ECOLOGICAL STATUS AND PROBLEMS OF THE DANUBE RIVER AND ITS FISH FAUNA: A REVIEW

The ecological status of the Danube River and its fisheries prior to 1988 has been summarised by Bacalbasa-Dobrovici (1989) for the first International Large River Symposium. Since then the situation has changed in many ways: the trends of river–floodplain disintegration initiated by the major river regulation schemes in the nineteenth and early twentieth centuries accelerated. Moreover, further hydropower dams were built along the course of the river and its major tributaries, further reducing the ecological integrity of the riverfloodplain systems in several stretches. On the other hand, a number of mitigation schemes were initiated e.g. in Austria, Hungary and Romania in order to compensate for the continuing losses of riverine landscapes. The measures taken to control water pollution were partially successful and water quality along the river has shown a general improvement. The overall deterioration of riverine habitats due to pollution, river engineering and land use is reflected in the high number of endangered fish taxa. The main cause for the decline of many species is the continuing loss of riverine littoral habitats due to river engineering Therefore the main focus of river management in several of the riparian countries is on the conservation of riverine biota, a stronger incorporation of ecological aspects in river engineering and the development of restoration programmes. Several 273 F. Schiemer G. Guti H. Keckeis M. Staras 1 Inst. of Ecology and Nature Conservation, Dept. Limnology, Univ. Vienna, A-1090 Vienna, Althanstr. 14, Austria 2 Hungarian Danube Research Station, Hungarian Academy of Sciences, H-2131 Göd, Jávorka U. 14, Hungary 3 Danube Delta Research Institute, Babdag Str. 165, RO-8800 Tulcea, Romania ECOLOGICAL STATUS AND PROBLEMS OF THE DANUBE RIVER AND ITS FISH FAUNA: A REVIEW international schemes have been proposed to undertake concerted action to improve the overall situation. There is a sound scientific basis for ecologically orientated river management along the Danube: over the past 20 years, environmental conditions, fish ecology and fisheries have been intensively studied at several research institutions by means of large-scale field surveys and experimental studies. These results allow the present ecological status to be redefined. New concepts for commercial and recreational fisheries as well as floodplain restoration were developed. The present contribution synthesises recent developments in fish ecology and fisheries of the Danube and concentrates on key management issues. THE DANUBE: ITS CATCHMENT, GEOGRAPHY AND HYDROGRAPHY The Danube flows over nearly 3 000 km from the Black Forest to its delta in the Black Sea, passing through Europe from west to east. It is an international river, flowing through nine countries – Germany, Austria, Slovakia, Hungary, Croatia, Serbia and Montenegro, Bulgaria, Romania and the Ukraine (Figure 1). Thus, the river connects the West, Central and East European countries. The Danube Basin can be divided into three regions. The Upper Danube extends from the Black Forest to the Devin Gate below Vienna, the Middle Danube from the Devin Gate to the Iron Gate where it passes in the Southern Carpatians and the Balkan Mountains and finally the Lower Danube through the Romanian and Bulgarian lowlands. The Danube Delta at the Black Sea is the second largest in Europe with an area of 5 640 km. The Upper Danube (Figure 1) is characterized by a steep gradient of 0.2-1.1‰, the Middle and Lower Danube by a low gradient, except for the cataracts of Iron Gate (Laszloffy 1967; Liepolt 1967). 274 Ecological status and problems of the Danube Figure 1. a) The Danube River basin b) Longitudinal profile of the slope (indicated are the main differences in slope) and the mean discharge (from: Waterway transport on Europe’s lifeline, WWF, Vienna, 2002). With a mean discharge of about 6 500 m s at its mouth the Danube is the second largest river in Europe and twenty-first in the world. The hydrological regime of the upper region is characterized by high runoff from the Alps. The main Alpine tributaries are the Lech, Isar and Inn. Maximum discharge rates in this zone are due to the runoff from snowmelt in the Alps between May to August. Increases in discharge to the Middle Danube come mainly from the Save (46.6 percent), Tisza (34 percent) and Drava (29 percent). Below the Tisza and Save there is a characteristic change in the seasonal runoff pattern, with a maximum in April and May and low discharge rates from August to January (Benedek and Laszlo1980; Hock and Kovacs 1987). The Danube exhibits high water level fluctuations in the range of several meters; for example in Mohacs (Hungary) fluctuations exceed 9 m. Large alluvial areas with extensive floodplains exist in unconstrained sections (Tockner, Schiemer and Ward 1998; Tockner et al. 2000). The width of the present inundation area downstream of Vienna and in the middle basin varies between 1 and 5 km, in the lower basin between 5 and 10 km. The multi-purpose use of the river is of vital importance for the more than 82 million people inhabiting its 800 000 km basin. The use of the catchment and the river itself has had strong impacts on the environmental conditions of the river-floodplain system (Khaiter et al. 2000; Bloesch 1999, 2001). HUMAN IMPACTS: WATER QUALITY AND