Any animal that is not well adapted to its environment will not survive long either as an individual or as a species. The fish is finely tuned to its environment, but because its en-vironment is one in which we can see little, hear poorly and smell not at all, it has taken a long time for us to begin to understand how well equipped a fish is.
How well can fish see?
The fish eye is basically similar to that of man and other vertebrates. Beneath the outer surface (the cornea) which is often partly covered by a clear fat-ty eyelid to assist streamlining, the central part of the eye is occupied by a hard, clear, spherical lens. This lens is virtually optically perfect and transmits light without distortion on to the sensitive cells of the retina which lines the back of the eye.
In some fish, notably those which depend on sight for finding their food, such as the trout, the lens is not perfectly spherical but slightly oval. The lens can be moved backwards or forwards by special muscles, and its shape means that it has two focal lengths. The ability to move the lens rearwards means that it moves closer to the retina at the back of the eye, and this gives the fish its forward vision. A trout can, therefore, focus at the same time on a distant object and one close by – seeing clearly the angler’s waders and at the same time the fly in front of its nose.
Rods and cones
The retina contains light-sensitive cells of two types – rods and cones. The rods are more sensitive and respond to poor light, while the cones respond to bright illumination and are responsible for colour vision. Fishes active in daylight have retinae with more cones than rods, while those which live in the deep sea or are active at night have fewer cones. Pike, trout and the carp family are daylight feeders and have numerous cones in the retina. These enable them to see in colour and in conditions of full daylight. Eels and burbot, however, which are mainly dusk or night time feeders, are equipped with many rods in the retina. Trout, however, can see in the twilight, for their vision switches from cones to rods as the light fades. In poor light, young salmon have been observed to sink into deeper water, so that food is thrown c into contrast against the brighter 1 surface of the water, and then to t watch and take items as small as £ water fleas (Daphnia) with ease.
Because fish do not have visible ears, it was long assumed that they could not hear. In fact, their ears are sensitive enough for them to hear well, although different groups can perform better than others. In the carp family, which includes most of our freshwater fishes – roach, dace, chub, barbel and carp – we have one of the most sensitive groups of fishes. In this family the inner ear itself is elaborated in various ways. The sounds are transmitted to the inner ear by a chain of small bones called Weberian ossicles.
This capacity to hear sounds so well (the North American catfishes, Ictaluridae, can detect a range of vibrations up to 13,000 cycles per se- cond) has been a major factor in the success of the large group of fishes, which includes the carp family, the characins, and the catfishes, which have colonized freshwaters. No doubt the turbid nature of most freshwaters, which are darkly col-oured by plant plankton, silt or rotting vegetation, has meant that such fishes find it necessary to take advantage of their hearing ability.
Even without the swimbladder con-nection, which is a feature of the carp family, other fishes can hear well. Many fish make noises by releasing small bubbles of gas, originating in the swimbladder, and schools of pilchard and herring can be detected in calm weather by the altered water surface where the bubbles burst. Others, such as wrasses, grind their teeth – and cod, haddock and gurnards use their swim-bladders to amplify the noises that they make.
One can only assume that these noises are made to be heard by others of the same species. Even swimming produces noises of a characteristic type, and it is likely that schooling fish are assisted in keeping together as a shoal by the continual rustle produced by the muscles of the other fish. It has also been established that some sharks can detect these swimming noises. Tape recordings of a struggling fish speared by a diver have been played with success to attract sharks towards the sound source.
All fishes have a good sense of smell, but those with eyes developed for poor light tend to have more elaborate and sensitive nasal organs. Most fish have two nostrils, the front one being raised on a short tube, while the posterior one opens flat on the surface of the head in front of the eye. The front nostril leads into a nasal pit which has a rosette of deeply folded skin heavily laden with sense cells. The flow of water in the rosette carries odour particles over these cells, and out through the hind nostril.
The freshwater eel has particularly well-developed nasal organs with a long rosette, and experiments have proved that it is very sensitive to smells. At a dilution of only one part in ten thousand million million of water, pure organic substances can be detected when only two or three molecules of the substance are in the olfactory organs.
Other fishes, such as the carp, are possibly less sensitive, although not much so. There is little doubt that they can detect and avoid injurious or unpleasant substances such as those that cause pollution. Much of a salmon’s movement, returning to its native stream to spawn, is believed to depend on its ability to detec the characteristic odour of the spawning stream.
This again implies an ability to detect only a few molecules of scent passing through the nostrils – and it means that the fish reacts positively to increasing amounts of odour as it gets closer to ‘home’.
Taste in man is a sense confined to the region of the mouth. But in fishes it is spread farther. The mouth of a fish contains large numbers of cells, known as taste buds, on the lips, the underside of the snout and around the mouth, especially on any barbules.
Pouting and cod, when feeding, swim around with their chin bar-bules extended forwards, sampling the sea bed for edible items as they move – and no doubt gudgeon and barbel also can taste an offered bait without taking it into their mouths. Gurnards have taste cells on the long pectoral rays which they use for searching out molluscs, crustaceans and worms.
It is known that carp have a sense of taste, and can distinguish salti-ness, sweetness, bitterness and acidity. There is also no reason to think that they do this less efficiently than man. Gurnards and other fishes detect the extract of the worms or clams on which they normally feed, but seem indifferent to sensations of sweetness or saltiness. Not surprisingly, some research at marine fishery laboratories has ex-plored taste sensitivity of fish in search of an artificial substitute which tastes like the natural food but stays fixed to the hook better!
The lateral line is a conspicuous sense organ in most of our fishes. It varies a little from species to species, but basically is noticeable as a series of scales along the side of the body bearing a tube on part of their length. It then continues as a sunken tube and has openings over the head, around the eyes, on the cheeks and on the lower jaw.
The sensitive part of the lateral line is a small cell, the neuromast. 2 When pressure within the canal I changes due to outside influences, g these neuromasts detect the altera- g tion and a message travels through I the dense nerve network of the canal to the brain. Experiments with captive fishes show that the lateral line system can detect a variety of vibrations. The North American catfish or bullhead can detect vibrations of between 20 and 100 cycles per second by means of these organs, and it seems that at the upper range where this system fades out the animal’s sense of hearing takes over.
It can be concluded that by means of its lateral line system, a fish can detect localized disturbances caused either by currents or by an approaching predator. This also acts as a distant touch sense. Change of water pressure from objects or prey can allow the fish to detect their presence without actually coming into contact.
A further aptitude found in fishes, and which is unique to them, is that some are capable of producing elec-trical discharges. It is most extreme in the electric ray or torpedo. Some tropical fish, such as the electric eel and the electric catfish, also produce a strong stunning current. Less noticeable is the low capacity elec-trical ability of the rays and skates. They have weakly developed electri-city-producing organs in the tail muscles, although they are rather reluctant to discharge their current, which is delivered at a voltage of about four volts. In the skates and rays this function is not fully understood, but possibly the electric field produced can serve as an early warning system to alert the fish of approaching potential predators. More probably, it serves as a species-recognition system keeping a school of rays together at mating time and ensuring that the same species mate.
Fish are well adapted for the con-ditions in which they live. With such a battery of acute senses on the side of the fish it is no wonder that the successful capture of specimen fish is a feat of skill and endurance.