It is important to observe the external factors when fly fishing. Weather, water clarity and temperature are some of the important factors. Reading water temperature is important for fishing success especially when there are no active hatches. This article will give you a primer on how to use this information.
Thermometers come in the old style analog and newer digital versions. I prefer the digital as it is easy to read with polarized glasses and the numbers are larger. There is generally also an indicator beep when the thermometer has read the temperature instead of guessing with an old style.
Once you have recorded the temperature you can determine your fishing strategy. You will know what time of day is best to fish (probably more use the next outing), what type of fly to use and how to fish it. Trout react and adjust their metabolism to the temperature of the water they are in. The extremes on either end and they will be slow moving. As the temperature settles toward 60 degrees the fish are the most active. There are 5 Ranges that I will refer to by number.
#1 = 50 degrees or lower
2. 50-55 degrees
3. 55-65 degrees
Zone 4 – 65-70 degrees
5. Greater than 70 degrees
Water temperature will change over the course of a day. Starting cooler and warming as the sun is on it and then cooling off again towards evening. Keep this in mind when choosing when to fish. Colder water is best fished with the sun on it and warmer water is best fished early or late in the day. The middle zone of 55-65 can be fished throughout the day.
The colder the water the more trout will shut down and slow their metabolism. Fish in the first two zones are almost dormant and couldn’t hit a fast moving bait. They are like a car on a cold day. They need to warm up first. To fish them it is best to use nymphs with as little action as possible. Zone 3 is the normal range for trout feeding. You can fish dries, wets, nymphs and streamers at normal speed. Increases in temperature will force the trout to seek water with a higher oxygen content. They will be looking for springs, tributaries or moving water. They will also prefer the shade to the sun – much like us. These fish are best sought with smaller flies and long leaders. If terrestrials are present this is a good time to use them.
This is all part of thinking like a fish. Trout need to seek their comfort zone for optimal survival. Use this guide and you will increase your catch.
The Fishing Geek features a Pocket Guide to Fly Fishingthat features a single page dedicated to water temperature. It includes all the information in the article.