Challenges for Diadromous Fishes in a Dynamic Global Environment

<em>Abstract</em>.-In this paper, we develop logistic stream temperature models for 17 selected sites in northeastern North America and evaluate the potential changes from warming climate under two scenarios (low and medium-high emissions). Classification of the magnitude of the (1) long-term (1980-2002) and (2) annual thermal regimes allowed examination of the relative spatial and temporal patterns of instream thermal variability across the 17 sites. At the regional scale, the classification identified three broad groups of rivers (cool, intermediate, and warm) reflecting geographical location and moderated by site-specific factors. The interannual classification identified four thermal year types reflecting increasing magnitude and variability in the annual thermal regime. The dominance of thermal year types and the frequency of occurrence indicated significant variability between years for all sites and within thermal regions. Under the two climate change scenarios, stream temperatures in the 17 sites are projected to increase by 2050. However, there are regional differences with intermediate and warm region rivers projected to be more affected, particularly under the medium-high emissions scenario. More significantly, the duration of weeks when temperatures exceed 20°C (taken as a threshold of thermal stress for Atlantic salmon <em>Salmo salar</em>) is projected to increase with variability in response between river groups. We comment on the ecological significance of these potential future increases in stream temperature and duration for Atlantic salmon in the region.