Silver eel migration behaviour in the Baltic

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1 ICES CM 6/J:26 Theme session J Is there more to eels than SLIME? Silver eel migration behaviour in the Baltic Håkan Westerberg, Ingvar Lagenfelt and Henrik Svedäng Abstract During the eel migration season 16 female silver eels (Anguilla anguilla L.) were tagged with data storage tags (Lotek LTD 241) and released on the Swedish east coast at longitude 6 o 3 N. Until now 8 ( %) of the eels have been recaptured and the data of 6 tags were possible to recover. In all 71 days of migration is documented and the data show a very consistent and regular behaviour, both between individuals and over time. Essentially all swimming activity was between dusk and dawn, starting when the light level at the surface equalled the civil twilight limit and stopping in the morning at approximately the same light level. In the daytime the eels stayed quiet at the bottom, in bottom depths varying between 2 and 36 m. The only exception to this diurnal behaviour was one eel during one day, moving in an area where the bottom depth was in excess of 3 m. In this case the eel continued migration during daylight, following the thermocline in 8-12 m depth. Two of the eels stayed stationary at the bottom from 1 to 4 full days before resuming migration. Nighttimes the swimming depth was very close to the surface. As much as 9 % of the time was spent within. m of the surface. This is close to the resolution of the pressure sensor. Short dives were made down to the thermocline depth or occasionally down to what probably was the bottom. The duration of such dives were typically to 1 minutes. The mean migration speed along the coast between release and capture site was around 16 km/day. This gives an average swimming speed of.4 m/s when the eels were active, which fits with swimming speeds recorded in telemetry experiments. This implies that the movements of the eels were relatively steady along the coast and that the depth in daytime can be used to give offshore distance, which in combination with the accumulated swimming time since release can be used to reconstruct the trajectory. Keywords: Migration, swimming depth, geopositioning, diurnal behaviour Contact author: Håkan Westerberg Swedish Board of Fisheries, Box 423, SE 4126 Göteborg, SWEDEN Hakan.westerberg@fiskeriverket.se

2 Background The spawning migration of the European eel (Anguilla anguilla L.), from rivers and streams in Europe to the Sargasso sea, is one of the most impressive feats of animal migration and orientation. The behaviour during the migration has been studied using telemetry tracking among others by (Tesch 1989), (McCleave and Arnold 1999), (Westerberg 1979). So far the navigation mechanism is unknown. Geomagnetic cues (Tesch et al. 1992a) and selective stream transport (McCleave and Kleckner 1982) have been proposed. Westin (Westin 1998) has claimed that a phase of imprinting is necessary and that eels translated and stocked as glass eels are unable to navigate properly. To test this hypothesis a tagging study was made in the Baltic, with the aim of classifying the life history of recovered eels using otholite chemistry (Tzeng et al. 1997). As part of this study some eels were tagged with data storage tags (DST), with the purpose to look for possible differences in the detailed behaviour of normally orienting eels and eels that had an abnormal migration. Of the 16 eels that were tagged with DST % were recaptured all having migrated with a normal speed and direction. The otholite analysis of those eels, together with the conventionally tagged ones, is not yet finished and will be presented separately. The purpose of this paper is to present the unique dataset of 6 silver eels monitored simultaneously during migration in he same area for 7 to 23 days. The most significant findings are the regular diurnal activity, the migration very close to the surface during the night and the occurrence of occasional exploratory dives of short duration. Those features are found to be remarkably similar in all the eels studied. Methods The tagging and release was made on the 31 of September in Kalmar sund, between Öland and the Swedish mainland (Figure 1). The eels had been captured in eel pound nets just north of the release place the same day. The tagging was made without anaesthesia and the length and weight of the eels were measured, together with the eye diameter and fat content to assess the degree of silvering (Pankhurst and Lythgoe 1983). All the eels with data storage tags were females and judged to be in the silver eel stage. The length was between 64 and 8 cm and the weight from. to 1.2 kg. In all 16 eels were fitted with Lotek data storage tags of the type LTD 241. The DSTs had a depth range of 1 or m with 1 % FS accuracy and. % FS resolution, corresponding to 1-2 m and.-.1 m respectively. The final depth readings were noted from the recordings of all recovered tags when the DST was lifted out of the water and varied from -.2 to -.4 m. Those values were used to correct the swimming depth time series. The weight in water of the tag is 3 g, or <.6 % of the body weight of the eels, which is well below the 2 % deemed as a maximum to allow undisturbed behaviour (Jepsen et al. 2). The DST tags were programmed to start logging at : UTC on the 1 of October, approximately hours after the release. Data were recorded each minute. The experiment was advertised, with the instructions to save recovered eels to allow study of the otholiths. This caused some tags to be frozen together with the eel, which destroyed the recording. Of the eight eels recaptured six gave readable data. 2

3 The approximate nightly temperature profile along the eel s trajectory was constructed from the depth and temperature values during dives. The time constant of the external temperature sensor of the tag is 3 sec. Typically the vertical velocity during descent and ascent was.1 m/s, which made the temperature-depth measurement relatively imprecise. The tags were fastened externally, anterior to the dorsal fin, using stainless suture. The eels were kept in moist seaweed prior to tagging and handled with a wet cloth to avoid injury to the epidermis. The whole handling procedure took about 1 minute and the eel was immediately released into the sea. The release was made together with a total of 6 eels tagged with coded acoustic tags (Vemco V13) in a square array with Vemco V2 data logging buoys. This tagging was part of a baseline study for an offshore windpower park (Westerberg and Lagenfelt ). As the release was made during daylight almost all the acoustically tagged eels remained at the bottom inside the array until dusk the same day. Active migration started around :3 local time (18:3 UTC). It is assumed that the eels tagged with DSTs behaved similarly. Results A total of eight (%) of the 16 tagged eels were reprted recaptured. All recaptures were made south of the release site. Table 1 summarises the recapture data and Figure 1 shows the place of recapture. Two recaptures were made in bottom set gillnets for cod, which normally don t capture eels, but where the external tag can entangle. The remaining captures were in eel pound nets, which is the main gear used in the Baltic silver eel fishery. Table 1. Summary of data for the recaptured eels. All eels were released on the 31 of September. Nr Length (m) Weight (kg) Recapture date Days Gear Data pound net yes pound net yes pound net yes gillnet no pound net first day missing gillnet yes pound net no pound net yes 3

4 Figure 1. Release and recapture points for the DST-tagged eels in the study. The red rectangle in the overview corresponds to the area of the main map. Activity pattern All the eels showed a predominately diurnal activity pattern passive at the bottom during daylight and actively swimming close to the surface during the night. Figure 2 shows an overview of the swimming depth pattern of all the eels. In one case (eel nr 8, the 7 October) the eel remained active during the whole day, but the mean swimming depth was around the thermocline during daylight. Both eel nr 1 and 6 remained at the bottom for one or more full days before resuming migration. 4 4 Date (October) Depth (m) Eel nr Figure 2. Swimming depth time series of all eels, shown on a common timescale. 4

5 Date (October) 6 Depth (m) Depth (m) Figure 3. Expanded view of swimming depth during 48 hours from two eel tracks. Sundown Sunrise Date Time UTC Figure 4. Average time of onset of activity (circles) and termination (triangles) for the six eels on a given date. The solid lines show time of sunset and sunrise corrected for date and approximate longitude of the eels. The error bars show the 9% confidence interval. Dashed lines show the time of start and en of civil twilight.

6 Figure 3 show examples of the swimming depth record in closer detail. Evidently the onset and termination of activity is easy to determine. This was done for the whole records and the average time of start and stop of activity has been calculated for each date, using data from all active eels that day. The result is shown in Figure 4. The activity is very symmetric with the local daylight cycle and there is a close correspondence with the time of end and onset of civilian twilight Diving behaviour A common feature for all the eels is the sporadic dives, which occurred between fairly long periods of swimming close to the surface. Figure gives an example of such dives. Typically the dives consisted of monotonic movement down and then up. The duration of the dives varied but were in most cases short and proportional to the maximum depth, 3- minutes for a 1 m dive, without prolonged periods at medium depths. The periods close to the surface could be quite long, up to 2 hours deviating less than. m from zero depth Depth (m) 1 1 Figure. Examples of exploratory dives of eel #1 around midnight October 9 to 1. Horizontal scale is hours, zero at midnight UTC. The dots show the measuring points with 1 minute intervals. To analyse the diving behaviour for the whole dataset a dive was defined as a period when the eel changed from a swimming depth less than 1 m from the surface to a deeper level during more than 2 minutes. A deep exploratory dive was defined as a dive with a maximum recorded depth larger than 4 m. The time and maximum depth of all such dives were compiled and the total number for each active nightly period is shown in Figure 6. The average number of deep dives ranges between 1 and 2 per night and there is no trend over the 12 day period with data. 6

7 Number of dives Tag Mean Date (October) Figure 6. The number of deep exploratory dives during the nightly activity period for individual eels and averaged for all the eels (red line). Error bars show the 9% confidence limit. To look for a diurnal pattern in dives the frequency of dives per hour was calculated and averaged individually for each eel for all the active nights with a full recording. The mean of those averages is shown in Figure 7. There is no clear pattern, but a tendency for a higher diving frequency in the early and late phase of the active period and a quieter period in the middle of the night. The moon was in the first quarter when the eels were released. On the 1 October 4% of the moon surface was lit and it became visible at 2 UTC, approximately 2 hours before sunrise. On the 11 of October 8 % of the moon was lit and it was visible until 21 UTC in the evening, or 4 hours after sunset. No aspect of the diving behaviour seems related to the moon. The maximum depth of the dives tends to aggregate around depths of strong vertical temperature gradients, thermoclines. Figure 8 and 9 give examples of this taken from two different eels. The only case of an eel showing activity during daytime is shown in Figure 1, with the accompanying nightly temperature profiles. It seems that the eel moves in an intermediate water layer between the main thermocline and a surface layer of slightly higher (on the second day lower) temperature. 7

8 Dives/hour tag Time (UTC) Figure 7. Diving activity pattern of individual eels averaged for all full nights of activity. Upper panel average for individual eels, lower panel the mean activity of all eels. Error bars show the 9% confidence interval. Temperature Time UTC Depth (m) Tag 1 night 6 Figure 8. Comparison of temperature stratification and maximum depths of dives. Eel nr 1 during the night between 6 and 7 October 8

9 Temperature Time UTC Depth (m) Tag night 6 Figure 9. Comparison of temperature stratification and maximum depths of dives. Eel nr during the night between 6 and 7 October. Temperature Time UTC Depth (m) Figure 1. Comparison of temperature stratification and swimming depth of eel 8 from October (red temperature profile) to 7 October (black temperature profile. 9

10 Swimming velocity The time of active swimming is well defined from the depth time series. For each eel the total recorded active time has been calculated. The shortest water route from the release point to the place of recapture gives the minimum distance travelled and with those parameters the minimum mean horizontal swimming speed can be calculated. The result is shown in Table 2. Table 2. Minimum mean swimming velocity assuming a straight trajectory. Nr Body length (m) Distance over water (km) Total swimming time (h) m/s BL/s Evidently the real trajectory is longer than the straight line track. Horizontal undulations and vertical excursions will add to the total distance. The vertical excursions can in principle be measured from the depth time series. A rough estimate shows an extra distance of approximately 3 to m/night or about km for a whole track. The horizontal meandering probably adds more, but can not be calculated. A larger uncertainty is introduced by water currents. The calculations in Table 2 gives speed over ground, but the true swimming speed is relative to the water. No current measurements are available, but in the area the currents may well be or 3 % of the eels speed over ground. The maximum vertical velocity recorded was.2 to.3 m/s. There is no clear difference in the maximum rate of ascent and descent. The 99 % percentile may be used as an estimate of the characteristic vertical velocity during dives. Table 3 shows the statistics of vertical velocity found for the different eels. Table 3. Statistics of mean instantaneous vertical velocity, averaged over the 6 sec sampling interval, calculated for a total of approximately 7 hours for each eel. The vertical velocity is counted negative for diving and positive for ascent. Nr Mean abs. velocity (m/s) Sdev Min (m/s) Max (m/s) 99% percentile of abs. velocity (m/s)

11 Geopositioning The light measurements could in principle be used to estimate the daily position of the eel. The precision of the positions obtained were however very poor and not useful for reconstruction the trajectories of the eels. One reason for the uncertainty is the special behaviour during dawn and dusk, when the eel usually made large excursions up and down between the bottom and surface. This caused large disturbances on the light curve during the most sensitive period for determining the time of sunrise or sunset. An alternative way to reconstruct the eel s trajectory is to use the known time of activity each night and the depth at the place where the eel spent the intervening daylight periods. If the mean swimming speed during each night is assumed to be equal to that for the whole track the length of the successive nightly movement may be calculated. Using this and the known depth at the endpoint in most cases a unique probable position can be found. Figure 11 shows the individual tracks constructed in this way. The eels seems mostly to follow the general coastline but with some distance. The median depth where the eels stopped during daylight was 12 m. Figure 11. The individual trajectories of the six eels reconstructed from daily distance and bottom depth at period of rest. 11

12 Discussion Several factors support the assumption that the behaviour seen in this study is representative for migrating silver eels in the Baltic. First the similarity between all individuals, second the stability of the activity pattern during the whole tracks. In addition many parameters fits well with earlier data. The migration rate of 1 - km/day seen in this study falls within the range found from earlier conventional tagging experiments (Martinköwitz 1961; Sjöberg and Petterson ; Trybom and Schneider 198) and the average swimming speed of. BL/s is similar to what has been observed in telemetry tracking in the Baltic (Tesch et al. 1992b; Westerberg 1979). The only eel which shows sign of abnormal behaviour is nr 6, with three stops of two to four days duration. This eel also had the lowest swimming speed (.36 BL/s) during the active periods. However, the conclusions drawn from the study would be the same even if this eel were excluded. The main findings are the strict diurnal rhythm, following the light condition, the periods of swimming very close to the surface, the ubiquitous exploratory dives and the migration along the coast but still at relatively large distance. This verifies and support observations from telemetry experiments, made with few individuals and sort duration. The quiet periods at the bottom during daylight has been observed earlier (Westerberg 1979). The regularity is striking. The weather during the experimental period was stable with a high pressure over the southern Baltic and predominantly clear skies. The direct light measurements from the DST are of little help as the threshold light level for the onset of activity seems to be below the resolution of the sensor and the measurements often were disturbed when the eel was buried in the sediment, or made rapid vertical movements up and down. The close relation to the time of local civil twilight (Figure 4) suggests that the cue for activity is light rather than a circadian rhythm. One eel continued migration also during daylight for one day. Such continuous migration is the most common behaviour seen in telemetry experiments in the open sea (Tesch 1989) and has also been observed in the Baltic (Tesch et al. 1992b). One explanation is that continuous migration occurs at larger bottom depths, and the diurnal activity is restricted to when the eels move close to the coast. A new observation is the long periods of swimming very close to the surface. The actual distance from the water surface is not fully resolved by the DST, but must have been just a few centimetres. The reason for this behaviour is unknown. A speculation is that there is a relation to the orientation mechanism. If the eel uses celestial cues it obviously has to be close to the surface. One mechanism that has been proposed for magnetic orientation requires a weak light to function also. This radical pair process depends on magnetic effects in the retina (Wiltschko and Wiltschko 6) and cannot work in total darkness. Starlight is sufficient however. The significance of the sporadic dives is also unknown. The possibility that the dives were a way of saving energy by alternating glides and powered movement up again, which is known in birds and elasmobranches (Weihs 1973), has been tested and disproved earlier (Westerberg 1984). The eels swim actively both down and up. Some dives probably go all the way to the bottom and could be a way for the eel to sound the depth. The majority seems to turn at some intermediate depth however. The data indicates that there is a relation between the turning depth and features of the stratification. The low resolution of the temperature profile obtained from the DTS data, and the uncertainty of what the true maximum depth of a dive is with a 12

13 sampling interval of 6 sec., makes a detailed analysis difficult. A further possible cause of the dives is predator avoidance. The maximum vertical velocity is significantly less than the typical horizontal swimming speed however. This means that the dives have a gentle slope which would be unlikely if it were an avoidance behaviour. It is also not known what information in the stratification is relevant for the eel. Temperature may be a secondary parameter and other water mass properties as salinity, smell or velocity may be the primary information. The possibility to reconstruct the trajectory of the eel between the points of release and recapture has great potential. Compared to telemetry experiments the tracks from DST data have relatively long duration and the possible disturbance from a tracking vessel is absent. Several individuals can be tracked simultaneously, allowing comparison of reactions to external factors as weather or moon phase. This means that such data can be very useful for testing hypothesis about navigation behaviour. The limited size of the present study makes such analysis difficult, although further analysis will be made of possible correlation with weather and hydrography. The potential of future large-scale studies, with several release sites at different times, seems very promising however. Acknowledgment This study was made as an extension of a tagging study financed by the research program Vindval. References Jepsen, N., A. Koed, E. B. Thorstad, and E. Baras. 2. Surgical implantation of telemetry transmitters; how much have we learned? Hydrobiologia 483: Martinköwitz, H Ergebnisse von Blankaalmarkierungen an den ostrügenschen Küste und Möglichkeit ihrer Nutzung für die Fangsteigerung durch neuartige Reusenkonstruktionen. Z. Fisch. 1: McCleave, J. D., and G. P. Arnold Movements of yellow- and silver-phase European eels (Anguilla anguilla L.) tracked in the western North Sea. ICES Journal of Marine Science 6(4):1-36. McCleave, J. D., and R. C. Kleckner Selective tidal stream transport in the estuarine migration of glass eels of the American eel (Anguilla rostrata ). I C E S Journal of Marine Science 4(3): Pankhurst, N. W., and J. N. Lythgoe Changes in vision and olfaction during sexual maturation in the European eel Anguilla anguilla (L.). Journal of Fish Biology 23(2): Sjöberg, N., and E. Petterson.. Blankålsvandring. Fiskeriverket Information (3):1-48. Tesch, F.-W., T. Wendt, and L. Karlsson. 1992a. Influence of geomagnetism on the activity and orientation of eel, Anguilla anguilla, as evident from laboratory experiment. Ecol Freshwater Fish 1:2-6. Tesch, F.-W., H. Westerberg, and L. Karlsson. 1992b. Tracking studies on migrating silver eels in the Central Baltic. Meeresforschung 3: Tesch, F. W Changes in swimming depth and direction of silver eels (Anguilla anguilla L.) from the continental shelf to the deep sea. Aquatic living resources/ressources vivantes aquatiques. Nantes 2(1):9-. 13

14 Trybom, F., and G. Schneider Markierungsversuche mit Aalen und die Wanderungen gekennzeichneter Aale in der Ostsee. Rapp. p.-v. Reun. Cons. perm. int. Explor. Mer. 9:1-9. Tzeng, W. N., K. P. Severin, and H. Wickstroem Use of otolith microchemistry to investigate the environmental history of European eel, Anguilla anguilla. Mar. Ecol. Prog. Ser. 149: Weihs, D Mechanically efficient swimming techniques for fish with negative buoyancy. Journal of Marine Research 31: Westerberg, H Counter-current orientation in the migration of the european eel. Rapp. P.-v. Réun. Cons. int. Explor. Mer. 174: Westerberg, H Diving behaviour of migrating eels studied by ultrasonic telemetry. Pages in H. P. Kimmich, and H.-J. Klewe, editors. Biotelemetry, volume VIII. Westerberg, H., and I. Lagenfelt.. Åltelemetri: Utgrunden och Lillgrund. Swedish Board of Fisheries, Project report. Westin, L The spawning migration of European silver eel (Anguilla anguilla L.) with paarticular reference to stocked eel in the Baltic. Fisheris Reserch 38: Wiltschko, R., and W. Wiltschko. 6. Magnetoreception. BioEssays 28((2)):