Here in North Carolina we are starting to face the extremely warm weather of summer as we usually do in June, and we are concerned with the warm water temperature issue while fly fishing for trout. Water temperature is very important anytime of the year but especially during the hot summer season.
We don't recommend catching and releasing trout when there is a reasonable doubt that the trout will survive after being released. When you practice catch and release, you should be aware of the stress caused by the warm water. Never guess at the water temperature, always carry a thermometer or two and check the water temperature almost every day that you fish, especially if there is a question about it being too warm or too cold.
The amount of dissolved oxygen in the water is directly related to the water temperature. You are probably aware that cold water holds dissolved oxygen much better than warm water. However, the effects of water temperature on the amount of dissolved oxygen doesn’t change at a constant rate. It changes at an accelerated rate.
For most species of trout, the trout are most active when the water ranges from about 50 to 65 degrees. Due to their metabolism level, the cold-blooded trout feed at the maximum rates within this temperature range. From 65 to 70, the amount of dissolved oxygen begins to change at an accelerated rate and it becomes a big factor. In simple terms, at 65 there's no problem and the trout continue to feed aggressively. At seventy degrees, there can be a problem with low dissolved oxygen depending on the type of water.
When the trout start to become oxygen starved, they will begin to cease their feeding activity and when the water temperature reaches about seventy-four degrees, the trout need to have highly oxygenated water just to survive. Again, keep in mind that these temperatures are guidelines since there is a major difference in the amount of dissolved oxygen contained in still water as compared to fast, turbulent water. Obviously, moving water will hold more oxygen than the still water and this can be a factor.
All of us who want the fish to survive want a safety margin. We don't want to catch a fish that shows signs of being stressed. They will experience the same feeling that you get when you climb a high mountain. At 10,000 feet the air is sufficiently thin and you will struggle for breath not looking for any more exercise, likewise, the trout will stop fighting as hard. If it's in your hands before being released, it won't be difficult to hold. When you put it back in the water (which should be done just as quickly as possible), it may hesitate or roll over and then slowly swim away. Compare this with a trout that isn't stressed and you will see it streak away like a rocket.
The slow reaction does not necessarily mean the trout will die just as you will not surely die when you get out of breath climbing the mountain, however, it does put the fish at some risk. There have been many times when I have wanted to give the trout mouth to mouth recitation, only to see them active as normal only minutes later. According to many studies on this subject, the great majority of trout will be just fine after being released if done properly. Far more will die from anglers taking pictures and/or mishandling the fish otherwise.
So to summarize: if there's any question about the water temperatures, fish at the highest elevations. Do not guess at the water temperature but take the water temperature frequently. If the water temperature is below sixty-eight degrees, feel free to catch and release all the trout you want. Just make sure you release them as quickly as possible. If the water temperature exceeds the magical sixty-eight degrees, the risk to the trout is too great and I would suggest you look for another location perhaps at a higher elevation.
Remember to be realistic, some anglers have blown the problem of fish stress and mortality completely out of proportion - some unintentional and some trying to impress anglers as the "savior of trout".