Predicting Trout Behavior

Trout are tough to figure out.

But, I’ve learned over time that one thing really, really matters: water temperature. That’s why I never leave for the river without my stream thermometer.

Since learning about this, I now know where to find trout, when in the day, and what fly/technique to use. I have to credit all this to a random conversation. I was talking with a grizzled veteran, whom I call The Trout Whisperer.

He had on him a small index card, on which was written in small handwriting various data on water temp, air temp, bug hatch schedules, etc. It was a cheat sheet to tell him what fly to use and when.

And, he told me about why temp matters.

Trout love 55 to 60 degrees Fahrenheit (details for trout, and other species, here). In that range, they will range very far for your fly. They will strike hard, and also, usually, because there’s a lot of bug life in the water, too.

So, both because of food availability and their charged-up bodies, they are aggressive. It’s a great time to fish a dry fly, or, for that matter, any fly.

Per a Tom Rosenbauer podcast, he believes that fishing really slows down when the water goes below 40 to 42 degrees. Fish are cold-blooded creatures, and when it’s cold and there isn’t much bug life, they minimize their movement to save energy. They’re in semi-hibernation to expend the fewest calories possible. A dry fly presentation isn’t going to cut it, usually.

That’s why trout in winter will be found in slower-moving water, usually in deep pools, since bigger bodies of water take longer to freeze. Moreover, they won’t chase your fly, so, you have to put it right on their noses, usually with a small fly dead-drifted. Sometimes, a slow-moving streamer works, too.

In the winter, noon to afternoon is the best time to fish, as the sun will warm up the water a few ticks. And, if it’s a sunny day, look for trout in the slow-moving shallows, basking in the sun.

Similarly, really warm water is tough on trout. That’s because the amount of oxygen goes down as water temps rise. Here’s a chart showing oxygen solubility vs. water temp.



Over 70 degrees, and the trout are basically gasping for breath. If waters really heat up, trout die. That’s why fishing for trout when it is super-warm endangers them: they’ll spend much oxygen battling the rod, and they won’t recover easily or may even suffocate to death.

It’s another reason why trout in warm water will be found in well-oxygenated water such as riffles, runs directly below riffles, or near a spot where there’s a feeder tributary or cool underwater spring. This fact also is one reason why dawn and dusk are the best times to fish in August.

So, what’s an angler to do?

First, before heading out, I go to the USGS site. They not only have flows for local waters, but for some, water temperatures, too. Even if your target river doesn’t have a temp reader on the gauge, there’s likely to be one nearby that will give you an approximate answer.

And, with the impending snow melt possibly blowing out rivers and making them cold, the water discharge data will be critical to know. So, plan ahead.

The Web site is good, but I find that a mobile app is better. I like to use River Data, a free app for the iPhone/iPad. I’m sure there’s a similar app for Android. It’s an app that takes the USGS data and displays them in a very mobile-friendly way.

Second, I take the river’s temperature periodically when I fish. I now bring a water thermometer to gauge where a river is overall, and, to hopefully find warm spots in the winter and cool spots in the summer.

So, hope that is helpful. Many things drive trout behavior. Water temperature is a really, really big one.

I now catch more fish, and bigger fish, since knowing all this.


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6 thoughts on “Predicting Trout Behavior

  1. What thermometer do you use? I've been thinking of getting one and I'm trying to decide if it's worth getting a digital one. Heading out to some small streams this afternoon – I'll send a report.

    1. I didn't give much thought to a digital therm. I just picked up the cheapest regular one I could find at a fly shop.

      I think the idea of "point and shoot" via a digital one sounds cool. Depending on range, it could quickly identify cool spots in the summer and warm spots in the winter. But, for me, the question would be: what's the weight?

      When I fish, I am out there for as long as I can, sometimes 12 hours. I like to range far from the crowds. I pack with me all my food and water, and so, I want to cut down on weight in other areas as much as I can.

      If you buy a digital one, LMK. Would love to know more about it.

      Good luck today!

    2. Check your local Dollar Store, Walmart, or somewhere like that. I got one titled as a "pool thermometer" (non-electric) for like $1.50. Been using it for about 10 years now.


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