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The spawning migration of European silver eel (Anguilla anguilla L.) with particular reference to stocked eel in the Baltic
Fisheries Research 38 (1998) 257±270
The spawning migration of European silver eel (Anguilla anguilla L.) with particular reference to stocked eel in the Baltic Lars Westin* Institute of Systems Ecology, Section Gotland, University of Stockholm, S-620 35, FaÊroÈsund, Sweden Received 7 October 1997; accepted 27 May 1998
Abstract Tagging investigations have been performed in the Baltic with previously stocked eels, anosmic eels, partly anosmic eels and natural eels as a control group. There were great differences between natural eels and previously stocked eels. The anosmic eels differed from natural eels in speed, direction and hibernation and agreed in these aspects more with the previously stocked eels. Among the partly anosmic eels, the group with left nostril blocked behaved like the anosmic eels while that with right nostril blocked behaved like the controls. This suggests that olfaction is essential for orientation. The stocked eels, which have never been in the Baltic before have lacked opportunities to imprint this orientation cue. In spite of that, the previously stocked eels succeeded in ®nding their way to the southernmost parts of the Baltic but usually only to areas very different from those of the migrating natural eels. This is tentatively explained by the occurrence of a second cue, temperature. The migration and orientation back to the spawning area in the Sargasso Sea may be a reversal of thermal and olfactory prehistory of the young stages during their period of active dispersal. The orientation capacity of eels in shelf areas like the Baltic is not a point to point event, but rather too much swimming with limited progress. Eels stocked in the Baltic drainage area have problems in ®nding their way out. As a consequence, the contribution to the spawning stock in the Sargasso Sea has to be questioned. # 1998 Elsevier Science B.V. All rights reserved. Keywords: European eel; Baltic Sea; Migration and orientation; Stocking; Imprinting; Olfaction; Salinity; Temperature
1. Introduction According to catch data for the Baltic Sea, which dates from the 1940s there has been a long-term decrease in the recruitment of eels (SvaÈrdson, 1976; WickstroÈm, 1986). Further, there have been recent reports of a more drastic decline in the immigration of
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glass eels in several western European countries (Moriarty, 1986; HagstroÈm and WickstroÈm, 1990). This long term decline seems to have been most evident in countries around the Baltic Sea, where eels are most probably recruited through the Fair Isle channel and across the North Sea to the SkagerackKattegat area and the recent more abrupt decline has reduced an already low level of recuritment of eels to the Baltic to almost nil (HagstroÈm and WickstroÈm, 1990). As a consequence, and as an attempt to enhance elver recruitment and eel stocks, heavy stocking pro-
0165-7836/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00162-3
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L. Westin / Fisheries Research 38 (1998) 257±270
grammes have been undertaken in the Baltic drainage area during the last 50 years, with eels originating from western Europe. As an example, the majority of the lakes in Poland were almost devoid of eels before the stocking programme began in the 1950's (Filuk, 1984; Moriarty et al., 1990). Today, the eel ®shery of Poland largely depends on stocking with imported glass eels (Moriarty et al., 1990). As the migration pattern of natural eels in the Baltic is well documented (Trybom and Schneider, 1908; Hessle, 1929; MaÈaÈr, 1947; MartinkoÈwitz, 1961; Ask and Erichsen, 1976; Westin and Nyman, 1979; Karlsson, 1984; Karlsson et al., 1988) the question arises as to how the eels from stocking programmes would migrate when encountering an unknown area. In tagging experiments examining previously stocked silver eels and control groups of natural eels, differences have been observed in their behaviour (Westin, 1990). In an attempt to clarify these, the present paper adds the results of two more tagging experiments with silver eels performed 1991 and 1992. 2. Material and methods The previously stocked eels in this investigation orginated from a formerly eel-free lake, Fardume TraÈsk (570 4600 N, 180 5700 E), situated on the island of Gotland in the middle of the Baltic proper. The lake was stocked with imported glass eels from France in September 1980 (Westin, 1990). At the lake outlet a ®xed wire-trap was built, which gave total control of the eel escape from the lake. The migratory peak in silver eel run is normally in autumn (Tesch, 1983) but in this lake it occurs in spring; presumably depending on low water ¯ow in early autumn. As a consequence, the silver eels in the tagging experiments in 1991 and 1992 were caught in May±June and held in captivity until tagged and released. All handling of the ®sh took place during anaesthesia with benzocaine and the tags used were Carlin tags. Most recaptures were reported from the commercial Baltic eel ®shery with fyke-nets. 2.1. The tagging experiment 1991 Eels from Lake Fardume were trapped in the outlet during May-June 1991 and kept in captivity in the
stream. On the ®rst days of August the eels were transported by helicopter to the AskoÈ laboratory (580 5000 N, 170 3800 E) on the mainland of Sweden, 60 km south of Stockholm. A total of 221 females were tagged with Carlin tags and released in the harbour of the AskoÈ laboratory. A control group of 219 females which consisted of natural eels caught 20 km from the release point by a commercial ®sherman, were tagged and released on the same occasion and at the same site. The day of release for the two groups was 5 August 1991. 2.2. The tagging experiment 1992 In 1992 a reverse experiments was carried out. A group of 589 natural silver eel females, caught by the same ®sherman on the mainland of Sweden as in 1991, was shipped to Gotland July 12. They were tagged and released from Ar (570 5600 N, 180 5600 E) on the northern coast of Gotland immediately outside the laboratory approximately 100 m west of the small outlet stream from lake BaÈste TraÈsk. The release was conducted between 14 and 17 July, 1992 together with 469 formerly stocked silver eel females from Lake Fardume. 2.2.1. Natural eels Altogether 589 silver eel females were tagged and divided into 3 groups. One control group of 230 specimens was tagged just before release. A second group of 208 eels was made anosmic by injection of Selastic1 744, primerless silicone, into their nasal cavities. Blocking the nostrils temporary deprive the eel of olfactory information by preventing water exchange in the nasal cavity (Tesch et al., 1989). As it is possible that the anosmic eels can be affected by the experimental conditions it was decided to create a third group. In this group of 151 eels, only one of the nasal cavities was blocked: in 81 the right and in 70 the left nostril. 2.2.2. Fardume eels These previously stocked eels were trapped in the outlet from Lake Fardume during May±June 1992. They were divided into two groups. One group of 209 specimens was transferred to brackish Baltic water of a salinity of 6±7 ppt and the other group of 260 eels was held all the time in fresh water in the stream of
L. Westin / Fisheries Research 38 (1998) 257±270
259
Table 1 Flow chart summarizing the experiment Origin and status
Treatment
Release
Recaptures shown in
Locality
Number
Lake Fardume previously stocked
Tagged
AskoÈ
221
Lake Fardume previously stocked
Tagged
Ar
469
JaÈrflotta natural eels
Tagged
AskoÈ
219
JaÈrflotta natural eels
Tagged
Ar
230
JaÈrflotta natural eels
Tagged nostrils blocked
Ar
208
JaÈrflotta natural eels
Tagged left nostrils blocked
Ar
70
JaÈrflotta natural eels
Tagged right nostril blocked
Ar
81
Tables 2, 3, 5 Figs. 3, 8 Tables 4, 5 Figs. 7, 8 Tables 2, 3, 5 Fig. 2 Tables 4, 5 Fig. 4 Tables 4, 5 Fig. 5 Tables 4, 5 Fig. 6 Tables 4, 5 Fig. 6
origin until tagged and released. The ¯ow chart summarizing the experiment is shown in Table 1. 3. Results 3.1. Tagging experiment 1991 3.1.1. Natural eels Recaptures were mostly from August±September and the recapture rate in 1991 was 27.4% (n60). After October there were no more reports of tag returns until July±September the following year, after hibernation (n16, 21%). The recaptures were all along the coast south of the release locality (Fig. 2). When leaving the Baltic, the recapture data È resund (the Sound indicated that outlet through O between Sweden and Denmark) was mainly chosen (Fig. 1). The tag returns after hibernation were also spread in a southerly direction from the release point È resund (Fig. 2). to the Baltic outlet in O 3.1.2. Fardume eels The Fardume eels gave 20 (9%) tag-returns but none during the period when the recaptures of the control group dominated in August (Table 2). They appeared later from September and onwards. In contrast to the control group recaptures were also reported from an area north of the release site. The recaptures
Fig. 1. Map of western Europe, south-west Baltic and the Baltic È resund. outlet through O
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L. Westin / Fisheries Research 38 (1998) 257±270
Fig. 2. Origin and recaptures of the control group of Silver eels released from AskoÈ 1991.
from the southern Baltic late in the migration season È resund area like were not to the same extent from the O the controls but proceeded in southwesterly direction (Fig. 3). A total of 14 tag-returns from the Fardume eels were recaptured in the following years (41% hibernation) almost all from the southern part of the Baltic, outside Denmark and Germany. The numbers of recaptures from the two groups on a time scale are shown in Table 2 and a comparison is also made of the two groups in Table 3 based on recaptures, days to
Fig. 3. Origin and recaptures of previously stocked Silver eels from Lake Fardume released from AskoÈ 1991.
recapture, distance swum, speed, direction and hibernation. 3.2. Tagging experiment 1992 3.2.1. Recapture rate during the first migration period From the natural eel experiments, the control group without olfactory impairment, had the lowest recapture rate, 14.3 % (n33). The highest value was from
Table 2 Recapture rate over time of previously stocked eels from lake Fardume compared to a control group of natural eels Origin
Control group natural eels Fardume eels previously stocked
Recaptures 1991 Number
August
September
October
November
December
Total n (%)
219
34
19
7
0
0
60 (27.4%)
221
0
4
8
7
1
20 (9%)
Recaptures 1992±1995 Hibernation (%) 16 (21.1%) 1992 14(41.2%) 1992±95
60
20
219
221
Control (natural eels) Previously stocked (Fardume eels)
Recaptures (n)
Number released
Group
9.0
27.4
Recaptures (%) 30 (n55) 75 (n17)
Days to recaptur(e) 202 (n55) 275 (n17)
Distance (km)
3.7
6.7
Speed (km/day) 10 2100 120 280
Direction (%) 16 (1992) 14 (1992±95)
Recaptures (Hibernation n)
41.2
21.1
Recaptures (Hibernation %)
203 (n15) 522 (n13)
Distance (km)
10 2100 114.3 285.7
Direction (%)
Table 3 Comparisons of control group of natural eels and previously stocked eels from Lake Fardume in the investigation 1991 as regards numbers, recaptures, time to recapture, distance, speed, direction (1N, 2S) and hibernation
L. Westin / Fisheries Research 38 (1998) 257±270 261
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L. Westin / Fisheries Research 38 (1998) 257±270
the group with one of the nasal cavities blocked, 20.5% (n31). The anosmic group was intermediate with 17.8% (n37) recaptures reported during the migration period in 1992 (Table 4). The lowest values for the two Fardume groups where from the eels held in freshwater with only 2.7% (n7) returns. However, of the recaptured Fardume eels 75% were from the brackish water groups with a return of 9.6% (n20) (Table 4). In these groups of distant recaptures, the Fardume eels which behaved in a different way are excluded (n14). They entered the small stream 100 m from the release point and went back into fresh water, where they were caught by electro®shing in October 1992. Four of them had lost the tags. Of the remainder, seven were from the fresh water group and three from the brackish water group. When pooling the results from the experiments of 1991 and 1992, it is obvious that the previously stocked eels differ in having few recaptures in the ®rst months after release and increasing late in the season; reversed to natural eels and a very low recapture rate compared to natural eels. P***<0.001 ( 2-test d.f.1). 3.2.2. Recaptures after hibernation The total number of recaptures during the second migration period (autumn 1993) from the four groups of natural eels was 24. The majority, 14, was from the anosmic group (27% hibernation), while the controlgroup gave four (11% hibernation) and the two groups with one nostril blocked yielded six more (16% hibernation). The Fardume eels gave eight recaptures during 1993, four from the saltwater- and four from the freshwater group (23% hibernation) (Table 4). The results from 1991 to 1992 give a hibernation rates for the formerly stocked eels at least twice as high. P*<0.05 ( 2-test d.f.1). 3.2.3. Distance, time elapsed between release and recapture and speed The distance was calculated as the shortest possible distance swum between release and recapture site. When calculating time and speed, altogether 12 (9%) eels have been excluded owing to uncertainty of recapture date or locality. Among the three groups which orginated from the natural eels, the control group had covered a mean distance of 296 km in a mean time of 43 days (n28). This group had the highest speed with a mean of 6.9 km/day. The anosmic
eels were slowest and had covered a distance of around 233 km in 46 days which gives a mean speed of only 5.1 km/day (n34). The values for the groups with one nostril blocked were rather different. Eels with right nostril blocked behaved more like controls in distance, 260 km and speed, 6.5 km/day (n17), while eels with left nostril blocked, 210 km and 5.3 km/day (n14), were similar to the anosmic group. The most diverging patterns were evident in the Fardume groups which are handled as one because the two groups were similar with regard to distance, time and speed. The eels from Fardume had accomplished a distance of 408 km on a mean time of 97 days. This gives the lowest speed among all groups with a mean of 4.2 km/days (n23) and signi®cantly slower than the controls, P***<0.001 (t-test, d.f.48). Another way to illustrate the differences between natural eels and the previously stocked eels from Fardume is to calculate the mean time between release and recapture and also include the hibernating eels. The mean time during 1988±1992 was for the control groups 2,7 months (n117) and 1984±1992 for Fardume eels 9.1 months (n96). Data from Westin (1990) was also included. From the tagging experiments performed the following divergences can be mentioned. The calculated mean speed in previously stocked eels is roughly half compared to natural eels. The time between release and recapture, hibernation included, is three times longer for previously stocked eels than for natural eels. The mean distance covered is twice that of natural eels. Recaptures in the southern Baltic up to four years after release. 3.2.4. Swimming direction Of the total of 32 recaptures in the control group, 30 were from a south-westerly direction (S) which leads to the Baltic outlets (Fig. 4). The Fardume eels had 15% of the recaptures in north to west direction (N) (n27). All recaptures from the second migration period were from the SW sector (S) (Table 4, Fig. 7). When the localities of recaptures from the two investigations (1991 and 1992) are taken into consideration, there are further differences between
37
20
7
81
70
208
209
260
Anosmic right
Anosmic left
Anosmic both nostrils Fardume Brackishwater Fardume fresh water
14
17
33
230
Control
Recapture (n)
Number released
Group
2.7
9.6
17.8
20.0
21.0
14.3
Recaptures (%)
105
94
46
40
40
43
Days to recapture
364
424
233
210
260
296
Distance (km)
3.5
4.5
5.1
5.3
6.5
6.9
Speed (km/day) 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2,
6.2 93.8 0 100 21.4 78.6 32.4 67.6 15 85 14.3 85.7
Direction, %
4
4
14
3
3
4
Recaptures Hibernation (n)
36
17
27
18
15
11
Recaptures Hibernation (%)
1; 2; 1; 2; 1; 2; 1; 2; 1; 2; 1; 2;
2 2 0 3 2 7 6 8 0 4 0 4
Direction, n
Table 4 Comparisons of the six different groups from the 1992 investigation as regard to numbers, recapture, distance swum, speed direction (1N,2S) and hibernation
729
545
324
220
543
268
Distance (km)
L. Westin / Fisheries Research 38 (1998) 257±270 263
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L. Westin / Fisheries Research 38 (1998) 257±270
Fig. 4. Origin and recaptures of the control group of Silver eels released from Ar, Gotland 1992.
the control groups and the eels orginating from Fardume. In the controls 90% (n112) are reported from the Swedish coast and thus only a few are caught in the southern parts of the Baltic compared to the Fardume eels where only 46% (n69) are reported from the coast (Table 5).
When arriving at the southernmost parts of the Baltic, the natural eels change direction to north È resund area while the eels from and proceed into the O the Fardume do not and are instead reported from the Baltic south coast from Poland, Germany and the Danish islands (Fig. 8). To conclude, there are differences in migration behaviour in previously stocked eels as follows. Higher frequency of recapture north of ther release locality than in natural eels. P**0.0068 (Fisher's exact test, Sokal and Rohlf, 1969). Fewer recaptures on the way to southern parts of the Baltic and more in the southern Baltic compared to natural eels, P***<0.001 ( 2-test d.f.1). General failure to È resund, instead turn north towards the outlet through O they proceeded to the southernmost parts of the Baltic in contrast to natural eels. P***<0.001 ( 2-test d.f.1). Recaptures from odd areas where natural eels are only occasionally reported, P***0 (Fisher's exact test). Recaptures on the other side of the Baltic outlets which have never been reported for natural eels. The anosmic eels (Fig. 5) differed and had a higher frequency of recapture north of the release locality 32% (n36) than in natural eels, P***0 (Fisher's exact test). In the anosmic group 14 eels were reported the following year (hibernation) of these 6 (43%) were still to be found in the NW sector (N). Also the eels with left nostril blocked (Fig. 6) showed a different pattern and had a higher frequency of recaptures north of the release locality than for natural eels, 21.4% (n14) (P**0.002. Fisher's exact test). This was the only group where all were caught on the Swedish coast and none in the southernmost
Table 5 È resund area in the different groups Comparisons of eel recaptures reported from the Swedish coast or from South Baltic including the O Group released Natural eels Control group released from the mainland, AskoÈ 1991 Control group released from Gotland Ar 1992 Total control groups Previously stocked eels released from the mainland, AskoÈ 1991 Released from Gotland Ar, 1992 Brackish water adapted Fresh water adapted Total previously stocked eels Anosmic eels Anosmic right Anosmic left
Caught on the Swedish coast
Caught in the Baltic outlets or southernmost parts of the Baltic
69 32 101 20 9 3 32 42 15 17
7 4 11 14 15 8 37 9 5 0
(91%) (89%) (90%) (59%) (38%) (27%) (46%) (82%) (75%) (100%)
(9%) (n76) (11%) n36 (10%) n112 (41%) n34 (62%) n24 (73%) n11 (54%) n69 (18%) n51 (25%) n20 n17
L. Westin / Fisheries Research 38 (1998) 257±270
265
Fig. 5. Origin and recaptures of anosmic Silver eels released from Ar, Gotland 1992.
Fig. 6. Origin and recaptures of partly anosmic Silver eels released from Ar, Gotland 1992.
parts of the Baltic (Table 5). The eels with right nostril blocked (Fig. 6) were all from the same direction, leading south (S) (n17) and there was no signi®cant difference between this group and natural eels. In spite of the few recaptures owing to small group-sizes with its right nostril blocked and left nostril blocked there are signi®cant differences between the two groups as regard recaptures north of the release locality (P*0.014. Fisher's exact test) and the migration pattern to the southern Baltic (P*0.0498. Fisher's exact test) Fig. 6, Table 5. In the anosmic group 82% are from the Swedish coast, with right nostril blocked 75% and in the group with left nostril blocked 100% was reported from the coast and none from south Baltic (Table 5).
1983). The majority of investigations on the orientation of eels have been performed in the Baltic. During the last decades a number of conventional eel tagging experiments has been reported and many tracking studies have been carried out in the southern Baltic, both with intact (Tesch, 1979; Westerberg, 1979; Tesch et al., 1991) and anosmic eels (Tesch et al., 1991). From earlier investigations of the eel migration in the Baltic the recapture rates are in the range 25± 50% (Ask and Erichsen, 1976). Previous investigations on Fardume eels never exceeded 10% (Westin, 1990). This was one reason to change the release locality from Gotland to the mainland in 1991; the ®shing pressure on the mainland, mostly by fyke-nets, is more intense compared to the Gotland area. The outcome of the 1991 experiment was that the controls gave expected values based on literature (35%) whilst the Fardume eels gave around 15%. This is a low value compared to the controls, but higher than in earlier experiments performed at Gotland. One tentative explanation for the low recapture rate is that the
4. Discussion The capacity of eels to orientate, and their migration has been attributed to many mechanisms (Tesch,
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L. Westin / Fisheries Research 38 (1998) 257±270
Fig. 7. Origin and recaptures of previously stocked Silver eels from Lake Fardume released from Ar, Gotland 1992.
Fig. 8. Recaptures from southern Baltic of previously stocked Silver eels from Lake Fardume, Gotland during 1984±1993.
previously stocked eels are migrating later in the season and are not following the same route as the natural eels and thus avoid capture. This assumption is supported by the catches of Fardume eels north of the release site, which amounted to 20% in the investigation of 1991. This is also supported by the differences in recaptures between natural eels and Fardume eels from the Swedish east coast. On their way to the Baltic outlets, 90% of the natural eels but only 46% of the Fardume eels are reported from the coast, thus never reaching the southernmost parts of the Baltic (Table 5). Most of the recaptures of Fardume eels are from the Danish islands. The southern coast of the Baltic from RuÈgen to Gdynia is an extremely unlikely location for the natural eels released in western Baltic (Karlsson, 1984). Ask and Erichsen (1976) had none of their 830 recaptures in this area (Karlsson, 1984), but 13% of the Fardume eels are reported from this part of the southern Baltic. (Fig. 8.). The low recapture rate of Fardume eels compared to natural eels can be partly explained by their migration
to an area alien to the natural eels, and as a consequence, there is a low ®shing pressure. Also, some of the tagged and released silver eels were temporarily deprived of their migrating instinct. The most striking example is the return of some eels to fresh water after being released in the Baltic as observed in the 1992 experiment from Gotland. From all the earlier mentioned tagging experiments in the Baltic it is well documented that when the natural eels reach the southwesterly tip of Sweden the directional choice is changed to a northerly one. There seems to be a border between this tip of Sweden and the island of MoÈn (Fig. 1), which is frequently transgressed by natural eels (94%) (Ask and Erichsen, 1976) while only 27% of the previously stocked eels cross this border (Fig. 8). Of the reported 55 recaptures of Fardume eels during 1984±1993 from the southern Baltic including the results reported by Westin (1990), 15 of them had, È resund however, crossed this border and entered the O like natural eels but 11 of them were caught in 1992
L. Westin / Fisheries Research 38 (1998) 257±270
and 2 of the 4 remaining were caught after a period of 4 years each. Excluding 1992, 91% of the remaining È resund 44 recaptures missed the outlet through O compared to 6% of natural eels. Ask and Erichsen (1976) pointed out that 8302 recaptures of natural eels none had been caught on the other side of the Baltic outlets. The eels from Fardume also differ in this respect. Two of them were caught some 70 km distant from the outlets through the Belts (north AArhus and Ebeltoft on Jylland Fig. 8). One interpretation is that they still had dif®culties in orientation. The control groups have given 121 recaptures of which 83.5% were from the migration period in the same year. The remainder were from the second year and no later reports have been received. The Fardume eels, on the other hand, (n97) gave 60% the ®rst year and 30% the following year but, in contrast to natural eels, there are reports two years after (6%) and also recaptures (4%) in year 3 and 4. As there are no tagging experiments performed with formerly stocked eels, it is dif®cult to ®nd universal applicability of my results. However, the eels in the tagging investigation off the Polish coast (Karlsson et al., 1988), orginated from the Masurian lakes and were thus dominated by stocked eels because the natural eel stock has practically ceased to exist in the Masurian lake area in Poland and the eel ®shery depends to a large extent on stocking with imported glass eels (Ciepielewsky, 1976; Leopold, 1976; Filuk, 1984; Filuk and Wiktor, 1988; Moriarty et al., 1990; Bartel and Kosior, 1991). A comparison shows that there was a low recapture rate, around 10% all from the western È land, Baltic from Fehmarn to north of the island of O È resund. Most of and 60% missed the outlet through O the recaptures during the ®rst year were late in the season. Furthermore there was a high hibernation rate (30%) and recaptures also in years two and three (Karlsson, personal communication). Agreements occurred between previously stocked eels from lake Fardume and Masurian lakes in all of six possible comparisons. The stocked eels from Fardume have been investigated since 1984 (Westin, 1990) and altogether 1300 silver eels have been tagged and released. The results show that there are great differences between stocked and natural eels as regards migration patterns. SvedaÈng and WickstroÈm (1997) studied the main energetic stores at the silver eel stage by analysing muscle
267
fat concentrations in female silver eels from marine, brackish and fresh water habitats in Sweden, including Lake Fardume TraÈsk. They found a very high proportion of lean eels (i.e, silver eels with a fat content below 20%) in Lake Fardume, and silver eels with low fat content occurred at all nine localities investigated. Their explanation of the aberrant migration behaviour in the stocked eels from Lake Fardume is that it is a question of food search, these eels prolonging their initial migratory phase to allow for essential feeding before leaving the European coastal water. But, if lean eels are migrating to odd areas in the Baltic, are failing È resund, are trapped to ®nd the normal way through O mostly very late in the season, are sometimes recaptured two to four years after starting the migration, and in some cases are trapped on the other side of the Baltic outlet, it is peculiar that natural eels did not ful®ll the expectations and do the same, because lean eels occurred at all study sites (SvedaÈng and WickstroÈm, 1997). Consequently, if all lean eels found are prolonging the migration for feeding, they ought to be descending from stockings in the Baltic. In a tracking experiment, Tesch et al., 1991 found that the behaviour of anosmic eels deviated considerably from that observed in intact eels. The swimming speed was approximately half that of the natural eels. Normal swimming speed is estimated at 1 knot (44 km/day) (Tesch ibid 1991), which agrees with earlier observations (MaÈaÈr, 1947). The choice of swimming direction varied between both individuals and during trackings. He stated that these observations in isolation would indicate that the orientation mechanism of eels is disturbed by anosmia. This observation, however, was contradicted by the already mentioned conventional tagging experiment on anosmic and a control group of untreated eels orginating from the Masurian lakes and released off the Polish coast at the same time, and in the same area, as the tracking experiments (Karlsson et al., 1988). The returns of the anosmic eels did not show any signi®cant difference in migratory behaviour compared to the control group. As the material in the tagging experiment off the Polish coast was dominated by previously stocked eels, caught in traps at outlets when migrating, the contradicting results of these combined tracking and tagging experiments off the Polish coast can now be understood. In an earlier tagging investigation (Karlsson, unpublished), the anosmic group behaved differently
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L. Westin / Fisheries Research 38 (1998) 257±270
È resund area. From from the control, and missed the O my point of view there was a disappointment because the expection was, according to the earlier investigation (Karlsson unpublished), that my anosmic eels, È resund like the would also fail to turn north into the O Fardume eels. However, the anosmic eels turned north È resund area. One and found their way into the O anosmic eel succeded in passing the outlet and was caught at the Swedich coast at HoÈganaÈs one year later. It was also a surprise that the Fardume eels this year È resund (1992) preferred mainly to turn north into the O (Figs. 1 and 8). As it was possible that the anosmic eels could be affected by the experimental conditions, a third group was created with only one nostril blocked. No differences were expected between the groups with left or right nostril blocked. Handled as one, the group was very heterogenous. Divided into two there are differences. Of the ®ve groups, the one with the left nostril blocked obviously had greater problems ®nding the Baltic outlet since this was the only group from which there are no reports from the south Baltic; all were reported from the Swedish east coast. This group mainly agreed with the anosmic eels: the same low speed, recaptures in the NW sector, shortest distance covered, shorter than in the anosmic group. Some hibernating members from the left nostril group like the anosmic eels, were still in the NW sector. The eels from the group with the right nostril blocked were more similar to the controls as regards speed and distance covered. This was the only group where none had been caught in the NW sector and also the group with highest percent recapture from southern Baltic. Although, the results are statistically signi®cant, this does not necessarily mean that they are biological signi®cant. Repetition of the `one nostril experiment' has, however, not been possible due to lack of funding. The nose of the eel is probably the most sensitive sense organ and for at least part of its spawning migration may play an important role (Tesch et al., 1991). There is experimental evidence which shows that silver eels can discriminate water of increased salinity by their sense of smell (Tesch, 1974; Hain, 1975). Whether olfaction alone can lead migrating eels out of the Baltic is dif®cult to prove but it does not seem to be the only orientation cue. The previously stocked eels from Lake Fardume and also the eels
from the Masurian lakes (Karlsson et al., 1988) seemed, when lacking an imprinted directional cue, to respond to a second mechanism which may, initially, direct eels relying only on this temporary loss of course, and later to a gathering in the southernmost parts of the Baltic. Something must have brought them there. In an investigation of diel activity of yellow and silver eels it was shown that their behaviour is different in one respect. When temperature decreased, the level of activity in yellow eels also decreased in contrast to silver eels where the activity immediately increased. This was interpreted as an avoidance reaction (Westin and Nyman, 1979). Since one cue, the imprinted olfaction, is absent in the previously stocked Fardume eels, the gradual cooling down of the Baltic from north to south during the migration season forces the eels to the least cold area of the Baltic, and they then end up in the south. This also helps to explain the È resund, which delayed migration. The outlet through O is in a northerly direction is generally missed (Fig. 1). Tosi et al., 1990 investigated in a laboratory study the relation of water odour, salinity and temperature to ascent in glass-eels. The outcome was that freshwater ¯ows are preferred to salt-water, temperature below the acclimation temperature are preferred to higher temperatures, and that odour attractivenes mainly acts by reinforcing preferred stimuli. The migration and orientation back to the spawing-place may simply be a reversal of the observed thermal and olfactory preferences of the young stages during a period of active dispersal. The salinity in the northernmost part of the Baltic is 1±2 ppt and increases ten-fold on the way to the outlets in the south. According to data collected over many years, salinities and temperatures in the Baltic show well developed gradients from north to south and east to west (Bock, 1971; Lenz, 1971). It is suggested that oceanic factors such as a slower Gulf Stream in the 1980s are responsible for the observed drastic decline in eel recruitment (Castonguay et al., 1994). However, the decrease in eel dates from the 1940s. To improve the eel stock heavy stocking programmes have been undertaken in the Baltic since then. The Baltic is almost exclusively populated by females and in this remote area the most fecund eel females are produced (SvaÈrdson, 1976). Compared to the Baltic drainage area, there is a scarcity of areas suitable for eel production in Europe.
L. Westin / Fisheries Research 38 (1998) 257±270
Is it the combined effects of heavy exploitation, environmental degradation and the deleterious effects of persistent pollutants (Larsson et al., 1991) which have resulted in an ongoing decline in high fecund Baltic eel females spawing in the Sargasso Sea leading to a decline in recruitment? The Baltic is an enclosed area and seems, according to this investigation, to act as a trap for stocked eels and consequently, the contribution to the spawning stock has to be questioned. Acknowledgements The author greatly acknowledges the ®nancial support given by Carl Tryggers Fund, Gunvor and Josef AneÂrs Fund, the Swedish Council for Forestry and Agricultural Research and the World Wide Fund for Nature. Thanks also to Ron Johnstone for correcting the English, Bibbi Mayrhofer for drawing the ®gures and the anonymous referees for valuable remarks. References Ask, L., Erichsen, L., 1976. BlankaÊlsmaÈrkningar vid svenska È stersjoÈkusten 1941±1968. Meddelande fraÊn HavsfiskelaborO atoriet Lysekil 199, 1±117 (in Swedish). Bartel, R., Kosior, M., 1991. Migrations of tagged eel released into the lower Vistula and to the Gulf of Gdansk. Pol. Arch. Hydrobiol. 38, 105±113. Bock, K.H., 1971. Monatskarten des Salzgehaltes der Ostsee. Dt. Hydrogr. Erg. H 12, 1±147. Castonguay, M., Hodson, P., Moriarty, C., Drinkwater, K., Jessop, B., 1994. Is there a role of ocean environment in American and European eel decline? Fish. Oceanogr. 3(3), 197±203. Ciepielewsky, W., 1976. The size, sex and age of seaward migrating eel from two Masurian lakes. Int. Counc. Explor. Sea (EIFAC) Symp. Eel Res. Manage. pp. 331±12. Filuk, J., 1984. Eel from the Vistula Firth its past, present and future state. Biul. Morsk. Inst. Ryb. 3-6, pp. 13±18. Filuk, J., Wiktor, J., 1988. Management of eel stocks in the Vistula and Szczecin lagoons in the light of the process of eel stocking. Biul. Morsk. Inst. Ryb. 5-6, pp. 36±44. HagstroÈm, O., WickstroÈm, H., 1990. Immigration of Young Eels to the Skagerrak-Kattegat Area 1900±1989. Int. Reveu Ges. Hydrobiol. 75, 11±20. Hain, J.H.W., 1975. The behaviour of migratory eels, Anguilla rostrata, in response to current, salinity and lunar period HelgolaÈnder wiss.. Meeresunters 27, 211±233. Hessle, C., 1929. De senaste aÊrens fiskmaÈrkningar vid svenska È stersjoÈkusten. Medd. Lantbruksstyr (In Swedish) 278, 25± O 38.
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Karlsson, L., 1984. Migration of European silver eels, Anguilla anguilla. Ph.D. Thesis. Uppsala University, Uppsala, 745, 1±13. Karlsson, L., Bartel, R., Tesch, F.-W., 1988. Migration and orientation of tagged silver eels released in the southeastern Baltic. ICES C. M. 1988/M:17, pp. 1±6. Larsson, P., Hamrin, S., Okla, L., 1991. Factors Determining the Uptake of Persistent Pollutants in an Eel Population (Anguilla anguilla L.). Environ. Pollut. 69, 39±50. Lenz, W., 1971. Monatskarten der Temperature der Ostsee. Dt. Hydrogr. Erg. H 11, 1±148. Leopold, M., 1976. Stocking as a main factor determining the level of eel catches in Poland. Int. Counc. Explor Sea (EIFAC) Symp. Eel Res. Manage. 14, pp. 1±13. È ber die Aalwanderung im Baltischen Meer auf MaÈaÈr, A., 1947. U Grund der Wanderaalmarkierungsversuche im Finnischen und Livischen Meerbusen in den Jahren 1937±1939. Medd. Statens Unders. FoÈrsoÈksanstalt. SoÈtvattensfisk, Drottnigholm 27, pp. 1±56. MartinkoÈwitz, H., 1961. Ergebnisse von Blankaalmarkierungen an der ostruÈgenschen KuÈste und MoÈglichkeit ihrer NuÈttzung fuÈr die Fangsteigerung durch neuartige Reusenkonstruktionen. Z. Fisch. 10, 653±663. Moriarty, C., l986. Elver catches in European rivers 1938±1985. ICES C.M. 1968/M:17, 1±12. Moriarty, C., Bninska, M., Leopold, M., 1990. Eel, Anguilla anguilla L., stock and yield in Polish lakes. Aq. and Fish Manage. 21, 347±355. Sokal, R., Rohlf, J., 1969. Biometry. W.H. Freeman, San Fransisco, p. 776. SvaÈrdson, G., 1976. The decline of the Baltic eel population. Rep. Inst. Freshwat. Res. 55, 136±143. SvedaÈng, H., WickstroÈm, H., 1997. Low fat contents in female silver eels: indications of insufficient energetic stores for migration and gonadal developement. J. Fish Biol. 50, 475± 486. Tesch, F.W., 1974. Influence of geomagnetism and salinity on the directional choice of eels. HelgolaÈnder wiss. Meeresunters 26, 382±395. Tesch, F.-W., 1979. Tracking of silver eels (Anguilla anguilla) in different shelf areas of the northeast Atlantic. Rapp, P.-v. ReÂun. Cons. perm. int. Explor. Mer 174, 104±114. Tesch, F.-W., 1983. Der Aal: Biologi und Fischerei. Paul Parey, Hamburg, p. 340. Tesch, F.-W., Westerberg, H., Karlsson, L., 1989. Tracking studies on migrating silver eels in the central Baltic. ICES C.M. 1989/ M: 21, 1±16. Tesch, F.-W., Westerberg, H., Karlsson, L., 1991. Tracking studies on migrating silver eels in the Central Baltic. Meeresforsch 33, 183±196. Tosi, L., Spampanato, A., Sola, C., Tongiorgi, P., 1990. Relation of water odour, salinity and temperature to ascent of glass-eels, Anguilla anguilla (L.): a laboratory study. J. Fish Biol. 36, 327± 340. Trybom, F., Schneider, G., 1908. Die markierungsversuch mit Aalen und die Wanderungen gekennzeichneter Aale in der Ostsee. Rapp P.-v ReÂun. Cons. perm. int. Explor. Mer 9, 51± 59.
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Westerberg, H., 1979. Counter-current orientation in the migration of the European eel. Rapp. P.-v ReÂun, Cons. perm. int. Explor. Mer 174, 134±143. Westin, L., 1990. Orientation mechanisms in migrating European silver eel (Anguilla anguilla): temperature and olfaction. Mar. Biol. 106, 175±179.
Westin, L., Nyman, L., 1979. Activity, orientation and migration of Baltic eel (Anguilla anguilla L.). Rapp R.-v. ReÂun. Cons. perm. int. Explor. Mer 174, 115±123. WickstroÈm, H., 1986. Growth of cultured eels stocked in two Swedish lakes. Vie Milieu 36, 273±277.
The spawning migration of European silver eel (Anguilla anguilla L.) with particular reference to stocked eel in the Baltic Lars Westin* Institute of Systems Ecology, Section Gotland, University of Stockholm, S-620 35, FaÊroÈsund, Sweden Received 7 October 1997; accepted 27 May 1998
Abstract Tagging investigations have been performed in the Baltic with previously stocked eels, anosmic eels, partly anosmic eels and natural eels as a control group. There were great differences between natural eels and previously stocked eels. The anosmic eels differed from natural eels in speed, direction and hibernation and agreed in these aspects more with the previously stocked eels. Among the partly anosmic eels, the group with left nostril blocked behaved like the anosmic eels while that with right nostril blocked behaved like the controls. This suggests that olfaction is essential for orientation. The stocked eels, which have never been in the Baltic before have lacked opportunities to imprint this orientation cue. In spite of that, the previously stocked eels succeeded in ®nding their way to the southernmost parts of the Baltic but usually only to areas very different from those of the migrating natural eels. This is tentatively explained by the occurrence of a second cue, temperature. The migration and orientation back to the spawning area in the Sargasso Sea may be a reversal of thermal and olfactory prehistory of the young stages during their period of active dispersal. The orientation capacity of eels in shelf areas like the Baltic is not a point to point event, but rather too much swimming with limited progress. Eels stocked in the Baltic drainage area have problems in ®nding their way out. As a consequence, the contribution to the spawning stock in the Sargasso Sea has to be questioned. # 1998 Elsevier Science B.V. All rights reserved. Keywords: European eel; Baltic Sea; Migration and orientation; Stocking; Imprinting; Olfaction; Salinity; Temperature
1. Introduction According to catch data for the Baltic Sea, which dates from the 1940s there has been a long-term decrease in the recruitment of eels (SvaÈrdson, 1976; WickstroÈm, 1986). Further, there have been recent reports of a more drastic decline in the immigration of
*Tel.: +46 498 224630; fax: +46 498 224567; e-mail: [email protected]
glass eels in several western European countries (Moriarty, 1986; HagstroÈm and WickstroÈm, 1990). This long term decline seems to have been most evident in countries around the Baltic Sea, where eels are most probably recruited through the Fair Isle channel and across the North Sea to the SkagerackKattegat area and the recent more abrupt decline has reduced an already low level of recuritment of eels to the Baltic to almost nil (HagstroÈm and WickstroÈm, 1990). As a consequence, and as an attempt to enhance elver recruitment and eel stocks, heavy stocking pro-
0165-7836/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00162-3
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L. Westin / Fisheries Research 38 (1998) 257±270
grammes have been undertaken in the Baltic drainage area during the last 50 years, with eels originating from western Europe. As an example, the majority of the lakes in Poland were almost devoid of eels before the stocking programme began in the 1950's (Filuk, 1984; Moriarty et al., 1990). Today, the eel ®shery of Poland largely depends on stocking with imported glass eels (Moriarty et al., 1990). As the migration pattern of natural eels in the Baltic is well documented (Trybom and Schneider, 1908; Hessle, 1929; MaÈaÈr, 1947; MartinkoÈwitz, 1961; Ask and Erichsen, 1976; Westin and Nyman, 1979; Karlsson, 1984; Karlsson et al., 1988) the question arises as to how the eels from stocking programmes would migrate when encountering an unknown area. In tagging experiments examining previously stocked silver eels and control groups of natural eels, differences have been observed in their behaviour (Westin, 1990). In an attempt to clarify these, the present paper adds the results of two more tagging experiments with silver eels performed 1991 and 1992. 2. Material and methods The previously stocked eels in this investigation orginated from a formerly eel-free lake, Fardume TraÈsk (570 4600 N, 180 5700 E), situated on the island of Gotland in the middle of the Baltic proper. The lake was stocked with imported glass eels from France in September 1980 (Westin, 1990). At the lake outlet a ®xed wire-trap was built, which gave total control of the eel escape from the lake. The migratory peak in silver eel run is normally in autumn (Tesch, 1983) but in this lake it occurs in spring; presumably depending on low water ¯ow in early autumn. As a consequence, the silver eels in the tagging experiments in 1991 and 1992 were caught in May±June and held in captivity until tagged and released. All handling of the ®sh took place during anaesthesia with benzocaine and the tags used were Carlin tags. Most recaptures were reported from the commercial Baltic eel ®shery with fyke-nets. 2.1. The tagging experiment 1991 Eels from Lake Fardume were trapped in the outlet during May-June 1991 and kept in captivity in the
stream. On the ®rst days of August the eels were transported by helicopter to the AskoÈ laboratory (580 5000 N, 170 3800 E) on the mainland of Sweden, 60 km south of Stockholm. A total of 221 females were tagged with Carlin tags and released in the harbour of the AskoÈ laboratory. A control group of 219 females which consisted of natural eels caught 20 km from the release point by a commercial ®sherman, were tagged and released on the same occasion and at the same site. The day of release for the two groups was 5 August 1991. 2.2. The tagging experiment 1992 In 1992 a reverse experiments was carried out. A group of 589 natural silver eel females, caught by the same ®sherman on the mainland of Sweden as in 1991, was shipped to Gotland July 12. They were tagged and released from Ar (570 5600 N, 180 5600 E) on the northern coast of Gotland immediately outside the laboratory approximately 100 m west of the small outlet stream from lake BaÈste TraÈsk. The release was conducted between 14 and 17 July, 1992 together with 469 formerly stocked silver eel females from Lake Fardume. 2.2.1. Natural eels Altogether 589 silver eel females were tagged and divided into 3 groups. One control group of 230 specimens was tagged just before release. A second group of 208 eels was made anosmic by injection of Selastic1 744, primerless silicone, into their nasal cavities. Blocking the nostrils temporary deprive the eel of olfactory information by preventing water exchange in the nasal cavity (Tesch et al., 1989). As it is possible that the anosmic eels can be affected by the experimental conditions it was decided to create a third group. In this group of 151 eels, only one of the nasal cavities was blocked: in 81 the right and in 70 the left nostril. 2.2.2. Fardume eels These previously stocked eels were trapped in the outlet from Lake Fardume during May±June 1992. They were divided into two groups. One group of 209 specimens was transferred to brackish Baltic water of a salinity of 6±7 ppt and the other group of 260 eels was held all the time in fresh water in the stream of
L. Westin / Fisheries Research 38 (1998) 257±270
259
Table 1 Flow chart summarizing the experiment Origin and status
Treatment
Release
Recaptures shown in
Locality
Number
Lake Fardume previously stocked
Tagged
AskoÈ
221
Lake Fardume previously stocked
Tagged
Ar
469
JaÈrflotta natural eels
Tagged
AskoÈ
219
JaÈrflotta natural eels
Tagged
Ar
230
JaÈrflotta natural eels
Tagged nostrils blocked
Ar
208
JaÈrflotta natural eels
Tagged left nostrils blocked
Ar
70
JaÈrflotta natural eels
Tagged right nostril blocked
Ar
81
Tables 2, 3, 5 Figs. 3, 8 Tables 4, 5 Figs. 7, 8 Tables 2, 3, 5 Fig. 2 Tables 4, 5 Fig. 4 Tables 4, 5 Fig. 5 Tables 4, 5 Fig. 6 Tables 4, 5 Fig. 6
origin until tagged and released. The ¯ow chart summarizing the experiment is shown in Table 1. 3. Results 3.1. Tagging experiment 1991 3.1.1. Natural eels Recaptures were mostly from August±September and the recapture rate in 1991 was 27.4% (n60). After October there were no more reports of tag returns until July±September the following year, after hibernation (n16, 21%). The recaptures were all along the coast south of the release locality (Fig. 2). When leaving the Baltic, the recapture data È resund (the Sound indicated that outlet through O between Sweden and Denmark) was mainly chosen (Fig. 1). The tag returns after hibernation were also spread in a southerly direction from the release point È resund (Fig. 2). to the Baltic outlet in O 3.1.2. Fardume eels The Fardume eels gave 20 (9%) tag-returns but none during the period when the recaptures of the control group dominated in August (Table 2). They appeared later from September and onwards. In contrast to the control group recaptures were also reported from an area north of the release site. The recaptures
Fig. 1. Map of western Europe, south-west Baltic and the Baltic È resund. outlet through O
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L. Westin / Fisheries Research 38 (1998) 257±270
Fig. 2. Origin and recaptures of the control group of Silver eels released from AskoÈ 1991.
from the southern Baltic late in the migration season È resund area like were not to the same extent from the O the controls but proceeded in southwesterly direction (Fig. 3). A total of 14 tag-returns from the Fardume eels were recaptured in the following years (41% hibernation) almost all from the southern part of the Baltic, outside Denmark and Germany. The numbers of recaptures from the two groups on a time scale are shown in Table 2 and a comparison is also made of the two groups in Table 3 based on recaptures, days to
Fig. 3. Origin and recaptures of previously stocked Silver eels from Lake Fardume released from AskoÈ 1991.
recapture, distance swum, speed, direction and hibernation. 3.2. Tagging experiment 1992 3.2.1. Recapture rate during the first migration period From the natural eel experiments, the control group without olfactory impairment, had the lowest recapture rate, 14.3 % (n33). The highest value was from
Table 2 Recapture rate over time of previously stocked eels from lake Fardume compared to a control group of natural eels Origin
Control group natural eels Fardume eels previously stocked
Recaptures 1991 Number
August
September
October
November
December
Total n (%)
219
34
19
7
0
0
60 (27.4%)
221
0
4
8
7
1
20 (9%)
Recaptures 1992±1995 Hibernation (%) 16 (21.1%) 1992 14(41.2%) 1992±95
60
20
219
221
Control (natural eels) Previously stocked (Fardume eels)
Recaptures (n)
Number released
Group
9.0
27.4
Recaptures (%) 30 (n55) 75 (n17)
Days to recaptur(e) 202 (n55) 275 (n17)
Distance (km)
3.7
6.7
Speed (km/day) 10 2100 120 280
Direction (%) 16 (1992) 14 (1992±95)
Recaptures (Hibernation n)
41.2
21.1
Recaptures (Hibernation %)
203 (n15) 522 (n13)
Distance (km)
10 2100 114.3 285.7
Direction (%)
Table 3 Comparisons of control group of natural eels and previously stocked eels from Lake Fardume in the investigation 1991 as regards numbers, recaptures, time to recapture, distance, speed, direction (1N, 2S) and hibernation
L. Westin / Fisheries Research 38 (1998) 257±270 261
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L. Westin / Fisheries Research 38 (1998) 257±270
the group with one of the nasal cavities blocked, 20.5% (n31). The anosmic group was intermediate with 17.8% (n37) recaptures reported during the migration period in 1992 (Table 4). The lowest values for the two Fardume groups where from the eels held in freshwater with only 2.7% (n7) returns. However, of the recaptured Fardume eels 75% were from the brackish water groups with a return of 9.6% (n20) (Table 4). In these groups of distant recaptures, the Fardume eels which behaved in a different way are excluded (n14). They entered the small stream 100 m from the release point and went back into fresh water, where they were caught by electro®shing in October 1992. Four of them had lost the tags. Of the remainder, seven were from the fresh water group and three from the brackish water group. When pooling the results from the experiments of 1991 and 1992, it is obvious that the previously stocked eels differ in having few recaptures in the ®rst months after release and increasing late in the season; reversed to natural eels and a very low recapture rate compared to natural eels. P***<0.001 ( 2-test d.f.1). 3.2.2. Recaptures after hibernation The total number of recaptures during the second migration period (autumn 1993) from the four groups of natural eels was 24. The majority, 14, was from the anosmic group (27% hibernation), while the controlgroup gave four (11% hibernation) and the two groups with one nostril blocked yielded six more (16% hibernation). The Fardume eels gave eight recaptures during 1993, four from the saltwater- and four from the freshwater group (23% hibernation) (Table 4). The results from 1991 to 1992 give a hibernation rates for the formerly stocked eels at least twice as high. P*<0.05 ( 2-test d.f.1). 3.2.3. Distance, time elapsed between release and recapture and speed The distance was calculated as the shortest possible distance swum between release and recapture site. When calculating time and speed, altogether 12 (9%) eels have been excluded owing to uncertainty of recapture date or locality. Among the three groups which orginated from the natural eels, the control group had covered a mean distance of 296 km in a mean time of 43 days (n28). This group had the highest speed with a mean of 6.9 km/day. The anosmic
eels were slowest and had covered a distance of around 233 km in 46 days which gives a mean speed of only 5.1 km/day (n34). The values for the groups with one nostril blocked were rather different. Eels with right nostril blocked behaved more like controls in distance, 260 km and speed, 6.5 km/day (n17), while eels with left nostril blocked, 210 km and 5.3 km/day (n14), were similar to the anosmic group. The most diverging patterns were evident in the Fardume groups which are handled as one because the two groups were similar with regard to distance, time and speed. The eels from Fardume had accomplished a distance of 408 km on a mean time of 97 days. This gives the lowest speed among all groups with a mean of 4.2 km/days (n23) and signi®cantly slower than the controls, P***<0.001 (t-test, d.f.48). Another way to illustrate the differences between natural eels and the previously stocked eels from Fardume is to calculate the mean time between release and recapture and also include the hibernating eels. The mean time during 1988±1992 was for the control groups 2,7 months (n117) and 1984±1992 for Fardume eels 9.1 months (n96). Data from Westin (1990) was also included. From the tagging experiments performed the following divergences can be mentioned. The calculated mean speed in previously stocked eels is roughly half compared to natural eels. The time between release and recapture, hibernation included, is three times longer for previously stocked eels than for natural eels. The mean distance covered is twice that of natural eels. Recaptures in the southern Baltic up to four years after release. 3.2.4. Swimming direction Of the total of 32 recaptures in the control group, 30 were from a south-westerly direction (S) which leads to the Baltic outlets (Fig. 4). The Fardume eels had 15% of the recaptures in north to west direction (N) (n27). All recaptures from the second migration period were from the SW sector (S) (Table 4, Fig. 7). When the localities of recaptures from the two investigations (1991 and 1992) are taken into consideration, there are further differences between
37
20
7
81
70
208
209
260
Anosmic right
Anosmic left
Anosmic both nostrils Fardume Brackishwater Fardume fresh water
14
17
33
230
Control
Recapture (n)
Number released
Group
2.7
9.6
17.8
20.0
21.0
14.3
Recaptures (%)
105
94
46
40
40
43
Days to recapture
364
424
233
210
260
296
Distance (km)
3.5
4.5
5.1
5.3
6.5
6.9
Speed (km/day) 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2,
6.2 93.8 0 100 21.4 78.6 32.4 67.6 15 85 14.3 85.7
Direction, %
4
4
14
3
3
4
Recaptures Hibernation (n)
36
17
27
18
15
11
Recaptures Hibernation (%)
1; 2; 1; 2; 1; 2; 1; 2; 1; 2; 1; 2;
2 2 0 3 2 7 6 8 0 4 0 4
Direction, n
Table 4 Comparisons of the six different groups from the 1992 investigation as regard to numbers, recapture, distance swum, speed direction (1N,2S) and hibernation
729
545
324
220
543
268
Distance (km)
L. Westin / Fisheries Research 38 (1998) 257±270 263
264
L. Westin / Fisheries Research 38 (1998) 257±270
Fig. 4. Origin and recaptures of the control group of Silver eels released from Ar, Gotland 1992.
the control groups and the eels orginating from Fardume. In the controls 90% (n112) are reported from the Swedish coast and thus only a few are caught in the southern parts of the Baltic compared to the Fardume eels where only 46% (n69) are reported from the coast (Table 5).
When arriving at the southernmost parts of the Baltic, the natural eels change direction to north È resund area while the eels from and proceed into the O the Fardume do not and are instead reported from the Baltic south coast from Poland, Germany and the Danish islands (Fig. 8). To conclude, there are differences in migration behaviour in previously stocked eels as follows. Higher frequency of recapture north of ther release locality than in natural eels. P**0.0068 (Fisher's exact test, Sokal and Rohlf, 1969). Fewer recaptures on the way to southern parts of the Baltic and more in the southern Baltic compared to natural eels, P***<0.001 ( 2-test d.f.1). General failure to È resund, instead turn north towards the outlet through O they proceeded to the southernmost parts of the Baltic in contrast to natural eels. P***<0.001 ( 2-test d.f.1). Recaptures from odd areas where natural eels are only occasionally reported, P***0 (Fisher's exact test). Recaptures on the other side of the Baltic outlets which have never been reported for natural eels. The anosmic eels (Fig. 5) differed and had a higher frequency of recapture north of the release locality 32% (n36) than in natural eels, P***0 (Fisher's exact test). In the anosmic group 14 eels were reported the following year (hibernation) of these 6 (43%) were still to be found in the NW sector (N). Also the eels with left nostril blocked (Fig. 6) showed a different pattern and had a higher frequency of recaptures north of the release locality than for natural eels, 21.4% (n14) (P**0.002. Fisher's exact test). This was the only group where all were caught on the Swedish coast and none in the southernmost
Table 5 È resund area in the different groups Comparisons of eel recaptures reported from the Swedish coast or from South Baltic including the O Group released Natural eels Control group released from the mainland, AskoÈ 1991 Control group released from Gotland Ar 1992 Total control groups Previously stocked eels released from the mainland, AskoÈ 1991 Released from Gotland Ar, 1992 Brackish water adapted Fresh water adapted Total previously stocked eels Anosmic eels Anosmic right Anosmic left
Caught on the Swedish coast
Caught in the Baltic outlets or southernmost parts of the Baltic
69 32 101 20 9 3 32 42 15 17
7 4 11 14 15 8 37 9 5 0
(91%) (89%) (90%) (59%) (38%) (27%) (46%) (82%) (75%) (100%)
(9%) (n76) (11%) n36 (10%) n112 (41%) n34 (62%) n24 (73%) n11 (54%) n69 (18%) n51 (25%) n20 n17
L. Westin / Fisheries Research 38 (1998) 257±270
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Fig. 5. Origin and recaptures of anosmic Silver eels released from Ar, Gotland 1992.
Fig. 6. Origin and recaptures of partly anosmic Silver eels released from Ar, Gotland 1992.
parts of the Baltic (Table 5). The eels with right nostril blocked (Fig. 6) were all from the same direction, leading south (S) (n17) and there was no signi®cant difference between this group and natural eels. In spite of the few recaptures owing to small group-sizes with its right nostril blocked and left nostril blocked there are signi®cant differences between the two groups as regard recaptures north of the release locality (P*0.014. Fisher's exact test) and the migration pattern to the southern Baltic (P*0.0498. Fisher's exact test) Fig. 6, Table 5. In the anosmic group 82% are from the Swedish coast, with right nostril blocked 75% and in the group with left nostril blocked 100% was reported from the coast and none from south Baltic (Table 5).
1983). The majority of investigations on the orientation of eels have been performed in the Baltic. During the last decades a number of conventional eel tagging experiments has been reported and many tracking studies have been carried out in the southern Baltic, both with intact (Tesch, 1979; Westerberg, 1979; Tesch et al., 1991) and anosmic eels (Tesch et al., 1991). From earlier investigations of the eel migration in the Baltic the recapture rates are in the range 25± 50% (Ask and Erichsen, 1976). Previous investigations on Fardume eels never exceeded 10% (Westin, 1990). This was one reason to change the release locality from Gotland to the mainland in 1991; the ®shing pressure on the mainland, mostly by fyke-nets, is more intense compared to the Gotland area. The outcome of the 1991 experiment was that the controls gave expected values based on literature (35%) whilst the Fardume eels gave around 15%. This is a low value compared to the controls, but higher than in earlier experiments performed at Gotland. One tentative explanation for the low recapture rate is that the
4. Discussion The capacity of eels to orientate, and their migration has been attributed to many mechanisms (Tesch,
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L. Westin / Fisheries Research 38 (1998) 257±270
Fig. 7. Origin and recaptures of previously stocked Silver eels from Lake Fardume released from Ar, Gotland 1992.
Fig. 8. Recaptures from southern Baltic of previously stocked Silver eels from Lake Fardume, Gotland during 1984±1993.
previously stocked eels are migrating later in the season and are not following the same route as the natural eels and thus avoid capture. This assumption is supported by the catches of Fardume eels north of the release site, which amounted to 20% in the investigation of 1991. This is also supported by the differences in recaptures between natural eels and Fardume eels from the Swedish east coast. On their way to the Baltic outlets, 90% of the natural eels but only 46% of the Fardume eels are reported from the coast, thus never reaching the southernmost parts of the Baltic (Table 5). Most of the recaptures of Fardume eels are from the Danish islands. The southern coast of the Baltic from RuÈgen to Gdynia is an extremely unlikely location for the natural eels released in western Baltic (Karlsson, 1984). Ask and Erichsen (1976) had none of their 830 recaptures in this area (Karlsson, 1984), but 13% of the Fardume eels are reported from this part of the southern Baltic. (Fig. 8.). The low recapture rate of Fardume eels compared to natural eels can be partly explained by their migration
to an area alien to the natural eels, and as a consequence, there is a low ®shing pressure. Also, some of the tagged and released silver eels were temporarily deprived of their migrating instinct. The most striking example is the return of some eels to fresh water after being released in the Baltic as observed in the 1992 experiment from Gotland. From all the earlier mentioned tagging experiments in the Baltic it is well documented that when the natural eels reach the southwesterly tip of Sweden the directional choice is changed to a northerly one. There seems to be a border between this tip of Sweden and the island of MoÈn (Fig. 1), which is frequently transgressed by natural eels (94%) (Ask and Erichsen, 1976) while only 27% of the previously stocked eels cross this border (Fig. 8). Of the reported 55 recaptures of Fardume eels during 1984±1993 from the southern Baltic including the results reported by Westin (1990), 15 of them had, È resund however, crossed this border and entered the O like natural eels but 11 of them were caught in 1992
L. Westin / Fisheries Research 38 (1998) 257±270
and 2 of the 4 remaining were caught after a period of 4 years each. Excluding 1992, 91% of the remaining È resund 44 recaptures missed the outlet through O compared to 6% of natural eels. Ask and Erichsen (1976) pointed out that 8302 recaptures of natural eels none had been caught on the other side of the Baltic outlets. The eels from Fardume also differ in this respect. Two of them were caught some 70 km distant from the outlets through the Belts (north AArhus and Ebeltoft on Jylland Fig. 8). One interpretation is that they still had dif®culties in orientation. The control groups have given 121 recaptures of which 83.5% were from the migration period in the same year. The remainder were from the second year and no later reports have been received. The Fardume eels, on the other hand, (n97) gave 60% the ®rst year and 30% the following year but, in contrast to natural eels, there are reports two years after (6%) and also recaptures (4%) in year 3 and 4. As there are no tagging experiments performed with formerly stocked eels, it is dif®cult to ®nd universal applicability of my results. However, the eels in the tagging investigation off the Polish coast (Karlsson et al., 1988), orginated from the Masurian lakes and were thus dominated by stocked eels because the natural eel stock has practically ceased to exist in the Masurian lake area in Poland and the eel ®shery depends to a large extent on stocking with imported glass eels (Ciepielewsky, 1976; Leopold, 1976; Filuk, 1984; Filuk and Wiktor, 1988; Moriarty et al., 1990; Bartel and Kosior, 1991). A comparison shows that there was a low recapture rate, around 10% all from the western È land, Baltic from Fehmarn to north of the island of O È resund. Most of and 60% missed the outlet through O the recaptures during the ®rst year were late in the season. Furthermore there was a high hibernation rate (30%) and recaptures also in years two and three (Karlsson, personal communication). Agreements occurred between previously stocked eels from lake Fardume and Masurian lakes in all of six possible comparisons. The stocked eels from Fardume have been investigated since 1984 (Westin, 1990) and altogether 1300 silver eels have been tagged and released. The results show that there are great differences between stocked and natural eels as regards migration patterns. SvedaÈng and WickstroÈm (1997) studied the main energetic stores at the silver eel stage by analysing muscle
267
fat concentrations in female silver eels from marine, brackish and fresh water habitats in Sweden, including Lake Fardume TraÈsk. They found a very high proportion of lean eels (i.e, silver eels with a fat content below 20%) in Lake Fardume, and silver eels with low fat content occurred at all nine localities investigated. Their explanation of the aberrant migration behaviour in the stocked eels from Lake Fardume is that it is a question of food search, these eels prolonging their initial migratory phase to allow for essential feeding before leaving the European coastal water. But, if lean eels are migrating to odd areas in the Baltic, are failing È resund, are trapped to ®nd the normal way through O mostly very late in the season, are sometimes recaptured two to four years after starting the migration, and in some cases are trapped on the other side of the Baltic outlet, it is peculiar that natural eels did not ful®ll the expectations and do the same, because lean eels occurred at all study sites (SvedaÈng and WickstroÈm, 1997). Consequently, if all lean eels found are prolonging the migration for feeding, they ought to be descending from stockings in the Baltic. In a tracking experiment, Tesch et al., 1991 found that the behaviour of anosmic eels deviated considerably from that observed in intact eels. The swimming speed was approximately half that of the natural eels. Normal swimming speed is estimated at 1 knot (44 km/day) (Tesch ibid 1991), which agrees with earlier observations (MaÈaÈr, 1947). The choice of swimming direction varied between both individuals and during trackings. He stated that these observations in isolation would indicate that the orientation mechanism of eels is disturbed by anosmia. This observation, however, was contradicted by the already mentioned conventional tagging experiment on anosmic and a control group of untreated eels orginating from the Masurian lakes and released off the Polish coast at the same time, and in the same area, as the tracking experiments (Karlsson et al., 1988). The returns of the anosmic eels did not show any signi®cant difference in migratory behaviour compared to the control group. As the material in the tagging experiment off the Polish coast was dominated by previously stocked eels, caught in traps at outlets when migrating, the contradicting results of these combined tracking and tagging experiments off the Polish coast can now be understood. In an earlier tagging investigation (Karlsson, unpublished), the anosmic group behaved differently
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È resund area. From from the control, and missed the O my point of view there was a disappointment because the expection was, according to the earlier investigation (Karlsson unpublished), that my anosmic eels, È resund like the would also fail to turn north into the O Fardume eels. However, the anosmic eels turned north È resund area. One and found their way into the O anosmic eel succeded in passing the outlet and was caught at the Swedich coast at HoÈganaÈs one year later. It was also a surprise that the Fardume eels this year È resund (1992) preferred mainly to turn north into the O (Figs. 1 and 8). As it was possible that the anosmic eels could be affected by the experimental conditions, a third group was created with only one nostril blocked. No differences were expected between the groups with left or right nostril blocked. Handled as one, the group was very heterogenous. Divided into two there are differences. Of the ®ve groups, the one with the left nostril blocked obviously had greater problems ®nding the Baltic outlet since this was the only group from which there are no reports from the south Baltic; all were reported from the Swedish east coast. This group mainly agreed with the anosmic eels: the same low speed, recaptures in the NW sector, shortest distance covered, shorter than in the anosmic group. Some hibernating members from the left nostril group like the anosmic eels, were still in the NW sector. The eels from the group with the right nostril blocked were more similar to the controls as regards speed and distance covered. This was the only group where none had been caught in the NW sector and also the group with highest percent recapture from southern Baltic. Although, the results are statistically signi®cant, this does not necessarily mean that they are biological signi®cant. Repetition of the `one nostril experiment' has, however, not been possible due to lack of funding. The nose of the eel is probably the most sensitive sense organ and for at least part of its spawning migration may play an important role (Tesch et al., 1991). There is experimental evidence which shows that silver eels can discriminate water of increased salinity by their sense of smell (Tesch, 1974; Hain, 1975). Whether olfaction alone can lead migrating eels out of the Baltic is dif®cult to prove but it does not seem to be the only orientation cue. The previously stocked eels from Lake Fardume and also the eels
from the Masurian lakes (Karlsson et al., 1988) seemed, when lacking an imprinted directional cue, to respond to a second mechanism which may, initially, direct eels relying only on this temporary loss of course, and later to a gathering in the southernmost parts of the Baltic. Something must have brought them there. In an investigation of diel activity of yellow and silver eels it was shown that their behaviour is different in one respect. When temperature decreased, the level of activity in yellow eels also decreased in contrast to silver eels where the activity immediately increased. This was interpreted as an avoidance reaction (Westin and Nyman, 1979). Since one cue, the imprinted olfaction, is absent in the previously stocked Fardume eels, the gradual cooling down of the Baltic from north to south during the migration season forces the eels to the least cold area of the Baltic, and they then end up in the south. This also helps to explain the È resund, which delayed migration. The outlet through O is in a northerly direction is generally missed (Fig. 1). Tosi et al., 1990 investigated in a laboratory study the relation of water odour, salinity and temperature to ascent in glass-eels. The outcome was that freshwater ¯ows are preferred to salt-water, temperature below the acclimation temperature are preferred to higher temperatures, and that odour attractivenes mainly acts by reinforcing preferred stimuli. The migration and orientation back to the spawing-place may simply be a reversal of the observed thermal and olfactory preferences of the young stages during a period of active dispersal. The salinity in the northernmost part of the Baltic is 1±2 ppt and increases ten-fold on the way to the outlets in the south. According to data collected over many years, salinities and temperatures in the Baltic show well developed gradients from north to south and east to west (Bock, 1971; Lenz, 1971). It is suggested that oceanic factors such as a slower Gulf Stream in the 1980s are responsible for the observed drastic decline in eel recruitment (Castonguay et al., 1994). However, the decrease in eel dates from the 1940s. To improve the eel stock heavy stocking programmes have been undertaken in the Baltic since then. The Baltic is almost exclusively populated by females and in this remote area the most fecund eel females are produced (SvaÈrdson, 1976). Compared to the Baltic drainage area, there is a scarcity of areas suitable for eel production in Europe.
L. Westin / Fisheries Research 38 (1998) 257±270
Is it the combined effects of heavy exploitation, environmental degradation and the deleterious effects of persistent pollutants (Larsson et al., 1991) which have resulted in an ongoing decline in high fecund Baltic eel females spawing in the Sargasso Sea leading to a decline in recruitment? The Baltic is an enclosed area and seems, according to this investigation, to act as a trap for stocked eels and consequently, the contribution to the spawning stock has to be questioned. Acknowledgements The author greatly acknowledges the ®nancial support given by Carl Tryggers Fund, Gunvor and Josef AneÂrs Fund, the Swedish Council for Forestry and Agricultural Research and the World Wide Fund for Nature. Thanks also to Ron Johnstone for correcting the English, Bibbi Mayrhofer for drawing the ®gures and the anonymous referees for valuable remarks. References Ask, L., Erichsen, L., 1976. BlankaÊlsmaÈrkningar vid svenska È stersjoÈkusten 1941±1968. Meddelande fraÊn HavsfiskelaborO atoriet Lysekil 199, 1±117 (in Swedish). Bartel, R., Kosior, M., 1991. Migrations of tagged eel released into the lower Vistula and to the Gulf of Gdansk. Pol. Arch. Hydrobiol. 38, 105±113. Bock, K.H., 1971. Monatskarten des Salzgehaltes der Ostsee. Dt. Hydrogr. Erg. H 12, 1±147. Castonguay, M., Hodson, P., Moriarty, C., Drinkwater, K., Jessop, B., 1994. Is there a role of ocean environment in American and European eel decline? Fish. Oceanogr. 3(3), 197±203. Ciepielewsky, W., 1976. The size, sex and age of seaward migrating eel from two Masurian lakes. Int. Counc. Explor. Sea (EIFAC) Symp. Eel Res. Manage. pp. 331±12. Filuk, J., 1984. Eel from the Vistula Firth its past, present and future state. Biul. Morsk. Inst. Ryb. 3-6, pp. 13±18. Filuk, J., Wiktor, J., 1988. Management of eel stocks in the Vistula and Szczecin lagoons in the light of the process of eel stocking. Biul. Morsk. Inst. Ryb. 5-6, pp. 36±44. HagstroÈm, O., WickstroÈm, H., 1990. Immigration of Young Eels to the Skagerrak-Kattegat Area 1900±1989. Int. Reveu Ges. Hydrobiol. 75, 11±20. Hain, J.H.W., 1975. The behaviour of migratory eels, Anguilla rostrata, in response to current, salinity and lunar period HelgolaÈnder wiss.. Meeresunters 27, 211±233. Hessle, C., 1929. De senaste aÊrens fiskmaÈrkningar vid svenska È stersjoÈkusten. Medd. Lantbruksstyr (In Swedish) 278, 25± O 38.
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