Water Temperature for Trout

Where trout feed, slow down, and get stressed, how to take the reading, and when to stop.

From the book Fly Fishing for Nerds — $9.74 →

Trout take their body temperature from the water. USGS puts the two facts that matter under that: temperature governs which animals can live in a river, and warm water holds less dissolved oxygen than cool water. The number to write down is the stream, not the air in the parking lot.

1. Where feeding is easy, and where it slows

The published ranges come from trout culture and from agencies watching wild fish. They come from different kinds of work, and they do not share a single cutoff. Treat them as the starting hypothesis, then measure your reach.

For trout raised as a crop, North Carolina State Extension calls 55–65°F the optimum for production, and says summer water should rarely pass 70°F and should not pass 75°F. SRAC Publication 223, the Southern Regional Aquaculture Center fact sheet, uses that same 55–65°F optimum, puts the minimum temperature for growth at about 38°F, and says that above 68°F a trout digests feed poorly. NC State Extension’s pond guidance says to cut feeding back when the water is below 45°F or above 68°F, and to stop if the fish show stress above 68°F.

Wild trout are the same animal in water you do not control. Use 55–65°F as the band where metabolism supports feeding well. Below about 45°F the feeding window is short, and it usually sits in the warmest part of the day. In the mid-60s, Trout Unlimited says trout get listless and sluggish. The time-of-day guide groups a fishing day from those same neighborhoods: cold at about 45°F and below, a wider day from about 45–60°F, a shorter plan from about 60–68°F, and a stop at about 68°F.

2. Why warm water is when you stop or move

Penn State Extension, writing about ponds, lists a maximum below 70°F for trout and dissolved oxygen above 6 mg/L. Warmer water simply holds less oxygen. Texas A&M AgriLife Extension’s dissolved-oxygen fact sheet gives the saturation comparison: water at 45°F can hold 11.9 mg/L, and water at 90°F can hold 7.4 mg/L. A warmer trout also demands more oxygen, so the margin shrinks from both sides.

A hooked fish spends that margin on the fight. Trout Unlimited describes the recovery problem directly: trout are cold-blooded, warm water carries less dissolved oxygen, and a fight in that water is hard to come back from. Sluggish fish, or fish stacked in deep holes and not eating, are TU’s sign that it is already too hot to fish for them.

Virginia DWR says trout are most comfortable in highly oxygenated water that does not exceed 68°F, and that fishing becomes dangerous for the trout once temperatures reach the upper 60s. Brook trout feel the heat sooner than rainbows. Brown trout can handle warmer water than either. All three can tolerate up to 77°F for a short period, and that temperature can be lethal if it lasts. TU adds a species line the Virginia note does not: water that suits rainbow and brown trout is not cool enough for cutthroat, bull trout, or mountain whitefish. If those are the fish in the river, use a stricter stop than 68°F.

Wyoming Game and Fish draws two lines of its own. Above 65°F, consider keeping a legal fish instead of releasing it. At 70°F, stop catch-and-release. Your state may post a closure with a different hour or a different number. A posted rule wins over any number on this page.

On water with no posted number, this site stops catch-and-release at about 68°F. That is Virginia’s comfort line, and it sits between TU’s “above 65, quit” and Wyoming’s “stop releasing at 70.” If the river is already there when you park, fish a higher creek, a spring, or a tailwater that is still cold, or go home. A few casts in the warm riffle still count as fishing those trout. A dam with a low outlet can release the cold bottom layer of a reservoir through the summer. USGS describes that pattern on its temperature page. Cooler on the map is a hypothesis until the thermometer on that water agrees.

If the morning started in the 50s and the afternoon crosses about 68°F, stop at the crossing. Land any fish you are already fighting as quickly as you can, and keep it in the water. Follow the handling Wyoming prints for hot weather: do not squeeze the fish or put fingers in the gills. A fish that cannot hold itself upright has a poor chance after release. End the day.

3. How to take a stream temperature

Carry a stream thermometer. The Trout Unlimited piece linked above is mostly that instruction: put one in the pack, and use it. Keep it with the nippers, so the number is in your hand and not in the truck.

• Put it in the moving water you mean to fish, and leave it there until the reading stops changing. A bottle you scooped and carried up the bank has already started to match the air.
• A sunny slack edge can read warmer than the run a rod-length away. Measure the run.
• Take another reading if you stay into the afternoon. Virginia DWR sees summer mornings in the low 60s climb into the mid-60s and upper 60s later in the day. On their trout streams, water temperature often peaks in late July and early August. That calendar is Virginia’s. The second reading is the part to copy.
• If a USGS gauge on the river reports temperature, use it to decide whether the drive is worth making. Then measure the reach you will fish. The river-flow guide is how to find that gauge page. The sensor and your pool can be miles apart.

PingWhen (paid, from $9 a month) can email you when a USGS gauge crosses a flow or gage-height level you set.

A guess from your hand is too coarse for a stop rule. So is the air temperature on the truck’s dash.

4. How temperature ties to the hour and the season

The hour matters on a stream that sits in the sun, because the water often warms as the day goes on. It matters less on a spring creek, or on a bottom-release tailwater that is still in the 50s at noon. The thermometer decides which kind of river you are standing on. Once you have the number, the time-of-day guide is the plan: late morning into afternoon while the water is still cold, morning and evening when a freestone river climbs through a summer day, and a short session in the warmest part of a winter day.

Season is the same variable on a longer clock. Summer is when the stop rule does the most work. Winter is the short window. The water may sit near the minimum SRAC gives for growth, about 38°F, or up in the 40s. The thermometer tells you which. Spring and fall often land in the 50s, the easy band, until a warm afternoon climbs out of it. Write the temperature next to the month. After a few trips that note is a better calendar than a generic hatch chart.

Rain can move the temperature and the color in the same afternoon. A storm may cool a small freestone and stain it before you arrive. The after-rain guide is the rise, the stain, and when to stay on the bank.

5. One line before you rig

Write the time, the water temperature, and the decision: fish this reach, move to colder water, or quit. If you stay, pick the lane with the reading-water guide and run one short test. If you quit, the line is the result. That is a finished session, and it is kinder to the fish than another hour of the same warm drift.

Water temperature, season, and where trout sit as the day changes are covered at more length in Fly Fishing for Nerds: A Data-Driven Guide to Finding, Hooking, and Landing Trout. Readers can take 25% off ($9.74) with code NERDS25 at https://leanpub.com/flyfishingfornerdsadata-drivenguidetofindingfishfly-fishing-for-nerds/c/NERDS25.

The Riverbank Checklist is the one-page bank routine, with a line for today’s hypothesis and the result. Temperature belongs in that note.