With vast areas of the province starting to experience warming temperatures and a lack of rain in the forecast, countless cool flowing freestone trout streams will start to experience a rise in water temperature. Most of us who fish know that when trout streams get too warm, the fishing goes downhill fast and the safety of the fish has to be taken into consideration when deciding your fishing destination.
The upper limits of the temperature range 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.
I typically carry a stream thermometer and take readings through out the day. knowing water temperatures can help in a few ways you can start to be able to know when certain hatches will occur and be able to keep better track of the water temperature of the stream or river that you are fishing and be able to make a call to continue fishing or give it a rest or start your day earlier when water temperatures are not affected by solar radiation. It’s use is simple submerge it in the water below 12” and let sit until the temperature stabilizes. I typically do this 2-3 times a day more when I suspect water temp
The long and the short of it is warm water contains less oxygen than cold water. As temperature rises and dissolved oxygen decreases, fish begin to experience higher levels of 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 77F, but stop growing at 73F.
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 lethal for the fish.
How do you know when the conditions remain comfortable enough to fish your target stream without creating a lethal situation for its residents? Unfortunately, studies vary and there doesn’t seem to be any one set of accepted limits. That said, there is a considerable consensus that all three major trout species (brook, brown and rainbow) begin to experience some level of stress at around 64F, with that stress increasing rapidly as the temperature rises further.
Water temperature is not the only determining factor of dissolved oxygen (speed of current also plays a factor, for example). Trout which spend extended periods of time living on these generally accepted thermal margins will likely have a greater tolerance outside these margins such as Trout in Lakes. However, 64F and above represents a valuable limit outside of which you should make a call to go or no go.
On days when temperatures soar, and especially during extended periods of high temperatures, the catch and release fisherman should pay specific attention to stream temperatures throughout the stream he or she is hoping to fish. When temperatures in moving water exceed 64F it is best to call it and return another day.