by
P.J. Sharkey
Marine Electronics
Killybegs, Donegal, Ireland
In recent years the Department of Agriculture and Fisheries in Ireland has embarked on a programme of investigations of how electricity can be employed for the management, development and exploitation of inland fisheries. This work is being carried out by the writer under the direction of Mr. C.J. McGrath, Fisheries Engineer, assisted by Mr. D.F. Murphy, Assistant Engineer.
A field laboratory has been fitted out at the Department's Salmon Hatchery at Glenties, Co. Donegal, and here applied research into the subject is conducted under actual working conditions. In that part of the work dealing directly with eel fisheries, or although concerned with other work found to be applicable in part to eel fishing as well, some results of practical interest to those concerned with the exploitation of eels were obtained, and the following account deals with these.
The particular activities in question were as follows:
- Electro-fishing of migrating silver eels in daylight
- Electro-fishing of yellow eels in daylight
- The fishing of migrating silver eels by conventional horizontal grid trap suitably modified to avail of the use of electricity
- Laboratory and field experiments on the use of electricity for the blocking and guiding of eels
- Fishing of migrating silver eels by fyke net assisted by an array of electrodes suitably positioned and energized
- Control of migrating silver eels by the use of floodlights
- The trapping of migrating elvers with the aid of electricity.
During the development work on an electric eel barrier, which will be described later in this document, a battery-operated electro-fisher with an output of 350 V was used to monitor the migration of silver eels in a generally fast flowing river system and to collect specimens for experimental work. Good catches were made, as the following sampling results indicate:
5 lb in 45 minutes (6.6 lb per hour)
8 lb in 40 minutes (12.0 lb per hour)
5 lb in 45 minutes (6.6 lb per hour).
However, the electro-fisher in use initially had a fixed frequency square wave form pulsed D.C. output at between 25–30 p.p.s. and it was observed that some eels were being missed because they became rigidly tetanized in their hiding places, from which they floated out after the fishermen had passed by. A solid state electro-fisher, with an output of 350 V, which was under development at the time and in which the pulse repetition frequency could be varied from 10 to 150 pulses per second, was, therefore, tried with improved results, as described hereunder, which could not be explained entirely by the experience gained in previous operations in selecting productive and avoiding unproductive stretches of water:
30 lb in 120 minutes (15 lb per hour)
16 lb in 60 minutes (16 lb per hour)
With this machine, best results were obtained at a p.r.f. of 10 per second. The lower frequency, combined with the triangular wave form pulsed D.C. output possible with this machine rather than square wave form pulse of the previous machine tried, produced less tetanus, presumably because of the shorter exposure time. With this energizing, the eels more frequently broke out from their cover while still in range of the landing net and were easy to capture.
A further improvement was achieved by using a shovel-shaped electrode equipped to act as a landing net. This arrangement had been suggested originally by Dr. D. Piggins of the Salmon Research Trust of Ireland for the capture and recovery of small salmonids in streams with gravel bottoms and had been developed by the writer for that purpose. The frame is made from ½ in diameter aluminium bars and is roughly triangular in shape with each side measuring about 16 in. The handle is attached at the mid-point of one side. This shape of net greatly facilitated the lifting of the eels, especially from among the stones and gravel, and under boulders on the bottom of river channels.
This experience would suggest that, if approved by the fishery authority, in suitable river systems migrating silver eels could be caught in daylight by the above method in commercial quantities. The fishing times per catch quoted above include the time spent moving equipment and men from place to place along the river banks. Initially, a four-man team was employed, but later the team was reduced to two: one man fished with the battery and pulser mounted on a carrying frame on his back, while the helper remained on the bank carrying a narrow-necked collecting tank into which he put the captured eels. The fishing technique employed was for the man with the electro-fisher to wade in the stream, fishing under overhanging banks and under large rocks and any other dark area where migrating eels would tend to conceal themselves during daylight hours. When drawn from the cover, the eels were caught in the landing net and thrown onto the bank of the river where the helper collected them and put them into his collecting tank. This man were a glove with a rough-textured or sand finish which facilitated the handling of the eels.
The experience gained in this operation suggested that there was little to fear that this fishing operation would interfere with stocks of salmonids. Young salmon and trout were encountered in large numbers in this fishing, but they were very rarely knocked out because of the low p.r.f. and short pulses.
In many of the smaller, less productive streams in Ireland, there are present quite large numbers of small yellow eels (3 in to 6 in in length) which could perhaps be removed and employed for the intensive stocking of the larger and more productive lakes and river systems. A technique has been devised which makes electro-fishing operations for these fish much more productive than they might otherwise be. Cotton wool waste is soaked in pilchard oil and placed in small sacks. Stones are also placed in the sacks to prevent their being moved by the river current, and they are then placed in pools or else are tied to the roots of trees along river banks adjoining stretches of deep flowing water. The eels are quickly attracted to these baits, and electro-fishing can commence with good results within a few hours of their being placed in position. The baits have been found to remain effective for a period of two weeks, and each location can be fished with profit daily; even in some cases, twice daily throughout this period. The electro-fisher equipment employed for the capture of silver eels can be used for this purpose also, except that a finer mesh landing net must be employed together with a faster rate of pulses per second of the order of 40–50.
A horizontal grid trap of the conventional pattern which had been constructed across a river channel for the capture of downstream migrating smolts was employed to recapture silver eels that were being used in experiments to test the effectiveness of an electric barrier in deflecting eels. This barrier had been erected across the river channel upstream of the grid trap, and the silver eels which had been captured previously for this purpose were released upstream of the barrier and when they moved downstream their reactions to it were studied at various energizing levels. It was found that although the eels were caught in the flume forming part of the trap and discharged by it into the holding tanks provided at each bank of the river for this purpose they, nevertheless, disappeared overnight unless removed from the tanks to a more secure place of detention. It was surmised that they made their way back up the flume and escaped over the sides. Some eels also had been seen to escape across the top of the flume when they first entered it while it was surcharged with water. To prevent this happening, the trap was electrified in a very simple manner by fastening 7/22 bare copper wire along each side of the flume at mid-water level and continuing this into and around the inside of the holding tanks. This was then energized at 25 V A.C. R.M.S. 50 ops. Thereafter, when eels were released onto the grids of the trap and were swept into the flumes they curled up and were swept into the trap boxes. Eels were retained successfully in these electrified boxes for two weeks. The energizing was then switched off, and the following day the eels were no longer to be seen.
It was noted that during the hours of daylight the eels made no attempt to escape from the holding tanks even after the power had been switched off.
Laboratory experiments were carried out in an 8 ft 4 in-long tank, 2 ft wide, made up of transparent acrylic panels. This was filled with water to a depth of 1½ ft. Electrodes consisting of lengths of 2 in diameter galvanized iron tubes were suspended across the width of the tank and half-way along its length, each tube being 2 ft apart from its neighbour.
Ten silver eels of various sizes, from 10 in to 27 in, were placed in the tank and allowed to acclimatize over a period of two weeks. The tests were then run as follows: one end of the tank was illuminated with a 100 kw floodlamp (white light). The eels were allowed to settle in the dark end of the tank of their own volition. This retreat to the dark end usually took approximately 15 minutes to complete. The barrier electrodes in the tank were then energized at 24 V D.C. from two 12 V accumulators in series. The spotlight was then turned to illuminate the tank and where the eels had congregated. The movement out of the illuminated zone toward the dark end would then commence in a few minutes. As soon as the eels encountered the barrier field, they withdrew very sharply. They made repeated and determined attempts to get through however. These attempts would be sustained for hours at a time, individual eels taking turns to attempt a break through. The general movement of the eels was slow and leisurely, but the attempts on the barrier were sudden and determined, usually made by one individual at a time after a patient searching of the fringe of the barrier field. After each attempt the eels withdrew with a convulsive reverse swimming action and retreated as far as possible from the field. With the spotlight off, the eels made no determined attempts on the barrier and apparently only encountered the field by accident.
Following the tank experiment, work was begun on installing an eel diversion barrier in a river. A suspension cable was slung across the river at an angle of about 30° to the flow, and, from this 2 in diameter galvanized tubular electrodes were suspended at 2 ft intervals. However, the barrier was not completed in time for the eel migration, and tests had to be confined to liberating hand-caught silver eels above the completed section of the barrier. A 30 ft section was energized at 24 V D.C. from batteries, and 10 eels were released some 20 ft upstream. Of the 10, three were lost sight of, two lead along the barrier to a trap at the downstream end, and five lead upstream along the barrier and escaped around the unfinished end. Arrangements were made to extend the barrier across the full width of the river channel, and field experiments will be continued during the migration of eels next winter.
An exploratory trial of the feasibility of guiding migrating silver eels by electricity in a major river channel was made a number of years ago on the River Shannon by the writer on behalf of the Electricity Supply Board of Ireland. At Killaloe, a commercial eel weir is operated just down-stream of a multiarch road bridge. Below each arch there are three fyke nets, designated from left to right as A, B and C. Experience over the years had shown that, at the arch selected for trials, net C caught nearly as many eels as A and B together. The success of the experimental guiding of the eels would be indicated by the extent to which the catch pattern could be changed. The electrode arrangement was designed to divert eels to net B.
The depth of water at the site was 10 ft, and three 10-ft lengths of copper bus-bar were used as electrodes, one strapped vertically to each side of the arch at the upstream end, and one standing vertically just upstream of the centre of the mouth of net B. The array was energized with 110 V full-wave rectified D.C. from a unit powered by 50 cps mains supply (i.e. 100 half-sine pulses per second, each having a duration of 10 milliseconds).
The arrangement was fished during the hours of darkness on two consecutive nights. There were five fishing periods, each of two hours' duration per night, with power on and off every second period. The catches in the five power-off periods were as follows:
| Net A - 119 lb | Net B - 161 lb | Net C - 260 lb | (This conformed with the normal catch pattern). |
In the five power-on periods the catches were as follows:
| Net A - 70 1b | Net B - 238 lb | Net C - 68 lb |
Further trials were made to determine whether better results could be obtained with higher energizing voltages. Energizing voltages of 200, 250 and 280 were tried, and the guiding effect was found to fall off as the voltage increased, until at 280 V it was negligible.
This adverse reaction of fish to stimuli exceeding certain limiting values compared with favourable reactions below these figures has been experienced in other investigations carried out since these trials were made. This limiting value has been found to vary from site to site and quite clearly is influenced by the particular conditions obtaining at each site. The usual experience is that when the limiting figure is exceeded the effect of the stimulus on the fish is not to induce the fish to react more strongly but rather to inhibit it from reacting at all and, by causing the fish to become disorientated, it is incapable of avoiding the zone of stimulus but instead is carried involuntarily by the water current deeper into the hostile environment of the stimulus.
As a result of the marked avoidance reaction to light shown by the silver eels in the tank experiments, it was decided to test this effect in the natural river. Accordingly, two 500 W floodlights (white light) were arranged under one arch of a road bridge which is situated just above the site of a horizontal grid trap. The lights were adjusted to illuminate one half of the river for some 20 yards above the bridge. The adjustment was such to give a fairly sharply defined line of demarcation between the light and dark areas. The downstream migrants were thus presented with a choice of a bright or dark bridge arch. The arrangement of the grid traps and boxes below the bridge is such that it is possible to ascertain which arch was favoured by the migrants.
The complete arrangement was in working order by mid-November 1967, but it would appear that the eel migration had occurred earlier during some three weeks over very heavy floods. However, large numbers of salmonids were taken in the trap corresponding to the illuminated arch, which would suggest that this method may be of some assistance in trapping salmon smolts. Trials will be made during the next smolt migration as well as during the eel migration. It is appreciated that the use of this method is limited to waters of shallow depth, free from discolouration, not usually associated with conditions obtaining during silver eel migrations.
Reference was made in Section 4 to electrifying the flume of a grid trap to prevent eels escaping after capture. This arrangement was in operation when elvers were migrating upstream and during periods of high tail water level at the horizontal grid trap they were able to get into the flume. Those that entered the energized part of the flume were washed down into the trap box serving that portion of the flume, with those entering an unenergized part were not affected. From this box they could be removed easily by dip netting. This method of trapping elvers will, therefore, be of interest to people collecting them for examination or transplanting.
1 Submitted at the Fifth Session of EIFAC as FI/EIFAC 68/SC I-4
