A few years back there was a series of studies conducted in New England streams on Atlantic salmon. The short-version is yes, temperature is the primary factor regarding hooking mortality for Atlantics. In water temperatures of upper 40s/mid-50s, hooking mortality was quite low, between 3 and 6 percent. Above 60 degrees, mortality increasd substantially; one trial at 62 degrees found over 20 percent mortality after release. On the Skykomish, I have yet to encounter temperatures above 60 degrees in August/September, but I haven't taken too many measurements.