Trout Water Temperature: The 68 °F Line, Limits and Summer Refuges

Trout feed best in water of 55–65 °F (13–18 °C), and the day to stop fishing for them begins at 68 °F (20 °C). Brook trout stress sooner, at 65 °F (18 °C). Brown trout hold on to 70 °F (21 °C). Lake trout want it colder than any of them, 48–55 °F (9–13 °C). The upper 70s kill. Below: the chart for five trout, why 68 °F is the line, how to take a reading that means something, where trout go in July, and the fall and winter bands.

A rainbow trout grows fastest at 63 °F (17 °C) and dies at 77–79 °F (25–26 °C). Fourteen degrees separate its best day from its last one, which is why trout anglers carry a thermometer.

The chart: five trout, four numbers each

Comfort is the feeding band, spawn the temperature that starts it, stress where the fish must find colder water; the stop line is where a caught trout starts paying for the fight with its life. Figures from New York DEC, the EPA, West Virginia and Michigan DNR and Wismer and Christie; the wider water temperature chart puts these five next to twenty other species.

Brook trout are the tightest: West Virginia DNR calls anything above 65 °F a stress factor. Brown trout are the toughest, with a growth ceiling of 66–70 °F (19–21 °C), which is why browns survive in rivers that lost their brookies. Lake trout die at 74 °F (23 °C), a temperature a bass finds pleasant.

Can trout survive in warm water?

Briefly, yes; for a season, no. The lethal readings sit in the upper 70s: 77–79 °F (25–26 °C) for rainbow, about 77 °F (25 °C) for brook trout, 79–81 °F (26–27 °C) for brown. A stocked pond that reads 78 °F (26 °C) in July has no trout left to protect. Between 68 and 77 °F a trout lives, but it stops growing and holds in whatever cold seam it can find.

The reason is arithmetic. A fish’s metabolism roughly doubles with every 18 °F (10 °C) of warming; Clarke and Johnston found much the same rule across 69 species. A trout at 68 °F needs about twice the oxygen it needed at 50 °F (10 °C), while warm water carries less of it. Add a fight and the fish runs an oxygen debt it may not repay; it can swim off and die hours later under a cut bank. That is why the catch and release numbers turn on the thermometer as much as on the hook. The EPA’s Region 10 guidance draws the same line for Pacific salmon and steelhead: a maximum of 64 °F (18 °C) where they migrate and rear, 43–50 °F (6–10 °C) for water holding eggs.

Why anglers stop at 68 °F, and what a hoot-owl closure is

Montana Fish, Wildlife and Parks turns the line into law. Under its drought policy, when a river’s daily high stays above the department’s trigger for several days running, FWP places a “hoot-owl” restriction on it: fishing closes from 2 p.m. to midnight, and the cool hours when the owls still call stay open. Full closures follow when flows drop as well. The list changes weekly through July and August; check FWP’s closures page before any summer trip in the state.

The rest of the country runs on the honor system, and the 68 °F line is the system. It is not a legal limit; it is the point where a trout you release stops being a trout you will catch again.

How to take a stream temperature that means something

A number from the wrong place or the wrong hour is worse than no number.

• In the current, not at the edge. The margin where you wade in runs warmer than the run the trout hold in. Drop the thermometer in knee-deep moving water, at the depth of the fish, for a full minute.
• Mid-afternoon, not dawn. A stream peaks in mid-to-late afternoon and bottoms out just after sunrise. A shallow bay warms 5–8 °F (3–4 °C) between dawn and mid-afternoon in spring, and a small open stream in July moves nearly as much.
• Trend beats level. A river falling from 70 to 66 °F (21 to 19 °C) after a storm fishes; one climbing from 62 to 66 °F (17 to 19 °C) under a heat dome is about to close.
• Move the thermometer. A tributary mouth, a spring seep or a shaded canyon reach can read several degrees below the main river a hundred yards (90 m) away. That difference is where the trout are.

Where trout go in summer

Trout do not tolerate heat; they leave it. Four places hold them when the main river reads 70 °F (21 °C).

Tailwaters. A dam draws from the cold bottom of a stratified reservoir, which can read 60 °F (16 °C) at 15 ft (4.6 m) while the surface is 82 °F (28 °C). The first miles below the dam stay in the comfort band all summer.

Springs. Groundwater comes out close to the region’s year-round average air temperature, far below a July afternoon. A seep the size of a bathtub can hold a pod of brook trout through August.

Tributary mouths. A small, shaded, fast tributary runs colder than the river it feeds. Trout stack in the plume where the two meet, often within a rod length of the seam.

Depth, in lakes. From late June a lake stratifies and trout live under the thermocline. New York DEC’s trolling table: 45–58 °F (7–14 °C) for steelhead, 48–55 °F (9–13 °C) for lake trout, 55–65 °F (13–18 °C) for browns and rainbows. Find that depth and fish there.

The summer decision, step by step

For a trout stream from late June through early September, on any day the forecast air high passes 85 °F (29 °C).

1. Check the closures page the night before. A hoot-owl restriction or a full closure makes the decision for you.
2. Read the water in the current where you will fish, and know the afternoon high from yesterday’s 3 p.m. reading.
3. Below 65 °F (18 °C) all day: fish normally. For brook trout, 65 °F is also the day’s ceiling.
4. Afternoon high of 65–68 °F (18–20 °C): fish, but shorten everything. Barbless hooks, the fish kept in the water, no photograph in the air, and a tippet heavy enough to end the fight in a minute: 4X fluorocarbon lands a 16 in (40 cm) trout, on a knot that holds.
5. Afternoon high above 68 °F (20 °C): fish dawn only. Start in the gray light, keep the thermometer clipped to your vest, and quit the moment the water touches 68 °F.
6. Morning reading already at 70 °F (21 °C): leave the trout alone. Drive to a tailwater, a spring creek or a lake you can fish deep. Or fish bass and carp, which feed hardest at 65–85 °F (18–29 °C).
7. Write down the reading, the hour and the result. A notebook of afternoon highs tells you when your home water closes next July.

Fall and winter: when the fishing turns back on

Fall is the trout’s reward for surviving summer. As the water falls back through 65 to 55 °F (18 to 13 °C), browns and rainbows feed hard in daylight again. The brown trout spawn starts as the water drops into 42–48 °F (6–9 °C), brook trout at 40–50 °F (4–10 °C), lake trout on rocky shoals at 45–55 °F (7–13 °C). Walk around cleaned, light-colored patches of gravel in the tailouts from October on: those are redds, and the eggs in them are next year’s fish.

Steelhead run on the same falling thermometer: New York DEC calls them most active at 45–58 °F (7–14 °C) and says the run slows below 40 °F (4 °C).

Winter is the metabolism rule run backward. Every 18 °F (10 °C) of cooling halves what a trout needs to eat, so a rainbow at 40 °F (4 °C) feeds about half as often as one at 58 °F (14 °C). Fish the warmest hours of early afternoon and the slowest water.

The other bands: bass, crappie, catfish, walleye and bluegill; the species pages carry each fish’s biology, and the trout hub tells the five trout apart.