Temps for Trout: How to Catch Fish that Behave Hot n Cold

Fish are cold-blooded, so their body temperature reflects that of the water in which they swim.

July 15, 2020 By Hilary Hutcheson

I’m guessing if pop music star Katy Perry had a dime for every time she was quoted in a fly-fishing magazine, she’d boast only one shiny Roosevelt. But if fly fishers had a dime for every time the lyrics to her song, “Hot N Cold” rang true for us in our relationship with fish, we’d be rich.

You’re hot then you’re cold

You’re yes then you’re no

You’re in then you’re out

You’re up then you’re down

Rather than let a vacillating trout lie, most anglers are keen on sleuthing solutions to the flux. And it turns out, one of the reasons our targeted species seem hot and cold to our advances is . . . actual temperature variances. Fish are cold-blooded, so their body temperature reflects that of the water in which they swim.

Click here to Download a PDF of the Temp Chart

“Since fish are strongly affected by water temperature, I’ll drive for hours to find the right temps,” says Todd Tanner, head instructor at School of Trout. “I know that if I’m fishing in conditions that are just warm enough or cool enough, there will likely be good fish activity and plenty of bugs.”

School of Trout is an in-depth, multi-day educational and cultural retreat for adult anglers held at TroutHunter fly shop in Island Park, Idaho. The ways in which warm and cool water and weather conditions contribute to trout behavior is on the syllabus, alongside equipment, entomology, knots, casting, reading water, wading, conservation, and more. The school offers sessions for beginners, intermediate, and advanced anglers and is taught by an all-star roster of instructors including Tom Rosenbauer, one of the world’s most iconic and admired anglers.

Rosenbauer has often addressed the topic in his many books, podcasts, and seminars. “Water temperature is especially important for fly fishers,” says Rosenbauer. “Whereas with bait fishing, a slow-moving fish might smell the bait and slowly inhale it, but with fly fishing you’re looking for actively feeding fish that you have to entice with the right presentation and pattern. You want temperature on your side.”

In The Orvis Encyclopedia of Fly Fishing, Rosenbauer explains that all fish have an approximate range where they will feed if the opportunity presents itself, and an optimum range where fish are constantly on the feed. For example, research suggests a rainbow trout’s feeding range is generally 40 to 68 degrees, and its optimum temperature range is 52 to 64 degrees. (It can be a bit warmer for brown trout, and several degrees cooler for brook trout, cutthroat, and bull trout.) “Your best opportunities with a fly are within that optimum temperature range,” says Rosenbauer. “But you can catch fish at the lower and upper ends.”

When fishing at the lower end of the optimum range, Rosenbauer suggests looking for sandbars, dark-colored weed beds, or slower current, all indicators of slightly warmer water. When conditions are at the warmer end, look for deep pools and shady banks, especially with ledgerock that can hint at cool groundwater seeping in. Springs that run into a river can offer cooler water in the summer and warmer water in the winter.

A study published by the North American Journal of Fisheries Management in 2010 indicates an increase in mortality of caught-and-released rainbow trout, brown trout, and mountain whitefish angled in waters equal to or above 73 degrees Fahrenheit, and determines fish stand a better chance of survival when caught in the morning or evening, when the water is cooler.

In Montana, state wildlife officials enact what’s called hoot owl restrictions as a means of reducing undue stress on trout living in above-average water temperatures. Hoot owl regulations require anglers to stop fishing between the hours of 2 P.M. and midnight once water temperatures have peaked at 73 degrees F. for three consecutive days.

“Sure, this is the School of Trout,” Tom Rosenbauer reminds the class, “But don’t be afraid to target other species like bass and pike in lakes when the rivers are too warm for trout. Those other fish can tolerate warmer conditions.”

In warmer months, lakes can develop a thermocline, in which the top layer of water can be 10 to 15 degrees warmer than the water 15 feet below. There’s high oxygen content just above the warm- and coldwater meeting point, and this point is often a honey hole for fishing success. If you find structure like weeds or rocks at the thermocline, the odds of finding fish are even better.

Conversely, when the mercury drops, the enzymes that digest a fish’s food work more slowly in cold water, so it can take longer for a fish to process a meal before eating again. This means longer pauses between bites. Also, subsurface ice, frazil, and anchor ice can negatively impact fish habitat.

Rosenbauer says fishing below or above the feeding temperature range can put fish in jeopardy. Scientists agree, offering data to support suggestions that anglers can negatively impact a fishery by targeting fish when temperatures are too hot or cold.

Experienced anglers can testify that locating fish when the seasons change is not always easy. Strong rainbow trout that occupied a stiff riffle in June will likely have vacated with the onset of autumn, headed to mellower feeding grounds. United States Geological Survey research shows trout are less likely to hang out in fast runs or riffles during the fall and wintertime, opting instead for slow, deep pools where they can conserve energy and find ample cover.

Behavior Lessons

As a fly-fishing guide and School of Trout instructor myself, it’s my job to help anglers learn where to cast their fly for the best chance at a fish amid certain conditions, as well as what fly patterns to choose. At the School of Trout, students are taught this information in a workshop setting before taking their new skills to the Henry’s Fork River. I recall one particular school day in which all 12 participants, armed with fresh information on mayflies, caddis, and stoneflies after that morning’s entomology class, exited the TroutHunter lodge and descended together onto the river’s edge.

The crunch, crunch of frost-encrusted, seasonally spent mullein plants under our wading boots as we marched to the river made it hard to hear discussions between the participants, but I could feel their excitement at being outdoors for this lecture. It was afternoon, and the sun glinted off our fly rods, offering a false sense of balminess in the 33-degree air.

I stood in a fast current with my back angled away from the group, demonstrating different fly presentations, including drifting a dry fly, feeding line, mending, and reach casting. As I executed a drift I opined was particularly exemplary, one of the participants voiced a hearty “Ohhh wow, awesome!” driving home my none-too-humble assumption that my fly presentation was spot-on. Then, another student offered, “Hey, whoa, yeah!” Within seconds nearly all the students had joined in to oooooh and ahhhhh. And in the next second I realized that none of these expressions had anything to do with my demonstration.

The group had stopped watching me, and were watching a number of actively feeding fish, visibly flashing underwater in the gin-clear, slow pool downstream from me. “There, again!” exclaimed an excited student, pointing at the pool. My eyes followed the finger in time to see the unmistakable silvery flash of a deep rainbow trout eating nymphs. Then I saw a second fish feed. Then another.

My structured workshop on getting a good drift was overshadowed by an organic lesson on fish behavior. Students were able to see firsthand how fish that had likely occupied the aerated riffle and turbulent pocketwater in the summer now set stakes in slower, deeper pools as seasons changed. We talked about how the trout were not chasing their food or rising to it, but moving laterally only slightly to dine on caddis larvae and black stonefly nymphs. The polite group thanked me for the lesson, although I may have heard one of our Southern guests add, “Bless your heart.”

There’s also data to back up what many anglers have hypothesized: Certain trout species require colder water than others for growth, migration, and feeding. What this means for fly fishers (in addition to better understanding the ecosystem in which we play) is that if we know what species of trout we want to target, we can decide where to find it based on known water temperature. For example, a Montana State University study published by the American Fisheries Society in 2007 compares the thermal requirements of rainbow trout and westslope cutthroat trout, with results indicating that when both species are found in a stream, the cutthroat will likely be in the colder headwaters, displaced by more abundant rainbow trout in the warmer water at lower elevations.

With the understanding that water temperature impacts the entity of fish, that different species thrive in different climates, and fish often change feeding grounds with the seasons, the next step is to venture into the weeds on how to properly angle for trout.

Someone call the doctor

Got a case of a love bi-polar

Stuck on a roller coaster

Can’t get off this ride

This stanza of Katy Perry’s “Hot N Cold” clearly represents a frustrated fly fisher who loves catching trout, but requires counsel on the disparity of water temperature if she is to find success and enjoy herself. Looks like I’d better commission my teenage daughters to create an iTunes playlist for this year’s School of Trout.

How to fish at the lower (colder) end of the optimum temperature range:

Legality—Always confirm your state’s wintertime regulations and open seasons.
Safety! — Ask yourself: Is this water safe for me to wade or row in? Is there a likelihood of hypothermia? Am I properly dressed?
Handle with care — The gills and eyes of trout are fragile and can freeze if the fish is taken out of the water in freezing air temperatures. This is another reminder to keep the fish in the water when netting and releasing it. Don’t touch fish with your gloves—the fabric may remove the important skin mucus that provides fish a barrier against disease.
Slow down, low down — Fish are working with a slower metabolism in colder seasons, so they don’t want to use energy by holding in fast water. Fish deeper, slower pools, slow down your presentation, drop nymphs to the bottom as well as testing different depths, and be patient since it takes longer for fish to digest their food in the cold.
Lighten up — Winter water is often clear, and slow-moving trout have plenty of time to inspect your setup. Consider going without an indicator, or use a smaller, less conspicuous one.
Sleep in — The fish won’t be active until the warmest part of the day, so there’s no need for dawn patrol.
Flies — Beadhead nymphs (caddis, mayfly, stonefly), worm patterns, soft-hackles, midge pupae, and adult midge clusters. Mild days with a few extra degrees of warming call for #16-22 Blue-winged Olives.

Old Temps for Trout — Trout feed most actively in water temperatures from 52 to 64 degrees F. When the water is colder or warmer, you’ll need to adapt your strategies to tempt them. You should also take special precautions to protect the fish in extreme conditions when the water is very cold and icy, or when it’s over 68 degrees. (Landon Mayer photo)

How to fish at the upper (warmer) end of the optimum temperature range:

Give a Hoot — Check regulations to make sure there aren’t temperature-related restrictions, like Montana’s hoot owl restrictions. Check the water temperature and answer: Is the water so warm that it is causing the fish undue stress if I catch it? If the water is over 68 degrees Fahrenheit, catch-and-release fishing becomes impractical if not unethical.
Troubled Water — Look for places where water is more turbulent, thus aerated, such as below waterfalls, lake waves,
pocketwater, rapids, and riffles.
Big Stick — Use rods and line weights that are heavy enough to bring fish in quickly without a long fight. The longer fish struggle, the harder it will be for them to recover in warmer, less oxygenated water.
Early Bird — Get up and at ’em early in the morning or late in the evening, during the coolest parts of the day when fish are most actively feeding and not stressed.
Signs — Look for springs and groundwater seepage, shady banks, and ledgerocks.
Flies — Early summer calls for large attractors, Salmonflies, Golden Stoneflies, and baitfish streamer patterns. In midsummer consider adult mayflies and wild attractor patterns. Late summer often demands smaller caddis and midge patterns, and fall calls for ants, beetles, hoppers, and sculpins.

Hilary Hutcheson started guiding fly-fishing trips as a teenager in West Glacier, Montana. Today she continues to guide the Flathead River system, and owns and operates her fly shop, Lary’s Fly & Supply, in Columbia Falls, Montana, where she lives with her daughters Ella and Delaney, her partner Ebon, and their three-legged Labrador Jolene. Check out her climate-solutions film Drop, which she wrote and narrated.