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More than meets the eye.

How and what fish see in water has long been a hot topic of debate among anglers. It affects everything we know about our sport, and the decisions we make (often subconsciously) about what lures to use, their size, and especially their colour.

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Despite differences in habitats, humans and fish have similar eyes. We share lens-covered retinas that house rod and cone cells. Rod cells perceive contrast in low light (basically black and white vision) and cone cells provide colour vision. Less than 10 percent of our cone cells respond to light from the blue end of the visible spectrum. The other two types of cones--red-sensitive and green-sensitive--occur in variable proportions in people who otherwise have normal colour vision. In fish, as there is some variation across the spectrum of species, the three cone types come in even more variable proportions. Some fish even have a fourth type of cone cell for sensing ultraviolet light, while some can sense polarised light.

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So, what can fish see, and how is it different to how we perceive vision? For humans, the medium is air, and for fish, the medium is water. Water is approximately 780 times denser than air. As light hits water, not only does the light bend, it is selectively scattered and absorbed. Most (78%) of the visible spectrum is absorbed within 10 meters. In clear water, the long wavelengths we see as red, yellow, and orange are lost first, while the shorter wavelengths of the visible spectrum--the colours we see as violet, blue and green--can reach farther, with blue penetrating clear waters to a considerable depth. How far light penetrates on a given day depends on the angle of the sun (time of day, time of year), the choppiness of the water's surface, and what is actually in the water column (algae, sediment), etc.

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What do fish see above the surface?

While we know fish have good in-water vision and are well adapted to their varying environments, just how much they can see outside of their medium has a bearing on how we present baits and lures, and even ourselves. There is a phenomenon known as "Snell's Window", which was first described by mathematician Willebrord van Roijen Snell in 1621. Snell's

Window is essentially a round portal from which the surface world is visible from underwater. The window is formed by the refraction of light as it enters the water, causing a 180[degrees] view of the world to be condensed into 97.2[degrees]. This is because light that enters or leaves the water within the "critical angle" of 48.6[degrees] will be refracted and can be seen underwater, while outside of the critical angle, it will be reflected away.

A fish can see anything within the 97.2[degrees] (twice the critical angle) sight window of the Snell's Window, but objects near the edge of the window will be severely distorted and out of focus. The greater the angle, the greater the distortion, while objects directly above the fish will appear almost completely undistorted. The deeper a fish is, the larger the diameter Snell's Window becomes (the diameter is 2.26 times the water depth of the fish), although the 97.2[degrees] angle remains constant. Beyond the edges of Snell's Window the surface of the water appears as a mirror, reflecting the bottom, though if the surface is choppy, the mirror effect is reduced.

As anglers, Snell's Window is important when fishing clear water, especially shallow water, or when one has to approach closely or the fish are particularly skittish. Keeping a low profile relative to the horizon can often mean the difference between catching and not. If you can stay below about 20 degrees above the horizon you may still be visible, but you'll be distorted and difficult to see. Avoid bright coloured clothing and try to limit your movement.

Some carp anglers fishing the clear and relatively shallow waters on Mud Island, often employ some pretty ingenious methods to remain undetected. I have seen people climbing trees to cast, lying down at the water's edge, or even erecting scarecrow-type figures days ahead of visiting their chosen fishing hole. I was interested a couple of years ago to witness a makeshift scarecrow along the banks of the Zambezi at the Barotse flood plains. Although not employed often, the method obviously works well in African waters, too. Even if you are not going to erect a scarecrow, try to find a natural backdrop that will mask or break your silhouette.

Obviously, surface lures and flies have a better chance of success if cast within Snell's Window, as the fish will be more able to see it. Anything outside that window, when seen from below, is a "mirror" that reflects the bottom. Remember too, that while you may conceal yourself adequately, waving a six foot fishing rod around can defeat your disguise. Learn to pitch or employ a side-arm casting technique.

How fish see us

Interestingly, a recent article entitled "Discrimination of human faces by archerfish (Toxotes chatareus)" (by Cait Newport, Guy Wallis, Yarema Reshitnyk & Ulrike E. Siebeck), sought to determine whether a species of fish, unlikely to have experienced any evolutionary pressure for human facial recognition, could learn to discriminate human faces.

The archerfish was used as a model for this behavioural experiment as it is known for knocking down aerial prey with jets of water. This fish relies heavily on vision to detect small prey against visually complex backgrounds, demonstrating significant visual cognitive abilities, especially as it is able to compensate for refraction. Part of the study was to determine if animals without a neocortex can learn to recognise human faces.

The neocortex is the top layer of the cerebral hemispheres responsible for higher-order thinking and accounts for about 76% of the human brain's volume. It is involved in functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and in humans, language.

Human facial recognition has been demonstrated in birds (pigeons especially), sheep, dogs, cows and horses--all creatures which possess a neocortex and have been domesticated and have possibly experienced evolutionary pressure to recognise humans. In their experiments, the scientists showed that archerfish can learn to discriminate a large number of human face images even after controlling for colour, head-shape and brightness by standardising these aspects.

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Fish present an interesting example as they can use colour patterns for recognition which are additionally affected by changes in water quality and lighting. Because different wavelengths are not attenuated equally in water, some colours within a pattern are affected more than others. Their experiment showed that archerfish can discriminate human faces without having any obvious selection pressure for this specific task, suggesting they have sophisticated discrimination abilities. The paper reports, "This is not surprising as so many behaviours fundamental to the survival of a wide range of species rely on accurate vision-based object recognition, including predator detection, mate selection, and feeding".

While the research answers more fundamental scientific questions, to fishermen, the results pose some interesting questions. We have long known that certain species--like bass--have the ability to learn, and the same is assumed of other predators like tigerfish. But just how cognitive do they become? If some species with well-developed sight used for identifying prey, possibly have the potential to recognise individual faces, how well do fish remember baits and lures? And what if they have been pricked by our offerings? Do fish that are regularly caught on a red spinner for instance, and released many times, become so attuned to the negative experience of it, that they have a distinct visual cue to avoid it?

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The flip side of the abovementioned learned responses, is how long can fish actually remember? One wonders if it is possible that a particular sport fishery which is under intense fishing pressure (like sections of the middle Zambezi), though managed with strict catch and release policies, begins to decline (not because the fish are not there), but because the fish have become wise to the ways of fishermen. It is an interesting tidbit to tantalise your brain while sitting peacefully at your favourite fishing hole. More so, if the reason you are not catching fish is possibly because you are ugly ... and the fish can see you!
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Author:Williams, Ant
Publication:African Fisherman
Date:Aug 1, 2016
Words:1395
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