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OCCURRENCE AND LIFE PERSPECTIVES OF BEAVERS IN THE SLOVAK SECTION OF THE DANUBE RIVER

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Karol PACHINGER

Zoological Institute, Faculty of Natural Sciences, Comenius University, Mlynska dolina, 84215 Bratislava, SLOVAKIA

CONCLUSIONS Beavers introduced in Austria migrated downstream the Danube river, some of them arrived to the Morava river and it tributaries in the Zahorska nizina lowland and some of them moved further looking for a place to stay in the Danube arms. Beavers have settled well in the Zahorska nizina lowland, and they are reproducing and migrating along all its flows. In spite of the fact that the Danube river arms seemed to be a system topically and trophically much more suitable for beavers, essentially fewer beavers have been observed there, and these have never subsisted a long time in that area (with a single exception). As a reason of up to present less presence of beavers, strong Danube floods and high extent of the Danube water level fluctuation, caused by closing the river to a limited space, are considered.

#### BEAVERS CASE STORY

Two beavers were introduced in 1976, and four in 1977 in the side river arms of the Danube flood(plain forests, near Eckartsau [1], approximately 15 km from the confluence of the Danube with the Morava river. Already in July 1977 a beaver running across the road near Jakubov ponds in the Zahorska nizina lowland was shot by accident. It means, that this animal had to overcome an approximately 50 km distance, firstly downstream of the Danube river and after that against the stream of the Morava river and then against the stream of the Rudavka river, up to the place of shooting. The preparatory of this beaver can be found in the collection of the Zoological Institute of the Comenius University, Bratislava. Since then, there was a certain vacuum in the reports of the occurrence of beavers until the winter 1986-1987, when some pickings were found in the Sandorf water reservoir, near the village Prievaly. Beavers had to cover a distance of approximately 125 km along water flows from Eckartsau up to Prievaly. At that time there lived a small population of beavers in the Marchauen reserve on the right side of the Morava river, and the beavers at Prievaly could originate from that place too. In this case they had to cover a distance of only about 85 km. Beaver pickings were found also at the locality Centuz, a former arm of the Morava river, in December 1987. Since that time, till now (February 1995) there are multiplied observations of beavers practically on all flows of the Zahorska nizina lowland. At the end of 1994 they were found even in the brook Brezinka in the village Brezova pod Bradlom. This brook springs in the Myjavska pahorkatina upland and the village Brezova pod Bradlom is situated under the northern slopes of the Male Karpaty mountains. This could mean that beavers have already settled all suitable biotopes in the Zahorska nizina lowland and they move further to the north. I have described a more detailed situation dealing with spreading of beavers in the Zahorska nizina lowland in the previous work [2]. At present we know that regenerated populations of beavers live permanently at lakes Centuz and Sandorf, and we suppose, that reproducting pairs can live also in other localities with a sufficiently deep water. The whole invasion of beaver to the Zahorska nizina lowland lasted eight years (1987-1994) and today beaver pickings are already no scarce at the rivers of the Zahorska nizina lowland and along the Morava river.

It is remarkable, that although the beavers migrated downstream the Danube river, they settled down in the Zahorska nizina lowland, where they began to reproduce. It means that they moved along the way: the Danube river - the Morava river - tributaries of the Morava river. All this happened in spite of the fact that there are (seemingly) essentially better conditions for the life of beavers in the Danube arm system between Bratislava and Komarno - a larger space, more food, fewer people, better shelters. Was it really only because the migrating beavers downstream the Danube river reached the tributary, the Morava river, and entered it? It seems, that the reality was different, because beavers were observed also in the Danube river underneath the confluence with the Morava river, and even earlier than in the Zahorska nizina lowland. Somogyi [3] informed in the journal Ziva that at least one, or more probably two beavers lived in a right-side Danubian arm, between Bratislava and the village Rusovce (directly in the area of the large Bratislava) in the period from February 1984 until April 1985. A hole and numerous pickings were photographed at the river bank. Their settlement at the given place were destroyed by the spring flood, which enforced the beavers to move downstream. In the summer 1985 and in the spring 1986 beaver pickings were found near the village Cunovo, closely to the frontier with Hungary; it means approximately 10 km downstream from the first finding place, and one year later, in 1987, fishermen caught a beaver into a net at the village Dunakiliti in Hungary, approximately 8-9 km downstream from the second finding place [4]. It is possible, that it was the same exemplar. In 1986-1988 beaver pickings were found in the Danubian river arm system, near the village Baka, on the island Kalapsziget, and on the neighbour islands. In the spring 1991 pickings were found in the Vodarensky ostrov island between Bratislava and the village Devin, and in the winter 1991-1992 a big beaver was living in the Fokovske rameno arm near Palkovicovo. This beaver - a pet of fishermen - was inspiring a great attention, because he was blind, he did not avoid humans, and it was possible to take a pictures of it. In the spring 1992, at the time of increasing the Danube water level, the beaver disappeared without any trace. In the winter 1992-1993 beaver pickings were found in the Danube river arms near Sulianska brana and since the winter 1993-1994 beavers are again present in the river arms between Bratislava and Rusovce, and also in the Biskupicke rameno arm, surrounding the state nature reservation Ostrov Kopac. This river arm is flooded with water after filling the Hrusov reservoir.

It can be seen that also in the Danube river downstream the confluence with the Morava river beavers have occurred, in an essentially lower number (seven registered occurrences( during ten years (1984-1994). It is too low in comparison with the Zahorska nizina lowland, where such a number of pickings could be found in 8 years occurrence that it is impossible to register all of them (although those are in the majority migrating exemplars). Beavers are living until now at the Sulianska brana (since winter 1992), between Petrzalka and Rusovce (since the winter 1993), and in the Biskupicke rameno arm (since winter 1993). The longest period beavers are living in the territory of the Vodarensky ostrov island (since 1991) and they have survived also large flood events in 1991 and in 1992. In the summer 1992 we have ascertained that except for a male adult there lives also one (or two?) juveniles in a den. It is probable that the flood in the autumn 1992 took away one adolescent juvenile and this have settled at the Sulianska brana, where it lives up till now. This was the last large flood in the Danube river. Vodarensky ostrov is the only locality in the Danube river, where at least adult beavers were not taken away by the floods. In the spring of 1993, after filling the Hrusov reservoir, there were no floods. It means that at present there are three inhabited localities downstream Bratislava, which were not affected by the high water level.

If we compare hydro-ecological conditions in the dead river arms of the Morava river, as well as in other flowing and stagnant waters of the Zahorska nizina lowland with the conditions in the Danube river and its river arm system, we have to state that beavers were not exposed to such destructive effects of floods, as in the Danube river. In the regulated Danube river the water increased during 24(36 hours by about 3(6 m, and the whole inundation area was changed to one huge stream of cold water, tearing down everything. These conditions could not be for the benefit of even such good swimmers as beavers. It is probable that the same number of beavers migrated downstream the Danube to its river arm system, as the number which arrived to the Morava river, but these beavers remained undiscovered, and the first flood flushed them away. I note, that the beavers could be better hidden before the sights of humans in the island jungle of the Danubian river arm system, than in the flows of the Zahorska nizina lowland.

The objection could be fallen that beavers were living in the European rivers (also in Asia and North America) since the end of the Tertiary until the end of the first half of the last century, and suddenly they do not like the floods of the Danube river, which are truly as old as the Danube river by itself. At this place it is necessary to say that since the Danube has been regulated, and its river arm system has been lowered to a fragment of its original area, and it has been closed to a 2-3 km wide space between the flood-protection dikes, the Danubian floods have a totally different character. In the past the water could spread to hundreds of river arms, and therefore the water level increased more slowly, it did not flood totally the whole area. It means that animals had time to move to elevated places and to survive the floods. Present floods - a destroying wild stream - in the closed within-dike area are in fact a non-natural phenomenon, caused by humans (of course, a total elimination of floods is unsuitable for flood(plain forests too). Of course, an adult beaver survives such a flood too. Although the stream will float it to a lower place, it will still find food, and after the drop of the water level a shelter at that place too. However, beaver youngsters are a long time dependent on their parents support; they live together with their parents in a den for two winters at least. These youngsters are not able to survive floods, which float them away from the den, and separate them from their parents. Except for this, in the past ten thousands of beavers lived in the whole Danube river and its tributaries, and neither the most terrible flood could endanger their numbers. At present we count beavers in the Danube rivers only per pieces. In Austria - in the side river arms - their number is estimated to 300-400 individuals.

Since the construction of the Danubian weir close to the village Cunovo and the derivation of the water by the by-pass canal to the hydroelectric power structure Gabcikovo the situation in the main channel and in the river arm system is changing essentially. Destructive flood waves are eliminated and as long as the floods in the river arm system will be simulated, it will be always under human control. At present it will be shown, whether and to what degree our present arm system will be suitable as a biotope of this rare animal. It is necessary to mention that a limiting factor for the occurrence of beavers is the existence at least 2 m deep water and the loamy consistence river banks, where they can dig holes. The river banks need to be so high, that the beaver can dig a hole, starting from the entrance of the den, which is always under the water level, to such an altitude, which the water does not reach, and where the beaver can build a dry nesting chamber.

#### REFERENCES

  [1] Kolla, H. P., Setter, M., 1990: Biber in den Donau-Auen östlich von Wien. Eine erfolgreiche Wiederansiedlung. Umwelt, Wien, p. 1-75.

  [2] Pachinger, K., 1994: Spontane Verbreitung des Bibers (Castor sp. Linneanus, 1758) im Flusstal der March und in der Tiefebene Zahorska nizina. Ekológia (Bratislava), 13, p. 209-214.

  [3] Somogyi, J., 1985: Beaver near Bratislava (in Slovak). Ziva, 33, p. 234.

  [4] Stollmann, A., Voskar, J., 1989: Returning of water beaver to the nature of Slovakia (in Slovak). Folia Venatoria, 19, p. 245-256.