ENDANGERED FISH SPECIES OF THE DANUBE RIVER IN AUSTRIA

Transcription

1 REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. 4, (1989) ENDANGERED FISH SPECIES OF THE DANUBE RIVER IN AUSTRIA FRITZ SCHIEMER AND THOMAS SPINDLER University of Vienna, Institute of Zoology, Department of Limnology A-I090 Wien, Althanstrape 14, Austria ABSTRACT The ecological affinity of fish species to different habitat types in the course of their life cycles is investigated in the free-flowing sections of the Austrian Danube. The results point to the great significance of the degree of hydrological integration between the river and its adjoining waters. In the breeding and fry stage, rheophilic species are bound to the river itself, but the preferred zones show great variations according to both fish age and species. A highly structured shoreline is important in providing a gradient of water current and a spectrum of food sources. Regulated embankments are characterized by an extremely reduced fauna with low population densities of eurytopic species. These results give evidence that the conservation of the characteristic species association is incompatible with the planned hydroelectric power dams in the remaining, free flowing zones of the Danube. The importance of the fish fauna as an indicator group for the condition of large river systems is discussed from the conservation point of view. KEY WORDS Rivers Ecological indicators Fish fry Ontogenetic niche shifts Feeding ecology Water currents INTRODUCTION This paper is concerned with aspects of nature conservation of the Danube, using the fish fauna as an indicator of ecological quality. The reduction of a riverine fish fauna, especially in larger river systems, is a worldwide phenomenon indicating the intensive usage of such habitat types (see Holden, 1979; Lelek, 1980; Welcomme, 1985). The Austrian part of the Danube-350 km of its total course of 2850 km-has undergone two major environmental changes: one in the last century following the water regulation scheme for flood control and for the improvement of navigation, and a second in the last 30 years resulting from the construction of hydroelectric power dams. The Danube in Austria represents a high potential for the generation of hydroelectric power and has been increasingly exploited since 1954 with the ultimate goal of forming a continuous chain of 11 hydroelectric power dams (Figure 1). Eight dams were completed by 1985, and at present, only two zones of free flow remain: the Wachau (33km) as a landscape of high cultural importance, and the section from Greifenstein (upstream of Vienna) to the Czechoslovakian border (69 km). During the past six years the preservation of these few remaining sections has been intensively discussed. Concern has been expressed by ecologists that further damming will result in a severe threat to the characteristic biota. In connection with this discussion and the planning of further hydroelectric power dams in and below Vienna, ecological studies have been intensified, including investigations of the fish fauna. This paper presents some aspects of conservation based on these studies. RESULTS In the Danube and its backwaters between Vienna and the Austrian-Czechoslovakian border, 50 fish species (45 autochthonous, 5 exotics) have been recorded during recent surveys (Schiemer, 1985, 1988; Wanzenbock ef at., 1989; see Table I). This represents a high proportion of the original fauna (53 spp.) of 088&9375/89/ $ by John Wiley & Sons, Ltd. Revised 1 March 1989

2 398 F. SCHIEMER AND T. SPINDLEK m a.s ' Hainburg ---o o a a o river km Figure 1. The Austrian part of the Danube with the locations of hydroelectric power dams (according to DOKW, Danube Electric Power Company). The inserted map outlines the course of the Danube River and the Austrian borders this zone. For a discussion of the distribution and origin of the Danubian fish fauna see Balon, 1964,1968; Balon et al., 1986; Jungwirth, This species richness is due to the habitat diversity of the alluvial system in this part of the river, which favours the occurrence of several ecological guilds. Many of the species found are considered to be vulnerable and endangered (Lelek, 1980). Figure 2 illustrates the ecological grouping (rheophilic, limnophilic, and eurytopic) using the Cyprinidae as an example. It is apparent that the rheophilic guild not only is the largest but also contains the highest number of endangered forms. For conservation management a good knowledge of the ecological requirements of the characteristic species is essential. The various fish species can be grouped according to their specific links to the different zones of the alluvial system.

3 ENDANGERED FISH SPECIES OF THE DANUBE 399 Table I. List of fish species found since 1983 in different habitats types (river, connected backwaters, disconnected backwaters) of the alluvia1 system of the Danube between Vienna and the Czechoslovakian border. : abundant, : common, : rare. River conn. disc, backw. backw. Acipenser ruthenus L. Salmo trutta forma fario L. Hucho hucho (L.) Esox lucius L. Rutilus rutilus (L.) Rutilus pigus Virgo (Heckel) Leuciscus leuciscus (L.) Leuciscus cephalus (L.) Leuciscus idus (L.) Scardinius erythrophthalmus (L. ) Ctenopharyngodon idella (Val.) Aspius aspius (L.) Leucaspius delineatus (Heckel) Tinca tinca (L.) Chondrostoma nasus (1.) Gobio gobio (L.) Gobio kessleri Dybowski Gobio uranoscopus (Agassiz) Gobio albipinnatus Lukasch Barbus barbus (L.) Alburnus alburnus (L.) Alburnoides bipunctatus (Bloch) Blicca bjoerkna (L.) Abramis brama (L.) Abramis ballerus (L.) Abramis sapa (Pall.) Vimba vimba (L.) Pelecus cultratus (L.) Rhodeus sericeus amarus (Bloch) Carassius carassius (L.) Carassius auratus gibelio (Bloch) Cyprinus carpi0 L. Hypophthalmichthys molitrix (Val.) Cobitis taenia (L.) Misgurnus fossilis (L.) Silurus glanis L. Anguilla anguilla (L.) Lota lota (L.) Gasterosteus aculeatus L. Lepomis gibbosus (L.) Stizostedion lucioperca (L.) Stizostedion volgemis Gmelin Perca fluviatilb L. Gymnocephalus cernuus (L.) Gymnocephalus schraetser (L.) Gymnocephalus baloni Holcik & Hensel Zingel zingel L. Zingel streber (Siebold) Proterorhinus marmoratus (Pall.) Coitus gobio L. Number of species

4 400 F. SCHIEMER AND T. SPINDLER RHEOPHILOUS L leuciscus e L cephalus L idus A aspius C nasus G gobio G albipinnatus - G uranosc B barbus A ballerus A sapa V vimba 6 EURYTOPIC R rutilus e + A alburnus L B bjoerkna T A brama C auratus gi C carpio LIMNOPHILOUS & S erythrophthalmus delineatus rc, tinca P cultratu A bipunctatus Figure 2. The cyprinid fauna and their ecological grouping in the Danube downstream of Vienna. white: not endangered; shaded: potentially endangered ( rare and intermediate acc. to IUCN); black: endangered (endangered and vulnerable) Species strictly bound to the river itself (e.g. R.p. Virgo, L. idus, B. barbus, V. vimba, G. schraetser, Z. zingel, 2. streber), Riverine species that, during certain periods of their lifespan, require zones of low flow, e.g. backwaters with open connections to the river (e.g. A. ballerus, A. sapa, C. nmus, A. aspius), Species bound to disconnected branches of the river (with a strong development of vegetation) (especially L. delineatus, M. fossilis, C. carassius, S. erythrophthalmus, T. tinca),

5 ENDANGERED FISH SPECIES OF THE DANUBE Eurytopic species found in all habitat types (e.g. E. lucius, R. rutilus, A. alburnus, B. bjoerkna, A. brama, S. lucioperca, P. fluviatilis). With regard to conservation, an important aspect is the integration of the aquatic system between river and backwaters. A faunistical comparison between different backwater types revealed that open branches contain the more diverse and endangered fauna (Schiemer, 1985). This is due to the fact that different ecological groups are represented. In addition to limnophilic and eurytopic forms, riverine species with specific links to the backwater habitat occur. The question arises as to whether these links are due to specific requirements for 1. Food, 2. Winter and flood refuge, 3. Spawning grounds, 4. Nurseries for the fish fry. An answer can be given by analysing the seasonal occurrence of the various species in the open backwaters. Figure 3 provides a schematic representation of results of the three years fishing programme using monofilament gill nets of various mesh sizes (15 mm, 20 mm, 26 mm, 40 mm, 50 mm) (Schiemer and Dudzinski, in preparation). 14 sampling series yielding a total of 8215 adult fish were analysed. Several patterns can be distinguished: 1. Species occurring throughout the year in the backwaters in consistent population densities (e.g. R. rutilus, A. bramu, A. alburnus) 2. Rheophilic species found in the backwaters only from autumn to spring (e.g. Abramis sap) 3. Rheophilic species often occurring in high numbers in the backwaters except during the spawning period (e.g. A. ballerus; to some extent Chondrostoma nasus) 4. Species occurring in the backwaters throughout the year, but with highest catches in spring and autumn (B. bjoerkna). It is obvious from our data that in some cases the link exists because of the specific trophic demands (e.g. A. ballerus, which is a zooplankton feeder) while other rheophilic species use the backwaters mainly as a winter and flood refuge (A. sapa)., I A.br., R.r. Figure 3. Patterns of seasonal occurrence of the adults of various species in backwaters connected to the river (A.b. = Abramis ballerus; B.b. = BIicca bjoerkna; A.s. = Abramb sapa; A.br. = Abramis brama; R.r. = Rutilrrs rutilus. Schematic representation of a three year sampling programme, based on 8215 adult fish analysed Important ecological links are due to specific requirements during the larval and juvenile stages. Detailed studies on the distribution pattern of fish fry provide clear evidence that open backwaters do not function as spawning places or nurseries for the rheophilic species as has often been suggested in the literature. These species are bound to the inshore zone of the river itself.

6 402 F. SCHIEMER AND T. SPINDLER I 50 "A n= I n=3237 L ?"W. il_ -rlel. _ ~ m Figure 4. Percentage composition of the fish fry population in five habitat types (from top to bottom: (a) disconnected backwaters, (b) connected backwaters, (c) small bays in the inshore zone of the river, (d) gravel banks, (e) artificial shoreline made of angular stones. The number of individuals analysed (n) refer to materal collected in 1985 (disconnected backwaters) and 1986 (all other habitats) black columns: rheophilic species 1 = B. barbus 2 = L. leucbcus 3 = C. gobio 4 = C. nasus 5 = R. pigus Virgo 6 = V. vimba 7 = L. cephalus 8 = L. idus 9 = G. schraetser 10 = A. aspius 11 = G. uranoscopus 12 = G. albipinnatur white columns: eurytopic species 13 = A. alburnus 14 = A. brama 15 = B. bjoerkna 16 = R. rutilus 17 = G. cernuus 18 = S. lucioperca 19 = E. lucius 20 = P. jluviatilis 21 = P. marmoratus 22 = C. carpio shaded columns: lirnnophilic species 23 = S. erythrophthalmus 24 = R. sericeus amarus 25 = T. tinca 26 = C. carassius

7 ENDANGERED FISH SPECIES OF THE DANUBE 403 From June to August 1985 and from May to October 1986 a large survey of the fish fry populations in various habitat types of the Danube downstream of Vienna was carried out, using lift nets and beach seines (mesh size 1 mm). The material (n = 9808 fish) was pooled to habitat types. Figure 4 compares the faunal composition at the different locations: connected and disconnected backwaters, naturally structured shoreline of the river (gravel banks and small bays), and artificial shoreline made out of angular stones (ripraps). Two important results emerge from this comparison: 1. The fry of rheophilic species are absent in both backwater types, being found exclusively in the naturally structured littoral zone of the river itself, and 2. The lowest population density of the O+ age class and lowest species diversity are found along the linear, articificial shorelines. In disconnected backwaters with dense aquatic vegetation the fish fry is composed of eurytopic and limnophilic species. Open backwaters are dominated by eurytopic forms, whereas riverine species are virtually absent. A high diversity and a large number of endangered species are found among the O+ population in small bays and in shallow sloped gravel banks of the river shore. A detailed study on the ecological requirements of the riverine O+ fauna gave clear evidence that both habitat types are essential for the development of a diverse fish fauna, because of ontogenetic niche shifts of the riverine fry within the first few months (Spindler, 1988). Significant changes in habitat preference indicate shifts in the requirements related to water currents, substrate, and food. During the first few weeks after hatching, all species are bound to sheltered habitats (i.e. small littoral bays) where water velocities are generally low, despite changing water levels of the Danube. With increasing age and body size, rheophilic species migrate to the adjacent shallow gravel banks. Thus in late Percent June n= 2937 I 1 n.832 p-7 -- ", - I_ mm c7 July August September Bay Gravelbank 35-40mm Figure 5. Habitat separation between eurytopic (white) and rheophilic (black) fish during the first months of life. The columns indicate the portion of the total population of fry found in adjacent bays and e ve1 banks. The average size of fish fry in June and September is indicated

8 404 F. SCHIEMER AND T. SPINDLER summer the populations of riverine fish exhibit a distinct spatial separation from the eurytopic ones (like bleak and roach), which remain in sheltered bay habitats (Figure 5). However, only shallow-sloped gravel banks, which provide the necessary gradient of low currents, are populated by riverine fry. Figure 6 provides current isopleths during three sampling periods and shows the respective numbers of species caught at two gravel banks of differing slopes. Ontogenetic niche shifts within the first few months of life can also be observed in the trophic requirements. Figure 7 illustrates the specific dietary changes with size for 4 species in the inshore zone of the river. B. barbus is benthivorous from the onset of feeding. In C. nasus the surface oriented feeding ( Anflug, drifting particles) is replaced by a substrate oriented mode. A contrasting pattern is found in A. alburnus, where the diet consists of zoobenthos in the early phase, changing to floating, suspended particles and further to a predominance of surface drift. In R. rutilus the initial diet consists mainly of meiobenthos, but expands to encompass a very broad spectrum within the next few months. From these results it is apparent that a rich structure of the littoral zone-with a close proximity of different habitat types-is essential. Comparing the habitat requirements of the riverine fish fry with the existing inshore structure of the Danube downstream of Vienna (see Wosendorfer and Leberl, 1987 ) it is obvious that only few, restricted stretches at present provide high quality conditions (Figure 8). SUMMARY AND CONCLUSIONS Our results show that the fish fauna is an excellent indicator of the ecological quality (e.g. habitat diversity, hydrological integrity) of large river systems, because of the various and ontogenetically rn m crn 10 lsec 40 SPP m crn rn m I I I I J Figure 6. Isopleths of water current velocity at two gravel banks with different slopes and the number of species of fish fry found at three sampling occasions

9 ENDANGERED FISH SPECIES OF THE DANUBE 405 rnm Lt Yo 100 n= 74 n=73 n=44 n=18 C.nasus n= n= 24 A. alburnus I Yo 100- ao n=26 n=25 n-27 n=16 R.rutilus Figure 7. Changes in food composition (in % volume) with body size (Lt = total length) in the fry of four species occurring in the river. Food items: 1 = detritus, 2 = meiobenthos, 3 = macrobenthos, 4 = zooplankton small, 5 = zooplankton large, 6 = airborne material variable requirements of well-adapted species. One has however to be aware that not isolated faunistical surveys but detailed studies of the population structure are essential. An analysis of the fry populations and its distribution pattern is particularly appropriate for river quality assessments and can provide guidelines for conservation management as well as for waterworks engineering. Faunal diversity and the large number of endangered species reflect the high quality of the few undammed stretches of the Danube in Austria. Our results clearly indicate that, in order to maintain and improve this situation, two requirements are essential: 1. A rich structure of the inshore zone of the river, which provides the necessary ecological gradients at fluctuating water levels to meet the demands of fish fry (with regard to substrate, food, and current) in the course of their ontogenetic development. 2. Open connections between the river and its backwaters to allow migration of species and the outdrift of organic material from the highly productive stagnant zones. The present situation can be improved by reopening back waters to the river and by a partial restructuring of the linear, articificial shoreline.

10 406 F. SCHIEMER AND T. SPINDLER 5 krn - Figure 8. (a) Danube river system between Vienna and the Czechoslovakian border; (b) shore lines of the river providing high ecological quality for the development of fish fry (black bars) and open connections (at mean water level) between the river and backwaters (arrows) It appears that those requirements of conservation are mutually exclusive to a further construction of hydroelectric power dams. Our results allow the prediction that further construction of hydroelectric power dams on the Danube will threaten a high percentage of the present Austrian fish fauna. A large number of characteristic Danubian fish is on the verge of extinction. ACKNOWLEDGEMENTS This research was supported by the Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich, project No. S-35/01. Further support was provided by the municipality of Vienna. We thank S. Dudzinski, A. Schneider, H. Wintersberger, J. Wanzenbock, R. Konecny, and H. Keckeis for their help in the field. The graphs were made by A. Kleych. REFERENCES Balon. E. K Verzeichnis, Arten und quantitative Zusammensetzung des tschechoslowakischen Donauabschnittes, Zool. An;., 172, Balon, E. K Urgeschichte der Donau-Ichthyofauna (vor dem EinfluD seitens des Menschen), Arch. Hydrobiol. (Suppl. Donauforschunn 3), 34, Balon, E: K., Crawford, S. S., and Lelek, A Fish communities of the upper Danube River (Germany, Austria) prior to the new Rhein-Main-Donau connection, Env. Biol. Fish., 15, Holden, P. B Ecology of riverine fishes in regulated stream systems with emphasis on the Colorado River, in Ward, J. V. and Standford, J. A. (Eds), The Ecology of Regulated Streams, Jungwirth, M Die fischereilichen Verhaltnisse in Laufstauen alpiner Fliisse, aufgezeigt am Beispiel der osterreichischen Donau, Osterr. Wasserwirtschuft, 36, Lelek, A Threatened freshwater fishes of Europe, Council of Europe Nature and Environment Series, 18, 269 pp.

11 ENDANGERED FISH SPECIES OF THE DANUBE 407 Schiemer, F Die Bedeutung von Augewassern als Schutzzonen fur die Fischfauna, Osterr. Wusserwirtschufr, 37, Schiemer, F Gefahrdete Cypriniden-Indikatoren fur die okologische Intaktheit von Flukystemen, Nutur und Lundschuft, 63, 9, Schiemer, F. and Dudzinski, S. in preparation. Seasonality of occurrence and food selection of common cyprinids in a backwater system connected,to the Danube. Spindler, T Okologie der Brurfsche in der Donuu,,bei Wien, Ph. D. dissertation, University of Vienna, 129 pp.,. Wanzenbock, J., Kovacek, H., and Herzig, B Uber das Vorkommen der Griindlinge (Gattung Gobio) in Osterreich, Osterreichs Fischerei, 42, Welcornme, R. L River fisheries, FA0 Fish. Tech. pup., 262. Wosendorfer, H. and Leberl, S Uferzonen der Donau von Wien bis zur Marchmiindung, Munuskript der Wusserstrujjendirektion, Abt 24-Biotopschutz u. Landschaftspflege, Wien.