Qualitative and quantitative changes in the ichtyofauna of the danubian reservoirs after the construction of the iron gates hydropower plant dams

Abstract

Human society reaps immense benefits from rivers and in the course of the last century man has considerably altered rivers, using them as waterways, for irrigation, energy production, water intakes, commercial and recreational fisheries, etc. As a consequence of anthropogenic effects, degradation of river courses has occurred and there is great need for preserving and restoring their physical, chemical and biological integrity. The Danube is an international river that connects 17 countries and has been under anthropogenic influence since ancient times. Intensive exploitation of fish in the early Holocene led to human settlements remaining for longer periods along the Danube. Numerous Neolithic archaeological sites demonstrate that fishing was the main economic activity of humans on the Danube. Alterations to the river's course in order to secure navigation through the Djerdap gorge were conducted already in the late 19th century, while significant changes in this part of the Danube occurred after the construction of the Iron Gate I and Iron Gate II dams, in 1970 and 1984, at Danube rkm 943 or rkm 863, respectively. This led to interruption of the Danube's longitudinal continuity and to the curtailment of migration routes of anadromous fish species that migrate from the Black Sea up the Danube for spawning. In the past, the beluga sturgeon (Huso huso) migrated all the way to Straubingin Germany (rkm 2327), and the Black Sea shad (Alosa immaculata) went as far as Budapest (rkm 1650). Overfishing and illegal fishing of sturgeon species for their highly prized caviar, water and sediment pollution, and physical alteration to the bottom and banks of the Danube, as well as the restriction of migration due to dam construction led to a significant decrease in population sizes of sturgeons and shads, which now have only 17.4 kilometers for spawning in the Serbian part of the Danube. At the same time, there was the introduction of non-native (allochthonous) fish species for aquaculture, which later became permanent members of the Danube ichthyofauna (silver carp - Hypophthalmichthys molitrix, bighead carp - Hypophthalmichthys nobilis, grass carp - Ctenopharyngodon idella). Construction of the dam has promoted the expansion of certain non-native fish species. Pontic gobies have taken full advantage of all the opportunities created by the formation of reservoirs and the emergence of new biotopes, as well as the changes in the structure of ichthyocenosis that occurred within those new ecosystems. The spread of non-native fish species can have a negative impact on native fish species. All this has led to significant changes in the qualitative and quantitative composition of fish in the newly created reservoirs on the Danube, as well as changes in the ichthyofauna downstream from the Iron Gate II dam. Changes in the ichthyofauna can be traced through the commercial catches of fish before and after dam construction, which indicate a decline of sturgeon species in the catch - of stellate sturgeon (Acipenser stellatus) after the construction of the Iron Gate I dam, and of Russian sturgeon (Acipenser gueldenstaedtii) after construction of the Iron Gate II dam. Only the beluga was present in the fishermen's catch until 2006, when a moratorium was placed on fishing of sturgeon species in the Lower Danube. In addition to having a negative effect on anadromous species, dams also have a negative effect on potamodromous species such as the sterlet (Acipenser ruthenus) and catadromous species such as the eel (Anguilla anguilla). Despite being a potamodromous species that migrates only within rivers, sterlet also showed a decrease in population size after the construction of the dams, with catches comprising younger age classes compared to the period before dam construction. After the construction of the Iron Gate I dam, mass migrations were observed of adult sterlet towards upstream regions with higher flow speeds and where sedimentation processes are less pronounced than in the reservoir. Sterlets belonging to higher age classes are rarely captured in reservoirs, whereas, before the dams were constructed, sterlets weighing 1-3 and even 4 kg were common in the daily catch. Eel harvesting records were kept up until 1968. Following that period, eels are only occasionally present in the catch of commercial fishermen, but there is no continuous monitoring of the species. The reduced flow rate and substantial oscillations in water levels are significant negative influences that have caused changes in the ichthyofauna after dam construction. Migratory fish species such as the barbel (Barbus barbus), that prefers a faster flow, migrated to the upstream part of the watercourse, whereas species such as the bream (Abramis brama) exhibited intensive expansion and growth in the newly formed reservoirs. The Action plan for sturgeon species management in fishery waters of the Republic of Serbia provides measures to be taken for the conservation and restoration of sturgeon species in Serbia. However, these measures have not been initiated so far, the development of sturgeon species aquaculture is only in its initial stages, there is no regular monitoring of sturgeon populations, the illegal fishing of sturgeon species continues in the Lower Danube, as well as fishing sterlet below permitted size (40 cm standard body length). Research aimed at the construction of fish passes at the Iron Gates I and II dams has only just begun, and a feasibility study is expected to start. The feasibility study requires the collaboration of scientific institutions, the Iron Gates I and II hydropower plants and the involvement of all stakeholders interested in solving this problem. Further development of sturgeon species aquaculture is necessary for reducing the pressure on natural populations and enabling their recovery. Joint collaboration of the Lower Danube countries is also needed for the protection of sturgeon species. Shads in Serbia are legally protected by a 1993 act, but there are no detailed data on the status of the populations of Black Sea shad (Alosa immaculata), and especially the status of Danube shad populations (Alosa tanaica). The Black Sea shad is a commercial species in Romania for which there is a prescribed fishing ban. The ban follows Black Sea shad spawning migration from the Danube Delta to the Iron Gate II dam, and alternates along the following sections: from the Black Sea to Danube rkm 43; from rkm 43 to rkm 238; and from rkm 238 to rkm 845.6 of the Danube. In Bulgaria, the fishing ban on Black Sea shad comes into force on April 15 and lasts until May 15. In this context, there is a complete discordance among Black Sea shad fishing regulations in Romania, Bulgaria and Serbia. This has resulted in the existence of sectors of the Danube, where the Danube forms the border between Romania and Serbia, with shad being harvested on the Romanian side but shad fishing being strictly prohibited on the Serbian half of the Danube. During the 1958-1989 period, the monitoring and regulation of commercial fishing, especially of sturgeon species and shads, was covered by a convention ("Convention Concerning Fishing in the Waters of the Danube") signed by Romania, Bulgaria, Yugoslavia and the Soviet Union. However, following political changes during the 1990s, this convention is no longer in effect. Wholesale purchase of fish from commercial fishermen was organized from the 1950s to the 1990s, when it was possible to conduct valid analyses in the assessment of fish resources. After the changes that occurred in the 1990s and the end of fish purchase from the users of fishery areas, problems arose in Serbia regarding the validity of fish harvesting data. Commercial fishermen are obliged to submit a catch list, but the credibility of this information is questionable. All this prevents any in-depth analyses regarding catch and economic gains in commercial fishing. In addition to untrustworthy statistics on commercial fish catches, there is the problem of illegal fishing which is hard to estimate, both regarding size of catch and the fish species that are harvested illegally. The decrease in the population size of sturgeon species and shads is followed by an increase in the number of non-native species in the catch of commercial fishermen. Since 1979, non-native species -Prussian carp, silver carp, bighead carp and grass carp - have become significantly more present in the catches of commercial fishermen, only to become 4 out of the 6 best-selling freshwater fish species on the Serbian market in the 2010-2011 period. Construction of the dams has had an impact on the population sizes of economically less valued fish species, as well as non-native species. Some of the introduced fish species have established (maintained) populations throughout the Serbian sector of the Danube, while others are only in limited areas of the Iron Gates I and II reservoirs (e.g. the black-striped pipefish - Syngnathus abaster) and in the potamon of fish communities in the coastal areas of the Danube and its stagnant waters (e.g. the Chinese sleeper - Perccottus glenii). The expansion of Pontic gobies is distinctive, with the monkey goby (Neogobius fluviatilis) completing its expansion before the construction of the large dams on the Danube and the formation of reservoirs, while other Pontic gobies inhabited the newly formed biotopes in the new reservoirs. It is safe to say that most of the benthic biotopes of the Iron Gates reservoirs are practically overcome with populations of different goby species. In addition to the negative effects of non-native species, there are some positive effects, as is the case with gobies, that represent a significant share in the diet of commercial fish species such as the wels catfish (Silurus glanis). It can be concluded that the construction of the dams of the Iron Gates hydropower plants caused qualitative and quantitative changes in the ichthyofauna of this sector of the Danube, which are most visible in the decreased number of anadromous migrants, sturgeons and shads, and the increased number of non-native species of fish that prefer the slower flow of water in reservoirs. The key problems related to sturgeons are the obstruction of spawning migration at Danube rkm 863 and the decrease in spawning areas of sturgeon species due to siltation of reservoirs. In that sense, it is necessary to start planning a feasibility study for the construction of fish passes at the Iron Gates dams which would allow the migration of these species upstream of the Iron Gates I dam for the another 879 km of the Danube, to the Gabcikovo dam (at Danube rkm 1833). Detailed analyses regarding the habitats of sturgeon species are also needed, to enable their protection. The lack of credible catch statistics from the reservoirs nowadays, as well as the absence of continuous monitoring, prevents any estimation of fish resources and their economic value.