The influence of broad scale climatic phenomena on long term trends in Atlantic salmon population size: an example from the River Foyle, Ireland

The effect of marine climatic conditions in the North Atlantic on the abundance of returning migrant Atlantic salmon Salmo salar in the River Foyle, Ireland was examined. Catches of Atlantic salmon from commercial netting stations significantly predicted a measure of population size independent of catches over a 48 year period, thus commercial net catches were used as a measure of relative population size over a longer period in this study. The North Atlantic Oscillation index in winter (wNAOI) provides a generalized measure of climate variation for the northern hemisphere and between 1875 and 2001, the wNAOI was a highly significant predictor of the 5 year running mean of catches of migrant Atlantic salmon returning to the River Foyle. When the index was < 0.151, the wNAOI correlated significantly and negatively with Atlantic salmon catches, with 70% of the variance in population size explained by variation in conditions in the marine environment between 1875 and 2001. When the wNAOI was > 0.151, however, this relationship uncoupled. The probability of catches exceeding the long-term median was 2.34 times lower in years where the wNAOI was above the 0.151 breakpoint than for years when it was < 0.151. The wNAOI exceeded the 0.151 threshold on 8 out of the 10 years prior to 2001. Models of climate change indicated that the NAOI is likely to increase significantly with time. If these models are correct, this study would lead to the conclusion that a decoupling of these broad scale climate effects on Atlantic salmon population size will become the norm. Data presented here suggests two consequences of this. Firstly that the value of the NAOI as a predictive tool for forecasting adult Atlantic salmon population size will be limited and secondly that the median population size will become lower in the future. (c) 2006 The Fisheries Society of the British Isles.