When it comes to finding fish that are actively feeding, a stream thermometer is as close to a crystal ball as it gets. Using a stream thermometer corretly is an affective fishing tool – LEARN HOW HERE.
What is a Stream Thermometer?
A stream thermometer is a thermometer—just like what you’d use on the Thanksgiving turkey—that’s designed for use by anglers, biologists, or anyone wanting to measure the temperature of water.
To take a temperature reading, you submerge a stream thermometer for 10 to 15 seconds, or as long as it takes to acclimate. Pull it out and take a look at the mercury. If you’re lucky, it’ll be within the prime fishing temperature range, which we’ll get into later.
Does Water Temperature Matter for Trout?
Yes, it does. And a lot more than you may realize. Let me share a quick story to show you what I mean . . .
Recently, I was fishing one of my favorite spots and wasn’t having any luck, although I had caught many fish there before. It’s a beautiful spot where a run dumps into a hole with lots of structure, just downstream of a feeder creek that enters the main river—perfect for swinging streamers.
After fishing hard for over an hour with no success, I decided to have a cup of coffee. Watching the steam rise from my mug into the crisp March air, which was in the low to mid-40s, I started wondering about the water temperature. So I pulled out my thermometer and measured the temperature where I was sitting right at the outlet of the feeder stream. 38 degrees . . . ugh! No wonder I hadn’t caught anything.
Then, I measured the water temperature in the main river above the feeder stream. To my astonishment, my stream thermometer read 46 degrees!
This simple experiment showed me that my favorite spot in warmer weather behaves exactly the opposite in winter and early spring. In the summer, the ultra-cold water from the feeder stream keeps my favorite hole fishing great whereas early in the season, it turns off the fish.
The key is finding the right temperature range.
How Water Temperature Affects Trout
Along with other environmental factors like water quality, the abundance of food, and adequate cover, fluctuations in water temperature have a massive effect on both a trout’s survival and day to day, minute to minute, behavior.
Here are a few reasons why:
- Trout are cold-blooded. A trout’s body temperature remains more or less the same as the water surrounding it. In cold water, trout metabolize food more slowly, causing them to eat less—there’s your first clue to solving the trout water temperature mystery.
- Water temperature affects dissolved oxygen levels. Basically, the warmer the water, the less dissolved oxygen it holds, and the harder it is for trout to breathe. With less oxygen present, trout in warm water become lethargic and their feeding activity decreases dramatically. There’s your second clue.
- Trout thrive within a specific temperature range. The exact range varies across different trout species and water bodies, but trout-friendly temperatures generally fall within 45 to 65 degrees. I’ve found 48 to 53 degrees to be the real sweet spot in my experience.
- Insect activity is influenced by water temperature. You may notice that mosquitos in your area get really bad when average daily temps hit a certain level in the spring. The same is true for the insects trout eat in streams. As you take stream temperatures over the course of a season, you can observe what insects hatch and at what water temperature. Keep notes and watch for trends to help inform your fly selection.
- Even small fluctuations affect the bite. Ever have one of those days when the bite suddenly turns off for no apparent reason? Especially if you’re fishing in water that’s on the edge of being too cold or warm, a small bump in temperature could be just enough to shut it down.
Are there Different Types of Stream Thermometers?
When shopping for a new stream thermometer, you’ll find two main varieties:
- Analog — Your standard mercury or alcohol-based glass tube thermometer. Analog fly fishing stream thermometers are fully submersible so you can check the water temperature at different depths.
- Digital — Similar to instant-read kitchen thermometers, digital fly fishing thermometers use a probe to measure the temperature of water, which is displayed on a digital screen.
Analog thermometers, like the River Traditions Stream Thermometer, are considerably more versatile and reliable than digital stream thermometers that are more expensive and more prone to failure. Options for quality digital stream thermometers are limited, and the one that did exist—the Orvis Digital Stream Thermometer—appears to be discontinued.
Will I Catch More Fish if I Use a Stream Thermometer?
The short answer is yes, but with one caveat:
You must remember to actually use your stream thermometer.
Keep your stream thermometer clipped to your vest or pack and get in the habit of checking the water temperature frequently. For example, whenever you change flies, take an extra 20 seconds to take a reading. If you notice a hatch getting started, or better yet, catch a fish, whip out your thermometer and look for that temperature in the future!
You don’t have to take detailed notes every time you check the water temperature—you’re fishing; not conducting a scientific study. But you should pay attention to the correlation between fish and insect activity and water temperature. Notice how the stream changes from month to month or how an influx of rain affects water temperature. All of this information is only attainable through experience, and if you put in the work, you can use it to your advantage to help you catch more fish.
What are the Best Temperatures to Catch Fish?
While we can make rough predictions about their behavior, fish can be unpredictable and it’s impossible to know exactly what they’ll do next. That said, fishing within specific ranges of temperature depending on your target species is the smart thing to do for better odds at hooking up.
To give you an idea of the preferred temperature ranges of different fish, here’s a quick chart for reference:
Stream Thermometer Tips for Fly Fishing Success
Curious how to put your stream thermometer to work? Here are some tips to help you get started . . .
- Choose stream structure to fish based on water temperature. From bank to bank and run to run, stream temperatures can vary depending on water depth and structure. If you’re floating a stretch of river in a drift boat, dunk your thermometer into a deep hole early in the day—if you get a reading that’s too cold for trout, you’ll know to avoid such water as you fish throughout the day.
- Find springs and cool water inlets in the summer. If you’re lucky enough to fish spring-fed creeks, a stream thermometer can help you find the coldest portions of the stream amid the highs of summer.
- Vary retrieve speed based on water temperature. Try to fish at the fish’s pace. If water temperatures fall within a trout’s prime range, those fish will be raring to go and can be targeted with a faster retrieve rate. If the water’s on the warm side, be patient and slow down your retrieve.
- Keep a temperature log. The real magic of stream thermometers is that they help you gain a more intimate understanding of a fishery. But to gain the many advantages such understanding brings, you have to make a reading stream temperatures a habit and stick with it over the course of the season and into the future. In a basic notebook, write down the date, the stream or lake, any fish you caught, and the water temperature. Keep it simple or get as detailed as you want, just don’t forget to reference this hard-earned data.
- Pay attention to temperature trends. Downward trends in water temperature are usually a bad sign. Upward trends are better—every tick the mercury climbs could send the trout into a feeding frenzy. But what you really want is stable temperatures. If for several days or weeks in a row, your favorite stream is running consistently within prime trout temps, do whatever it takes to get out there. That’s what sick days are for, right?
Although it’s called a “stream” thermometer implying use in running water, these handy devices can also be used in stillwater lakes and ponds as well as inshore and offshore saltwaters. No matter where you’re fishing, carry a stream thermometer to help you catch more fish.