has a strong genetic component (Gjerde 1984), environment also plays a significant role through effects on growth (Saunders et al. 1983a). Growth at various times during the post-smolt year may be important for achieving maturity (Duston and Saunders 1999, Gudjónsson et al. 1995, Scarnecchia et al. 1991). Alternatively, some investigators have suggested that climate variations that extend migrations beyond the normal return distance affect the proportion of grilse in the return (Martin and Mitchell 1985).
The North Atlantic Oscillation (NAO) is the dominant mode of atmospheric variation in the North Atlantic (Dickson et al. 2000) and has been associated with the effects of climate variation on the survival and maturation of Atlantic salmon. The spring thermal habitat areas associated with post-smolt survival of central European salmon stocks are derived from large ocean areas where the distribution of SST is affected by the NAO (Dickson and Turrell 2000). Likewise, the winter thermal habitat associated with the abundance of specific age components in the Northwest Atlantic are also derived from areas where SST distribution is correlated with the NAO (Friedland et al. 1993). However, it would be premature to suggest that the NAO is the only mode of climate forcing that affects salmon. For example, high-frequency fluctuation in currents in the Barents Sea appears to create a lagged linkage between Icelandic and Russian salmon stocks (Antonsson et al. 1996). Other atmospheric indices might be useful in developing hypotheses about transoceanic and global stock synchrony and in explaining salmon population trends (Klyashtorin 1998).
The unprecedented decline in Atlantic salmon abundances over the past few decades raises concerns about the effect that climate change may have on Atlantic salmon. With climate at the core of many of the factors contributing to the decline of stocks, the effect of further shifts, beyond the reactive norms to which salmon populations have adapted, now poses the threat of a range shift for the species. If climate changes are compounded by other anthropogenic factors affecting the health and size of the stock (e.g., through habitat effects on the freshwater part of the life cycle), local populations may be driven to extinction.