers and regions, and pinpoint mechanisms that might be at work, at sea or in fresh water. This approach will require advanced statistical methodology and will incorporate additional time series for climatic (density-independent) and biological (mainly density-dependent) variables. The use of the combined national catch data is strategic, since it will generate a particularly wide latitudinal spread and a correspondingly powerful template for analysis and interpretation. Biological interpretation of population-specific findings will generate insights required to predict the impacts of current climate change scenarios on Atlantic salmon populations. Prediction will facilitate future management of the resource.