THE VISION OF FISH.
NO point comes up for discussion more frequently among anglers than the question of what fish see under water. It is a point on which many other questions turn, particularly those of dressing flies for trout and salmon ; and it is one which, if it could be definitely decided, might help us to understand many things which are now obscure or unintelligible, as, for instance, the reasons which impel a trout or salmon to take a particular fly at a particular time or place, and to refuse the same fly at another time or place. Fishermen therefore will read with peculiar interest the account of a series of experiments conducted by Dr. Francis Ward, and described by him in the current number of the Salmon and Trout _Magazine. Dr. Ward has been making his experiments for many months, with a pond of his own and special apparatus for photographing objects from below the surface of the water, and he has established, or helped to establish, some very striking facts. He has shown that an object may not only look perfectly different above and below water, but also may appear differently coloured when placed in different positions in regard to the observer's eye, looking at it from under water. Further, he has made some extremely interesting observations on the coloration of fish under water as compared with their surroundings. A brown trout, for instance, has the power of changing its colour, or the location of dark and light on its body, to suit its surroundings. If a trout lies resting above pebbles on the floor of a stream, it develops blotchy patches- of light colour about its body which harmonize exactly with the blotchy surface of the pebbles, so that the shape of the fish becomes confused with its surroundings and so practically invisible. If this trout, again, is startled, it darts away, and as it moves becomes a mirror of its surroundings ; it becomes dark in dark water and light in bright water. Dr. Ward, one hot dry day last August, when fishing was out of the question, watched the fish in the clear water of the Lancashire Hodder. He found that he could detect the sea-trout easily enough, but the brown trout were invisible until they were startled, when a conspicuous fish barred like a perch would dart up-stream. Some of these fish he was able to keep in sight, and he found that the barring quickly disappeared and he lost sight of them ; they had become mirrors pure and simple. It is doubtless this capacity for mirroring their surroundings which helps to protect fish from their enemies. A dace or roach lying perfectly motionless among weeds is not the least like the fish as we see it when it lies on the bank, pulled out of the water. The roach that we see is a fish with blue- green scales above, silvery scales below, and red fins. The roach which the pike sees, or rather looks at without seeing, is a uniformly shaded form which may be green, grey, or red according to its surroundings. The pike in the same way, no doubt, by keeping perfectly still, is invisible to the roach which may be destined for its meaL It is only when the fish turns or moves, as the dace may move, for instance, to pick up a worm, that the flash of light from his flank betrays him, and he becomes visible to the man looking for him from the bank or the pike waiting to dine among the weeds.
One of the most fascinating of Dr. Ward's many experi- ments has been his series of tests of what is actually seen by an eye under water, gazing at such different objects as a swimming bird, a standing heron, and a trout fly. Ho smut to have sat under water and looked up through it at various moving and stationary creatures and angler's lures, and he has also contrived to take some very remarkable photographs. The question arises at once, of course, whether what a man sees is what a fish sees ; whether, for instance, a fish may not be able to see objects further or in a different way, as a cat can see in the dark when a man cannot. But here Dr. Ward, though he naturally cannot tell us positively and precisely what a fish does see, argues convincingly enough that in essentials men and fish see alike. In the first place, the laws of light, reflection, and refraction must be the same for all light-receiving organs of vision. In the next place, fish notice the same things which men notice, such as the flash of a turning dace under water or the presence above water of au angler stalking on the hank of a stream. It follows that it is reasonable to compare the two powers of vision as regards the colour and form of the objects they perceive. Take, then, the field of vision of a man looking up from under water. The field of vision separates itself, as it were, into two portions, an outer and an inner field. The inner field is an inverted cone of light, of which the eye is the apex. The outer field is the surroundings of this cone. So that the eye under water may be compared to that of a man seated in a room which is lighted only by a circular skylight in the centre of the ceiling, with this difference, that in a room the higher the ceiling the narrower would be the angle from the man's eye to the outside edge of the skylight; whereas under water the deeper the fish lies, the larger and wider the cone becomes. This is because the angle from the eye to the surface of the water is always the same, namely, 48 degrees. Inside this cone, then, everything which the eye under water sees is seen against a brilliant light ; outside the cone the background against which an object is seen may vary immensely in character. If the water is absolutely still, the surface forms a perfect mirror, so that the stones and weeds at the bottom of a pool would be exactly reflected in it, just as if the ceiling of the imaginary room outside the centre skylight were a mirror which exactly reflected the pattern of the carpet. It is this property of reflection, it would seem, which makes it possible for a bird like a heron to stand in water unperceived by fish swimming underneath him. The heron's leg from the knee-joint down- wards, say, is exactly reflected in the under-surface of the pool, and if it happens to be standing near or among the stems of reeds or other plants growing in the water, the bird's legs and the reeds are practically undistinguishable. But it is not only the surface of the water which mirrors its sur- roundings. The breast of a gull swimming on the surface outside the cone-of light takes the exact colour of the surface
it breaks ; and a cormorant, diving for fish under water, appears green among green weeds and brown among brown rocks. All this is as it appears to the eye outside the cone of light. In the cone of light it is different. Colours disappear and the object which enters the cone becomes a silhouette. The swimming gull, for instance, becomes sharply defined as a bird, and the trout-fly drawn across the cone turns from a blur into precise outlines.
Here we seem to be confronted with suggestions which may possibly have some influence on the future of angling, at all events, as regards the making of flies and lures. In fishing the dry or floating fly for trout it looks as if shape and size rather than colour were the things which the trout must notice. In fishing the sunk fly for either trout or salmon, again, it would seem that the colours could only be brought to the notice of the fish by presenting the fly in a particular way. If the fly is passed between the fish and the light the fish will only see a shape. If, on the other hand, the sun is shining from behind the fish so that the light falls on the parts of the fly which the fish sees, then the fish can detect differences of colour and can notice the flash of a gold or silver body. This is surely the explanation of a fact which all salmon anglers must have noticed, and which many, perhaps, have accepted as a wrinkle of local experience rather than as a matter of scientific reasoning. That is that some pools fish better in the evening than the morning. What happens is that the fish lie at a particular spot in the pool— the " taking place," as the angler comes to know it. If the fly is worked round to the fish, then, with the light coming from the far side of the river, it may not be possible for the fisherman ever to show the fly to the fish except as a grey silhouette. If, on the other hand, the sun is shining from a different quarter of the sky the fly may be worked round so as to come to the " taking place " as a flashing, brilliant thing, as bright and attractive as herring fry turning and twisting from its chasing foe. The salmon looks, is attracted, and is hooked. Here at least we have what looks like a sequence of facts which can be accounted for by sound reasoning; though to be sure there are dozens of things in fishing for which other reasoning, equally sound, fails to account—as for instance, the fact that a salmon will take one fly and not another, or will refuse the same fly a dozen times and take it the thirteenth time. But to be able to account for everything in fishing by sound reasoning would be to destroy me of its chief fascinations.
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