CONCLUSIONS Some specialists used a very simple method for an evaluation of fishery before and after damning of the Slovak stretch of the Danube River. Before the damning the Danube fishery was in the best state. Now, after the damning both the ichthyofauna and the fishery have been dramatically and drastically destroyed. It must be said that the truth is somewhere in the middle of this evaluation and that there exists a large potential of an favourable improvement of the habitat especially for many young fish specments, and in the old Danube conditions are far closer to the river's natural state than the old high velocity main channel with the fortified river banks and navigation.
The changes in hydrological regime had an adverse impact on the Danube fishery as registered already by Heckel [3] and Kornhuber [5].
The construction of the barrages on the submountain part of the Danube and the deepening of the Danube river bed for navigation purposes changed considerably the hydrological regime of the Slovak section of the river. The substantial changes in transport of bedload and its depositions occurred over a major part of the river causing a progressive sinking of the river bed.
In the section between Bratislava and Hrusov, the branches during periods of low discharges used to drain totally and dried completely. In the middle and lower part, branches got smaller during m3.s-1riods of drought. The water flows into the branches only by discharge of 2,300 m3.s-1 and higher. At the discharge of 5,000-6,000 m3.s-1 the branches were filled up to the banks (3-5 days per year) and only at the discharge of more than 6,000 m3.s-1 the whole inundation area between the dikes was flooded (2 days in average).
It had a negative impact on the water level in the branch system and subsequently on fisheries, whose output decreased progressively. Later a significant deterioration of water quality resulting from extensive industrialisation occurred as well. In 1973 the water quality at the river inlet on the Slovak territory reached the 4th degree in quality (alphamesosaprobia). Besides the above mentioned factors, there was an extensive catch of some fish species.
On the other hand, intentionally or accidentally new species, as Oncorhynchus mykiss, Coregonus lavaretus, Ctenopharyngodon idella, Aristichtys nobilis, Hypophatalmichthys molitrix, Ictalurus nebulosus, Gasterosteus aculeatus, Micropterus salmoides, Lepomis gibbosus, which were previously unknown, were introduced or appeared in the Danube. Thus, in the 70s there were 61 species belonging into 15 families in the Slovak-Hungarian sector of the Danube [4, 5].
The annual fish production could be characterised similarly. In the main channel its average value was 18 kg/ha and it was about 200 kg in the branches. The three most numerous species were Alburnus alburnus, Rutilus rutilus and Gymnocephalus cernuus. Their ratio was 89.10% of the total abundance and 80.66% of the total ichthyomass, respectively. The ratios of economically preferred species were 0.49 - 0.55% of the total number and 3.15 up to 3.30% of the mass of the whole ichthyocenoses. The secondary species represented 0.88 - 6.27% of the total number and 6.89 - 11.54% of the total ichthyomass. The accompanying species represented 98.63 to 93.18% of the number and 89.96 - 85.16% of the mass. The number of species varied between 19 - 29.
The ichthyocenoses were in an unbalanced state, with a predominance of non-predator species over predator species, in a very unfavourable ratio. This had negative consequences, such as the decrease in production, progressive decrease of valuable species, and decreases in economic value of ichthyocenoses and total catch [2].
The reservoir, which has been created behind the Cunovo weir, is in comparison with the original project shorter, and in the lower part narrower. The daily fluctuation of the water level is minimal, with the level at 130-131 m a.s.l. The hydroelectric power plant is being operated at current mode. It means that there is a relatively short retention time of the water and a fast exchange of its volume. Essentially, the reservoir represents a several times enlarged Danubian river bed. Due to the lower flow velocity some sedimentation of the suspended load and bedload occurs in certain parts of the reservoir. The water temperature shows some differences, but in principle it corresponds to the original values. No temperature stratification occurs.
As far as quantitative composition of macrozoobenthos in concerned, the abundance of macrozoobenthos has reached in average 22,563 specimen on m2 in the upper part of the reservoir, and even 76,368 specimen on m2 in the lower part of the reservoir. The biomass reached in average 167.9 g.m2 in the upper part of the reservoir and 335.1 g.m2 in the lower part of the reservoir. These values of the biomass confirm high productivity of the Danube in the area of the reservoir [8].
The occurrence of economically preferred species such as Stizostedion lucioperca and Silurus glanis will increase. The species Esox lucius, Cuprinus carpio and Tinca tinca will be dependent on the water purity and the steadiness of the water level, especially during the spawning, early development and growing periods. An increase in the number of Acipenser ruthenus can be expected too. Most likely, the species Leuciscus cephalus, Abramis brama, Vimba vimba and Chondrostoma nasus will be the predominant fish.
Old river bed of the Danube
After the damning of the Danube the actual old river bed between Cunovo and Palkovicovo has smaller discharge, lower flow velocity and higher water purity as a consequence of partial sedimentation in the reservoir. Its bottom has become more stable during the season and consequently the fauna of macrozoobenthos is richer now. The food base for fish is also richer, although the structure of benthos is being gradually rebuilt.
An intensive sedimentation of the drifted components occurs in the lower part of the old channel due to the broad and shallow river bed and slower flow. Due to this fact, the ratio of the oligochets increased and the number of amphipods and trichopters decreased [7].
During the monitoring in 1993 the presence of 35 fish species in the old river bed was confirmed, including rheophile species: Oncorhynchus mykiss, Salmo trutta m. fario, Coregonus lavaretus, Abramis sapa, A. ballerus, Chondrostoma nasus, Leuciscus cephallus, L. idus, Vimba vimba, Silurus glanis, Lota lota, Gymnocephalus baloni, Gymnocephalus schraetser, Stizostedion lucioperca, S. volgense, Zingel streber, Z. zingel, Proterorhinus marmoratus, Cottus gobio, and other species [5], (Table 1). The changes in the ichthyofauna concern only some habitats of the littoral zone of the old river bed.
River branch system
Due to a certain sedimentation in the reservoir the transparency of the water flowing into the branch system increased moderately. The turbidity caused by inorganic particles (suspended load) decreased substantially. The water has some vegetation turbidity due to mild phytoplankton. The growth of higher water plants will be prevented by higher vegetation turbidity and by deepening of the branch system. The number of predatory phytophile fish will increase. The danger of eutrophisation, anaerobic conditions and fish destruction is lower. The water flow will be guaranteed.
The monitoring in 1993 has confirmed the presence of 29 fish species occurring in the system, despite the short period of monitoring and limited number of monitored localities. The regular occurrence of 55 fish species is supposed. (Table 1)
The conclusions of the research project underestimated the importance of the reservoir, and in particular, the importance of filling the already dried branches near Podunajske Biskupice and Rusovce with water, as well as the creation of extensive spawning and feeding areas in the upper part of the reservoir. Nevertheless - and this is decisive - they did not focus solely on conservation of already substantially changed conditions of the river in the whole area of the inland delta, but recommended half-intensive system of fishery production in the branches.
The recent results of the artificial water discharge in the left-side inundation area confirmed the standpoint that existing technical equipment enables to solve all problems connected with the derivation of the river. The artificial water discharge into the branch system enabled to revitalise dead branches of the parapotamal and plesiopotamal type, and to maintain them filled up with water, according to the needs and their character during the whole years.
The intake structure of the branch system makes it possible to arrange water levels in the branches to control the flow and time according to the water temperature, during which the spawning early development of young specimen and their feeding can take place. From the phylogenetic adaptation of fish point of view it is important that it develops a food basis and reduces the mortality of young specimens, especially in the winter period. Thus, the conditions of the fishery will be improved in this section of the river.
New solution of the system of branches offers thus the possibility of effective investment into the fishery and fish production through a controlled application of appropriate fish fries and catch them at the end of the season. The value of the production and the possible catch will increase at least three times due to the changes of the species composition of ichthyofauna in favour of economically preferred species and they will become comparable the fish production in ponds of the second class.