How Light Affects Fish

A photo period is defined as the amount of sun light in a twenty four hour period. It is influenced by the amount of cloud cover and length of sun light on a daily basis. Seasonally, summer months have longer photo period days, and the angle of sunlight is more direct. The fall, winter and spring months include both shorter daylight periods and lowered sunlight angulations. In addition, there are longer shadows in winter than during the mid summer times. Seasonal variances influence the amount of light entering the water. Most animals have an internal clock, called the circadian rhythm, which is modified by the light/dark cycle every 24 hours. The eyes play a primary role in this, but body cells have some reaction to light levels as well. The body cells in fish are especially sensitive to changing light conditions daily.

As you may know, light diffuses through water differently than through air. Reds and orange with longer wavelengths are absorbed first, with 25% of red light being absorbed in the first meter. Violet doesn’t become indistinguishable until below 100 meters. Fish which live in predominantly shallow waters are more sensitive to the longer wavelengths of light such as red and orange. As depth increases, the visual spectrum narrows considerably, starting with the reds and orange. The majority of fish can distinguish colors quite well, but the maximum distance they can see is approximately 15 meters. Light rays striking the water’s surface at a right angle travels through it with little deviation. The angulated sunlight is less illuminating underwater because some of it is reflected away at the surface and the rays penetrating the water bend upwards, thus allowing less light to penetrate. Infra-red rays tend to affect fish more because they are elongated and penetrate cloud cover more readily. Would this be an argument for adding the color red to a lure because red is more visible in shallow water?

Fish eyes are different from humans. Human eyes have a cornea, an iris, a lens, a pupil, and a retina. The human retina contains rods which allow us to see in dim light and cones which perceive colors. Humans have two types of cones, fish have three…one for each of the colors red, green and blue which connect to nerve cells that transmit images to the brain. Humans have pupils that expand or contract to alter the amount of light entering the eye and eyelids, both of which help to prevent damage occurring due to bright light. Fish (exceptions are some shark species) do not have eyelids, and in most species their pupils are fixed and cannot alter. In bright light, the rods retract into the retina and the cones approach the surface, and in dim light, the opposite occurs. But unlike our pupils that change quickly, this process takes time in fish. Scientific studies on salmon have shown that it takes half an hour for the eye to adjust to bright light, and an hour to adjust to dim light. A fish’s retinas are specially adapted for seeing in dim light. As stated above, during periods of bright daylight, the rods become buried in between pigment cells lining the retina. In contrast, the cones which are adapted to sensing bright light move to the surface of the lining in response to increased illumination. The lower part of the retina, in the majority of fish, possesses more cones, and fewer rods, than the upper surface. This is due to the fact that the upper part of the eye receives more dispersed light than the lower half of the eye. The rods and cones in the eye shift according to the changes in light. This process is also anticipated according to the time of day, and the fish “expects” dawn and dusk, and the eyes will automatically begin to adjust accordingly. This is due to the circadian rhythm. This is one reason why during each 24 hours a regular period of light/dark is essential for fish.

Light also has profound effects upon the internal processes of fish. Light has a great influence on the development and metabolism of fish. An increase in light may produce a significant acceleration in egg development and production. In many fish species, the maturity of the gonads depends on the duration and intensity of light. If fish do not receive the correct amount and intensity of light, they may not develop properly and normal metabolism is altered. There are studies on spawning in some species being triggered by changes in the day/night cycle, and the hatching of eggs and the growth rate of fry can be impacted significantly depending upon the presence and intensity of light. The health of fish is closely connected to the intensity, types, and sudden changes in light.

Fish are more alert during bright sunlight conditions because they are more visible as prey. Food supplies for fish are most abundant in the shallow shore zones which are located in areas of more intense light penetration necessary for photosynthesis. Fish may only feel safe in these shallow zones during low light conditions. Fish will try to find shallows during times of low lighting which is usually early and late in the day or at times of seasonal low light conditions, occurring in the fall, winter and spring. Don’t forget light affects insects as well and are most active during low light periods and search out shaded areas to rest when the sun is the brightest. During mid-summer a fish’s presence in the shallows may be restricted to times of dawn, dusk or overcast days. A good place to find fish during bright light conditions is in darker areas adjacent to the shallow shore zones or in heavy cover in shallow water. During the winter, spring and fall you will most likely find fish spending more time in the shallows than they do during the summer season. Dark overcast and rainy days can draw fish into the shallows to feed, and dusk and dawn are also prime times for fishing these shallows. As the light intensity increases, fish will move to deeper water to adjacent channels and drop-offs. The activity patterns of fish are also related to light. Vision-orientated predators are most active during the day, often with peaks of feeding in the early morning and evening when food sources become more active. Piscivorous (fish eating) fish are most active at dawn and dusk. The movement of microorganisms and phytoplankton, the food source for baitfish, is also affected by light intensity. At dawn and dusk the food source is usually higher in the water column for both baitfish and feeding fish.

Remember, there are no set rules in fishing. One must be ready to think out of the box at times when fish are not responding to deep water presentations. Many active bass stay shallow in the hottest, brightest days of summer. The secret is to fish the right cover (wood and grass) with the right lures. These covers provide shade and ambush points. Plastics worked very slowly are best. Many of us just don’t have the patience to slow down to work plastics they way we should in hot weather. Making accurate, quiet presentations into bass-holding pockets is very important. A push pole may be needed when fishing near heavy cover near the shorelines. Surface lures are not just for morning and evening. Bass will violently crush surface lures in the middle of the hottest, brightest day of the year as long as you present the lure quietly where they are hiding. Bass don’t visit shallow water just in the summer, but in winter on bright sunny days, some of the biggest bass will be holding in shallow water where the water temperature has warmed, especially if the shoreline has any pebbles or rocks. The key again is to fish slowly with jigs and suspending jerkbaits. Yes, fish are very sensitive to light but don’t forget they find a way to ambush prey on the brightest of days.

Always enjoy your posts. Good read

Same here, Bonefish. Much thanks for all your insightful posts.

I will add, I just finished well in tourney and my biggest fish were caught in the heat of the day, VERY shallow, in the shade.

[QUOTE=bigwormy;519319]I will add, I just finished well in tourney and my biggest fish were caught in the heat of the day, VERY shallow, in the shade.[/QUOTE]

Those big fish will surprise you sometimes where they find to lay.

Are you a biology major? I loved biology in school. It was my favorite subject in HS and College.

You touched on the phytoplankton a bit but I'd like to know more about how the zoo plankton and phytoplankton respond to changing light conditions.

Remember that green leafy submergent plants produce the most oxygen during the brightest times of the day. So there is a lot of Dissolved Oxygen being given off by these submerged plants in the hot summer months. Shallow water also gets more oxygen from the air especially along a windward shoreline with lots of wave action. I think that maybe the waves increase the surface area and allow more oxygen to go from the air into the water by osmosis.

During the summer months in the middle of the day I'm seeing a whole lot of fish suspended out over the deepest parts of the lake I fish. This lake or old strip pit is about 60 something feet deep at the deepest basin. This area is quite large too. These fish are suspending at about 15 to 35 feet down from the surface over 60 ft of water. I see a lot of fish down even at 40 ft and wonder if there is a thermocline or not? Maybe these are catfish or carp that don't require a lot of dissolved oxygen and can live in the thermocline or below it for a time? I'm not really sure what they are. But I know that trolling a bandit 200 crank bait with 200 or 190 ft of ten pound test Stren Mono out will catch a nice 2.5 lb bass or two. And I've caught some nice 9 and 10 inch bluegills. I also will catch some nice crappie and bass along the drops off in the deeper water. These crappie are suspended down around 12 to 15 ft and over 30 ft of water which is near a feeding shelf that extends out from the shoreline about 50 yards and is about 7 to 8 ft deep before it drops down into the deeper waters. The submerged weeds stop growing at around 7 to 8 ft deep due to the turbidity of the water. There is a lot of algae in the water and the shoreline is full of green algae that grows on the surface.

Also the deeper water is full of something that shows up clearly on the Humminbird 898 C SI unit around 28 ft deep all the way down to the bottom in 60 ft of water. This is when using Max mode and lessening the filter and noise settings to min. Increasing the sensitivity makes even more debris show up on the sonar 2d and down imaging screens.