I ventured out on a hot June evening, three days into an official heat wave, rigged for dries. I found another angler in the spot I intended to fish and we struck up a conversation on the bank. “Nothing showing,” he said. I aimed my infrared thermometer at the water and was surprised to see it register 68.5 degrees. No wonder. My thermometer told me it was going to be a late night.
I’m not sure why these aren’t really marketed toward anglers, but the infrared stream thermometer is one of the most important tools I carry on the water. There are many reasons to check water temperature and as summer progresses the most important reason is approaching; fish mortality. When water temps hit 70 degrees, it’s a good idea to give the trout a break.
I used to carry a mercury thermometer, the type marketed to anglers by fly fishing tackle companies. It was better than not having a thermometer, but it was a pain to use: it needed to be submerged which meant carrying it on a lanyard or getting my hands wet. In winter, that’s an issue.
They can be difficult to read, especially in low light. “Lace it up in your boot” was one suggestion I read. Brilliant – if you have telescopic vision or Houdini-like flexibility, or if you want to confirm for the underwater world what they already know.
Consider the potential hazards of mercury and how easy it is to lose a thermometer in the stream. According to one public health website, one gram of mercury can contaminate a 20-acre lake. Most mercury thermometers contain more than that amount of mercury. I wonder how many get lost in the stream by anglers each season. Whether the mercury is truly a concern or not, I don’t really know, but it’s a non-issue for me personally because I found a better solution.
The model of infrared thermometer I carry is actually marketed as a tool for sommeliers and wine enthusiasts, but no miracles are required for it to measure the surface temperature of a trout stream.
I carry it on a retractor attached to the exterior of my chest pack. I removed the pen clip that came with it to avoid untimely snags on line and leader and secured the retractor to the thermometer with a zip tie. It’s where I need it, when I need it, and as a result, I use it often.
Here are few examples of the value of taking frequent stream temperature readings:
- Fish metabolism is closely related to water temperature. Temps can give you an indication of how the fish might be feeding and what water types they will inhabit.
- Temperature changes give an indication of what to expect from the fish: rising temperatures, even if the water is relatively cold, can trigger bite windows. The reverse is true of dropping temperatures when the water is on the colder side. Dropping temps after a hot spell can trigger fish activity. In other words, when fishing is slow, the thermometer can tell if that’s to be expected or if you might need to consider changing flies and tactics.
- Certain hatches are dependent on temperature. The thermometer can be more reliable than the calendar and fishing reports. I had a good hatch of Hendricksons all to myself in mid April with 47 degree water. Not exactly a science experiment, but it suggests that the rate at which water warms on a sunny spring day can be just as important as the actual temperature.
Is it accurate? I recently compared the readings of the infrared thermometer to a mercury thermometer and a digital thermometer with metal probe. The results were, as I used to often say was “good enough for government work.” Yes, it requires batteries and that means you have to change them periodically, but so do my headlamp and the SUV I use to get to the river. It’s a tough life we live….