Though there isn’t a “perfect prescription” for catching trout, there are some guiding principles for fishing smarter. Understanding water temperature and its impact on trout behavior is definitely at the top of that list. The more you monitor water temperatures, the weather, and the surrounding environment, the better you will understand what is going on under the water. You will start to learn where the fish are and why they are eating or not eating. At the end of the day you will catch more trout!

Water Temperature and Trout Behavior
As we begin to look at water temperature and its impact on trout behavior it’s important to first understand a little about a trout’s metabolism. Unlike warm blooded creatures that regulate their body temperature with an internal thermostat, trout are cold blooded and adapt to the temperature of their surroundings. Whether they’re holding in a deep pool in the cold of winter or a shallow riffle in the heat of summer, trout are able to acclimate to the water around them by cooling or warming as needed.

Just as temperature variations will cause trout to undergo metabolic changes within, these same temperature changes will cause external shifts in the trout’s behavior as well. As always, it’s a trout’s primary objective to expend the least amount of energy to obtain the most amount of food. This is especially true with the cold temperatures of winter when a trout’s body slows and energy levels are at a minimum. To deal with this, trout will move into the slower sections of the river to conserve energy. They will also become highly selective in what and how much they eat. To put it simply, the margin of error becomes very slim for the fly fisherman during these colder months.

The opposite is true in the warmer spring and summer months when the water temperatures increase and the trout become more active. During this time the trout will move into the faster water, become more opportunistic regarding their food selection, and will be less hesitant to chase down a meal. This is when the hard work of honing your skills in the winter will pay off.

Water Temperature, Dissolved Oxygen and Trout Behavior
It wouldn’t make sense to talk about how water temperature impacts trout behavior without addressing the reason why trout are so affected by these changes. Just as humans need oxygen to live, trout do too. The only difference here is that the oxygen trout utilize is dissolved in water (dissolved oxygen) and in much smaller quantities than the oxygen found in the air.

Water temperature, dissolved oxygen and trout behavior relate to each other in this way. The colder the water the more dissolved oxygen is present. The warmer the water the less dissolved oxygen is present. This is an interesting paradox for trout because in the colder season, when trout are the least active and need the least amount of dissolved oxygen, the highest concentration of dissolved oxygen exists. In the warmer seasons, when trout are the most active and need the most dissolved oxygen, the least amount is available to them. Because faster moving water holds a higher level of dissolved oxygen than slower water you will often find trout moving into these faster riffles and bubbly plunge pools during the warmer months to help offset this deficit.

General Water Temperature and Dissolved Oxygen (DO) Ranges
35-37°| Bottom Temp Range | High (DO) Content | Trout will seldom feed
38-39°| Low Temp Range | High (DO) Content | Trout will selectively feed
40-54°| Middle Temp Range | High-Medium (DO) Content | Trout will consistently feed
55-60°| Optimum Temp Range | Medium (DO) Content | Trout are on the feed
66° +  | Upper Temp Range | Low (DO) Content | Refrain from fishing

What Drives Water Temperature?
To better understand how water temperature impacts trout behavior it’s important to have a general idea of what causes heat to transfer to and from water. The following factors are some of the biggest influencers of this transformation: radiation, conduction, and convection.

The first and biggest contributor of heat transfer to water is solar radiation. It is here thermal energy (heat) is transferred to the water’s surface through sunlight. As the sun’s rays penetrate the water, heat is dispersed throughout the water column. The depth of this zone is based on how far the light permeates the water as well how many particles and other suspended solids are present (turbidity). In general deeper bodies of water will warm slower then shallower ones.

The next influencer of water temperature is heat conduction to and from the earth’s atmosphere. Since heat always flows from a higher temperature to a lower temperature, when the air is cold, warm water will transfer energy to the air and cool off, and if the air is hot, cold water will receive the energy and warm up. Since water temperatures are less susceptible to fluctuation than air temperatures, water temperatures will often stay consistent even when air temperatures rise or fall. I have noticed this during winter evenings on the river when the water temperatures stay warm even after the sun goes down and the air temperature begins to drop considerably.

Another significant factor that can impact water temperature is convection. When the wind blows across the surface of the water it can significantly increase the rate at which water warms or cools. Wind cannot however push the temperature of water above or below the temperature of the air itself. It can only accelerate the rate at which it reaches the air’s temperature.

Note: Keep in mind that water sources that feed the river system are also major contributors to water temperature. Some of these could be groundwater, springs, tributaries and precipitation (snow and rain). Something else to consider is whether or not a river is a freestone or a tailwater. A freestone river is fed by snowmelt and can vary greatly in temperature depending on the time of year, snowpack, etc. Tailwaters on the other hand are dam fed and are usually more consistent in temperature. This is because the water flowing out of the dam (if it’s a bottom draw tailwater) is from the deeper part of the reservoir where water temperatures are less subject to air temperature fluctuations.

Key Takeaways
Below are a few practical steps that will help you begin to apply some of what we have discussed. Remember, this approach is more work upfront, but the rewards will pay off down the road as you begin to better understand the connection between water temperature and trout behavior.

  • Note the air temperature of your destination before leaving on your trip so you can compare it to the actual water temperatures from the river. This will help you start to understand the correlation between air temperature and water temperature.
  • Take temperature readings on the river. Notice some of the natural elements that may be impacting water temperature. Is there a difference between the water temperature in sunny areas versus shaded areas? Does the water temperature increase or decrease after a rain shower or a windy front comes through?
  • As you monitor these environmental driven temperature changes start to track how these may be impacting trout behavior. Did feeding increase or decrease during or after these periods?
  • Lastly, document your findings over multiple trips. One time isn’t enough to come to a conclusion on anything. You will need time to uncover patterns and trends.

Additional Resources

Fundamentals of Environmental Measurements
https://www.fondriest.com/environmental-measurements/parameters/water-quality/water-temperature/

Seasonal Strategies – Pat Dorsey
https://www.patdorseyflyfishing.com/seasonal-strategies/