Trout are from the family Salmonidae, which also includes salmon, char and even grayling and whitefish. All fish in the Salmonidae family require cold, clean water for survival. They evolved in richly oxygenated cold-water habitats in the Northern Hemisphere and are unable to survive when water temperatures rise and stay above 21 degrees celsius (70 degrees Fahrenheit).
On warm days, make sure the water is cold enough for trout. Here, Tim Linehan, of Montana, can tell the water is cold because he’s standing in it. Most experienced trout anglers can tell by feel if the water is a good temperature for trout. Photo by Chris Hunt.
Some years back, I was fishing a small stream on a stretch of state land in eastern Idaho for native cutthroat trout. It was a hot summer day — much warmer than normal.
I fished for an hour or so, and, while I saw some trout moving in some deeper water, I wasn’t having any luck. The little creek was small enough to where I didn’t need to wade. But, as I approached a deep pool, I realized I’d be able to make a better fly cast from the other bank.
So, I stepped into the stream with the intent of wading across. That’s when I realized why I hadn’t had any luck. The creek was much too warm for these fragile native fish.
All trout require cold, clean water for survival. At certain temperature thresholds, trout begin to get lethargic.
As the water gets warmer, trout actively seek out cooler waters, like spring seeps or inflowing tributaries that deliver colder water to larger creeks or rivers. For most trout, when water rises above 21 degrees (about 70 degrees Fahrenheit), the conditions can prove fatal, especially if the warmer temperatures persist.
Trout Are Fragile
I’ve fished for trout my whole life. They are, by far, the fish I have pursued the most, particularly with a fly rod. Trout can be very resilient. They can handle extreme floods. They can even survive drought, so long as enough water persists.
But trout cannot handle warm water. They have evolved over millions of years ago in cold-water habitats, and they require cold water to survive and thrive. Trout genetics are diverse, but they have not evolved to handle warm water.
Across the globe today, trout only live in waters that are cold and quite clean.
How cold? Water temperatures below 18 degrees celsius (about 65 degrees Fahrenheit) are ideal for most trout (and for most members of the Salmonidae family).
Trout are more active in colder water. They feed regularly throughout the water column when they are swimming in cold water. They reproduce better in cold water, too.
What Happens When Water is Too Warm for Trout?
When trout find themselves in water above 18 degrees celsius, they become lethargic. Their feeding activity slows significantly. They become harder to catch.
As water gets even warmer — above 21 degrees or so — trout begin to actively seek out “thermal refuge,” or colder waters. Some trout will find spring seeps that pipe cold water into streams from the creek bed. Others will stack up in the mouths of colder tributaries.
If trout can’t find colder water, they will eventually die.
On one of my local trout streams in Idaho in the northwestern U.S., I know I can find trout on hot days if I fish just downstream from the mouth of a small spring that brings ice-cold water into the creek.
But, in the interest of keeping the native trout healthy, I am very careful about limiting my catch on hot days. With so many fish nosed up into the colder water of the spring, fishing can be very good. I believe it can be too good.
Can Catching Trout in Warm Water Kill Them?
According to a 2022 study conducted by the Idaho Department of Fish and Game, trout mortality is 69 percent higher if they are caught in water that’s 22 degrees (about 73 degrees Fahrenheit) or warmer compared to trout that are caught in temperatures below 18 degrees (65 Fahrenheit).
That same study indicated that catch rates for trout in warmer water is 77 percent lower for trout. They don’t feed nearly as much in warm water, because their focus is simply on survival.
In other words, even if you release trout you catch in warm water, it can still still prove fatal for the trout.
As an angler, it’s up to you to determine if it’s safe to catch trout in warm water, especially if you intend to release the trout you catch. Ethically, when water temperatures rise above 18 degrees, you shouldn’t fish for trout.
Warmer water contains less dissolved oxygen. When trout bite a fly or a lure in warm water, it’s more difficult for them to recover from the fight. Their gills are processing less oxygen. Even if they’re released, they could very well die from the encounter with you.
Carry a Thermometer
As an angler, you can easily tell when water is too warm to fish. Simply carry a water thermometer with you. They’re affordable, and they can immediately tell you if it’s safe to fish.
I carry mine on a little tab on my fishing vest. If I have the slightest worry about water temperature, I dunk the device in the creek and take a reading.
You can do the same thing, and then consider the following:
- If the water temperature is between 10 and 15 degrees (about 50-60 Fahrenheit), you should go fishing. Conditions are perfect!
- If the temperature is between 15 and 18 degrees (60-65 Fahrenheit), go fishing but be mindful, and test the water temperature every 30 minutes or so.
- If the water temperature rises above 18 degrees, consider doing something else.
Trout are found the world over, but no matter they swim, they’re always going to be found in cold water. As anglers, we have a responsibility to be the stewards of the fish we pursue.
When water temperatures climb into the upper teens (celsius), it’s our responsibility as anglers to stop fishing and allow trout to seek out water that is colder and more oxygenated.
If you plan to catch and release trout, don’t put them under undue stress by fishing for them in water that’s precariously warm.
Why do trout require cold water?
Trout evolved in cold-water environments, and their genetics and morphology have not adapted to warmer waters.
Are salmon averse to warm water, too?
Yes. Salmon and char are in the same family as trout, and they, too, are evolutionarily suited to handle warm water.
Why is warm water bad for trout?
Trout require lots of dissolved oxygen, and cold water is high in dissolved oxygen. As water warms, oxygen levels drop. It’s simple. If you catch a trout in warmer water, it doesn’t catch its breath as fast as it recovers.
Think of it in these terms: if you run a mile at sea level, the air you breathe has lots of oxygen. You can recover and catch your breath fairly quickly. If you run a mile at a high elevation, it’s harder to catch your breath and recover because the air contains less oxygen. That’s what warm water is like for trout.
Why do other fish do better in warm water?
Other fish, like bass and carp, evolved in warmer water with lower levels of dissolved oxygen. Some fish, like gar, snakeheads and arapaima can actually gulp air in the absence of dissolved oxygen in the warm waters where they live.
What trout do better in warmer water?
Anecdotal research suggests that brown trout are more tolerant of warmer water than other trout species, like rainbow trout or cutthroat trout.