The brook trout you can catch most easily in August are often the ones you should leave alone. When a river warms past the high sixties, the survivors pull into cold pockets — a spring seep at the tail of a pool, the mouth of a colder tributary — and they crowd in because it’s the only water they can breathe. They’re easy to find and easy to fool, and hooking them is close to the worst thing you can do to them. The trip worth taking goes the other direction: up, to water that’s cold all the way through, where catching a brook trout isn’t a death sentence for it.

Reading the Temperature Before You Drive

Brook trout are the most heat-sensitive of the common trout. The numbers are worth carrying in your head. They grow best between about 55 and 61°F. Cellular stress sets in around 69°F, and by the time water holds at a two-month average near 70°F, brook trout mostly aren’t there at all. The lab work from Chadwick, Nislow and McCormick shows those two numbers — the temperature that physiologically stresses the fish and the temperature that marks the edge of where the species can live — are essentially the same. They can’t just tough it out.