The message — with huge implications for a region where salmon have been worshiped, eaten and fought over for millenniums — is that neither the fate of the fish nor the trajectory of climate change will be linear or neat. Threats to salmon abound and concern runs deep on all sides, but the pattern of 2015 suggests more than ever that location matters, with salmon doing better in some waterways than in others, and certain types of salmon thriving the most. Human intervention may help, wildlife experts say, but they are still not entirely sure what measures will work best, particularly as microclimate conditions differ so greatly.

“Hedge your bets, spread your bets,” said William Stelle, the West Coast regional administrator at the National Oceanic and Atmospheric Administration’s fisheries department, summing up the new philosophy. If it looks as if the water in certain parts of the salmon’s range is going to be too hot for them to withstand, Mr. Stelle said, then the new approach is to find or restore places with cooler water. “You have to bring it down to the local conditions — what can you do in that place?”

The Environmental Protection Agency and the State of Oregon recently announced a three-year plan to map such “cold water refugia” in the Columbia and Willamette River systems. The United States Forest Service has developed a database of stream temperatures in the Northwest with plans to roll out similar models for other regions around the country looking to map their coolest streams.

The new thinking also points the spotlight on Portland and other places that have pioneered or advanced the idea, and where fishery managers say that cold-water refuges are showing signs of early — if tentative — success. Last fall, for example, scientists documented salmon spawning in Crystal Springs Creek, five miles from the downtown high-rises of Portland, Oregon’s largest city, for the first time in a half-century.