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.
In general, a trout needs water between 35 and 75 degrees to survive. So, if your stream thermometer is telling you that a river is 80 degrees, you will not find any trout there. This water may, however, hold some warm water species such as bass and bluegill.
Within a trout's survival temperature range is its "optimum" temperature range. This is the range of temperatures where a trout is most active. Its metabolism speeds up, and its body tissues demand nutrition for growth. This preferred range differs slightly for each species. Temperatures between 50 and 65 degrees are prime for brown and rainbow trout. Brook trout and cutthroat trout evolved in slightly colder conditions, so they prefer slightly colder water. Temperatures between 60 and 63 degrees are optimum for most trout.
Several factors play a part in making optimum conditions optimum. As previously noted, water temperature directly affects a trout's metabolism. Water temperature also affects the oxygen content of the stream. The higher the temperature, the less dissolved oxygen in the water. Keep in mind that just like humans, trout require oxygen to fuel their activities. Water temperature also gives an indication of the status of the food chain. As water approaches the mid 50s to low 60s, the food chain is activated (aquatic insects begin to move around and hatch), a trout's metabolism gears up, and the water is rich in oxygen. It's a good time for the trout. And it's a good time to go fishing.
After the water temperature dips below 45 degrees or so, a trout's metabolism grinds to a near halt. They tend to hang out on the bottom expending very little energy and needing little food. At this temperature, a dry fly is practically useless to you. A dry fly requires a trout to rise to the surface. This is one of the most energy-intensive means of feeding for a trout. Trout in cold water are sluggish, so your best bet for catching one is usually to drift a tiny nymph almost directly into its mouth.
As the water heats up in the summer, trout seek colder water. They will choose deep shady undercurrents, linger at the mouths of cooler tributaries or locate cool natural springs. Keep in mind: Your stream thermometer can help you locate natural springs. In warm conditions, trout will also be found in riffles and rapids. Although this water may be warm, it contains more oxygen than slower moving water.
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