With the warmer summer months at hand, water temperatures across the west are starting to climb. Discover how water temperatures affects trout behavior to make you a better fly fisher. If the water is really cold fish may not move very far or at all for a fly. Too warm and fish can be killed by simply catching them.

Discover how water temperatures affects trout behavior

It’s a cold wintery day with water temperatures in the upper 30’s. You’ve been fishing all day with nary a bump to show for your efforts. Are water temperatures to blame? There are a multitude of factors to consider but water temps are one of the most critical.

When the water is below 40 degrees, trout are reluctant eaters. Their metabolism slows way down and the need for caloric intake is much reduced. This results in fish that have to be convinced to eat a fly by basically bonking them on the nose with it. They will rarely move more than a few inches to eat and therefore can be tough to catch. The caveat to this is, if there are prolific hatches during the colder months , then fish will definitely move more than a few feet to feed while the hatch is taking place.

As water temps climb into the mid to upper 40’s, trout become more active. The bugs are as well. This leads to more opportunities to catch fish. Now, they will move a greater distance to eat, may be more focused on surface presentations and larger bugs can be found.

Discover how water temperatures affects trout behavior

Ideal water temps for trout occur between 55 and 58 degrees but are happy and healthy into the low to mid 60’s. . At these temperatures the fish are most comfortable, will move farther for a tasty snack and generally be at their best health. They will be found in most water types and will move into the faster water at these temperatures. Food will be plentiful and the fish happy.

The water feels warm to the touch, almost too warm. You check the temp with your handy streamside thermometer and are alarmed that it reads 69 degrees. Why the alarm you ask? When water temps exceed 68 degrees, the amount of dissolved oxygen dramatically decreases. While fish will still be very active eating insects when we catch them they will fight to get away. During this fight, lactic acid builds in the muscles of the fish similar to humans during exercise. Unlike humans, trout can not reduce the lactic acid and will succumb to it a short time after release.

Ideal water temperatures as seen on a stream thermometer

Mortality from water temperatures happens as the water approaches 78 degrees. While these temps are rare in the Northern Sierra, they can and will happen resulting in a die off.

What can we do to protect trout while practicing catch and release this as it pertains to fishing in the upper end of the water temp spectrum? First, carry a thermometer and use it frequently. Second and most importantly, stop fishing when water temps exceed 68-70 degrees and let the fish be until water temps drop back into a more suitable range. The fish will be happy that you did!


Jay Clark - Professional fly fishing guide for the MIddle Fork Feather River and Truckee River.

About The Author

Jay Clark is a lifelong outdoorsman that enjoys being near the water. Starting out with fishing as a childhood pastime, Jay has honed his skills over the years, evolving from a hobbyist to a seasoned fly fishing pro. Jay’s passion runs deep – offering guided fishing excursions along the Middle Feather River, Truckee River, and serene stillwater spots in the Northern Sierra.

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