Rainbow trout are cold-water fish and need low, oxygen-rich water temperatures.
Compared to other trout species, rainbow trout can survive in “warm” water that does not exceed 80 degrees. But these temperatures affect feeding activity, growth, and survivability.
Water Temperature and Trout
If you’ve ever noticed a pond turn green in the summer (like the one in front of my house), it’s because warm water cannot hold dissolved oxygen. On the contrary, cool temperatures in the winter create a clear pond with higher oxygen levels.
When it comes to trout, they need clear, oxygenated water year-round. Despite their resilience, all trout are sensitive to temperature changes. Temperatures beyond 70 degrees can be fatal.
The ideal temperature range for rainbow trout to feed is 52-64 degrees Fahrenheit. Trout will feed below this range but at a slower rate as the temp drops. In my experience, 37 degrees is the threshold where lethargy will turn to lockjaw. They won’t bite.
Rainbow trout might feed over 64 degrees but with as little energy expenditure as possible. If you see flashing trout in water this warm, remember that they might not be feeding. Instead, they’re stressed and circulating water in their gills for oxygen.
Unsurprisingly, feeding temperatures and growing conditions share the same range. Anything below 50 degrees will slow a rainbow trout’s growth, and anything over 70 degrees will stop growth.
Even as eggs, temperature affects growth. Water in the mid-40s causes eggs to hatch seven weeks after fertilization. Temperatures in the mid-50s hatch eggs three weeks after fertilization. In the wild, consistent temperatures are vital for a successful spawn season.
For fragile fish, rainbow trout and brown trout have impressive survival rates in warm temperatures. Studies have shown rainbow trout surviving in water up to 80 degrees. This tolerance is the main reason for their stocking success in different types of water.
How Hot Is Too Hot To Fish?
Though rainbow trout are hardier than other trout species, any water temperature over 67 degrees is too warm to fish.
While many anglers claim 70 degrees as their limit, I set the rod down when the water is over 65. In my experience, temperatures this high cause intense stress during the fight. Even with proper handling and a quick release, you will find trout belly up downstream because they couldn’t recover.
Next time you’re at the tackle or fly shop, consider purchasing a thermometer. First, seeing the correlation between water temperatures and fish activity makes this hobby a science rather than an art. Second, knowing if it’s too hot to fish will not be a guessing game.
Finding Trout On Hot Days
Just because you’re in the middle of a hot summer doesn’t mean you can’t fish. Start fishing in the morning to find trout on a hot day. The night’s cooling temperature will last into the mid-morning for safe fishing.
Consistent shade also offers cooler temperatures and a refuge for trout. But the optimal setting for a summer-time climate is a spring-fed stream. Look for tributaries and creeks that have spring sources. Trout congregate below these feeder creeks for comfort and food.
How Cold Is Too Cold For Trout?
Comfortable temperatures for trout are between 35-65 degrees. However, every Wisconsin winter, I find plenty of trout in sub-30-degree water temperatures.
Though they’re not feeding, trout in these temperatures await spring’s thaw with the rest of us.
What Trout Is Most Tolerant To Warm Water?
But high tolerance doesn’t come without consequences. With a hardy disposition, rainbow and brown trout displace natives in local streams, such as brook and cutthroat trout.
As a cold-water fish, rainbow trout have the highest tolerance for warming temperatures. But water over 67 degrees is unfishable, and any water approaching 80 degrees is lethal.
Next time you go trout fishing, carry a thermometer to measure stream and trout health. This way, you can fish with ethics and confidence.