Fly Fishing For Trout - Warm Water Temperatures
One of the most frequent type of questions we get at Perfect Fly during the hot summer
months relates to how warm water temperatures affects trout and trout fishing. First of all,
anytime you are fly fishing for trout in either cold or hot weather conditions, you should
have a thermometer along with you. Knowing the water temperature can be very
important for several reasons. It's best to always have one with you. By the way, there
are both analog and digital thermometers. The best types are the digital models.
One of the first things we advise anglers to do is to fish early in the mornings when the
water temperatures are the coolest. By early, we mean from not long after sunrise to
about 9:00 AM. This is earlier than most fly anglers get up in the mornings but it may be
the only time the trout are active and the only time you can catch them without adding
In the early mornings, the low water temperatures and low light conditions are usually
favorable. When the water temperature is around sixty-eight degrees F., there's nothing
wrong with fly fishing for trout but you need to know the warmer temperature of the water
does affect the trout. In general, when water reaches the high sixties it becomes much
more difficult to catch trout. This is especially true if there is low water conditions which
are often associated with warmer than normal water temperatures. .
Low water conditions can be responsible for the water temperatures rapidly changing. It
takes less time for shallow water to adjust towards the air temperatures than it does deep
Another handy item of equipment that helps out in most fly fishing situations involving
warm water conditions is a GPS. They can provide the elevation. Higher elevations
almost always have cooler water. Of course, you don't need a GPS with elevation data to
get to higher elevations of the streams you are fishing. It's just good practice to know just
how the elevation changes affect the water temperatures for future reference. We always
have a handheld GPS handy.
Another question we often get is "at what water temperature should anglers stop fishing
for trout". Provided reasonable care is taken, there's nothing wrong with catching trout on
the fly with water temperatures up to the low seventies if you care to try. The problem is,
it probably won't be easy. When temperatures reach the low seventies, the fish almost
cease feeding. The problem isn't the water temperature as such. It's the low oxygen
levels associated with the warm water.
The temperature of the water and the oxygen content are inversely proportional. It
doesn't change on a straight line basis. It changes on a sharp curve. The higher the
water temperature, the lower the oxygen level.
Studies done on the Firehole River in Yellowstone National Park have shown that when
the water is in the low seventies, the trout will actually lose weight. They simply don't feed
as much. The solution is to find cooler water, but if you insist fishing in water in the high
sixties or low seventies (which we do not recommend because of the increased difficulty),
find the highly oxygenated areas of the water - water falls, plunges, etc.
Regarding the question of whether or not it hurts the fish to catch them in water as high
as let's say seventy degrees, the answer is no - not if reasonable care is taken. By that
we mean, if the fish is released quickly after being caught, they should be fine. When
water temperatures are in the low seventies, it's best to leave the fish alone. The point at
which high temperatures can hurt the fish varies from species to species, area to area
and exactly how much stress is put on the fish during the catch and release. As a general
rule, we would say that when the water is close to seventy degrees, it is best to stop
Some believe that trout caught in warm water swim off and then die. That's possible of
course, but not probable. When a fish that is overextended, so to speak, revives and
swims off, it's usually fine. Don' try to revive them. Just release them quickly. What does
hurt the fish, especially at high water temperatures (and air temperatures after being
caught) is keeping them out of the water a long time. Far worse is mistreating them. I see
photos all the time where people have laid trout on the ground (usually by their rod) and
photographed them. That's not good.
All fish have a slimy coating on them that helps protect them. It helps prevent fungus for
one thing. It's easily removed from their outer body. Handling them with dry hands and/or
a dry landing net can do it. Always wet your hands and net before landing fish. Laying
them on the ground or any dry object can remove portions of it.
We should also mention that you should not fight a fish a long time on ultra light tippets
under warm water conditions as this adds to the stress. If a large trout is caught and
fought for a long time, it can be easily over stressed. So, it's possible to do that. If you go
early, check the water temperature, and fish at higher elevations, you can do well in most
marginal warm water situations. Just use common sense and take care of the fish.
Copyright 2013 James Marsh
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