While enjoying our night before Christmas with my son Stephen and his family, I asked him if he was going to go trout fishing during the Holidays. He said that he would try to get out on Monday, January 2. I suspect that will present a fair opportunity as the water temperature should be above 45 degrees and most probably near 50 degrees, due to the unusually warm weather this year here in North Carolina; and I am well aware of the water temperature and the effects that it has on trout.
The changes that occur as the water becomes colder and colder are exponential. As the temperature drops from 45 degrees down to 40 degrees in five degree increments, the way that the trout react is very noticeable. Likewise, there’s a very steep curve representing the decline in the way the fish react from 40 degrees down to 35 degrees.
At 35 degrees the fish may not even be able to survive. The differences in the range I just mentioned are huge. It's probably ten to fifty times the difference in how the change occurs in the 5 degree increment from 45 down to 40 degrees.
Remember, the amount of food the trout eat in cold water versus warmer water isn't really a direct factor in catching them. Fishing for fish you can't see that are holding in isolated areas usually in deeper water and often on the bottom in areas beneath fast water where the current is slow reduces your ability to get a fly in close to the trout like it must be presented to get them to eat it. They only need to eat one fly to catch them and if presented right, they will do that in cold water, especially in water above 45 degrees.
Good luck Stephen!