5

R app. P.-v. Réun. Cons. int. Explor. M er, 174: 5-6. 1979.

P R E F A C E

F. T h u r o w

Institut für Küsten- und Binnenfischerei der Bundesforschungsanstalt für Fischerei, Wischhofstrasse 1, D-2300 Kiel, Bundesrepublik Deutschland

Investigations on eel have only recently become a subject of wide international cooperation, which began when the European Inland Fisheries Advisory Com­mission started to promote joint efforts. An ad hoc working group on eel fishing gear and techniques was established, and subsequently a workshop on com­parative ageing was held in Montpellier, France. A further step was taken when EIFAC approached the International Council for the Exploration of the Sea for assistance in defining fields and ways of cooperating. As a result the Joint Symposium on Eel Research and Management was organized and held 9-11 June 1976 in Helsinki. Arrangements were planned by a steering committee consisting of J . Boëtius, J . Dahl, C. J. M cGrath (Rapporteur), F. Thurow, K. Tiews (Con­vener), and R. Welcomme. C. J . McGrath (EIFAC) and F. Thurow (ICES) were appointed chairmen of the symposium. Five consecutive panels were estab­lished and led as follows:

(1) Appraisal of the present status of eelfisheries I. Boëtius

(2) Measures for the improvement and main­tenance of eel fisheries I. Boëtius

(3) Age and growth under natural and artificial conditions C. L. Deelder

(4) Migration and reproductive phase F.-W. Tesch(5) Conclusions and recommendations F. Thurow

Papers submitted and other relevant information were presented by the panel leaders and discussed by the participants. This led to the conclusion that an assessment of the state of exploitation and of the effect of elver stocking was urgently needed. The Baltic, the North Sea, the Atlantic between the English Channel and Gibraltar, and the Mediterranean were tenta­tively identified as management units, with the areas extending seaward as well as inland. I t was recom­mended that EIFAC and ICES working groups co­operate to improve ageing by comparative reading of

otoliths of known age and by analysis of otolith struc­tural development. Further, the organization of an international expedition to the supposed spawning areas of the European eel was finally recommended. A full account of the discussions at the symposium has been published in EIFAC Tech. Pap. No. 28.

Fifty-four reports were made available to the sym­posium. This volume contains 22 articles based on 25 symposium papers. They are arranged in approx­imately the same sequence as the panels.

Many referees have contributed to the preparation of this volume. They have unselfishly rendered their assistance and used much of their time to aid this work.

My personal thanks go to the Technical Editor, Mrs J . Rosenmeier. Much of what is achieved in editing this volume is due to her efficient work.

The following reports, partly published elsewhere, were also prepared for the symposium:

Anon. 1976. First report of the W orking G roup on stocks o f the European eel. ICES CM 1976/M: 2, 33pp . (mimeo).

Bieniarz, K ., Epier, P., Cedrowski, A., and Sokolowska, M . 1976. Eel culture in artificial conditions. Rocz. N auk rol., H-98-4.

Boëtius, I., and Boëtius, J . Fecundity of the European eel. Dana. (In press).

Boëtius, I., and Boëtius, J . Estimate o f an energy budget for m igration and spawning of female European eels. Dana. (In press).

Ciepielewski, W. 1976. The size, sex, and age of seaward migrating eel from two M asurian lakes. Rocz. Nauk rol., H-97-2.

Deelder, C. L. 1976. Remarks on the age determ ination of eels with length back calculations. Aquaculture, 9 : 373-379.

Dembinski, W ., and Swierzowski, A. Selectivity o f eel pound-nets and the size structure o f eel populations m igrating downstream. (To be publ. in Rocz. Nauk rol.)

Dembinski, W ., and Swierzowski, A. Effectiveness o f electric eel fishing in Poland. (To be publ. in Rocz. N auk rol.)

Dembinski, W ., M ayer, I., and Swierzowski, A. Catches and selectivity o f an electrified eel seine net. (To be publ. in Rocz. Nauk rol.)

D eufel,J., and Strubelt, T . 1976. Running o f eel stocks in the Lake of Constance. Österr. Fisch., 29 (11/12): 189-195.

Descamps, B., Foulquier, L., and G rauby, A. E tude comparée de

6 F. Thurow: Preface

la croissance des anguilles en fonction de la tem pérature dans deux bassins en circuit ouvert.

H errm ann, G. O n the eel yields of inland fishery in the Federal Republic o f Germany.

Koops, H ., and Kuhlm ann, H . Prelim inary note on the growth of the European eel in a brackish therm al effluent.

Lam arque, P. 1976a. Types de courant électrique à utiliser pour la capture optim ale des anguilles. Piscic. Fr., 47: 30-37.

Lam arque, P. 1976b. A ppareil pour la mesure et la pesée d ’anguilles anesthesiées électriquem ent. Piscic. Fr., 47: 38-39.

Leopold, M . State o f eel m anagem ent in Poland.Leopold, M . Stocking as a m ain factor determ ining the level of

eel catches in Poland.Leopold, M . Basic problems of eel exploitation.Leopold, M . The effect o f trophic and biological conditions on eel

production and management.Leopold, M . Efficiency and prospectives o f eel m anagement.M cG rath, C. J . R eport o f the EIFA C W orkshop on age deter­

m ination of eels, M ontpellier, M ay 1975.M cG rath, C .J . R eport o f the second m eeting of the EIFA C ad

hoc W orking G roup on eel fishing gear and techniques, Dublin, M arch 1974.

Passakas, T . 1976. Further investigations on the chromosomes of Anguilla anguilla. Folia biol., 24 (2) : 239-244.

Peters, G. 1977. The papillomatosis (cauliflower disease) o f the European eel (Anguilla anguilla) : fluctuations in the rate of incidence in the Elbe and their causes. Arch. FischWiss., 27 (3): 251-263.

Saint Paul, U . 1977. Young eels in N orth Sea river estuaries caught for stocking purposes. Arch. FischWiss., 28 (2/3): 123-135.

Serene, P. M arché de l ’anguille en Europe. E tude d ’approche.Stott, B. O n the present state o f the eel resources in England

and Wales.Svärdsson, G. 1976. T he decline o f the Baltic eel population.

Inst. Freshw. Res. D rottningholm , 55: 136-144.Swierzowski, A. 1975a. General analysis o f eel catches in rivers

and lakes o f the drainage basin o f N arew river. Rocz. Nauk rol., H-96-4.

Swierzowski, A. 1975b. R hythm and intensity o f silver eel catches in the drainage basin o f N arew river. Rocz. N auk rol., H-96-4.

Swierzowski, A. The effect o f a smooth rectified electric current on the behaviour and metabolism of eel. (To be publ. in Rocz. Nauk rol.)

Veen, T . van, Frem berg, M ., H artw ig, H . G., and M üller, K . 1976. Photoreception and circadian rhythm in the eel. J . Comp. Physiol., I l l : 209-219.

22

R app. P.-v. Réun. Cons. int. Explor. M er, 174: 22-31. 1979.

AN E X P E R I M E N T A L E L E C T R I C A L G U I D A N C E S Y S T E M F O R E E L S AT K I L L A L O E E E L W E I R O N T H E R I V E R S H A N N O N

C. J . M c G r a t h

D epartm ent o f Fisheries and Forestry, Dublin, Ireland

D. P. O ’ L e a r y

Electricity Supply Board, Stephen Court, 18/21 St. Stephens’ Green, Dublin 2, Ireland

P. J . S h a r k e y

M arine Electrics, Donegal, Ireland

and

D. F. M u r p h y

D epartm ent o f Fisheries and Forestry, Dublin, Ireland

Silver eels are fished a t Killaloe on the R iver Shannon by an array of conventional fyke nets. There are special problems at the site which make the operation inefficient, and various efforts to improve the efficiency by m echanical means have failed. This paper describes attem pts to improve the efficiency of the Killaloe eel weir by electrical means, and incidentally to develop electro-mechanical means of capturing silver eels in river reaches where elaborate structures could not be built economically. Effort was concentrated m ainly on developing an electrical guidance system which would direct eels from a wide cross-section of river to a single net m outh. This would be followed up by a stunning or narcotizing device a t the net m outh to prevent escape through the wide meshes. Farther down the net a similar device would be installed to increase the efficiency of the non-return valve. The m ain sections o f the paper deal w ith the design and development o f the electrical guidance system, w ith particu lar emphasis on the unique type o f energizing - alternating pulses of sawtooth and rectangular wave forms. A t the time of writing, only one full-scale river trial o f the guidance system has been made and this proved abortive due to unexpected site problems. Plans for further trials are outlined.

IN T R O D U C T IO N which had been driven into the bed of the river im-Historical records indicate the importance of eel mediately downstream of Killaloe road bridge,

fisheries in the River Shannon in former times (Went, The catch of eels in this weir for each year of the 1950). See Figure 4. Over the years many of the period from 1955 to 1975 is set out in Figure 6. This original rudimentary eel weirs built on this river represents a maximum catch of eels in the waters system were demolished to improve navigation. In upstream of Killaloe of 2-48 kg per hectare and a 1928 the river was harnessed for the generation of minimum catch of 0-54 kg per hectare. These yieldselectricity, and in 1940, following the vesting of the of eels for the area of water involved were obviouslyfisheries in this river in the Electricity Supply Board much less than was to be expected, and investigationsof Ireland, the organisation built a new eel weir at were set in train which confirmed the suspicion thatKillaloe intended to capture in one structure the run a contributory cause of the low yield obtained wasof silver eels formerly caught by the large number of the inefficiency of the catching devices that had been small eel weirs. In general these weirs covered small provided at Killaloe.parts only of the width of the river. A sizable number Over the years a number of steps have been taken were located in that stretch of the river which was to improve the efficiency of this weir (O’Leary,downstream of the point of diversion of the major 1971). These have been less successful than expected,flow of the river to a head-race to supply the hydro- The most recent measure taken by the Electricity electric station at Ardnacrusha. Supply Board in collaboration with the Department

The new eel weir at Killaloe consisted of a series of of Agriculture and Fisheries to find a solution to the “coghill” or fyke nets of conventional type extending problem has been the employment of electricity. It across the full width of the river (Fig. 5). These nets was decided to carry out experiments at Killaloe withwere supported by a framework of steel uprights the electrical guidance systems for fish under de-

An experimental electrical guidance system for eels at Killaloe eel weir on the River Shannon 23

L . N eag h

BELFAST

L. Al len

L.Ree

D U B L I NA T H LO N E

P O R T U M N A

f L D e r g ^* / K I L L A L O E

L I M E R I C K

K I L O M E T R E S

Figure 4. Shannon catchm ent, Ireland.

velopment at the Department’s field research labo­ratory at Glenties Salmon Hatchery, Go. Donegal. This paper describes the development and nature of these guidance systems, and the difficulties that have been experiencedj|_to date in putting them into practice.

C A T C H M EN T DATA

The area of the catchment of the River Shannon upstream of Killaloe eel weir is 10 618 km2 and the water surface area of the rivers and lakes within this catchment is estimated to be 41 400 hectares. The average annual flow of the river a t Killaloe is 175 m3/ sec. and that which occurred during the eel run in the period October to February 1965-1975 was 250 m 3/sec., but a typical winter flood occurring during this period might be as much as 700 m 3/sec., approximately. Typical winter temperatures of the River Shannon at Killaloe range from 5-5 °C to 9-5 °C, and summer temperatures range from 13°C to 21 °C. The average annual rainfall is 1016 mm, of which 54-10% appears as run-off in this river.

FA C TO R S IN F L U E N C IN G T H E EEL R U N A T

K IL L A L O E

Lough Derg is a very large elongated and deep lake about one km upstream of Killaloe, with its main axis running SW-NE (Fig. 7). Owing to large-scale peat bog development work in the hinterland upstream of Lough Derg, unusually large amounts of suspended solids and peat silt occur in the influent river to the lake, but these are settled out in the upper end of the lake. There is therefore a significant difference in the clarity of the water between the influent and effluent flow. Eels migrating downstream and passing

Figure 5. Eel weir a t Killaloe.

24 C. J . M cGrath, D. P. O ’Leary, P. J . Sharkey, and D. F. M urphy

CATCHKgs.

S| ï | ÿ |i||

Figure 6. C atch o f eels, 1955-1975, a t Killaloe eel weir.

M OU NT SH AN NON

HOLY

LoughDerg

L e g e n dA V E R A G E W I N

( K m / H o u r )

7 8

SHOAL

;i* K ILLALOE B R .iji ( e e l w e i r )

Figure 7. D irection and speed of the wind as observed near the outlet from Lough Derg.

(Fig. 7). The wind creates a wave action which stirsup the silt on the bed of the shoal, thereby making thesupernatant waters turbid, whereupon the eels readily pass over and continue their migration downstream,

through Lough Derg experience a transition from An examination of the eel catches in Figure 8 inwaters of low clarity to those of high clarity as they relation to the rotation of the wind as shown inapproach the exit of the lake. It would appear from Figure 7 demonstrates this.observations made that eels moving downstream The hydroelectric station at Ardnacrusha is locatedthrough the lake tend to congregate in the deeper 11 km downstream from Killaloe. The quantity andwaters at the lower end of the lake and to remain pattern of water flow a t the eel weir are accordinglythere until conditions are such as to encourage further governed by the operation of the power station, andmigration. bear little relationship to what would occur naturally

Near the outlet from Lough Derg there is a shoal if the river were unregulated. The Ardnacrusha Powerextending across the width of the lake over which Station is used primarily to meet peak demands,the eels must pass (Fig. 7). The water depth here does During the eel migration season the peak tends tonot exceed 3-5 m. All indications point to the eels’ occur at 5:30 p.m. and to taper off to a minimumreluctance to move out from the deeper waters through around midnight. The corresponding flow variationthe shallower waters above the shoal until the latter is from 400 m 3 per sec. to 10 m 3 per sec. This meansbecome significantly discoloured. This discolouration that sometimes when heavy rainfall and generaloccurs when a strong wind blows from the northwest meteorological conditions are suitable for eel migra-

An experimental electrical guidance system for eels at Killaloe eel weir on the River Shannon 25

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III&

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ill

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27 28 29 30 31 1 2 1 3 1 4 1 5 6 7 8 1 9

DE CEMBER 1975 JANUARY 1976

Figure 8. Catch a t Killaloe as compared w ith w ind and w ater flow.

tion, the river flow at Killaloe may undergo an un ­natural reduction.

During low water flow in the river and when the lake surface is calm, it appears that the light from the moon can penetrate the clear water of the lake down to the bed of the shoal described above. Under such conditions the eels would appear to be reluctant to pass across the shoal. Eel runs normally begin to increase during the period approaching new moon and begin to decrease again during the period ap­proaching the first quarter of the moon.

FA C TO R S IN FL U E N C IN G T H E E FFIC IE N C Y O F

K IL L A L O E EEL W E IR

As is the normal practice in the operation of eel weirs of this type, it is necessary to lift the eel nets

at Killaloe at intervals during fishing operations in order to empty them. During a heavy run of eels it may be necessary to lift the nets many times during a night’s fishing. Escape of eels is inevitable during this operation while the nets are out of the water, and the greater the intensity of the run the greater the number of eels that will escape during this period.

From 1932 to 1934 the river bed upstream and downstream of Killaloe eel weir was dredged to a depth of 7 m. The bed immediately under the bridge and for a short distance upstream and downstream from it was not dredged and is now at a depth of 3-36 m. The configuration tends to ensure that the eels migrating downstream will concentrate close to the bed as they pass through the bridge openings and so likewise at the entrance to the eel nets immediately downstream from them. During strong flows in the river it is not possible to keep the sole-rope of the net on the bed, and at such times a considerable number of eels probably escape under the sole-rope. If during fishing the lower end of the net becomes unfastened from the supporting uprights practically no eels are caught.

There is street lighting at night along the roads on both banks of the River Shannon in the immediate vicinity of the town of Killaloe, as well as on the bridge. Inevitably a certain amount of background lighting is produced on the water surface of the river. This illumination also tends to drive migrating eels towards the bottom of the river and thereby enhance any tendency to escape under the sole-ropes.

When the demand for water by the power station falls off, flow in the river also drops and the nets sag and no longer remain fully extended. In this condition they are very inefficient for the capture of eels.

Further, owing to the large volume of water which must pass through the Killaloe eel weir to meet the demands of the power station, the upstream end of the nets must have large meshes (50 mm side) which are believed to be another contributory factor to the inefficiency of operation.

The eel run at Killaloe commences in the month of October. This coincides with the leaf fall and also with the decay of aquatic vegetation. The presence of this debris in the water at such times rules out the possibility of using small-mesh nets.

EFFO R TS TO IM PR O V E E F FIC IE N C Y

Experiments carried out for this purpose employing large-sized nets are described by O ’Leary (1971). A single large net was substituted for the three standard nets (Fig. 10) in one arch. The large net could be emptied without lifting the mouth as must be done

26 C. J . M cGrath, D. P. O ’Leary, P. J . Sharkey, and D. F. M urphy

with standard nets. This should have increased the catch by eliminating periods of approximately 15 min­utes every two hours when fishing was suspended. However, the large net proved to be less efficient, presumably because eels escaped through the large meshes which had to be provided near the mouth to give a suitable filter factor.

A screen consisting of 25 mm bars at 50 mm centres approximately forming a ramp 150 mm high at an angle of 45° was tried in the bed of the river at the entrance to a net. This screen seemed to inhibit the run of eels into that particular net.

A solid sill 250 mm high was placed at the entrance to one net. This sill definitely inhibited the run of eels into that particular net.

Heavy lead weights were attached to the sole-rope of the net. These made no apparent difference to the catching power of the net, but they did make the handling of the net more difficult.

The possibility of finding another suitable site for an eel weir between Killaloe and the power station was examined. The results of this survey were dis­appointing. The best that could be done was to set up an experimental fishing unit at Clonlara in the head-race to the power station where it was possible to fish only 20% of the cross section of the head-race canal. The catches made in this unit tended to con­firm that the weir at Killaloe was inefficient. The experiments were not very rigorously conducted, but such results as were obtained indicated that the catch­ing efficiency of the Killaloe operation might be as low as 26%.

E L E C T R IC A L G U ID A N C E

Efforts were made to find another solution to the problems, and it appeared that electrical guiding and blocking techniques offered possibilities as an aid to increasing the efficiency of the existing eel-weir installation at Killaloe or as an adjunct to a unit such as that at Clonlara, where circumstances dictated that a small part only of the cross section of the water way could be covered by nets for the capture of eels.

The principal applications of electrical aiding worth further investigation appeared to be the following :

(1) Concentrating eels on their approach to the net mouth.

(2) Preventing escape under the sole-rope at the net mouth.

(3) Preventing escape through the larger meshes near the net mouth.

(4) Preventing return through the valve of the net.

Since 1965 P. J . Sharkey has been experimenting with means of electrically guiding downstream migrant

fish such as salmon smolts and eels at the field labora­tory operated by the Department of Fisheries and Forestry at the State Salmon Hatchery at Glenties, Co. Donegal.

A N G LED SCREENS

First a line of vertical electrodes was placed in the Owenea River at an angle of 35° to the direction of flow, leading to a trap at the bank. A horizontal counter-electrode was laid on the bed along the toe of the bank upstream of the trap. The array was energized with half-wave rectified current at 15 volts. The intention was to lead the downstream migrants to the trap as they avoided the negatively charged vertical cathodes and were attracted to the horizontal anode.

However, observation of eel behaviour under test showed that while some individuals swam up- or downstream parallel to the cathode line, displaying cathodic avoidance, others who came too close to the cathode were incapacitated and swept past the trap by the flow of water.

The experiment was repeated in a laboratory tank 2540 mm in length to allow closer observation of the behaviour of eels in this type of electric field. I t was found that the half-wave rectified energizing was essentially tetanizing, or at least, produced pseudo forced swimming with rapid onset of fatigue. Either of these effects would defeat the purpose by resulting in the eels being washed downstream past the trap.

IN V ESTIG A T IO N O F EFFE C T S O F W A V E FORMS

In order to determine what wave form would be most suitable for use with the angled deflector screen, an energizing unit capable of producing a variety of wave forms was built (Fig. 9). I t was found in tank tests with live eels that direct current produced by full-wave rectified smoothed A.C. induced true forced swimming towards the anode. I t should be noted that the tank used for these tests had been designed specifically to produce a perfectly homogeneous field. This made it possible to ensure that the voltage gradient along the axis of the test tank was such that the body voltage tapped off by an individual fish was precisely what was required to produce forced swim­ming. In a series of tests it was determined that the required body voltage was 2 volts (i.e. half the McMillan (1928) constant of 4 volts to produce narcosis).

PR O B L E M O F R E P R O D U C IN G L A B O R A T O R Y CONDITIONS

IN FU L L -S C A L E R IV E R TRIALS

However, it was found impossible to produce a homogeneous field in the river without using elec­

An experimental electrical guidance system for eels a t Killaloe eel weir on the River Shannon 27

SWITCH B SELECT

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MOMENTARY CONTACT PRESS SWITCH ON I f LYING LEAD

OUTPUT PIN JACKS AND OSCILLOSCOPE TERMINALS,

Figure 9. M ultiple outlet tank energizing unit.

trodes so large that they would be too costly and would either be washed away or seriously alter the hydraulic flow condition in the river. It also be­came clear that even if a homogeneous field could be produced in a river, it would be impossible, because of the wide variation in the length of eels, to energize it at any one level which would induce in all eels the body voltage required for forced swimming.

When river trials of the angled screen were resumed using direct current produced from the full-wave rectified and smoothed A.C., probe tests of voltage gradient in the water confirmed that the field was heterogeneous, and that voltages sufficiently high to incapacitate even the smaller eels, and resulting in their being washed past the trap, occurred in various places.

Because of these difficulties, trials in deflecting eels with an angled screen requiring the eels to be diverted from areas of higher velocity to areas of lower velocity were abandoned.

T H E PR O G R A M M E D E L E C T R O N IC T R A P

CONCEPT

Once it was realised that under full-scale river conditions the only electrical field likely to be achieved was the heterogeneous one, consideration was focussed on how this limitation could be used to advantage. The first conclusion reached however, was that the actual trap device should be so located that fish that were momentarily incapacitated would be washed by the water flow towards it. One of the obvious defects of the angle screen had been that those fish in which forced swimming toward the bank-side trap was in­duced, were swept back into the centre of the river once they entered an area where the energizing level was high enough to incapacitate them.

These considerations led to the following ideas:(1) putting the trap at the point of highest velocity;(2) deliberately producing a heterogeneous field in which any fish travelling downstream would at one stage or other pass through an area where it would be subjected alternatively to stimuli appropriate to

28 C. J . M cGrath, D. P. O ’Leary, P. J . Sharkey, and D. F. M urphy

its size, producing forced swimming towards the anode and subsequent narcosis. In the latter state the fish would tend to be carried by water currents to the section of highest velocity where the trap would be located.

T O P O G R A P H IC A L L Y PRO G RA M M E D E L E C T R O N IC T R A P

A conventional fyke net arrangement 0-5 m wide was accordingly built in the centre of the river in a reach where the bed deepened fairly uniformly from each bank to the centre. In this reach the velocity tended naturally to be slack near the banks and higher at the centre. A single line of vertical anodes was mounted on the river bed along the centre line heading upstream of the trap, and two horizontal cathodes were laid along the bed near each bank.

This arrangement was energized with direct cur­rent (smoothed half-wave rectified A.G.) at a voltage level calculated to produce in the water electrical gradients of sufficient intensity to induce up to 2 volts of body voltage in smaller eels. At this level of ener­gizing, larger eels would be subjected to voltages which would narcotize them.

Observations during trials of this array indicated that some concentration of eels towards the central row of anodes was occurring. However, the actual number of eels trapped was small, because the afflux at the mouth of the trap militated against eels’ entering it. Nevertheless the concentrating effect was sufficiently encouraging to justify trials of a similar guidance system at another site where more suitable flow conditions obtained and there were already a number of traps available by means of which com­parative fishing tests could be carried out and a large run of eels over an extended period of time permitting such tests to be carried out.

The Electricity Supply Board agreed to collaborate in the mounting of such an experiment at the eel weir at Killaloe.

E L E C T R O D E A R R A N G E M E N T F O R E E L GUID A N CE

A T K IL L A L O E

Figure 10 shows the arrangement of electrodes de­signed for use at Killaloe. The central array forms the anode, and is in line with the centre of the middle net of a group of three net units. I t was argued that if anodic attraction is achieved, then the catch of eels in the centre net should increase while the catches in the two side nets should decrease. Because the water is deep a t this site a horizontal cathode simply laid on the bed would not produce a field of sufficient strength in the middle and upper layers of water. Again because of the depth, and because the bed is

CATHODE

CATHODE

FLOW

Figure 10. Plan of electrical guidance system.

hard, it would have been difficult and costly to form the anode array simply by driving tubes into the bed. The solution to both problems was found by con­structing the gate-like arrays shown in Figure 11. These were cheap to manufacture and easy to install, as basically they float in the water and require only such restraint as will keep them aligned in water where the velocity does not exceed 2 m per sec. They also have the advantage that the surface area can be readily increased by welding on additional members. They have worked out well in practice and have already survived winter floods in the river.

T H E E N E R G IZ IN G U N IT F O R K IL L A L O E

Experience at Glenties had suggested that the form of energizing should meet the following re­quirements :

(i) the energy should be direct current to secure anodic galvanotaxis and avoid tetanus;

(ii) gradients in the electric field should vary from 0-01 volts/cm to 0-08 volts/cm to produce forced swimming in all sizes of eels likely to occur at this site.

It was anticipated, correctly, that the field produced at Killaloe would tend to be homogeneous owing to the large area of electrodes and the dispersion of cur­rent over a large volume of water. I t likewise became clear that with an homogeneous field it would not be possible to meet criterion (ii) above at this site.

An experimental electrical guidance system for eels at Killaloe eel weir on the River Shannon 29

KI LLALOEBRIDGE

1 I M g a S: :■ : : % W m -

?j«,

Figure 11. Section, showing electrical guidance system.

Tfø sawtooth concept: Time programming

The solution devised for this problem was to vary the energizing level on a time rather than a topo­graphical basis. This could be achieved with a saw­tooth wave form, in which the energizing voltage pro­duced in the river rises from zero to whatever level is needed to produce forced swimming in the smallest eel. A sawtooth generator was built and various mark/space ratios were applied to eels in the test

tank. Three ratios as shown in Figure 12 were selected on the basis of observed effects, and the prototype is capable of producing any one of these three as required.

The orientation pulse

In an homogeneous field produced by the electrode geometry chosen for Killaloe, the voltage gradients would tend to run normal to the water flow. Tank

6 0 m s

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R R.F. 5

SAWTOOTH AND SQUARE WAVE PULSE REGIME

SCHEMATIC

RIPPLE FREE P .S .U .

c / b FUSES ETC.

C O N T IN O U S RATING

P.S.U. S A W «. SQUARE

VOLTAGE STABLE

GENERATORS

RATIO CO N TRO L

RECTANGULAR

PULSE G E N E R A T O R ;

VARIABLE D UTY

CYCLE, VARIABLE

O U TPU T LEVEL

S A W T O O T H

PULSE G E N E R A T O R ;

VARIABLE PULSE REP.

FREQUENCY. VARIABLE

O U TPU T LEVEL

P O W E R AMPLIFIER

O N E KILOWATT

DARLIN GTON

Figure 12a. Energizing unit for Killaloe; schematic.

+ 60 V

ELECTRODE LOAD; M A X 5J\, !

R1TR1 TR2 TR 3 -1 0

Drive input from sawtooth v or square unit < n j

PARA

R3

PU L SE PO W ER AM PLIFIER

Sei Sw.

_n__n_

12100UF Supply

Supply> Short

10 K > Interval p- Timer

BFY 57

3 -9K 1-5K

To Drive f Saw Pre Am]

Long In te rv a l T im er Gen

500UF!

S A W

COMPOSITE PULSEJ~L RATIO CONTROL UNITT o D r i v «

Figure 12b. Energizing unit for Killaloe; amplifier and control unit.

30 C. J . M cGrath, D. P. O ’Leary, P. J . Sharkey, and D. F. M urphy

+14 V

TR2

TR3

R6"

TR1

VRlR2

SAWTOOTH GENERATOR

+ 14VD2R6R1

TR1 TR3TR2UJ.T.

UR2^> p u l s e ! 1

PULSE WIDTH LEVELVRl

-1 4 V

SQUARE WAVE DRIVER W ITH 8:1 DUTY CYCLE.

Figure 12c. Energizing un it for K illaloe; sawtooth, and rectangular pulse generators.

and field experiments had shown that eels (in common with other species) align themselves along equipoten- tial lines to minimize the voltage their bodies would be subjected to in the water, as a result of which little if any induced swimming towards the anode could be expected other than accidentally as in the case of a fish already pointing towards the anode.

Accordingly it was felt necessary to inject into the energizing regime an orienting pulse which would, by inducing anodic curvature, turn the eels towards the anode, so that the next sequence of sawtooth energizing would drive them towards the central axis.

Rectangular pulse energizing was chosen for this function, and the unit was designed to produce three variations of this form with pulse durations of 10, 20, and 40 milliseconds and repetition frequencies of 20, 10, and 5 pulses per second (Fig. 12).

The unit is so designed that it can give any desired combination of sawtooth and rectangular pulses. Two regimes can be achieved: (1) 5 seconds of sawtooth pulses followed by 1 second of rectangular pulse, and(2) 10 seconds of sawtooth pulses followed by 1 second of rectangular pulse. The duration of rectangular orientation pulses in the two programmes is deliber­ately kept low to minimize the risk of tetanus.

PRO BLEM S AND F U T U R E R E SE A R C H

I t was hoped that preliminary results of trials at Killaloe would be available for publication in this paper, but this has been made impossible by problems encountered on site and not yet fully overcome. It was found that the electrical resistance of the array was very low - only one ohm. W ater conductivity was 330 micromhos at 15 °C. As a result the maximum voltages that could be achieved were 35 volts with sawtooth and less than 20 with rectangular wave. The former may have been adequate but the latter was much too low. Clearly, the rating of the supply unit was inadequate for the loading encountered.

Further problems were encountered with corrosion of the electrodes and a build-up of a coating, which may be algal in origin, resulting in a severe loss in voltage at the metal/water interface.

There are various options open which might over­come the problems encountered. Corrosion losses can be reduced by remaking the electrode arrays in stain­less steel. The load problem can be overcome by building a more powerful power supply unit, capable of producing 60 amps (instead of 20). The flexibility of the electrode mounting system allows changes in the array geometry, and it is now felt that this should

An experimental electrical guidance system for eels at Killaloe eel weir on the River Shannon 31

be availed of to place the cathodes at an angle to the direction of flow and to locate them somewhat farther upstream in relation to the line of the anode. This would ensure that eels on first entering the energized area of water would be subjected to a longitudinal body voltage which would produce the desired anodic taxis and the resultant concentration of eels along the central axis.

It is also proposed that the efficiency of the Killaloe eel weir be increased by attempting to prevent eels from escaping under the sole-rope, through the large meshes, and out of the sock.

I t is felt that all these ends can be achieved by applying alternating current at a level capable of inducing narcosis on approach to the critical areas. Reliance will be placed on water flow to wash the incapacitated eels into and down to the tail of the net. To prevent escape under the sole-rope a pair of ground-mounted transverse electrodes will be placed

on the bed, one a t the net mouth and one at 1 m upstream of it. The section of net a t the mouth and valve will be energized by electrodes made of copper braid woven circumferentially around the net.

The results of these arrangements and of the guid­ance tests will be the subject of a further paper to be published a t a later date when sufficient data have been accumulated on the system in operation to permit statistical analysis of the results and to arrive a t firm conclusions.

R EFER EN C ES

M cM illan, F. O . 1928. Electric fish screen. Bull. U .S. Bur. Fish (44) (Docum ent 1042): 97-128.

O ’Leary, D. P. 1971. An account o f some experiments in fishing eel nets in Ireland. E IFA C Cons. Eel Fish. G ear and Tech. EIFA C Tech. Pap. (14): 129-142.

W ent, A. E. J . 1950. Eel fishing a t A thlone; Past and present. Jo u r. Royal Soc. Antiq. Ireland 80 (2): 146-154.