## MONITORING OF ICHTYOCOENOSES IN THE SLOVAK PART OF THE DANUBE INLAND DELTA BEFORE AND AFTER OPERATION START OF THE GABCIKOVO BARRAGE SYSTEM

####
Jaroslav CERNY

Institute of Zoology, Slovak Academy of Sciences, Dubravska cesta 9, 84206 Bratislava, SLOVAKIA

CONCLUSIONS The principal changes of conditions for the development of hydrocoenoses, inclusive of fish, caused by cutting off the branches, decreasing of Danube bottom due to excessive excavation of gravel and shortening of floods duration appeared already before operation start of the Gabcikovo Barrage System (GBS). These factors gradually affected the dynamic processes in the Danube inland delta. From the point of view of fishery, these changes caused a decrease of catch in the Czechoslovak stretch of the Danube since 1975.

Diversion of the Danube main stream into the diversion canal of GBS in 1992 resulted in the interruption of natural processes in the inland delta. A great resource of energy, water and nutrients for the inland delta ecosystem has been liquidated.

In the first period after the operation start of the GBS, the communication between the branch system and the former main stream has been stopped as a consequence of the changed hydrological regime. Dividing of the branches into sections changed their character from the lotic one to lenitic one. A number of smaller oxbows was isolated. Since 1994, the further changes are partly prevented by supplying the remnants of the branch system by water taken from the by-pass canal. At present the branches are permanently discharging and they have again a predominantly lotic character. However, from the point of view of fish, it would be desirable to improve the migration possibilities of fish through the individual dams constructed across the branches and to connect the branches in some places with the original Danube river-bed.

A monotone littoral has arisen in the original river-bed with considerably reduced water levels. It has shifted more to the middle of the river-bed, what has resulted in disappearing of a number of natural shelters for fish. The fish abundance and the species diversity has decreased in the littoral of the former main stream.

The share of the frequent, eurytopic, mostly phytophilous species with a wide ecological valence increased in the ichtyocoenoses. The reophilous species predominating in branches in the past even in those periods, when the communication with the main stream was interrupted, have become subdominant or receding.

Abundance and ichtyomass of the economically valuable species, above all of the predators, have decreased considerably. This is not only a consequence of the ecological changes, but also of the increasing pressure of anglers and violating of the fishery law. The illegal fishing by all possible modes (casting nets, gill nets, explosives) has very spread in the Danube and is targeted mainly on the economically valuable fish.

Before the start of supplying the branch system by the water from the diversion canal, the strongly limited discharge in the earlier discharging branches supported increased sedimentation of deposits. The branches were endangered by gradual silting, which could result in liquidation of suitable spawning grounds for the lithophilous and the psamophilous fish and, consequently, in a decrease of the species diversity of fish. The two-year experience with supplying the branch system by water from the diversion canal indicates a positive fact, that the discharging at least the main branches moderates the negative effects of the GBS operation on the ichtyocoenoses.

Absence of the floods decreases the number of habitats suitable as spawning grounds, what causes secondary changes in the structure of the ichtyocoenoses and decrease of their productivity. From this reason it is important to provide artificial flooding of the within-dike area in the frame of the operation regime of GBS.

Further efforts to renaturation of this unique ecosystem should be oriented to preservation of biodiversity and maintenance of its dynamic balance. A prerequisite of it is a solution of communication of the original Danube river-bed with the branch system.

#### INTRODUCTION

The aim of monitoring was to characterise the fish fauna in the Danube inland delta before and after the start of Gabcikovo barrage system (GBS) operation. The main goal was to ascertain the species diversity and the abundance of fish with a special regard to the rare and endangered taxa. The delay of the GBS operation start to 1992 made possible to extend the monitoring period of the still non-disturbed state on three years (1990-1992). In autumn 1992, the Danube main stream was diverted into the diversion canal and, in the river branch system, the dams were constructed across the branches in 1993-1994. Than supplying of branches by water taken from an intake structure built up at Dobrohost began. The data concerning the ichtyocoenoses obtained from the studied localities in 1990-1992 are considered to be characteristic of the intact state, while those from 1993-1994 may reflect eventual changes caused by the impact of GBS operation

#### METHODS

In 1991-1994 the ichtyocoenoses were monitored in the following localities: Istragov - main stream, Istragov - upper side of the "Foki" dam, Istragov - Palkovicovske rameno river branch and Sporna Sihot. The fishing was carried out by means of an electroshoker of the type ZB 6 or ML BDS 50 under the voltage of 220-300 V and intensity of 0.5-3.0 A. The main Danube stream was fished along the littoral or on the both sides of the groynes. In the Danube branches the fishing was performed from a boat in the intervals of 20-60 minutes. The length of the fished stretches was of 50-60 m. The width of the fished stretch in the littoral of the Danube main stream was of 5 m along the banks and 3 m along the groynes. The fishing was performed once to twice a year. The caught fishes were measured with an accuracy of 1 mm and weighted with an accuracy of 1 g and than put back into the water. Along with the fishing, also the conditions on each habitat were recorded (character of the flow, vegetation, character of the bottom, offer of hiding places etc.).

The number of caught fishes and their weight were recalculated per unit of catch effort (CPUE) per 15 minutes or per 100 m of a stretch fished by means of the electroshocker.

The Shannon-Weaver's index of diversity [12] and the Sheldon's index of equitability were calculated for each locality and sample. The species were classified according to their occurrence constancy and dominance into classes presented by Losos et al. [10]. The proposal of the Red List of Fishes compiled by Holcik [6, 8] was used for evaluation of endangering degree of individual fish species, based on the IUCN scale: Ex - extinct species, E - critically endangered species, V - vulnerable species, R - rare species, I - indeterminate species. The Schiemer's classification [14] was used for characterisation of species according their relation to the stream intensity and the Balon's classification [3] modified by Holcik [6] was used for their characterisation in relation to the spawning substrate. Individual types of branches were classified according to Roux et al. [11] and Holcik [6].

#### CHARACTERISTICS OF MONITORING AREAS

The aquatic habitats in the within-dike area vary from an expressively lotic habitats to the lenitic ones. Down to Sap, the Danube main stream has a character of a montane river. However, there are many ecotones in its vicinity, in which the reophilous fauna contacts with the limnophilous one and a few meters further the river branches are already considerably isolated and their contact with the living branch system takes a short time. The monitored areas were selected from a variable habitat scale existing in the within-dike zone so to make possible to record the changes caused by the GBS.

The situation in the within dike zone before start of GBS operation had been worsen considerably due to gravel excavation and construction of dams in the Danube upper stream (Austria, Germany). The Danube reacted on the decreased quantity of deposits by erosion of its bottom and was cutting deeper into the alluvial sediments. It resulted in reduction of inundation and shallowing and silting of the Danube branches.

Fishing in the monitoring area Istragov was carried out on a stony groynes in the Danube main stream at the rkm 1817.5 and on the upper and lower side of the "Foki" dam constructed of quarry stone and separating the Palkovicovske rameno branch and the Danube main stream at the rkm 1816. At the time of the higher water levels, this branch supplies the whole branch system of Istragov. The substrate on the upper side of this dam was gravely near the low estuary of this branch into the main stream. Other parts were boggy, without vegetation. Before start of GBS operation this river branch, it represented a typical eupotamon along the upper side of the dam, although it was not discharging at low water levels in the Danube.

At the high water levels in the Danube (over 500 cm on the flow gauge in Gabcikovo), the overflowing of the "Foki" dam and the communication of both arms occurred. At average water levels the water discharge was limited on some damaged stretches of the dam. The substrate on the lower side of this dam was gravely in the vicinity of the broken parts, on the left side boggy. Due to the frequent washing of the stretch underneath the dam, the bottom was not covered by the vegetation and the branch had here character of a parapotamal.

In consequence of the GBS operation start, the water levels in the Danube old river were never more so high in 1993 to may communicate with the Palkovicovske rameno branch or to be higher that the "Foki" dam. The branch is considerably reduced at present (its total length reaches ca. 250 m), shadowed and dried up in its upper part. The fishes are retreated now into a lagoon arisen between boulders of the rocky dike. At present, its estuary into the original Danube river-bed is stuffed by the deposits and the branch has a character of a pleisopotamal. In contrast to the preceding years, the dam boulder foot was not discharging at all or only in spots, slightly to indistinctly. The fish were concentrated at these places.

In the Danube main stream, the fishing took place on upper and lower side of the stony groynes. The groynes point controlling the streamline was exposed to the direct pressure of the main stream. The groynes lower side was in a relatively calm place. In consequence of a considerable decrease of discharge in the main stream, the prevailing part of the groynes has been found on dry land and only its last fifth reaches into the river. On the lower side of the groynes foot, a sandy to gravely shore appeared, which does offer shelters for fish. Some overflooded boulders covered with a relatively rich perifyton and offering to the fish shelter and food basis remained only in the vicinity of groynes point and along its upper side. Monitoring area Sporna Sihot was represented by a closed branch remainder of the pleisopotamal type, on the level of the rkm 1703.5, it means under the estuary of the tailrace. At a higher water level, this locality communicates with the Klucovske rameno branch. During a longer isolation (at the average water level in the Danube) the water surface decreases in result of a considerable shallowing and under an extremely warm weather dries up entirely. The bottom is boggy, overgrown by green algae. Potamogeton and Ceratophyllum represent dominant components of the submersed vegetation. The littoral is shallow and boggy, the southern bank is covered by Phragmites communis, the northern one is steeper, overgrown by trees (Salix, Fraxinus, Acer negundo).

#### RESULTS

In 1990 we registered altogether 21 fish species in the Istragov area [4]: R. rutilus, A. bjoerkna, A. ballerus, A. alburnus, A. aspius, C. carpio, C. auratus, B. barbus, Ch. nasus, L. cephalus, L. idus, P. fluviatilis, S. lucioperca, S. volgense, S. glanis, G. baloni, G. cernuus, P. marmora tus, L. gibbosus, L. lota, E. lucius.

#### Istragov - the upper part of the "Foki" dam

In 1991, we registered here 15 fish species belonging to seven families. The eudominant species was C. auratus. It reached a dominance of 29.2 % in August, 69 % in October and 50.26 in average. CPUE in this locality reached 49.2 individuals per 15 minutes and 34.9 individuals per 100 m respectively. Average abundance and ichtyomass reached a value of 1103 ind. per hectare and 70.004 kg.ha-1 respectively. Diversity reached 1.64 in the August sample, 1.26 in the October sample and 1.86 in the total sample. The highest equability was exhibited by the August sample. The total sample equability reached 0.68.

The eurytopic species prevailed in the ichyocoenosis. Among the reophilous species we recorded B. barbarus, G. cernuus and L. lota. Majority of species belonged to the guild of species not protecting spawn and to the subtype of phytophilous species. Among the critically endangered species we recorded the wild form of carp - (C. carpio) and among the indeterminate species some juvenile Ch. nasus.

In 1992 we recorded 10 following species of fish: A. aspius, A. bjoerkna, A. alburnus, C. auratus, R. rutilus, S. erythrophthalmus, E. lucius, P. fluviatilis, G. cernuus, a L. lota. The samples from the beginning and end of August consisted of seven species each (H = 1.10 and H = 1.39 respectively) and of six species in October (H = 1.00). Most species belonged to the family Cyprinidae (six species), Percidae were represented by two species and Esocidae and Gadidae by one species each. Only in beginning of August the guilds of reophilous and limnophilous fish were represented by one species each (Aspius aspius and S. erythrophthalmus). In the end of August and in October the fish fauna consisted only of the eurytotic species. The species R. rutilus (dominance 5.4 to 18.1 %), C. auratus (14.3 to 67 %) and E. lucius (8.4 - 51.8 %) were eudominant. A. bjoerkna and P. fluviatilis were dominant other species were subdominant.

The phytophilous and phytolithophilous fish not protecting the spawn were represented by four species each (each of them representing 7.14 % of individuals) and, at the same time they represented the largest part of the ichtyocoenosis. The lithophilous and lithopelagophilous species not protecting spawn were represented by one species each (A. .aspius and L. lota). Occurrence of no critically endangered, vulnerable, rare or indeterminate species has been confirmed.

In 1993, we recorded occurrence of 12 following species: A. bjoerkna, C. auratus, L. idus, R. rutilus, S. erythrophthalmus, S. glanis, E. lucius, L. lota, P. fluviatilis, G. baloni, L. gibbosus a P. marmoratus. The sample from the end of September consisted of nine species (H = 1.66) and that from the beginning of November of 10 species (H = 1.28). The total diversity of both samples reached 1.92. The most species belonged to the family Cyprinidae (5 species), two species to the family Percidae. The families Siluridae, Esocidae, Gadidae, Centrarchidae and Gobiidae were represented by one species each.

The eudominant species were C. auratus (dominance in the end of September 29.33 %, in the beginning of November 1.58 %), R. rutilus (29.33 % and 60.31 % respectively) and P. pluviatilis (24.00 % and 16.66 % respectively). A. bjoerkna was eudominant in the beginning of November, although we did not record it in the end of September. Other specie were subdominant. The values of CPUE per 100 m were high as in the end of September as in the beginning of November (100.0 ind. and 167.0 ind respectively). The average CPUE per 100 m found here in 1993 is the highest among the studied localities. The reophilous fish was represented by three species, while the limnophilous one by one species only. Other species (66.66 %) were eurytopic.

The richest in species was the guild of phytophilous and phytolithophylous species not protecting the spawn (4 species - 33.33 % and 3 species - 25.00 % respectively). The spawn not protecting lithophilous and lithopelagophilous fish, the phytophylous fish laying spawn to selected places in the substrate and the polyphilous and speleophilous fish were represented by one species each.

Among the endangered species we recorded G. baloni. In the period 1989-1992, this species was always a dominant to edudominant component of the ichtyocoenosis at the lower side of the "Foki" dam. After diversion of the Danube, the discharge through this dam decreased and C. baloni disappeared from this place. Among the rare species we recorded L. lota.

In 1994 we recorded the following eight fish species: A. bjoerkna, L. idus, R. rutilus, E. lucius, P. fluviatilis, L. gibbosus, P. marmoratus a A. alburnus. The eudominant species were L. idus and R. rutilans, other species were subdominant.

The reophilous species were represented only by L. idus, other species were eurytopic. The phytophilous and phytolithophilous fish was the richest in species. We did not record any endangered species in this locality.

#### Istragov - Palkovicovske rameno branch

In 1991, we recorded 19 fish species belonging to 5 families. C. auratus, G. baloni and B. barbus were eudominant. CPUE per 15 minutes reached 71.5 ind. in August (68 ind. in October) and CPUE per 100 m 48.4 ind. (47 ind. in October).

Similarly as in the previous locality, the fish abundance was very high, but the ichtyomass relatively low. The diversity was high and fluctuated from 1.8 to 1.93 during the monitoring period. The diversity of the total sample reached 2.22 and equability 0.75.

The eurytopic species surviving under the flow velocity of 0.0 to 0.5 m.s-1 and the reophilous species dominated in this locality. Majority of the recorded species (11) belongs to the spawn not protecting fish and to the sub-group of phytophylous species. We recorded the occurrence of the critically endangered wild form of carp (C. carpio)in this locality.

In 1992 we recorded 14 fish species in this locality: A. bjoerkna, A. alburnus, C. auratus, G. albipinnatus, L. cephalus, L. idus, R. rutilus, E. lucius, P. fluviatilis, G. cernuus, G. baloni, L. lota and P. marmoratus. In the sample from the beginning of August we recorded 12 among them (H = 1.94), in end of August seven (H = 1.67) and in October six (H = 1.33). The family Cyprinidae was the richest in species (7 species), Percidae were represented by three species and Esocidae, Gadidae and Gobiidae by one species each. The reophilous fish were represented by four species in the beginning of August and by two species in the end of August and in October. All other species were eurytopic. Limnophilous species were not recorded in this locality.

In 1992, P. fluviatilis (dominace of 14.3 - 35.9 %), R. rutilus (2.1 - 14.1 %) and C. auratus (16.3 - 53.2 %). A. bjoerkna, G. baloni and P. marmoratus were subdominant, the remaining species were recedent.

The spawn not protecting phytolithophilous fish (5 species = 35.7 %) and phytophilous fish (3 species = 21.4 %) were richest in species. Two species belonged to the spawn not protecting lithophilous fish. The spawn not protecting lithopelagophilous and psamophilous fish, the spawn protecting polyphilous and speleophilous fish were represented by one species each.

The occurrence of no endangered, rare or indeterminate species was confirmed. This monitoring locality represents a spawning ground and a suitable microhabitat for the endangered G. baloni.

In 1993, eight fish species were recorded on the lower side of the "Foki" dam: C. auratus, L. cephalus, L.idus, R. sericeus, R. rutilus, P. fluviatilis, L. gibosus and P.marmoratus. Among them five species were in the sample from the end of September (H = 0.80) and six species in the sample from the beginning of November (H =1.15). The diversity of the total catch reached 1.12.

The richest in species was the family Cyprinidae (5 species), the families Percidae, Centrarchidae and Gobiidae were represented by one species each.

In the end of September and in the beginning of November, CPUE per 100 m (29.98 ind. and 33.97 ind. respectively) reached approximately only one third of its values found out at the upper side of dam. Also the average CPUE in 1993 was very low (31.96 ind.).

P. fluviatilis (73.33 % of individuals in the end of September and 27.45 % in the beginning of November) and R. rutilus (20.00 % and 56.86 % respectively) were eudominat. Other species, except for the recedent R. sericeus, P. marmoratus and L. gibosus, were subdominant.

In the end of September, we recorded two reophilous species and three eurytopic species. The limnophilous species were not recorded. All species found in the beginning of November were eurytopic.

The phytolithophilous fish not protecting the spawn (3 species = 37.50%) was the richest in species. The lithophilous and phytophilous the spawn not protecting fish, the ostracophilous fish laying the spawn into shelters, and polyphylous and speleophilous fish protecting the spawn were represented by one species each.

We did not record occurrence of any critically endangered, vulnerable or rare species. In the end of September we recorded L. idus considered as an indereminate species.

In 1994 we recorded nine fish species: L. cephalus, L. idus, A. bjoerkna, E. lucius, A. aspius, R. rutilus, P.fluviatilis, L. gibosus and S. erythrophthalmus. The family Cyprinidae was the richest in species (6 species), the families Esocidae, Percidae, Centrarchidae and Gobiidae were represented by one species each.

In comparison with the previous years, CPUE per 15 min. was low. The highest CPUE (17.6) was recorded in R. ritulus. Two species were reophilous, one limnophilous and other ones eurytopic. The phytolithophilous fish not protecting spawn were richest in species. We did not registered any endangered species.

#### Istragov - main stream

This locality was monitored since 1991, when we recorded 14 species of 6 families. The species, Cottus gobio and Chondrostoma nasus were eudominant. Average CPUE reached 25.8 ind. per 15 minutes or 20.9 ind. per 100 m. The high abundance and a relatively low ichtyomass indicate predominance of the juveniles in this locality using the shelters directly in the main stream. Diversity index of the whole catch was 2.26 (2.13 in August and 1.58 in October) and equability 0.85.

Eight species were reophilous (C. gobio B. barbus, Ch. nasus, G. albipinnatus, L. cephalus, L. idus, L. lota, G. baloni) and six eurytopic (A. alburnus, C. auratus, C. carpio, E. lucius, P. fluviatilis and P. marmoratus).

One species (G. baloni) is considered as rare and three species as indeterminate (Ch. nasus, L.idus and L. lota).

In 1992 we registered 12 species of 7 families. The occurrence of A. alburnus, C. auratus, C. carpio and Ch. nasus recorded in 1991 was not confirmed. On the contrary we recorded R. rutilus and B. barbatus absenting in 1991. As a result of conspiciously lotic conditions in this locality, the reophilous C. gobio and P. marmoratus were eudominant here. We recorded two indeterminate species (L. idus and L. lota) and one rare specie (G. baloni).

After start of GBS operation in 1993, we recorded, in two catches, six species (E. lucius, R. rutilus, L. lota, G. baloni, P. marmoratus, C. gobio) belonging to six families. Predominance of the eurytopic species indicated decrease of the conspicuously lotic habitat properties in a wider surrounding of this locality. Occurrence of the reophilous L. lota and G. baloni indicated a relative preservation of microhabitats in this locality. The constant dominance of C. gobio indicated a high water clearness in the Danube main stream.

In 1994, we did not catch any adult fish, only the juvenile individuals of R. rutilus. Only 10 % of the fished stretch was represented by a partially water-locked stony dam, its major part was dry and the littoral was boggy.

#### Sporna Sihot

In 1991, we found 13 species of four families (C. auratus, C. carpio, A. brama, A. ballerus, A. bjoerkna, A. alburnus, S. erythrophthalmus, R. rutilus, R. sericeus, P. fluviatilis, S. volgense, L. gibbosus, P. marmoratus) among them eight were found in the beginning of September and 10 in October.

C. auratus representing even 68 % of individuals in September and R. rutilus (17.3 % in September and 29.4 % in October) were eudominant. The CPUE reached 131 ind. per 15 minutes or 437 ind. per 100 m. Average abundance and ichtyomass reached 8749.8 ind. per ha and 35.960 kg.ha-1. The eurytopic species predominated in the ichtyocoenosis, the limnophilous ones were represented by S. erythropthalmus and the reophilous by A. alburnus.

The diversity index reached a value of 1.05 in September, 1.48 in October and 1.71 in both samples together. Equability reached 0.5 in September, 0.64 in October and 0.68 in both samples.

Up to 80 % of individuals of all recorded species belong to the phytophilous species not protecting their spawn. One species was ostracophilous. Among the fish protecting spawn and laying it into built nests, the polyphilous (L. gibbosus), phytophilous (S. volgense) and speleophilous fish (P. marmoratus) were represented by one species each. According to the trophical relation of the adults, majority of the recorded species belong to the zoobenthophagous fish. Among the endangered species, the wild form of carp (C. carpio) may be expected, but so as in other localities, the wild form can be hardly distinguished from the cultural one. Among the endangered species we recorded S. volgense.

In 1991, we recorded five species: R. rutilus, A. bjoerkna, C. auratus, E. lucius and S. erythrophthalmus. The fishing was possible only in the beginning of August, because the branch dried in later months in result of extremely high temperatures in summer 1992 and low water level in the Danube. The diversity index reached a values 1.36. The family Cyprinidae was the richest in species, three species (60 %) were eurytopic and one limnophilous (20 %). The reophilous species were not recorded in this branch. The eudominant R. rutilans was represented by 41 % of individuals and A. bjoerkna 32.7 %. C auratus was dominant and other species (E. lucius and S. erythrocephalus) were subdominant.

The phytophilous fish not protecting spawn was represented by four species (80 %) and the phytolithophilous fish not protecting spawn by one species (R. rutilus). We did not record any endangered species

In 1993 we recorded 9 fish species: A. bjoerkna, A. alburnus, A. aspius, C. auratus, R. sericeus, R. rutilus, T. tinca, M. fossilis a E. lucius. Seven of them were recorded in the end of September (H = 1.51) and six in the beginning of November (H = 1.35). Diversity of both samples was 1.49.

Cyprinidae were richest in species (7 species = 77.77 %), Gobitidae and Escocidae were represented by one species each.

CPUE per 100 m reached 70.74 ind. in the end of September and 124.98 in the beginning of November. Both values were high, because the ichtyocoenois consisted of the juvenile individuals, mostly of 1-2 years old fish. The average CPUE per 100 m in 1993 was equal to 124.98 ind.

C. auratus (39.13 % in the end of September and 19.12 % in the beginning of November), R. sericeus (29.88 % and 45.81 % respectively), A. alburnus (10.86 % and 27.09 % respectively) and E. lucidus (17.39 % in the end of September, in November not recorded) were eudominant. Other species, except for the recedent R. rutilus recorded in the beginning of November, were subdominant.

The limnophilous fish were represented by two species in the end of September, but in the beginning of November they were already not recorded. Remaining six species caught in September and all species caught in November were eurytopic.

The phytophilous fish not protecting spawn were represented by five species (55.55 %). Two species were phytolithophilous. Lithophilous and ostracophilous species laying eggs in a shelter were represented by one species each.

In 1994 we recorded 5 species: A. bjoerkna, A. aspius, C. auratus, R. rutilus, a E. lucius. Cyprinidae were represented by the largest number of species (4) and Esocidae by one species. CPUE per 15 minutes was low, because this branch did not communicate with the main stream in 1994.

R. rutilus was eudominant, other species were subdominant. All species were eurytopic. Most species belonged to the phytophilous species not protecting spawn.

In comparison with the previous years, the species diversity of fish decreased. Index of diversity reached the value of only 1.21. We did not record any endangered species.

#### DISCUSSION

#### Istragov - inner side of the "Foki" dame.

Among 21 species recorded here in 1990, we confirmed occurrence of 15 species in 1991 and only of 10 species in 1992. In 1993 we recorded 12 species and the average CPUE per 100 m was the highest among all monitored localities and for the whole period on investigation.

We explain the increasing of the species diversity and the high abundance by the close contact of this locality with the Danube main stream and by a continuing sporadic communication after the diversion of the main stream in 1992. Also reduction of the total area and denundation of articulated banks of the branch are of a considerable significance. It caused concentration of the major part of the ichtyocoenosis from the whole branch into the only possible shelter represented by the stony dam "Foki".

In 1994 we confirmed occurrence of 8 species. Two of them (A.alburnus and L. idus) were reophilous. The reactions of G. baloni are very interesting. It represented originally a constantly dominant or eudominant component of the ichtyocoenosis in the discharging parts on the upper side of the dam in Palkovicovske‚ rameno branch. After the diversion of the Danube main stream, this species moved to its lower side. In the last year, when the seepage through the damaged parts of the dam had stopped, G. baloni disappeared at all.

This locality gives an evidence of the limiting influence of the hydrological regime of the Danube and a great significance of the communication between the branches and main stream for a high diversity of ichtyocoenoses. The close contact of the branch with the Danube main stream enables existence of relatively stable and diversified ichtyocoenosis.

#### Istragov - Palkovicovske rameno branch

Among 21 species recorded here in 1990 we confirmed occurrence of 19 species in 1991 and of 14 species in 1992. This locality represented a spawning ground and a suitable microhabitat for the endangered G. baloni. In spite of the difficult identification, we suppose a part of the juvenile C. carpio to belong to the original wild form of this critically endangered species.

After start of GBS operation, the discharging of the Palkovicovske‚ rameno branch was stopped. It resulted in decrease of number of the recorded species (8 species in 1993) and diversity index. The relative abundance of individual species changed considerably. After 1992 we did not record any critically endangered or endangered species. In 1994, among 9 species two species were reophilous (L. idus and L. cephalus), one limnophilous and other species were eurytopic. In comparison with the previous years, the values of CPUE per 15 min. decreased considerably.

A relatively poor fish community along the out side of the "Foki" dam reflects changes caused in this locality by the diversion of the Danube main stream. They were manifested by the loos of communication between the branch and main stream. In the last years, at high water levels, this branch was discharging and at average water levels the water was flowing through cracks in the "Foki" dam. Such places represented a typical habitat and a spawning ground for G. baloni, whose occurrence was not recorded in this locality since 1993.

#### Istragov - main stream

In 1991 we recorded 14 species of 6 families in the littoral of the main stream and 12 species of 7 families in 1992. After start of GBS operation, the species diversity decreased in 1993. Six of the 12 recorded species were reophilous and six eurytopic. It indicated a relative preservation of microhabitats in this locality. The continuing dominating of C. gobio indicated a high degree of clearness in the Danube main stream.

In 1994, we did not catch any adult fish, only juvenile individuals of R. rutilus. Only 10 % of the fished stretch was represented by a water-locked dam. Its major part was denuded and dry, the littoral was boggy.

This locality represents the littoral zone of the Danube main stream and therefore, from aspect of the fish occurring in the whole eupotamal, the number of recorded species is not complete. After start of GBS operation, the water discharge and the flow velocity decreased considerably, shore line shifted to the river bed middle and the littoral shallowed and silted in this locality. The lack of shelters and monotony of the littoral caused probably a conspicuous decrease in abundance, values of CPUE. The species diversity decreased from 16 species recorded in 1991-1992 on 6 species in 1993 and on only one species in 1994. Lack of shelters and silting of the littoral caused that this locality has been abandoned not only by the reophilous but also eurytopic species. However, these changes can be related only to the observed habitats of the littoral zone of the former Danube river-bed. It can be supposed that majority species absenting in this eupotamal locality in 1993 and 1994 finds a refuge in the medial part of the former mains stream, in the deeper backwater of the littoral or in its submerged parts faced by boulders.

Among the endangered species, the endangered C. baloni indeterminate L. idus and L. lota disappeared from this locality. However, the decreasing population size of further reophilous species, e. g. Ch. nasus is alarming.

#### Sporna Sihot

Although this locality is not influenced by the construction of GBS, a considerable decrease in species diversity in this river branch was recorded during the investigations. A high dominance of the juvenile individuals was characteristic for this locality in 1991. We recorded here 13 species.

At high water levels, like in 1991, the branch communicates with other water of the inundation system of Medvedov and in spring it represents a suitable spawning ground for the major part of the Danubian fish. The Medvedovske‚ rameno branch is also a known spawning-ground for the critically endangered C. carpio, which was recorded here in 1991 together the endangered species S. volgense and indeterminate species A. ballerus. In 1992, we did not record these species here and we confirmed occurrence of only five species. It was probably connected with the fact that the branch had not communicated with the main stream in spring and it had dried because of extreme temperatures during the vegetation season of 1992 and low water level in the Danube.

Since 1993, when 9 species were recorded (in 1994 already only 5 species) we are observing disappearing of the ecologically stenovalent species, above all of those requiring a high content of oxygen, and increase of relative abundance of tolerant euryzopic species (R. rutilus, C. auratus), which survive even the extreme over-warming of the shallowed branch and tolerate low oxygen levels.

The extremely warm vegetation season and the low water levels in the Danube in the last years accelerate the process of silting and terestrization of this branch. The gradual reduction of its surface leads to the branch shallowing and drying. It results in easier catching of fish by the waterfowl. The anglers use this locality as a source of bait-fish. This branch can function in future as a spawning ground exclusively if its regular communication with the system of branches in the surrounding is possible.

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