Heatwaves and trout fishing

Another summer, another heatwave is hitting the Pacific North West. Temperatures over 110 are being predicted for the interiors of Oregon and Washington states. Globally this is not an isolated occurrence, with newsworthy heatwaves hitting northern Europe earlier in the summer.

I am not going to discuss the heatwaves in general. Other websites are more qualified, what I am interested in is how these hot weather events impact the trout fisheries, and what can we do in the short term to help.

Firstly, trout need cold water

When water temperatures get too high, trout struggle to get enough oxygen. If temperatures remain elevated for too long they die. Put simply, higher temperatures, warmer water, and fewer trout. It was estimated, that the recent heatwave in the United Kingdom killed tens of thousands of fish. The same stories repeat wherever heatwaves occur.

This chart below shows the temperature preferences of different species of trout. It can be a good idea to take a thermometer to the river with you to quickly know the temperature. Alternatively, the temperature for many trout rivers is published online. This website by USGS is a great place to start, and I am certain similiar resources exist for other countries..

Optimal Feeding temperatureTemperature trout become stressedLethal temperature
Brook trout44-64f (6-18c)65f (18c)70f (21c)
Cutthroat trout39-59f (4-15c)60f (15.5c)68f (20c)
Rainbow trout44-67f (6-19c)68f (20c)75f (24c)
Brown trout44-67f (6-19c)68f (20c)75f (24c)
Lake trout40-59f (4-15c)65f (18c)

Temperature preference and stress points for trout

Not enough water

In most catchments, heatwaves see a reduction in the flow of rivers, this is because hotter temperatures speed up evaporation and streams start to dry. This is compounded by the fact that heat waves are usually associated with high atmospheric pressures that further depresses spring flows.

To add to the water woes, the demand for water usually increases during heatwaves. Farmers want to use more to save their livelihoods, residential areas use more to keep cool. So there is less water, but more demand for it. This causes water levels to drop even faster.

In general, hot weather speeds up evaporation, reducing the flows of many freestone rivers. This in turn raises temperatures

This chart below, courtesy of the UGSG shows this summer’s water temperature of a Montana freestone river. Notice the sharp increase in water temperature that coincides with the start of the recent heatwave.

Too much water.

There is one exception, heatwaves can increase flows. Heatwaves can cause rapid melting of high elevation snow packs and glaciers which can cause significant floods in rivers which are fed by them. These will become increasingly rarer as the permanent snowpacks continue to melt and the glaciers retreat.

For example, due to a recent heatwave, I just read that the Greenland icesheet lost 6 billion tons of water for three days in a row. That is a big number that I struggle to comprehend. In other words, Greenland is losing a volume of water equal to a full Lake Mead every 5 days. Unsure just what impact that will have on the Arctic Char and Atlantic salmon populations.

Is too much water bad for trout? Will, it is better than too little water. But trout do best in stable flows. Violent floods can injure trout and wipe away the aquatic insects that trout feed upon.

Increased fire risks

Heatwaves result in wildfires, and wildfires spread faster during heatwaves.

Out of control wildfires cause obvious damage to forests and the animals that live in them. But how about the river life?

First some good use, the water usually protects the fish and aquatic life from the initial heat of the fire. So they might survive the initial burn.

The bad news is that the first heavy rains wash all that ash, mud, and muck into the streams. There the ash can block the gills of fish and smother aquatic life. The ash runoff after a fire can be devastating for trout. For example, the ash runoff from the 2019 Dollar Ridge fire killed all the brown trout in the Wild Strawberry stream.

Impact on trout fishing

I do not want to discuss broader impacts for too long and want to concentrate on how it affects us the fishermen in the short term. What can we do to help the trout, and can we still go trout fishing in such heat?

My personal opinion. Stay home until the heat subsides. Getting melted in high temperatures is not enjoyable and the trout have their best chance to survive if left alone.

Avoid catch and release fishing when water temperatures are high.

Hot summer temperatures is not a good time for catch and release fishing because the survival rate is significantly lower than normal conditions.

This is a simple one when water temperatures are high (see the previous chart), trout are already under stress. A prolonged fight for survival against fishermen is often enough to knock them over the edge. Either fish to keep, or chase a different species.

This is how I like to think about it, imagine being forced to sprint a mile in 100 degree heat. I suspect most of us will be exhausted after that. But, that is not all. After the sprint, while still panting for air you are forced to hold your breath for a minute. It all becomes too much, and many trout do not survive.

If you still plan on going trout fishing in warm water conditions. Finish the fights as quickly as possible, and leave the trout in the water at all times and expect almost any bleeding to be fatal.

Where to trout fish during hot weather?

Other than flying to the southern hemisphere, for some winter fishing. There are a few options that remain viable for trout fishing even in hot weather.

High mountain lakes and streams

Higher elevations, such as mountain lakes are excellent options to fish in hot weather. The higher above sea level the cooler it becomes. This is why some mountain lakes still have ice even in June.

So mountain lakes are a good way to escape the heat and catch some trout on even the hottest days.

Spring creeks are a consistent temperature year round

The next option is to fish spring creeks. Spring creeks get their water from aquifers that are nearly always cold. This means spring creeks can be a lot cooler than freestone rivers even in the same areas.

While the water temperature in spring creeks might be cool. The air surrounding them can be hot and humid. Bring water, and dress apporiately for the temperatures. Wearing waders in such temperatures is just asking for heatstroke.

Water temperature of a spring creek throughout a heatwave. The average temperature was about 15c or 59f.
Water temperature of a nearby freestone river, during a heatwave. The red lines mark the stress point and the lethal temperature for brown trout. The average water temperature was about 20.5c or 69f


Tailwaters that have their source deep within a lake are often much cooler than surface water. This is because deep water is nearly always colder than water higher in the column.

This makes tailwater another great option to fish in hot weather.

What to do with trapped trout in drying rivers?

I quite often come across trout trapped in isolated pools as their river dries. If you come across such fish it is a good idea to inform your local fisheries department.

Sometimes they have resources that allow them to capture and relocate fish caught in drying rivers. I have also assisted in a few relocations myself. It is a very good feeling saving wild fish from nearly certain death.

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