Salmon do not feed in freshwater. Their stomach atrophies and it is physically impossible for them to digest. If they did feed they would probably be easy to catch, instead the fly angler is faced with the proposition of fishing for a fish that cannot eat with something that it could not eat even if it were capable. What this means to the angler is that instead of imitating food items as he might for trout, he must provide a stimulus that causes salmon to react by taking the fly. This he does by studying temperature, water height, river size and the flow to come up with a likely solution.
Fish are cold blooded.
At low temperatures they are slow moving and require large attractions to turn them on, at higher temperatures they can expend more energy and they become more sensitive, therefore small flies are attractive. If the temperature gets too high the dissolved oxygen content of the water is reduced and the fish become lethargic. At water temperatures of mid seventies Fahrenheit or higher adult salmon become very uncomfortable and are easily stressed. Any increase in temperature may cause fatalities. From the angler's point of view therefore a range of temperatures between 32 degrees F and 70 degrees F is acceptable.