How Sunlight Affect The
Copyright 2020 James Marsh
by James Marsh
February 2020 Issue
You may have heard that fish avoid the sunlight because they don't have eye lens and that the bright
sunlight hurts their eyes? I have read that many times over the years; however, it's one of those things that's
only partially true. Fish don't have eye lens but the light from the bright sun doesn't hurt their eyes. Common
sense should tell you that. If it did, they would avoid the bright sunlight and many species of fish don't avoid
Although some species of fish do avoid the bright sunlight, it's not because it hurts their eyes. They avoid it
for other reasons. They prefer to hide from their predators in the dark, shaded areas of a stream, or they
prefer to hide as predators themselves. Many species of fish have excellent low light level vision compared
to their prey. These fish prefer to feed during low light levels. The brown trout is one of those species of fish.
The eyes of a fish are round. Their two round lenses allow them to see clearly under water. The human eye
lens is flat and when opened underwater, things become blurry because of that.
Different species of fish have different feeding habits. Large brown trout feed more like largemouth bass
than rainbow trout. There are exceptions, but the largemouth hide in cover, under a log or in a thick bush
and use the element of surprise to attack their prey which consist mostly of other fish. They prefer low light
levels because it gives them the advantage over fish they like to dine on. This has been proven over and
over during large bass tournaments. When there's a low pressure system, versus a high pressure system,
the overall number of fish weighted in is always much higher. Low pressure systems bring clouds and
sometimes rain, whereas high pressure systems typically bring clear, bright blue-bird skies. There aren't any
brown trout tournaments as such, but I would assure you the results would be similar. Brown trout prefer to
feed at night and in low light levels.
During a bright, sunny day using polarized glasses, try spotting trout from some of the high banks along the
streams in the park. Remember, the higher you are the easier it is for the trout to spot you, so stay hidden
from them. See how many rainbow trout you see out fully exposed to the sunlight versus how many large
brown trout you spot. I venture to say you will find some rainbows, but not the first large brown trout. Just to
qualify what I mean by large brown trout, lets say one over 14 inches. The only time you may spot a large
brown trout under these same conditions is during the spawn when they lose much of their caution. You will
often spot rainbows directly exposed to the sunlight, even during the middle of the day. Sometimes they will
be in a feeding lane eating emerging aquatic insects and sometimes just looking for food coming
downstream along the bottom. Sometimes they will be holding shallow and other times they will be holding in
deeper water. Often, when you spot a rainbow in the bright light, it will be its shadow on the bottom
you are actually looking at.
A large brown trout captures its prey more like a cougar or leopard. It relies on low light conditions to
hide and pounce on other fish and crustaceans. They too, use the element of surprise much like a
largemouth bass. You will find the large brown trout well hidden during a bright sunny day. They will be
underneath the crevice of a large boulder, undercut banks, or anywhere they can find to hide out of the
direct sunlight. They are mostly nocturnal, meaning they feed mostly at night, or in very low light conditions.
Often they feed very early or late in the day, and sometimes during dark, cloudy days. They may tend to
roam around and feed in dingy or off colored water during the day, but rarely, if ever, under clear water
The rainbows are more like the cheetah than a cougar or leopard. They don't hide and attack their
prey. They will feed right out in the open water and they will do so smack in the middle of the day in bright
sunlight. Now, I should stop and say that this doesn't mean that rainbow trout will eat more or even as much
on a bright clear day as they will on a dark cloudy day. They will generally eat more on the low pressure
system days, but it isn't because of the light's effect on their eyes. It's because most species of aquatic
insects prefer to hatch under low light conditions. This doesn't mean that the aquatic insects don't hatch on
bright clear sunny days, because they do. They just generally don't do so with the same intensity as they do
on cloudy days. You will often find a particular species hatching during a time span of a week or two when
there aren't any low pressure days. They just string out the hatch to last longer and there's almost always
less insects hatching. Of course, this is also affected by the time of year (low sunlight levels) and time of day
the particular insect hatches. The rainbows seem to care less. They will feed on emerging aquatic insects in
the bright sunlight or under the shade of cloud cover.
When the weather is hot and someone tells you that you should only fish the shaded areas of a trout stream
during the day, they may well be lowering, and possible even eliminating, your odds of catching rainbow
trout. That's poor, misleading advice.
Most trout streams, certainly the ones in Great Smoky Mountains National Park, are flowing and usually
rather fast. The water temperature is the exact same in the shady spots as it is in the bright sunlight. If you
don't believe this, just measure it for yourself. I have many times and I always find there is absolutely no
difference in temperature. By the way, I also have a very expensive, professional water thermometer that I
have used to test water temps although that isn't something I carry around to use fishing. I am just pointing
out the instrument is accurate to within a tenth of one degree F.
Any angler that has fished very much knows that rainbow trout will feed in the open water in the direct
sunlight at times. I would venture to say that I have caught at least a thousand rainbows doing just that and
probably many more. I couldn't say that about the brown trout, although the small brown trout will sometimes
feed much like the rainbows. I have caught plenty of larger, wild, brown trout on the surface on a dry fly (not
in the Smokies) from streams like the Big Horn, but thats not usually the case. I don't remember catching
many large browns feeding on insects during the bright sunlight but it happens sometimes. They will do that
when the large Golden Stoneflies and/or Salmonflies are depositing their eggs during the day on some
streams. Even then, you will find the brown trout will feed much more aggressively during low light conditions.
Large brown trout feed on other fish, nymphs and crustaceans. They don't normally do this on the surface
in bright light.