It was early July, and the Pale Morning Duns were thick enough to bring the biggest fish up to the surface. The hatch, per my buddy, was incredible, and if I’d made it to the river just a few hours earlier, I’d have been right in the thick of it.
The fishing’s always better the day before or after you leave the water, but there’s not much you can do other than fish it hard when you’re there. So, we set off for an afternoon float, and I kept my eye out for fish in eddies that might be snacking on spent mayflies.
The fish weren’t looking up, but we pulled a few out of deeper runs on nymph rigs. After the third or fourth, I realized the fish my buddy had released was having a tough time swimming off. I stuck my hand in the water and was alarmed at how warm it felt. A quick dip of the thermometer showed a reading of 69°F.
This was in a tailwater, and we were maybe six or seven river miles down from the dam. The water warms up here in late summer, but not early July. It never occurred to me that the water would be that warm, that early in the year. As much as we didn’t want to, we stowed the fly rods and soaked in the scenery.
A Fish Murderer
We never found a great PMD hatch on the rest of that trip, and I mentioned both the warm water and lack of bugs in one of the online forums I’m a member of. Another member came at me pretty aggressively, calling me a “fish murderer” because I said I quit fishing for trout when water temps hit 68°F.
That’s the number I’ve always been taught is the cutoff for trout fishing. Mark Taylor, who covers coldwater conservation and trout fishing for Trout Unlimited, says that 70 degrees is the cutoff point. The folks at Wild Trout Trust, a UK-based conservation group, pegs 68 as the magic number. And a new paper highlighted at KeepFishWet creates the idea of a “sliding scale” for safe angling temps, with 61 being the cutoff for rainbow, brook, and cutthroat trout, while browns tolerate temps up to 66.
Montana, which is one of the few states to enact and enforce fishing bans based on water temperatures, only implements their hoot-owl restrictions when water temperatures spike at or above 73°F for three consecutive days. The North American Journal of Fisheries Management also says 73 is the cutoff for a marked increase in fish mortality during catch-and-release angling. Trout’s Fly Fishing marks 67°F as the temperature at which you should stop fishing. Yellowstone National Park closes fishing when water temperatures exceed 68°F.
I could find more examples, but you get the point. There seems to be only a consensus that warm water temperatures negatively impact trout, where exactly that starts feels far from settled.
What should you do?
Which brings us to an important question, especially in early August when many low-lying rivers are probably too warm to fish: when should we stop fishing for trout? It’s undeniable that warm water has lower amounts of dissolved oxygen. Trout caught in warm water have a harder time recovering and a higher chance of dying after release in water with less oxygen.
I wish I had an answer. All I know right now is that it’s important to take care of trout in any way we can. For me, that means I stop fishing when water temps hit 68°F. That number seems to be the most commonly accepted. I’m completely open to changing that threshold, though, if we can ever settle on one.
For now, I’ll enjoy some high-country fishing and leave the low-lying creeks alone. They’ll be back in shape in a couple months, anyways. And perhaps that’s the real lesson here: regardless of whether 67 or 68 is the magic number to stop trout fishing, these warm temps push us to explore the places that are really only fishable a few weeks out of every year.