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Deep Sea Senses.

"How do fish in deep water find food?" "What senses do they use?" "What can fish see in the total darkness far beneath the surface of the sea?" Questions like these can not fail to be of interest to sea anglers. I know that I frequently feel that I am groping in the gloom for answers, particularly when the fish have been slow to bite. Often we know very little about the senses and behaviour of even the commonest fish and indeed, even in these days of space age science, it can be difficult to find out what fish taste, smell, feel and see.

Two studies by American and Norwegian scientists, on life in the depths (well below the reach of even a well filled multiplying reel), give us fascinating insights into what it is like to engage in the battle for survival beneath the waves. Firstly, Dr Bruce Robison has used a one man submarine called "Deep Rover" and an underwater robot "Ventana" to explore deep water. With a mix of direct observation and remote video recording it has been possible to paint a completely new picture of what goes on 500 metres down. In years gone by many of the delicate creatures which live in the cool, dark water were unknown. They were destroyed long before they reached the surface in the crude nets used to take samples but now they and their way of life, are coming to light.

It seems that, even in the murky depths, once human eyes have become accustomed to the darkness, it is just possible to tell that up is lighter than down, so perhaps the sensitive vision of deep water fish enables them to see much more than we used to think. In addition there is often much more light down there than the feeble remains of sunlight from the surface, because many of the animals themselves glow in the darkness. Any disturbance may cause the particles suspended in the water to give off a ghostly light. Some fish have their own faintly glimmering lamps which camouflage them against the feebly lit background. Other creatures (as a defence) plaster their attackers in a shining sticky gunge, revealing them to larger predators. A different tactic is used by certain worms which squirt luminous smokescreen, behind which they can escape. Many other animals simply flash brightly when disturbed so that the movements of a large fish or squid can trigger an underwater firework display. All of this suggests to me that, even at lesser depths, your dark coloured plastic eel may, at times, be clearly visible, one way or another, to hunting pollack or coalfish - but of course you knew that!

By no means all deep water animals are small and feeble, on the contrary some are huge. One colonial "jellyfish" rather like a Portuguese Man o' War can be forty metres long and there is a drifting sea squirt which produces a blanket of ooze as big as a drifting island. There is so much of this wispy living matter that the water is often like slimy, stringy, spiderweb soup.

Big fish were often attracted to the lights of Dr Robison's submarine, possibly because they resembled the 'natural' light given off by moving prey. The best way to avoid being eaten, it seems, is to keep still and many smaller animals do this. One species of fish is described as a "shadow stalker" and spends its life standing upright, tail down and head up. It peers upwards with great goggling eyes looking for the silhouettes of potential prey against the faintly glowing backdrop above. Curiously, it appears that one effective way of blending into the background is to be coloured red. Red skin, it seems, absorbs all the faint blue-green light making the animal invisible. This strategy is similar to the silver mirror camouflage of fish from the brightly lit surface layers.

It is difficult for human observers to see anything but vague shapes in the darkness. Swimming fish and squids simply look like streaks in the water. When they are disturbed or threatened they often change shape by curling up into circles or balls and hanging motionless - trying to mimic the shape and behaviour of bad tasting jellyfish or comb jellys. The scientists have actually watched hake attack fleeing fish but ignore others which had curled up, so don’t forget to keep your lure on the move.

The second bit of science was carried out by a group of boffins looking at a deep water fish from a mere 400m off the west coast of the USA. The sablefish which they studied were about six pounds each and were caught alive in traps. It was known that these fish probably find their food by its smell because they readily take stationary hooks baited with squid. The idea was to find out just how sensitive they were to the amino acid soup present in squid extract.

The fish were kept in large sea water pools and fed on whole squid in varying amounts. Before they were fed, squid juice was poured in so that the fish came to associate it with the presence of food. Once they smelled the juice (containing a potent mixture of amino acids which both attract the fish and stimulate feeding) the sablefish swam much quicker, twisting and turning in their search for a meal, so it was easy to tell whether they could detect the scent.

The results of these experiments are quite astonishing. The fish, if they were feeling hungry, responded to concentrations of squid extract as low as 14 parts in a billion. This means that the fish could potentially detect bait at a distance of several kilometres. Similar work with cod has shown that they need over a thousand times more amino acid dissolved in the water before they can sense it. It is also known that thornback rays are almost as sensitive as sablefish to attractive smells. All of these fish are put completely in the shade by the good old silver eel which has been shown to sense one substance at a concentration of about one part in a million billion.

In most of the species studied it is known that hungry fish respond to much lower concentrations than well fed specimens. This confirms the common observation of anglers that the presence of lots of natural food often makes it much more difficult to get a bite from anything.

If you have any comments or questions about fish, methods, tactics or 'what have you.'get in touch with me by sending an E-MAIL to - docladle@hotmail.com