River-sea thermal differential experienced by salmon post-smolts is not a proximal driver of marine survival.

Wild Atlantic salmon (Salmo salar) populations from many North Atlantic rivers have declined over recent decades. As these declines are thought to be driven largely by changes in the marine phase of the life cycle, there is a drive towards investigating causes and developing indicators for marine survival. Warming of rivers and seas is altering the smolt thermal environments and migration phenology. This may be causing increasing mismatch between in-river cues used by salmon to optimise the timing of their downstream migration and the suitability of thermal and trophic conditions encountered on sea entry. In this study, recently mobilised data on smolt migration timing, adult marine return rates and freshwater and marine temperatures are used to assess the potential of river-sea thermal differential as a driver of marine survival for a set of southern European Atlantic salmon populations. Shifts in smolt migration phenology appear to have buffered these populations against increasing freshwater temperatures, but post-smolts are migrating into a warming coastal environment. There was no evidence for significant trends in river-sea entry thermal differential, and a significant correlation with marine return rates was found for only one of the seven study populations. For these populations, this suggests that thermal differential at smolting is not a consistent driver or predictor of marine return rates and would not form the basis of a generally applicable indicator of marine survival.