Dr. W. Dembiński
Instytut Rybactwa Sródladowego
Dr. A. Chmielewski
Fishing in lakes by means of the conventional seine nets can be very profitable despite the fact that the efficiency of the whole process is rather low, i.e. of the order of 0.34 – 0.7 (the latter figure, cited by one author only, is probably too optimistic). This efficiency is understood as being the ratio of the number of fish actually caught to the total number of fish encircled by the net.
More effective seine nets therefore are very desirable, in view of the trend towards further intensification of fishery management of lakes.
It is generally believed that by the use of electricity one will be able to improve upon the results being achieved by the use of the net itself. It will be remembered that this was the case with the electrified trawl for eel.
With seine nets the problem is that the encircled fish escape in large numbers during the last phase of fishing, when the wings are being drawn together, and the concentration of fish is becoming too great.
Experiments under laboratory conditions showed that fish could not be highly concentrated when one of the partitions in the tank was the electric field in water. Fish tried to escape through this field despite this being very unpleasant, painful or even fatal.
Preliminary tests have been carried out with the electrified net partition, 30 m long, placed in between the wings of the seine net. An alternating current of 50 Hz was switched on when there was still 400 m of wings to be drawn (Fig. 1). The results were 700 – 1 500 kg of fish per one operation. Fishermen were pleased with the effect of the electric field. A number of fish were, however, strongly shocked and - as it takes two hours to draw the wings 400 m long - all the fish were exposed to the danger of electric current for such a long time. This is undesirable. The experiment showed also that the electrification of the wings of the seine nets would produce similar menace for fish.
The following ideas and requirements had been laid as the foundation of the project:
It is no good to electrify seine nets in their present form. The seine itself should be modified for the purpose of electrification.
The area covered by the electric seine net should be as big as in the case of the middle-size conventional one (10 – 20 ha).
Fish should fall into the net bags (from which they are not able to escape) during the whole process of drawing and not only in the last phase, after being concentrated by wings, as is the case with the conventional seine net.
The electric field in water should be produced at the mouth of the net. By immobilizing the fish, the escape is prevented. In this way the number of fish falling into the net bags during the whole process of drawing should be substantially increased.
Fish should be under the influence of the electric current for as short a time as possible. In order to ensure this, the end and side of net bags must be placed 15 – 20 m away from the electric field.
The proposed electric seine net is illustrated in Figure 2.
It consists of a number of net bags attached to the frontal part of the seine net and of rather short wings. The length of the frontal part is about 100 – 150 m. The wings are 50 – 75 m long. The net bags are fitted with special devices (“hearts”) which make it impossible for fish to escape. The length of the net bags is 25 – 30 m. Electrodes are installed only in the frontal part and are energised by the petrol-driven engine a.c. generator, 2 kVA, 230 V, 8,7 A type PAB2-1/230 or two of them in parallel, if necessary. This apparatus has been already used in Poland in the so-called “classical” arrangement for electric fishing and as the source of electrical energy for the electric trawl for eel.
It is assumed that the net will be hauled (Fig. 2) by two 5 hp engines mounted on two anchored boats. The haul will cover a distance of 1 –2 km. With the width of the frontal part being 100 m the area fished by the net will be 10 – 20 ha. The pulling power of a 5 hp motor is about 600 kg. Thus a resistance of the net of up to 1000 – 1200 kg could be catered for. The previous experience with nets of this size showed that it should not be as high as this. The drawing rate is expected to be about 1 – 2 km/h. Four to five people will be enough to operate the whole gear.
It is assumed that the electric seine nets will appear in two versions, differing one from another in mesh sizes and some parameters of the electric field in water. One of the versions will be for catching eel, another for catching carp, both to be used in lakes intensively stooked with these species.
Fig. 1 The conventional seine net with the electrified net partition (in between the wings).
Fig. 2 Electric seine net.