I'm curious to what the answers will be. My inclination is to say, there probably isn't any low limit, just common sense. I'd say the colder, the less effective the smoking process will be. Safety issues have to be addressed. If your trout is cured, then very low is probably ok with proper sanitation, or if the trout is to be cooked after smoking. I remember seeing three things over the years. 1. Big unheated smokehouses with small smokey fires to smoke country hams (had to be cold in there in the winter). 2. Smokehouse plans where smoke supply is from a separate and remote fire and the smoke travels to the smokehouse through underground piping. and 3. The Iron Chef where Morimoto smokes some fish in a smoker carved out of a chunk of ice (Egg Nog Battle).