With vast swaths of the country currently in the grips of what seems to be an interminable heat wave, countless cool flowing freestone trout streams have turned into something altogether different. Even freestone streams with strong cold water influences and spring creeks that normally remain temperature stable throughout the year have seen soaring temps with fish abandoning their normal feeding and holding lies in search of cold refuges. Most of us who fish know that when trout streams get too warm, the fishing goes downhill fast. Fish are either nowhere to be found or aren’t actively feeding.

For streams that straddle the borderline between the temperatures at which trout thrive and those at which they suffer, it’s possible to find fish that are actively feeding, but for which you shouldn’t be fishing unless you intend to keep said fish. The trouble for many fisherman can be determining where to draw the line. When it comes to trout, how hot is too hot?

The upper limits of the temperature range within which trout will feed, grow and remain unstressed by thermal conditions varies by species, however not all that significantly. These upper limits — which may be as high as 80 degrees depending on the species — can be misleading. These limits characterize thermal conditions under which trout that are otherwise unstressed will die should those conditions persist for a certain period of time (typically 24-48 hours). These limits can be misleading because they don’t provide much information about how high water temperatures that haven’t reached this lethal range can affect a fish that is about to be further stressed by being hooked and played by a fisherman.

Warmer water contains less oxygen than colder water. As temperature rises and dissolved oxygen decreases, fish begin to experience stress. These stresses begin to set in well before the water temperature reaches lethal limits. For example, rainbow trout are said to be able to survive in temperatures up to and exceeding 77°F (24°C), but stop growing at 73°F (23° C). It stands to reason that a fish, one which is already oxygen stressed while positioned carefully in current that minimizes its energy use, will be dramatically more stressed after being hooked and attempting to fight its way to freedom. In fact, in many cases, a fish otherwise properly handled and released under thermally stressful conditions may be likely to not survive.