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H. Mohr

Institut für Fangtechnik, Bundesforschungsanstalt für Fischerei Hamburg, Federal Republic of Germany

Most fishing methods are based on the exploitation of behaviour patterns peculiar to the fish sought after. Schooling is perhaps the most important feature, because it concentrates fish naturally for mass capture by suitable gear. But in the case of eel, schooling can be observed only in elvers or glass eel, when they enter the rivers, and this stage will not be considered in this report, which deals only with the adult or at least the larger sized eel. Eels live in different habitats widely distributed and special methods have to be employed in order to concentrate them so as to catch them in numbers. I may here refer to the gear classification of Prof. v. Brandt and repeat, that the more usual applied techniques for this purpose are:

  1. To direct the eels by weirs or fences, which lead them into traps;

  2. To attract them by shelters, which serve also as a trap;

  3. To attract them by food placed in a trap or attached to hooks on lines.

All this gear belongs to the so-called passive or stationary gear, and this means that the fish has to be active in order to enter the trap to take the bait. On the other hand, for moved or active gear it is favourable if the fish is passive or lethargic. In wintertime, when hidden motionless in the mud, eels are - or were at one time - pierced with spears or forks. During the warm season, in daytime, when burrowed not so deep in the bottom and so active that they leave their hiding place just before the approaching gear, eel are trawled or seined, where a flat and smooth bottom allows these fishing operations to take place.

The application of electrical gear will not be considered in this context, as fish are stunned by these means and the anodic reaction, which is induced even in a decapitated fish, is not behaviour in the proper sense of the term.

It has already been mentioned that phases of swimming activity are often the deciding factor in the capture of eels. Therefore it might be useful to examine in detail these periods of activity and the factors which cause them. But we have to consider separately the activity of the yellow eel and that of the sea-going silver eel, because different motivations prevail in these different stages of development in the eel.

As regards the yellow eel, an experiment was conducted recently in our Institute. The results I may discuss in a few words:

The eels were kept in an artificial rectangular pond, which was divided by a diagonal lattice into two unequal parts. Each contained shelters for the eel. When the eels were in motion, they were guided by the diagonal screen through a funnel, into the other part of the tank. When passing through the funnel, they activated an electro-mechanical counting mechanism and their passage was indicated on a clockwork-driven graph recorder. The period during which their activity was monitored by this means extended from 14 June to 14 August. The results obtained were as follows:

Usually the eels were active only at nighttime. As the season progressed, the length of the period of activity increased with the increase of night and the decrease of light intensity. In the diurnal rhythm two periods of maximum swimming activity were recorded, the highest peak occurring after dusk and the lowest before dawn.

It was also noted that the atmospheric pressure influenced the activity of the eel with peaks at any tendency of the pressure to fall; the actual value of the pressure was of no importance.

An extreme restlessness was always observed during thunderstorms, even during daytime. As the change of atmospheric pressure occurring during thunderstorms often did not exceed that of other days, this high activity might have been due to the electro-static charge.

Within the range 14° – 21°C the influence of temperature was not noted. It is known that below 5°C eels become somewhat inactive and at temperatures less than 9°C food bait is ignored. This limits the period of eel fishing with passive gear in our climate to that from May to October/November, and makes the rearing and fattening of eels in artificial ponds problematical.

The moon phase is obviously an influence also. Maxima of activity coincide with waning and especially the new moon, minima with full moon. Because of the relatively few data - the experiment lasted only two months - it cannot be decided if this is an effect of the moonlight or an intrinsic rhythm associated with the moon constellation.

It has been established by several investigators that the spawning migration of the silver eel depends mainly on the direct influence of the moon and it does not matter if it is visible or completely covered. Peaks of activity and greatest catches occur always in autumn with waning moon and drop nearly to zero at full moon.

In this context it should be mentioned that in recent years in the Baltic and in Eastern German rivers, chains of underwater lights were used as barriers in order to guide migrating silver eel into the traps.

The fact that the eel enters voluntarily into traps which simulate a shelter has already been discussed in the paper presented by Prof. v. Brandt. The motivation is not only a negative phototaxis but also a strong thigmotaxis; the desire is that the whole body will come into contact with a solid surface and this induces the eel to enter a shelter or to burrow into the bottom. When both possibilities are provided in a tank, the eel first digs into the bottom, but when it becomes accustomed to the environment it prefers the more convenient tubes and other shelters.

In the introduction it was mentioned that next to leading weirs the attraction of bait is the most important incentive to the concentration of eels. For this it is necessary that the eel be hungry and the bait be attractive; swimming activity alone is not sufficient. This leads to the question: which stimuli attract - or scare - eel, and more particularly, which are the most important sense organs in the eel?

In many other fish vision is the superior sense for the detection of food and they can be caught by dummies, which superficially resemble their food organisms. As indicated by its small eyes during the feeding stage, which are incapable of colour discrimination, and by its nightly activity, this is not so for the eel.

In recompense for poor vision the eel has the most efficient and sensitive smell organ, as well as the most complicated, of all fishes investigated in this respect.

In the course of conditioning experiments, Teichmann found that an eel can discriminate certain chemicals in a dilution of about one part per ten raised to the eighteenth power. That means it can detect this chemical, when 1 cm3 is distributed in sixty times the whole water of the Lake of Constance, the largest lake in Germany. Moreover, it means that a single molecule can stimulate the fish. In this respect the eel is similar to the dog.

Eel fishermen know well that eels are only attracted by very fresh natural bait and that stories of eel feeding on human corpses or caught in rotten horse sculls are fairy tales. The smell of the most attractive bait for eels often has for the human nose a peculiar odour, such as smelt, which smells somewhat like cucumbers; shrimps and pier worms which smell like iodine and so forth. As eel-bait in commercial fisheries is often lacking, it would be useful to have a chemical substitute. In some experiments we prepared less attractive food with chemicals that smelled like smelt or iodine or like fish meat. We tested many amino-acids and other substances, but up to now without obtaining a really positive result. In journals of sport-fishermen one finds numerous advertisements of lures such as “Ready Bites”, “Gypsies Secret”, “Ali Baba” and of other obscure and mystic names, but I daresay that this is all humbug. These substances usually have the smell of anise, musk, civet, castor valerian and even faeces - odours which indeed have a function in the sexual sphere of mammals, but which are of no biological meaning for the eel. In our experiments some of the lures even proved poisonous to eel, others were an effective repellent.

In another series of experiments it was investigated if chemicals which were used for preservation of the netting or which accidentally often came into contact with the gear, were repellents. Indeed some tar products and chemicals used to finish off netting or to fix the knots were first avoided, but were washed out after a few days and did not scare the fish further. Tubes of polyethylene were not accepted as shelters, because its solvents are soluble in water. These facts may explain the observation that new gear is usually inefficient for some days.

Despite their sensitive odour perception, eel, while in their refuges, were very resistent to strong and unpleasant odours. By means of an arrangement of tubes which served as eel shelters and into which chemicals could be introduced, a test was made of how strong a solution had to be before the eel was forced to leave the tube. I must mention that during the experiment the eel could not see the operator. It was discovered that the concentration of many acids of caustic solutions and copperacetate, which is said to be fish repellent, had to be of the order of 1:10 000 or 1 per 1 000 even, before the eel would leave the tube.

It was found that these concentrations were such that they could easily be tasted by the human tongue. This may explain why eels stay in waters contaminated to such an extent that their meat becomes unpalatable.

Among the most efficient deterrents was ammonia, which was effective at concentrations of the order of 1 per 100 000, and also strong irritants for mucous membranes (which are used as lachrymants and tear-gas) like Allyliscthiocyanate and Phenacyl chloride. These chemicals induced an avoidance reaction at a dilution of 1 per 1 million. It should be pointed out that these were unconditioned reactions, whereas the sensitive reactions mentioned earlier, in repect of which the eel was found to react in a manner comparable to the dog, were conditioned reactions.

Lastly I would like to mention some of the experiences noted of the behaviour of the eel when put into tanks or aquaria. Whereas elvers and very small pigmented eel will feed immediately, larger eel refrain from food for up to 4 weeks after their capture. Little by little they become somewhat tame, but habituation to the experimentor is less obvious than in most other fish. Homing to a certain tube or other refuge does not usually take place. This can easily be demonstrated by using eels of different colour or by tagged ones. Yet on a larger scale homing was observed by Tesch, who found that eels returned to their original habitat when released at distances up to 180 km away in the southern North Sea.

Newly caught eel when placed in a tank obviously feel rather uncomfortable and are nervous and gregarious. They gather together into a mass or try to force themselves into the same tube. They do not bite each other and do not bully the smaller ones.

During the following 4 weeks, however, when they become acclimatized and feel secure from danger, their behaviour changes to grossly selfish habits. They do not tolerate another eel in the same tube, although there would be sufficient room, and smaller eels are driven out of their shelters and bitten seriously. This different behaviour in poor conditions and in comfort is a feature the eel has less in common with the dog, but more in common with man.


O'Leary-asked about the effect of light colour.
Mohr-according to the investigations of Wunder the eel is colour-blind as it has no conesin its retina.
Moriarty-are the fish movements in any way affected by the electrical field of the detecting device?
Mohr-the counting mechanism was released by strings: there was no electrical field.
O'Leary-is any information available on the effect of noise, because at the Electricity Supply Board Eel Weir at Killalow there was considerable noise due to the vibration of steel bars used in the traps?
Mohr-eels tended to show movement away from the source of vibrations.
McGrath-is any information available on the effect of sudden changes of water velocity on eels?
Mohr-very little information is available and this might make a basis for further study.
O'Leary-the maximum velocity in a river usually occurred about 0.3 D (D = depth). In small runs of eels the maximum intensity of the run was practically on the bottom. During periods of heavy floods and muddy waters the eel run appeared to be uniform over the cross sectional area.
Vickers-is any information available on the effect of artificial lights on eels kept in total darkness for the 24 hours of the day?
Mohr-information on this subject is not complete but it appeared that eels will feed during the normal night period even though kept in darkness over the full 24 hours.
v. Brandt-commercial considerations often compel fishermen to take their gear out of the water before the entire swimming activity had ceased. It is therefore not possible, due to marketing conditions, etc., to reap the full benefit of scientific information on this matter. He also recommended that a trap should be 14 days in the water before an assessment could be made. Chemicals used in the manufacture of a trap will then be washed away and it should be possible to get a true assessment.
Deelder-the vibration from larger meshes in nets was adequate to deflect eels.
v. Brandt-the problem of nets was difficult. With large meshes you must have a heavier material. With small meshes the material can be lighter. This will affect the assessment of the effect of vibrations.
McGrath-they have found by experience that galvanized mesh perch traps work more effectively when painted black, than when left unpainted.

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