What is the most essential tool anglers should never go without, that informs your choice of line, where and when to fish, presentation approach, retrieval style, and fly selection?

Answer: water thermometer

Using a water thermometer and understanding how water temperature effects feeding behavior is critical to success. Conditions such as rising or falling barometric pressure, northerly and easterly winds, or a full moon can slow down or temporarily stop the bite, but water temperature is the key to determining where and when fish feed.

Water temperature is the most significant factor affecting trout feeding behavior.

When I was out fishing recently, early spring water temperatures hovered at 49 degrees F. Trout were more active and feeding 3-5 feet below the surface late in the afternoon when water temperatures were at their highest.

This week due to abnormally warm weather, lake temperatures increased from 49 to 62 degrees F (a 13-degree increase). This affected both the depth at which trout were feeding and the time the bite started.  Trout were willing to leave the safety of cover to feed under bright sunny conditions. All the fish I caught were in the top two feet on the back end of my retrieve. Through the day, trout were actively feeding on prolific emerging chironomid pupae that ascended vertically up the water column.

Caught on Vickie’s UV Midge Pupa

Water temperature also affects various fish species differently. For example, at the two shallow lakes I tested this week, once the water temperature warmed up to 68 degrees, trout went off the bite and I caught only bass and bluegill. Optimum water temperatures for several freshwater species are listed below:

Type of Fish

Lower Limit (degrees F)


Upper Limit





Brook Trout




Brown Trout




Largemouth Bass




Rainbow Trout




Smallmouth Bass




Note how the upper temperature ranges vary by trout species. Anglers must be careful when landing fish when water temperatures are near the upper limit.  As temperature rises and dissolved oxygen levels decrease, fish will experience increased stress as they are hooked and played. The risk of trout mortality increases when playing a fish in higher temperatures.

What water temperature is too hot?

Studies vary on what temperature is lethal to trout. However, there is consensus that rainbow, brook, and brown trout begin to experience stress at 68 degrees F which rapidly increases as the water temperature continue to rise. For many anglers, 68-70 degrees F is the stopping point for fishing trout. At such times it is best to limit fishing to early mornings and late evenings when water temperatures are cooler.

How to use your water thermometer:

  • Find springs and cool water inlets: The cool, oxygenated spring water springs acts like a fish magnet, especially during the warmer months. When you locate lower water temperatures, this may prove a good place to target.
  • Vary your retrieve speed based on water temperature: When water temperatures are within the trout’s optimal range, your retrieve can be a faster. If the water is warm or too cold, slow your rate of the retrieve.
  • Keep a temperature log: The most critical data I collect is water temperature in my fishing log. After the course of many seasons, the data collected provides valuable insight in feeding behavior and presentation strategies upon returning to the lake the following season.
  • When you see significant drop in water temperature due to incoming storms, the change in water temperature can put the trout off the bite until the lake stabilizes. I have witnessed lakes going off the bite for up to a week due to a severe cold front moving through.

The bottom line is, I find using a water thermometer helps me catch more fish by providing valuable insight on the best presentation strategy.


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