Fly Fishing in Rockies - Temperatures and Trout

From REI

A good stream thermometer is one of the most important pieces of fly fishing gear you can own. Why? Since trout are cold-blooded animals, their body temperatures are the same as the water around them. Their metabolisms and feeding behavior are linked directly to this temperature. If you have a knowledge of the relationship between water temperature and trout behavior, a stream thermometer can give you invaluable clues on when and where to fish a particular stream and even indicate what types of flies to use.

Temperature Ranges and Trout

Water Temperature Chart (F)

Here's a brief reference chart for diagnosing stream temperature and trout behavior:

Water Temperature Fluctuations

Remember that stream temperatures fluctuate over the course of the day on most freestone streams. Design your fishing strategies accordingly. In spring, water temperatures will be cold in the morning, warming towards ideal temperatures by midday, and then dipping back down again at night. In the summer, water temperatures will be optimal in the early mornings and evenings. Water temperatures may be uncomfortably high for trout during the middle of the day.

Acclimatized Trout

The temperatures outlined in this clinic are provided as a general guideline only. Trout in special conditions can develop tolerances for their specific environments. For example, fish who live in a "tailwater" environment like Utah's Green River have become accustomed to much colder-than-average water temperatures. (A tailwater is a river that runs from the outflow of a dam. This outflow is drawn from the lower levels of the reservoir behind the dam, so it is always cold.) The Green River trout have become conditioned to rise to dry flies in water that would have other trout shivering.

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