## ADAPTATION OF SELECTED SPECIES OF BIRDS ON ENVIRONMENTAL CHANGES IN THE BY-PASS CANAL OF THE HYDROELECTRIC POWER STRUCTURE GABCIKOVO AND THE ADJACENT INUNDATION OF THE DANUBE

#### Peter AC

Department of Psychology, Philosophical Faculty, Comenius University, Gondova 2, 818 0l Bratislava, SLOVAKIA

CONCLUSIONS The Danube with its system of branches creates an inland delta in the upper part of the common Hungarian - Slovak section, representing a significant locality with a relatively considerable diversity of natural components. The construction and operation of the Hydroelectric power structure Gabcikovo, as a fundamental interference with the environment, have formed new conditions for the existence of biota. The study aimed at analysis and comparison of spatial relations of selected species of birds attached to the aquatic environment in the main stream of the Danube, the branch system and the diversion canal of the Hydroelectric power structure Gabcikovo. The observations, with the application of a line method, were made in the sections determined in advance, in week intervals, from January to August 1994. Totally 54 bird species attached to the aquatic environment have been recorded. The obtained results indicate a partial purposeful adaptation of the observed species to the changed conditions. The occurrence of birds in the relatively natural environment of the main stream of the Danube and in the branch system is directly dependent on the aquatic regime and its dynamics. Differences were found in the preference of localities due to seasonal cycles. In the areas of the water dam with a higher tourist traffic the process of adaptation of some species of birds, especially from among Anseriformes, Lariformes, and Ralliformes (shortening of flight distance) has been started, characteristic of the water surfaces in urban aglomerations.

#### INTRODUCTION

During the past the Danube river has created a dynamic system of branches by its sediments, continually changing itself under the influence of the water flow. There existed a mosaic of aquatic terrestrial and semiterrestrial habitats in the area. Their diversity and carrying capacity manifested themselves in the species diversity occurring in this ecosystem, which also served as an important corridor and gathering site of migrant species. Because of its unique and particular surviving functions this area is recorded as a site of Important Bird Areas of the Slovak Republic, and since 1993 it can be found in the list of important wetlands of the Ramsar Convention too. Through historical ages, there have been changes in the ecosystem of the investigated area, which have been proportionate with the rising technical possibilities of humans. Flood control, the adaptation of the river for shipping, and later overdredging of gravel led to changes in the river's character and a significant loss of branch system dynamics. Forestry gradually replaced the majority of the natural flood plain forests by managed forests with a different structure. Lastly, and may be the most important human impact was the construction and operation of the Hydroelectric power structure Gabcikovo (HPSG). Data about the biota status has existed before the start of the construction, creating the opportunity to compare the new status with the past. Birds of aquatic habitats were selected as indicators for the evaluation of the changes. We investigated their spatial relationships and habitat preferences.

#### CHARACTERISATION OF SITES AND METHODS

Sites were selected as being representative of the landscape as it was known before the start of the construction and operation of the HPSG (the main Danube river and the branch system) and the diversion canal of the HPSG.

#### The main Danube river

For this site constant water flow is characteristic. The banks are almost completely made up of large quarry stone along the full length of the river, with occasional shallow gravel banks. After the main river was dammed near the village Cunovo, the current declined to approximately 1/7 - 1/5 of the primary. The water level proportionately declined, and the river narrowed itself. Gravel banks were revealed along the majority of the affected river section. Some of these revealed areas are more than one hundred meters wide.

#### The branch system

During the 20th century the branches were isolated from the main river, primarily the infusion mouths. As the river bed eroded, the water flow through the branches gradually dwindled so that the water in the branches was mostly stagnant in the years immediately preceding the start - up of the HPSG. Seasonal floods and minimal water level dynamics existed because of the natural regime of the river. The seasonal cycles of the spatial relationships of the biota also adapted themselves. Branch banks are forested by shrubs and trees of willow (Salix sp.), poplars (Populus sp.), and in certain areas reed (Phragmites communis) is growing. Submerged macrophyta were rare in the past because of the dynamic water regime. Shallow gravel or mud banks were revealed during periods with low water levels.

#### The diversion canal of the HPSG

The canal is an artificially constructed water route flowing at a higher level than the surrounding landscape. It serves for shipping, and leads water to the hydroelectric power plant. Its banks have been built of asphalted gravel dikes. There are asphalt roads on the top of the dikes. Data was obtained using the line method, from January to August 1994. Data obtained in the same way each year, from 1985 to 1993 was used for comparison purpose with the year 1994 (the year after the putting the dam into function). The investigated section of the main Danube river was delineated by river marking from 1821 - 1835 kilometres in a total length of 14 km. Within the branch system transects led through newly constructed small dikes. The total length of the transect was 5 km. In the diversion canal we obtained data from the top of its dikes, the full canal length is 17 km.

#### RESULTS

Table 1: List of observed bird species in investigated sites

Species | HPSG | D | Species | HPSG | D
--- | --- | --- | --- | --- | ---
Gavia stellata (Pontopp.) | + | - | Mergus merganser L. | - | +
Podiceps nigricollis Brehm | + | - | Somateria molissima (L.) | + | -
Podiceps cristatus (L.) | + | + | Circus aeruginosus (L.) | - | +
Podiceps ruficollis (Pall.) | + | + | Milvus migrans (Bodd.) | + | +
Phalacrocorax carbo (L.) | - | + | Haliaeetus albicilla (L.) | - | +
Ardea cinerea L. | + | + | Pandion haliaetus (L.) | - | +
Ardea purpurea L. | - | + | Fulica atra L. | + | +
Egretta alba (L.) | - | + | Gallinula chloropus (L.) | - | +
Egretta garzetta (L.) | - | + | Charadrius dubius Scop. | - | +
Nycticorax nycticorax (L.) | - | + | Numenius arquata (L.) | - | +
Ixobrychus minutus (L.) | - | + | Actitis hypoleucos (L.) | - | +
Platalea leucorodia (L.) | - | + | Tringa ochropus L. | - | +
Ciconia ciconia (L.) | - | + | Tringa nebularia (Gunn.) | - | +
C. nigra (L.) | - | + | Tringa totanus (L.) | - | +
Cygnus olor (Gm.) | + | + | Philomachus pugnax (L.) | - | +
Anser sp. Briss. | - | + | Vanellus vanellus (L.) | - | +
Anas platyrhynchos L. | + | + | Recurvirostra avosetta L. | - | +
Anas penelope L. | - | + | Larus canus L. | + | +
Anas stepera L. | - | + | Larus argentatus Pontopp. | + | +
Anas acuta L. | - | + | Larus ridibundus L. | + | +
Anas querquedula L. | + | + | Larus minutus Pall. | - | +
Anas crecca L. | + | + | Sterna hirundo L. | - | +
Aythya ferina (L.) | + | + | Chlidonias nigra (L.) | - | +
Aythya nyroca (Güld.) | - | + | Alcedo atthis (L.) | - | +
Aythya fuligula (L.) | + | +
Bucephala clangula (L.) | + | +
Clangula hyemalis (L.) | + | -
Mergus albellus L. | - | +

D - main river and branch system of the Danube
HPSG - the Gabcikovo Barrage system
+ - presence
- - absence

As can be seen in the list of observed birds species in the investigated sites, during the investigation we recorded 52 bird species of aquatic habitats. From this number we selected 8 easily identifiable, regularly occurring species. Their minimal and maximal relative numbers in the investigated sites are tabulated herein. If spatial relationships of selected species living in the natural landscape sites are compared to the species found in the artificial canal, it can be ascertained that the species Phalacrocorax carbo (L.), Egretta alba (L.) and Haliaeetus albicilla (L.) were the most dependent upon the main river and branch system of the Danube. During the winter of 1994, the species of genus Anas and Fulica atra L. preferred the open water and asphalt banks of the artifitial diversion canal. During the breeding season these species migrated and lived in the branch system until the juvenils became independent. The diversion canal of the HPSG attracted birds mostly during the winter, it did not offer conditions for breeding. Our findings are presented in Table 2. Table 2: Occurrence of selected bird species in investigated sites (relative numbers of individuals per km2)

Species |  | Jan. | Feb. | Mar. | Apr. | May | Jun. | Jul. | Aug.
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Phalacrocorax carbo(L.) | D | 0.2-1.1 | 0.3-18. 3 | 1.1-3.7 | 3.6-9.2 | 3.9-9.7 | 0.3-26.3 | 0.6-1.9 | 6.3-26.8
| G | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0
Ardea cinerea (L.) | D | 0.2-1.2 | 0.1-0.5 | 0.1-0.5 | 0.8-1.5 | 0.1-1.3 | 0.1-1.3 | 0.2-1.6 | 0.9-3.9
| G | 0.3 | 0 | 0 | 0.1 | 0.1-0.9 | 0 | 0 | 0.1-0.4
Egretta alba (L.) | D | 0.8-2.2 | 1.2-1.9 | 0.8-1.3 | 1.1-2.5 | 0.3-0.6 | 0.1 | 0.1-0.2 | 2.2-7.5
| G | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0
Anas platyrhynchos L. | D | 12.4-39.5 | 11.5-21 | 4.2-15.8 | 0.3-6.4 | 0.8-5.3 | 0.4-1.3 | 0.8-4.5 | 10.3-15.6
| G | 56-289.1 | 1.8-88.2 | 3.2-4.5 | 0.2-1.3 | 0.2-0.7 | 2.1-12.9 | 0.7-2.5 | 7.1-13.5
Cygnus olor (Gm.) | D | 0.1-0.5 | 0.1-1.5 | 1.2-2.9 | 1.3-2.9 | 0.3-0.5 | 0.1-0.3 | 0.1-0.2 | 0.1-0.3
| G | 0.8 | 0 | 0.4-0.9 | 0.4 | 0 | 0 | 0 | 0
Larus ridibundus (L.) | D | 0.3-1.4 | 1.8 | 0.2-0.3 | 0.2-0.5 | 0.6-1.9 | 0.6-6.4 | 0.6-2.1 | 1.0-4.3
| G | 0.4-5.6 | 0.3 | 0.8-5.4 | 1.0-1.2 | 0.2-0.5 | 0.5-5.0 | 0.1-0.7 | 0.7-5.6
Fulica atra L. | D | 1.7 | 0.3-4.2 | 0.3-2.6 | 0.3 | 0.1 | 0 | 0.1-0.3 | 0.6-.9
| G | 0.4-10.2 | 0.4 | 0.9-3.5 | 0.2 | 0 | 0 | 0.1-0.2 | 0
Haliaeetus albicilla (L.) | D | 0.1-0.4 | 0.1-0.4 | 0.1-0.2 | 0.1 | 0.1 | 0 | 0 | 0.1
| G | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0

D - main river and branch system of the Danube
G - the Gabcikovo Barrage system

#### DISCUSSION

By comparing the already existing results to our own data, and the data published from the period preceding the operation of HPSG, we noted several significant changes. In contrast to the past (Ac, Bohus, not published data) we have observed rising of the Fulica atra L., Gallinula chloropus (L.) and Podiceps ruficollis (Pall.) breeding population within the ranch system. This phenomenon could be attributed to population trends, but it is more likely connected with the stabilised water level, which enables a successful breeding, as well as with phytocenological changes (submerged macrophyta developing). Again, in contrast to the past (Ac, Bohus, not published data) during the summer of 1994 and 1993 more adults of Ardea purpurea L. were recorded in the branch system. An abundance of this species rose to approximately 20 specimens after the juveniles became independent. Not a long time ago [1, 2, and Matousek, 1961 and Kux, Randik 1961 in 3] this species nested on the Slovak side of the Danube, but as a result of the destruction and the changed character of the breeding sites, nesting takes place only on Hungarian side of the river now. During the investigated period this species utilised the area only as a trophic base. According to our records from recent period, the shallower branches were an important trophic base during low water levels, mostly during post breeding translocations and migration of Ciconia nigra L., Ardea cinerea L. and Egretta alba (L.). In 1993 this important function did not occur due to the stabilised water regime, which was accomplished by water supplied to the branches by the HPSG. As a result, the above mentioned species did not group themselves in common gatherings, but they occurred only individually. During the summer of 1994 enormal reduction in the water supply of the branch system caused a partial to absolute emptying of the lower section of the branch system, in which 142 Egretta alba (L.), 70 Ardea cinerea L. and 68 Ciconia nigra (L.) gathered during August 1994 were found. Significant changes in the spatial relationships of Phalacrocorax carbo (L.) were recorded. After the damming of the Danube and the emptying of its branch system during the winter 1992/93, the species were concentrated in the reduced main Danube river (Ac, Bohus, not published data). This was probably connected with faciliated predacity due to diminished water volume. Following the rise in the fish population in the storage lake of the HPSG, this lake is a preferred feeding area of the species now. A certain area of the Slovak section of the branch system has revolved as a night resting place for this species after a ten years hiatus. In the case of the Haliaeetus albicilla (L.) we recorded a significant preference for the least frequented sites of the branch system and the main river of the Danube, where there were no finished reinforced roads unlike the majority of the area, at this time. During our investigation period we recorded an unusual rise in the abundance of Cygnus olor (Gm.). During the fifties this species was not considered as an avifauna member of the area [1, 2]. The phenomenon is most probably connected with species population trends. Part of this population has been probably artificially translocated from old gravel pits in the city of Bratislava.

Some of the records, which we set great store by, should be compared with the status of the Danubian inundation area of the Hungarian section. There are strong indications that the changes noted by us are connected with alternations in the water regime in the Hungarian Danube area, but we have not had the opportunity to judge or follow their impact in this area.

#### REFERENCES

  [1] Balat, F., 1963. Ptaci fauna Zitneho ostrova. Biologicke prace IX/7, 84 pp.

  [2] Balis, M., 1952. Kvantitativny vyskum vtactva luznych lesov Podunajska. PhD. thesis, Comenius University, Bratislava, 117 pp.

  [3] Hudec, K., et al., 1972. Fauna CSSR. Ptaci 1. Academia Praha, 536 pp.

. Note: In the study the research findings of RNDr. Mirko Bohus, from Faculty of Naturas Sciences, Comenius University, were used as well.