Interesting correlation in the North Atlantic where research also points to climate change as a factor in the decline of atlantic salmon.
I don't have the reference to hand, so am only going by memory here. But basically it is believed that salmon feed in sea water at a very specific temperature - no doubt the production of plankton at the bottom of the food chain is the key, as the 'Nature' piece mentions. As a result of climate change, the area of the Atlantic that is at this crucial temperature has shrunk to about one-third of its former size. Some have suggested that this is the main reason for the reduction in numbers of salmon returning, which has occurred even where rivers have maintained consistent levels of smolt production.