0.7190 0.7633 0.7200 0.7046 0.4917 0.5750 0.5250 0.4800
A significant deviation from HWE at two different loci was detected in each investigated population, except the Curonian Lagoon population. The latter had only one significant deviation from HWE at the locus Aro121. Such two loci with deviations from HWE were characteristic of the group of naturally recruited eels and three loci were detected in the group of introduced European eels.
On the basis of the FIS value (0.3084), which reflects the degree of inbreeding in a population, we drew the conclusion that non-random mating is characteristic of the European eel population in Lithuania represented by all individuals sampled in the sea and lakes in the course of this investigation (Table 5). This FIS value can also reflect A. anguilla restocking nuances. Glass eels migrate from spawning sites in the Sargasso Sea to Europe in shoals, consisting of presumably akin individuals, and accumulate in river entries. There they are caught by fishermen and are later translocated upstream, into other river basins or even other countries within great distances.
Table 5. Evaluation of inbreeding (F
IS) and genetic differentiationand ( FST )within naturally recruited and introduced European eel populations using F statistics Populations of European
eel
FIS FST
Naturally recruited 0.3046 0.0000
Introduced 0.3128 0.0238
Overall 0.3084 0.0228
Overall genetic differentiation among the sampled eel populations from all the studied water bodies was evaluated too. Since the FST value at all loci was 0.0228 and the p value was 0.0281, it seems that there is small but significant genetic differentiation among the 4 examined populations (Table 5). Similarly, small but significant genetic differentiation was detected within the naturally recruited and introduced European eel groups. Since the FST
value calculated for the naturally recruited and introduced European eel groups was 0.000 and 0.0238, respectively, no genetic differentiation is likely to exist in the group of naturally recruited European eels. However, there is small but significant genetic differentiation within introduced fish groups. The lack of genetic differentiation within the group of naturally recruited European eels suggests that the Baltic Sea and the Curonian Lagoon samples could belong to a genetically uniform population.
One of the reasons determining a rather high calculated value of the inbreeding coefficient FIS
(0.3084) in the Lithuanian eel population, which is represented by the individuals caught in the Baltic Sea, the Curonian Lagoon, as well as Lakes Siesartis and Dringis, can be a decline in heterozygosity. The latter phenomenon can be explained by mating specificity and a non-random distribution of individuals at spawning sites that have an effect on the formation of rather homogeneous shoals of glass eels, consisting of individuals of similar genotypes. It might be suggested that the majority of naturally recruited eels reaching the Lithuanian coastal waters yearly belong to such a genetically uniform shoal. This particular model of spatial distribution of this species might be one of the main reasons causing the high FIS value.
Significant differentiation between most pairs of these populations was confirmed using the Raimond and Rousset test. Samples from the Curonian Lagoon and the Baltic Sea formed the only significantly non-differentiated (p=0.1120 > 0.05) pair among the investigated samples (Table 6). This fact means that they represent a single population consisting of presumably naturally recruited eels.
Table 6. Significance of genetic differentiation between pairs of populations (Raimond &
Rousset test )
Populations Coronian Lagoon
Baltic Sea Lake Dringis Lake Siesartis Coronian Lagoon -
Baltic Sea 0.1120 -
Lake Dringis 0.0243* 0.0153* -
Lake Siesartis 0.0140* 0.0022* 0.0008* -
* - significant differentiation
It is known that the introduction of glass eels into Lakes Dringis and Siesartis was performed repeatedly over several years. Presumably, they were introduced from different countries or at least from different locations within the country. As a result, eel populations are likely to have different genetic pools in the lakes under study (p value was 0.0008 < 0.05, Raimond and Rousset test).
Figure 2. Dendrogram based on Nei (1972) genetic distances. Numbers 1, 2, 3 and 4 indicate Curonian lagoon, lake Dringis, Baltic sea and lake Siesartis populations, respectively.
The dendrogram based on standard Nei genetic distances (Figure 2) indicates that the Curonian Lagoon and the Baltic Sea samples form one cluster which also reflects the closest relations between these groups of European eels. What is more, these groups consisting of naturally recruited eels could be considered as subpopulations of a single population. The dendrogram, as well as the Raimond & Rousset test confirms our suggestion that glass eels introduced into Lake Dringis and Lake Siesartis were transported from different locations or countries in different time periods and, as a result, they exhibit higher genetic distances.
Discussion
During this study the genetic structure of the European eel populations sampled in the Baltic Sea, the Curonian Lagoon, Lake Dringis and Lake Siesartis was examined using microsatellite assay. The percentage of introduced eels among samples taken from the Baltic Sea and the Curonian Lagoon was determined using otolith microchemistry analysis by a group of Taiwanese and Lithuanian researchers (Shiao et al. 2006). It was ascertained that introduced eels constitute 2% and 20% of eel populations in the Baltic Sea and the Curonian Lagoon, respectively, whereas individuals collected from the lakes were introduced totally.
Although there is no reliable information regarding the countries from which glass eels were introduced into Lake Dringis and Lake Siesartis, it is known that the restocking of Lithuanian inland waters with glass eels from England and France was performed on a regular basis over several decades (Shiao et al. 2006). Furthermore, there are reliable data indicating that all eels,
which inhabit Lake Baluošai were introduced (Shiao et al. 2006). Eels inhabiting Lakes Dringis and Siesartis are likely to have been introduced in a similar way.
Results of the study suggest that sampled eels from two lakes might be considered as representatives of different populations. However, the Baltic Sea and the Curonian Lagoon samples could be ranged as subpopulations of a single population. As it was suggested earlier, the significant genetic differentiation between the populations of the two lakes might be caused by the following factors: different age of individuals (which means that glass eels were caught and translocated into two lakes in different years); different restocking time and different sites from which glass eels were collected. It was demonstrated that naturally recruited European eels differed genetically from the introduced ones. Thus, there is small but significant genetic differentiation in the A. anguilla population consisting of the groups of naturally recruited and introduced European eels. Due to some methodical and sampling differences it should not be stressed that the calculated FST values were higher than those of other studies (Wirth & Bernatchez 2001, Dannewitz et al. 2005). However, previous studies did not examine genetic differentiation between the groups of naturally recruited and introduced European eels. Hence, the comparison of the FST value with the results of alternative studies is complicated.
Wirth and Bernatchez (2001) found genetic differentiation within the European eel stock, which could be determined applying the isolation by distance (IBD) pattern in A. anguilla.
However, some other studies proposed contradictory explanations, stating that there was no genetic differentiation determined by the IBD pattern between different European eel populations, whereas the presence of the temporal genetic structure in the European eel was determined (Dannewitz et al. 2005).
The purpose of the European eel restocking is to increase its resources in a particular country (Ringuet et al. 2002; Wickström 2005). However, some countries take a different approach to the European eel restocking strategy (Limburg et al. 2003; Sjöberg et al. 2005). For example, in Sweden eels are released not only into inland waters but even into the sea itself. Therefore, the purpose of fish restocking is not only to enhance local fishery but also to sustain A.
anguilla population by means of stock protection (Sjöberg et al. 2005; Wickström 2005). As A. anguilla otolith microchemistry studies describing spawning migrations of the Silver eel
are controversial (Limburg et al. 2003; Sjöberg et al. 2005; Wickström 2005), the question as to whether introduced European eels reach the Sargasso Sea or lose their way somewhere in the Baltic Sea still remains unanswered. Furthermore, the effect of crossbreeding of restocked and naturally recruited eels on this species in the Sargasso Sea could be examined too. If Wirth and Bernatchez’s (2001) hypothesis concerning the different timing of reaching the Sargasso Sea by eels from different geographical regions of their distribution range is correct, it is plausible that the European eels introduced into a particular country could reach the Sargasso Sea at the same time as the naturally recruited individuals.Our findings on the existence of genetic differentiation among eels could be in agreement with the conclusions drawn by Dannewitz et al. (2005) due to the existence of temporal genetic variation, which means that samples consisting of individuals of similar age have similar genetic structures. As there are different populations with different age structures in Lake Dringis and Lake Siesartis (Ložys, unpubl. data), it is plausible that our FST values reflect temporal genetic variation in the studied populations too.
To sum up, the initial investigations into genetic variability and genetic structure of the European eel population representing naturally recruited and introduced individuals do not support the hypothesis on the existence of complete panmixia in A. anguilla species.
Consequently, we think that the restocking strategy of this fish should be viewed with precaution. More investigations must be carried out to find out whether introduced European eels are able to reach the Sargasso Sea and if there is genetic differentiation caused by the isolation by distance in this fish species.
Acknowledgements
Financial support was provided by the Ministries of Education and Science of Republic of Lithuania and Republic of Latvia and by the Mutual Fund of Lithuania-Latvia-Taiwan (Republic of China).
References
1. Aljanabi, S. M. and Martinez, I. 1997. Universal and Rapid Salt-extraction of High Quality Genomic DNA for PCR-based Techniques. Nucleic Acids Research. 25 (22):
4692-4693.
2. Altun, T., Tekelioğlu, N., Nevşat, E. and Sağat, Y. 2005. Some Growth Parameters on European Eel (Anguilla anguilla L., 1758) Fed with Different Feeds. Journal of Fisheries
& Aquatic Sciences 22: 215-219.
3. Bardonnet, A. and Riera, P. 2005. Feeding of Glass Eels (Anguilla anguilla) in the Course of Their Estuarine Migration: New Insights from Stable Isotope Analysis. Estuarine
Coastal and Shelf Science 63: 201-209.
4. Dannewitz, J., Maes, G. E., Johansson, L., Wickström, H., Volckaert, F. A. M. and Järvi, T. 2005. Panmixia in the European Eel: a Matter of Time... Proc. R. Soc. B 272: 1129-1137.
5. Ellerby, D. J., Spierts, I. L. Y. and Altringham, J. D. 2001. Fast Muscle Function in the European Eel (Anguilla anguilla L.) During Aquatic and Terrestrial Locomotion. The
Journal of Experimental Biology 204: 2231-2238.
6. Ellerby, D. J., Spierts, I. L. Y. and Altringham, J. D. 2001. Slow Muscle Power Output of Yellow- and Silver-Phase European Eels (Anguilla anguilla L.): Changes in Muscle Performance Prior to Migration. The Journal of Experimental Biology 204: 1369-1379.
7. Genç, E., Şahan, A., Altun, T., Cengizler, İ., Nevşat, E. 2005. Occurrence of the Swimbladder Parasite Anguillicola crassus (Nematoda, Dracunculoidea) in European Eels (Anguilla anguilla) in Ceyhan River, Turkey. Turk J Vet Anim Sci 29: 661-663.
8. Hansen, M. K., Ingebrigtsen, K., Hayton, W. L. and Horsberg, T. E. 2001. Disposition of
14C-Flumequine in Eel Anguilla anguilla, Turbot Scophthalmus maximus and Halibut
Hippoglossus hippoglossus After Oral and Intravenous Administration. Diseases of Aquatic Organisms 47: 183-191.
9. Jing, Q. J., LI, Y. P. 1999. Random Amplified Polymorphic DNA Analysis of Eel Genome. Cell Research 9: 209-216.
10. Katoh, M., Kobayashi, M. 2001. Aquaculture and Genetic Structure in the Japanese Eel
Anguilla japonica. Proceedings of the Thirtieth U.S.– Japan Meeting on Aquaculture.
Sarasota, Florida, 3-4 December. UJNR Technical Report (30): 87-92.
11. Kirk, R. S., Morritt, D., Lewis, J.W. and Kennedy, C. R. 2002. The Osmotic Relationship of the Swimbladder Nematode Anguillicola crassus with Seawater Eels. Parasitology 124:
339-347.
12. Laffaille, P., Guillouët, J., Acou, A. and Legault, A. 2005. The Increase of Female Silver Eels (Anguilla anguilla) Proportion: a Possible Response to the General Decline of the European Eel Recruitment. ‘Fish and Diadromy in Europe-International Symposium’
Abstract Book. Bordeaux: Tilippe Camoin, 65.
13. Limburg, K. E., Wickström, H., Svedäng, H., Elfman, M. and Kristiansson, P. 2003. Do Stocked Freshwater Eels Migrate? Evidence from the Baltic Suggests “Yes”. American
Fisheries Society Symposium 33: 275-284.
14. Lokman, M., Young, G. 2000. Induced Spawning and Early Ontogeny of New Zealand Freshwater Eels (Anguilla dieffenbachii and A. australis). New Zealand Journal of Marine
and Freshwater Research 34: 135-145.
15. Madsen, H., Buchmann, K. and Mellergaard, S. 2000. Trichodina sp. (Ciliophora:
Peritrichida) in Eel Anguilla anguilla in Recirculation Systems in Denmark: Host-parasite
Relations. Diseases of Aquatic Organisms 42: 149-152.16. Maes, G. E., Volckaert, F. A. M. 2002. Clinal Genetic Variation and Isolation by Distance in the European Eel Anguilla anguilla (L.). Biological Journal of the Linnean Society 77:
509–521.
17. Mank, J. and Avise, J. C. 2003. Microsatellite Variation and Differentiation in North Atlantic Eels. Journal of Heredity 94 (4): 310-314.
18. Marco-Noales, E., Milán, M., Fouz, B., Sanjuán, E. and Amaro, C. 2001. Transmission to Eels, Portals of Entry, and Putative Reservoirs of Vibrio vulnificus Serovar E (Biotype 2).