Recently I was fishing with a fly fisherman who has fished with me for two seasons. And, he had read some of my trout books. So he took the water temperature and he said, “Jay, the temperature is almost to 50 degrees.”
“It’s that high, this is the first time this spring that it is that high,” I said.
So I took the water temperature too. I have a string tied on my thermometer and to my jacket, so it was dangling below my knee in the water. After having lost a couple thermometers you learn to tie everything to your fishing jacket. I made four casts and then looked at my thermometer. I called back to Don. “You’re right, it is at 50 degrees, and we should catch some trout.” In three hours that afternoon, when fishing Tainter Creek, Don caught and released five trout and I caught and released seven trout. We had a good afternoon fishing on one of my favorite streams.
It was March 27 and the air temperature was going up to 56 degrees and since all the snow was gone from the stream bank the water temperature was at 50 degrees, and under cloud cover, the trout were hungry.
You ask, “At what stream temperature do trout know it is time to feed?”
“Believe me when I tell you this is not an easy question to answer. For thirty years I though trout would feed at 40 degrees as the first opportunity to hit your lure. And, of course that came about by recognizing that water is heaviest at 38 to 39 degrees. So when the water in the stream hits 40 degrees some trout will hit your lure. It takes a lot of record keeping to find thing out about trout and water temperature.”
Last year I was fishing Tainter Creek with my son Dan, in the same place where Don and I was fishing it this year. And, the water temperature was at 43 degrees. The trout started to hit. In an hour and a half Dan and I caught 10 trout. As we were walking back to my pick-up. I told Dan, “This is the first time I have fished at the temperature when it has gone from 42 to 43 degrees. And, it’s significant because I knew trout would hit when the temperature went to 43 degrees.”
“Why did you know that?” Dan asked.
“Well you know I take the water temperature in my home every four years on Feb. 29. I put four ice cubes in a glass of water with my temperature gauge. I watch as the temperature goes down to 42 degrees and stays there until most of the ice has melted then it slowly rises to 45 degrees. So I knew if it rises to 43 degrees something is going to happen in the stream. And this is the first indication to trout that spring is just around the corner at 45 degrees. So you can forget about what I wrote about trout feeding when the temperature is at 40 degrees. You know a mischievous trout will hit at any time. But we want to know the lowest temperature at which trout will hit. And it is 43 degrees when most of the trout will hit your lure.”
Then Dan responded, “So you are saying that we should wait for the 43 degree rise in stream temperature.”
“Well it worked today. And I believe it will work every day in this driftless country. So if you are fishing in Trout Central you now know trout will hit when the temperature goes to 43 degree and up from there to 65 degrees.”
Then Dan asked, “If that is true. Then should the temperature in the stream move up rapidly or slowly?”
“Thanks for asking that question. For a long time I didn’t think it mattered. But when I look at my book of statistics I can easily see that if the temperature is rising slowly, instead of fairly fast, trout will hit best. So if it goes up five degrees that is too much of a change. I like it to increase about one degree every hour. And, my statistics prove that is best for fishing. So the angler should take the stream temperature every hour.”
Then Dan asked the question every angler should ask. “How do trout know if the temperature is rising only one degree?”
“Experiments have shown that trout can tell the difference if the water temperature will rise up one-tenth of a degree. So before your stream thermometer will show that the temperature is dropping trout know it is going down. And, they have quit feeding and are sitting in the bottom of the hole waiting for temperature to rise again. That is what happened to me on the opening day this year, January 5th, when the water temperature was at 42 degrees. And, it stayed there. I fished two beautiful holes and saw a lot of trout that wouldn’t even look at my lure. For trout, water temperature rules.”