5

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

PR EFA 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. McGrath (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. It 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.

134

R app. P.-v. Réun. Cons. int. Explor. M er, 174: 134-143. 1979.

C O U N T E R - C U R R E N T O R I E N T A T I O N IN T H E M I G R A T I O N OFT H E E U R O P E A N EEL

H å k a n W e s t e r b e r g

Oceanographic Institute, Box 4038, S-400 40 Gothenburg, Sweden

T he behaviour o f m igrating eels (Anguilla anguilla L.) was studied in the Southern Baltic w ith the help o f ultrasonic transmitters. C urrent measurements were m ade during the tracking. For parts of the track the direction chosen by the eel was found to be parallel to the current direction a t the dep th of swimming.

I t is suggested th a t: (1) The eel detects its orientation relative to w ater currents by perception of an internal potential gradient induced by movement through the earth ’s magnetic field. (2) The basic behaviour is a positive rheotaxis, bu t in the presence of olfactory stimuli o f freshwater origin this is reversed.

IN T R O D U C T IO N

During the first decades of the century Johannes Schmidt located the spawning area of the European eel in the southern Sargasso Sea. Since eels are dis­tributed from North Africa to northern Scandinavia, this means that they travel a great circular distance of 5000 to 7000 km, with a mean course from 270° to 220°, depending on the point of departure. The adults probably die after spawning and the larvae are carried more or less passively with the surface currents to the eastern shores of the North Atlantic.

The route of the migrating eels and the mechanism by which they find their way to the spawning area are unknown. As the eels start migrating they stop feeding, thus they are not caught on hooks. Their shape makes them unsuitable for capture by net or trawl and consequently very few eels have been taken in the open sea. The observations of migratory be­haviour made in inshore areas are summarized by Harden Jones (1968) and by Deelder (1970).

There are several possible explanations for how the eels find their way back to the Sargasso Sea: true navigation with the help of stellar observations (Edelstam, 1965) or by perception of the geomagnetic field (Tesch, 1972), and orientation against chemical gradient by olfactory clues (Teichmann, 1959) or along electrohydrodynamically induced electric fields (Rommel and McCleave, 1973a) Alternatively it could be the result of passive drift (Harden Jones, 1968).

A factor in common to all points of departure for the spawning migration is that the larvae have been

carried there by the current system of the North Atlantic. A hypothesis close at hand is then that the same currents could guide the migrating adults. Based on published fishery statistics and tagging experiments I suggested (Westerberg, 1973) that the observations from the Baltic could be interpreted as a positive rheotactic orientation moderated by hydrographic conditions, possibly by means of olfactory stimuli similar to those discussed by Creutzberg (1961) in relation to migrating elvers.

Essentially the idea was that migration is activated by water in which odour specific to fresh water is weak. If the odorous substance is degradable this will be characteristic of the water brought in to the coastal region during wind-induced upwelling, independent of salinity. The upwelling situation gives a coastal current parallel to the coast which is directed into the Baltic on the western side. Orientation against the current should bring the migrating eels out of the Baltic. On the eastern side the situation is re­versed.

This theory explains the general counterclockwise migratory direction shown in tagging experiments and the strong correlation between increases in catch of silver eels and a wind direction that leads to upwelling (Svansson, 1975). To test this hypothesis silver eels tagged with ultrasonic transmitters were tracked and the currents and hydrographic conditions were measured simultaneously. The results are presented below and indicate that the current provides the directional information for migrating silver eels in the Baltic.

Counter-current orientation in the migration of the European eel 135

Transducer Syntactic foam

Hg - battery

Figure 79. The ultrasonic transm itter assembly. Scale 1:2.

M A T E R IA L AND M ETH O D S

Eels for the experiments were caught in fyke nets in the same areas where they were to be released. The time between catch and release was less than one day. The eels selected were in the silver eel stage and probably females. In order to make the experiments reasonably comparable they were performed during the same lunar phase. Since it is well known that in this area the best catches of eels are made early in the first quarter of the lunar month, this was the time chosen for the experiments.

The ultrasonic transmitters were made by Chipman Instruments Ltd. The operating frequency was 50 kHz and the source level approximately 52 dB re 1 [x bar, 1 m. The acoustic signal was pulsed and the pulse frequency a function of the ambient temperature. The transmitter was attached to a standard eel tag with epoxy resin, and in order to reduce the negative buoyancy to approximately 3 g it was fitted with a piece of incompressible syntactic foam (Fig. 79). This assembly was fastened to the eel just in front of the dorsal fin.

Tracking was done with a sonar (Simrad SJ3) modified to work as a receiver only, with a range usually better than 1 km (Fig. 80). The ship was

Figure 80. Example o f detection range of transm itter. W ith the sonar in position A directed towards B the transm itter was audible inside the shaded area. The transm itter was located a t 45 m depth under a sharp halocline.

manoeuvred close to the eel three or four times per hour and a position fix was obtained by Decca Navigator. The Danish chain (7B) was used, and the random fixing error varied from less than 100 m in daytime to 700 m in the middle of the night (Anon., 1973). Uncertainty in relative position of eel and ship was insignificant compared with the fixing errors. In a preliminary experiment the tracking was done from a rubber boat with a hand-held hydrophone. Positions were obtained by radar from an anchored ship.

The pulse frequency was calibrated against tem­perature prior to the final assembly of tag and battery, using a battery held at constant temperature. The calibration obtained in this way during the experi­ment showed a temperature that was too high com­pared with the known bottom temperature, when the eel appeared to be stationary on the bottom. Later investigations made clear that the pulse fre­quency of the transmitter was determined in three independent ways. Apart from the dependence of temperature caused by a thermistor the frequency changed with the internal resistance and terminal voltage of the battery, both of which depended on the temperature and the degree of battery discharge. This gave in addition a complicated response to a rapid change in ambient temperature. The time constant of the thermistor was approximately 30 s, but the thermal time constant of the battery was of the order of 100 s, depending on the thickness of the epoxy coating. As the thermistor and the internal resistance had opposing effects on the pulse fre­quency, the result was an apparent overshoot in temperature.

A series of calibrations of tags similar to those used in the experiments, made with a battery at the same temperature as the transmitter, gave a mean slope of the calibration curve. This slope, together with the observed temperatures and frequencies at times when the eel was considered to be stationary on the bottom, was used instead of the original calibration to compute the temperatures. With the pulse-counting procedure used, the temperature resolution obtained is ± 0-3°, but the absolute accuracy is less certain owing to this indirect calibration.

The temperature stratification was measured with an NIO salinometer and the currents with gelatine pendulum current meters (Haamer, 1974). Moorings with five to ten current meters were deployed during the tracking and recovered later. The current measure­ments are nearly simultaneous and instantaneous pro­files. The accuracy of direction is ± 10° and of speed, ± 1 cm/s, with a threshold current of 2 to 4 cm/s for speed and 1 to 2 cm/s for direction.

2 000 m 1 500 m v

136 Håkan Westerberg

Table 56. Details of tracking experiments

Experim ent 1 2 3

Start o f trackingD a te ............................. 30 Sep 73 16 Sep 74 18 O ct 74Tim e ( M E T ) 1630 1900 1600

End of trackingD a te ............................. 30 Sep 73 19 Sep 74 20 O ct 74T im e ................................ 1830 1700 0700

Tracking periodH o u r s ............................... 2 70 39

Length of eel(m) ....................... 0-70 0-75 0-85

W eight of eel( k g ) ............................. 1-0 1-0 1-5

R ESU LTS

Some general data for the experiments are sum­marized in Table 56. The area of investigation is shown in Figure 81.

E X P E R I M E N T 1

The aim of this experiment was primarily to try out techniques with the acoustic tags, so the tracking was limited in time. It took place off the northeast coast of Bornholm. The wind was southwesterly Beaufort force 8, veering to the west and decreasing to 5 or 6 during the experiment.

N 57°

Kalmar

56 °

Malmö

16°

Figure 81. Location of the experiments.

52' 5 3 ’

1.0 km0.5

1800 h

N 55° 14*

^ 4 0 m

5 cm/s

J 7 0 0 h

20 m

Figure 82. Trajectory o f eel in experim ent 1. T im e marks with 10 min. intervals. C urrent measured after end o f tracking at the same dep th as the eel’s approxim ate swimming depth.

One eel with a transmitter inserted in its stomach was released around midnight and stayed in the same place on the bottom throughout the night. It was suspected that the transmitter could have been re­gurgitated, so the next eel was fitted with the trans­mitter externally as described above. This eel was released in the afternoon, before sunset, and was fol­lowed for two hours. Figure 82 shows the trajectory. The eel swam in the lower part of the thermocline in the temperature interval 9-5°-10-5°C and at a depth of 25 to 30 m. The current in this depth range, measured after the end of the experiment, was 5 cm/s with a direction of 120°. The mean swimming speed was 30 cm/s.

E X P E R I M E N T 2

Before the experiment began, winds were moderate, easterly and southeasterly. After the first night of tracking the wind became southwesterly and in­creased to force 5. This is the wind direction that gives rise to upwelling in the area. In this case the upwelling was probably weak.

This eel was followed for three days and nights and showed very regular diurnal activity. It swam actively during the night until approximately two and a half hours after sunrise. During the day it remained sta­tionary on the bottom and started to be active again

Counter-current orientation in the migration of the European eel 137

06 OOj

'740919 000.0

Still 0 7 5 0

74 09 19

Figure 83. Trajectory o f eel in. experim ent 2. Times given in local time (M ET). Time marks each hour. Broken lines on each side of the trajectory show the w idth o f the Decca random fixing error (63% probability level).

about one hour before sunset. The start and end of the active period were gradual. During the first and last hour the eel swam more slowly and sometimes stopped on the bottom for short periods.

Figure 83 shows the eel’s trajectory. In Figure 84 the swimming depth along this trajectory is shown, as deduced from the temperature readings from the transmitter and from the observed temperature stra­tification. The main observation is that the eel seemed to follow the thermocline rather than a constant depth. The preferred temperature interval was 13°— 14°C. Sporadic dives of short duration were observed every night. This behaviour was not recorded syste­matically during the first night, so the depth trace is incomplete in this respect. Furthermore, it was noted that the eel dived if the tracking ship or some other vessel passed immediately overhead. The dives that probably resulted from this are marked with an asterisk in the figure.

Figure 85 presents the results of the current measure­ments made during the tracking. The swimming depth of the eel was estimated as in Figure 84, and

from each current profile the two or three measure­ments nearest to this depth are included in the dia­gram.

The hourly mean speed relative to the bottom is shown in Figure 86. The Decca fixing error introduces an uncertainty that ranges from 4 cm/s in daylight to almost 40 cm/s at night. It is seen that the observed variations in swimming speed at night were well within this limit. The only significant variation in mean speed was the decrease during the first and last hour. In the same diagram the estimated current component parallel to the swimming direction is shown. The typical swimming speed relative to the water was approximately 0-5-1 body length/s.

E X P E R I M E N T 3

Before this experiment there was a long period of strong and moderate northerly winds supposed to have resulted in downwelling and currents moving south. At the time of release, the wind changed to the south and increased to force 6 during the first night.

138 Håkan Westerberg

I! I II19 22 01 04 0718 21 00 03 06 19 22 01 04 07 h

10 20 30 40 50 km

Figure 84. V ertical tem perature section along the track of the eel in experim ent 2. The thick line shows swimming depth rela­tive to tem perature structure. Vertical bars indicate the observation point and uncertainty in depth due to uncertainty in tem­perature measurements. The upper horizontal scale is local time (M ET). Arrows show place and time of tem perature soundings. Vertical broken lines m ark discontinuities in the section, where the eel has been stationary during daytim e. Asterisks indicate th a t the vessel passed overhead.

30'20 '74 0916 18 30

74 0917 00 30

20 km

20 c m / s74 0919 0 0 4 0

•15 m0810 17.5 m19 0 9 O g

1915 29

Figure 85. Vector diagram s o f currents in the depth range where the eel was swimming, measured along the trajectory o f experi­m ent 2. T he origin o f the vectors is a t the place o f measurem ent. Times in M E T . N um bers a t the points o f the arrows give the dep th o f the measurements. Solid lines surrounding the trajectory show the limits o f the random fixing errors (63% probability level).

Counter-current orientation in the migration of the European eel 139

Velo city re la t ive to ground

cm/s

80

60

AO

20

0

2019

î $ i00 06 12

JJ®18 24 06

-®12 h

- t im e

cm/s

80

60

A0

20

0

20

rn d®- 2A 06

t im e

Figure 86. The hourly m ean swimming speed of the eel in experim ent 2 relative to the bottom . The broken line shows the ap ­proximate current speed parallel to the direction of swimming. The black pa rt o f the lower line indicates time between sunset and sunrise.

This gave rise to a northeasterly surface current. Later the wind decreased and veered to the northwest, which resulted in a reversal of the current direc­tion.

The trajectory of the eel is shown in Figure 87. Atmospheric conditions put the Decca Navigator out of use from 1900 h the first day to 1300 h on the second day. During this period the position was taken by radar. The uncertainty of position was estimated to be 1 km maximum.

The water was almost homothermal from the surface to the bottom. The temperature indicated by the transmitter never differed significantly from that of the surface mixed layer, and it gave no information on the swimming depth. The signal strength received was at a maximum some distance from the eel, and decreased at closer range. It was also noticed that the signal became stronger when the ship rolled so as to point the sonar transducer upwards in the direction of the eel; thus it is probable that the eel was close to the surface. These phenomena were apparent except for the last 5 to 6 hours, when the signal strength increased steadily when the eel was approached and the swimming depth was probably greater.

The current measurements from the two most shal­low meters of each measuring station are shown in Figure 88.

Figure 89 shows the hourly mean speed relative to the bottom. During the period of Decca failure the uncertainty is more than 50 cm/s, which is the same order of magnitude as the computed speed. Otherwise the uncertainty is the same as in experiment 2. The short time variations are in general less than the un­certainty due to the random error of position. It should be pointed out that this uncertainty due to position error is not additive. The magnitude is in­versely proportional to the averaging time, hence, trends and averages over long periods are signif­icant.

There is no clear difference in activity between day and night. If the interpretation of swimming depth is right the surface current can be subtracted to obtain the swimming speed through the water. As seen in Figure 89 this would be approximately 40 cm/s grad­ually decreasing during tracking. This is less than 0-5 body length/s and a smaller speed than observed in experiment 2.

D ISC U SSIO N O F FIE L D E X PE R IM E N T S

E X P E R I M E N T A L C O N D I T I O N S

The possibility that experimental conditions may have influenced the eels’ behaviour cannot be ex­cluded. The most obvious factor would be the hydro­

140 Håkan Westerberg

2200'7410 20 0100

.190004 00!

07 0 0 ‘ 160 0

13Ô03 0 m 4 0 m 50 m 60m

1000

0 700

04 0 0 / !

741019 0100.

S ta rt 15 45

74 10 18 !10 km

Figure 87. Trajectory o f eel in experim ent 3. Notation as in Figure 83.

dynamic drag of the transmitter. If we look at the mean speed through the water, computed over the middle 10 hours of swimming for each night, it was 58, 67, and 43 cm/s in experiment 2. Eel No. 3 swam continuously and the successive 10-hour means were 41, 35, 29, and 26 cm/s. This points to a gradual exhaustion or an increased irritation from the ultra­sonic tag.

The frequency of the tag was 49 kHz, which is well above the audible range for eels (Diesselhorst, 1938). Eels have a high sensitivity to electric fields (Rommel and McCleave, 1973b), and it is not impossible that the fields generated by the tag could be perceived by the eels* A crude experiment was made with a tag hanging down in a tank among a few eels swimming

around slowly. No marked reaction was observed when the tag was switched on and off. The presence of the tracking ship was another disturbing factor. It was noticed that when the ship came too close the eel dived. If the eel tried to avoid the ship by swimming away from the sound this could mean that it was systematically chased in one direction. During ex­periment 2 this possibility was tested by manoeuvring the ship into different bearings relative to the eel. The direction of approach did not influence the course of the eel. I assume that the experimental situation did not influence the tracking results qualitatively, especially with regard to the choice of direction as representative for the behaviour of mi­grating eels.

Counter-current orientation in the migration of the European eel 141

7410 20 0030

20 cm /s

6m

06 40

1415

741019 0740

741018 1530

10 km

Figure 88. Currents along the trajectory o f experim ent 3. Notation as in Figure 85.

E N V I R O N M E N T A L F A C T O R S behaviour, but some negative conclusions can be The purpose of the experiment was to investigate drawn with confidence,

the relationship between environmental factors and Navigation based on visual observations is excluded, the orientation of the eel. The number of trials is In experiment 2 the eel swam a similar distance withclearly insufficient to permit generalizations about the same amount of meandering the first two nights.

Velocity relative to ground

cm/s

8 0

6 0

40 20

- n

18 oo 06 12 18 00 06tim e

Figure 89. T he hourly m ean swimming speed of the eel in experim ent 3. Notation as in Figure 86.

142 Håkan Westerberg

During the first night the weather was clear, whereas during most of the second there was a thick fog preventing any sighting of stars.

There is no indication that the eels followed the bottom topography. Figure 84 shows the depth va­riations along the track of eel 2. The total depth varied from 30 to 50 m. The distance between the estimated swimming depth and the bottom varied from 10 to 35 m. The track of the eel in experiment 3 was even more independent of bottom topography, as seen in Figure 87.

The results are also difficult to interpret by the hypothesis that the eels try to follow a constant course, coastline permitting. If the behaviour of eel No. 3 is representative, it shows that eels can swim for several hours along approximately straight tracks in open water, but following a variety of courses. In experiments 1 and 2 the eels moved more along the coast and the result could be explained as an attempt to head west or southwest. However, the relative independence of bottom topography discussed above makes this unlikely.

R H E O T A X I S

Tides are neglible in the Baltic, and the currents in the area are essentially governed by meteorological conditions and topography. There is no fundamental time scale which can be used to decide how represen­tative the single current measurements are. Especially with variable or weak winds the currents can change rapidly. Several of the current measurements were made just before the eel started swimming or after it stopped on the bottom. Furthermore, the probable error in positioning makes it necessary to average the swimming direction over relatively long periods of time. These factors, together with the imperfectly known swimming depth, make comparison between the current measurements and the swimming direction difficult.

A survey of Figures 85 and 88 may give the im­pression that the currents were frequently parallel to the eel’s course. The current can, however, be both against and along the swimming direction. The first alternative is best represented by the third night of experiment 2. The wind was steady and the three current profiles show much the same approximately easterly direction. The general heading of the eel was westerly and against the current. The second night also shows a swimming direction against the current when one compares the measurement after the eel stopped swimming with the direction during the last hours of swimming. The measurement at midnight shows currents toward the southeast, and at the same time the eel appears to have moved northwest in a loop. This loop is within the Decca fixing error and could have been an artefact, in which case the mean

speed of the eel was low and the direction uncertain during the time of the current measurement. These two nights, together with the single measurement of experiment 1 which also showed a swimming direction against the current, are cases with probable upwelling along the coast.

Experiment 3 was made during downwelling, and the tendency seen in Figure 88 was for the current and swimming directions to coincide. The most striking observation is that when the current changed direction by almost 180° between 1415 h the second and 0030 h on the third day, so did the eel. The first night of ex­periment 2 also shows a swimming direction along the direction of the current below the mixed layer. In this case the probable swimming depth was close to a depth where the current changed direction by 180°, so the comparison is even more uncertain than in the rest of the material.

In conclusion the experiments give the impression that rheotaxis may be important in the migration of eels. Especially in experiment 3 rheotaxis seems to give an explanation for the otherwise inappropriate swim­ming direction into the Baltic.

P E R C E P T I O N O F C U R R E N T S

Even if these observations make it seem probable that the eel utilizes the direction of the current as a reference, asking how this can be done is by no means a trivial question. A necessary condition is that the eel in some way perceives the solid earth. Visual contact is ruled out in all cases except in daytime close to the bottom. The geomagnetic field, which is almost stationary relative to the earth, could be used. In laboratory experiments it has been demonstrated that eels can show a preference for some particular direction in an environment that has no inherent orientation, and that the distribution becomes random if the geomagnetic field is removed (Branover et al., 1971). A receptor of magnetic fields is unknown in eels. One possibility is that the eel uses electric re­ceptors and detects the electric field generated by its movements through the stationary magnetic field. That eels have electric receptors of sufficient sensiti­vity was demonstrated in experiments by Rommel and McCleave (1973b) using externally applied fields. They were, however, unable to demonstrate any clear sensitivity to a change in strength of the geomagnetic field. In these experiments the movements of the eels were restricted.

Rommel and McCleave (1973a) suggest that the electric field generated in the ocean by current systems could serve as an orientational clue. However, this field is not uniquely determined by the local velocity, but depends on the distribution of velocity and conductivity in the surroundings. In areas with complicated current patterns the local geoelectric

Counter-current orientation in the migration of the European eel 143

field can have an arbitrary direction in relation to the current direction. This makes the external geoelectric field ambiguous for navigational purposes.

A more direct coupling to the local current direction is obtained if the eel is sufficiently insulated electric­ally, for instance by the subcutaneous fat layer. The electric field induced within the eel will then be determined by the orientation and velocity of the eel alone. The direction of the induced voltage gradient is perpendicular to the direction of the magnetic field and to the velocity of the eel. This means that a voltage drop along the eel will be proportional to the transverse velocity component of the eel, with a polarity which is determined by whether the eel moves to the right or to the left. In a horizontal position and with a current moving in the horizontal plane the eel can determine its relative orientation with respect to the current, but it is unable to decide the absolute direction from the voltage drop along its length axis. In order to determine its orientation relative to magnetic north it has to move vertically or to keep the body at an angle to the horizontal plane, unless the voltage difference can be perceived dorsoventrally.

AC K N O W LED G EM EN TS

I should like to thank all those who helped me during the expeditions with RV “Svanic” , as well as Kristina Hansson for all her work with the diagrams.

R EFER EN C ES

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