The present study reviews the parasite fauna of fishes in the Aral Sea, before, during and after the recent regression and salinization crisis. The native fish fauna was much depleted compared to the nearby Caspian Sea, comprising only 20 spp., all having a freshwater origin. The parasite fauna was similarly poorer, both in total species number and when considering parasites infesting individual species. Some fish species and parasites were introduced during the 20th century. Species introduction of fish and progressive salinization in the latter half of the 20th century interacted in various ways, occasionally favouring the spread of parasites. During the salinization parasites with endoparasitic lifecycles endured longer than many ectoparasites, the latter being constantly exposed to the salty water. But all parasites eventually suffered when the salinity tolerance of their free-swimming larvae was exceeded. Predation on zooplankton by introduced fish also temporarily impacted the free larvae of crustacean parasites, causing a decline. Alternatively, introduced fish that were prey to larger species could act to transmit parasites. All the parasites in the southern Large Aral Sea ultimately disappeared, either because their hosts were gone or because of an inability to endure high salinity. Many of the original fish species have now naturally repopulated the reconstituted Small Aral Sea, forming the basis of a renewed commercial fishery. As a result, some parts of the original parasite fauna have also reappeared together with some newly introduced species. The present study highlights the complex, sometimes unexpected, manner in which hosts and parasites can interact during a progressive ecological crisis. We emphasize that they must be an integral part of any sustainable ecological management of lakes and reservoirs. In the framework of revived fisheries and emerging aquaculture, we suggest a system for sound monitoring and control of fish parasites in the Aral Sea system.

The purpose of this review article is to describe changes in the ichthyofauna of the Aral Sea in the 20th and 21st centuries and comparison of the structure of fish catches in the Aral Sea of the 20th century with those in the Small Aral of the 21st century, as well as a generalization of data on the biology of fish species anew introduced into the Small Aral. In the first half of the 20th century, 20 species lived in the sea, of which cyprinids predominated. After a series of acclimatization measures were carried out in order to enrich the ichthyofauna, 17 new species of fish appeared in the Aral Sea, but the composition of commercial species changed a little. Until 1960, the Aral Sea was in a quasi-stable state. The regression of the Aral Sea began in 1961 and led to the extinction of most species in the ichthyofauna. At the time of the division of the Aral Sea into two reservoirs, only seven species of fish remained in both of them, of which only one species was native. The Small Aral, after dividing the former reservoir into two parts, stabilized its borders. The stabilization of the level of the Small Aral and its freshening led to the return of freshwater fish to the sea. This process had a favorable effect on the size of catches. Currently, the catch volume is about 8000 tons. The future of the ichthyofauna and fish catches depends on what project for the reconstruction of the Small Aral will be chosen. If the option of a singlelevel reservoir will be chosen, the prospects for the fishing industry in the Aral Sea will be more favorable than if the option of a two-level reservoir will be chosen.

We review the past, present and possible future of the Aral Sea system in context of the human caused regression crisis that resulted in the drying out of the larger part of this original brackish water sea. The results are put into the context of other threatened saline lakes and the general water crisis in the world due to overexploitation of water resources and climate change. We cover the geographic history and hydrology from the origin of the sea 17,000 years ago to the present. The original biota including animals, higher plants and algae are covered in full detail, and tracked through the regression crisis. We put special emphasis on fish and fisheries because of their economic importance for the surrounding populations. We also review the side effects of the regression in terms of human health and changes to the terrestrial environment and local climate. We explain the dramatic improvements to the fauna in the northern Small Aral Sea following the construction of dams to retain its waters and discuss future options to further improve this restored water basin. We contrast this with the progressing hypersalinization of the remnants of the southern Large Aral Sea, which faces conditions that will eventually render a "Dead Sea" condition hostile to all metazoan life. We end by highlighting the partial restoration of the Small Aral Sea as an example of how much restoration can be achieved for relatively little financial expense and in a short period, when good ideas, kind hearts and hard work operate together for the benefit of the environment and our human society.

Based on published data and studies carried out by the authors over more than 30 years, an analysis of the mollusk fauna of the Aral Sea is made. The species composition of mollusks that lived in the sea during the Holocene is considered and clarified. The main reason for changes in the species composition of this group is shown to have been a decrease or increase in the salinity of the environment associated with changes in the sea level. Long-term changes in the salinity of water, in our opinion, could have become the main reason for the scarcity of the mollusk fauna of the Aral Sea. We assume that the transfer of juvenile mollusks by birds might have played an important role in the replenishment of the fauna with new species. This is probably true for mollusks of the family Cardiidae and Ecrobia grimmi. Another source of replenishment of the fauna of the Aral Sea must have been due to species inhabiting the rivers flowing into the Aral Sea and the surrounding lakes. The process of faunal changes during the last regression of the sea is discussed in detail. The availability of data on the salinity at which the mollusks became extinct makes possible a retrospective evaluation of the estimates of limits of salinity tolerance for the existence of the same species obtained by different methods. The future of the fauna of the mollusks of the Aral Sea is considered in the implementation of different scenarios of sea rehabilitation.

This work is an attempt to trace changes in the fish fauna, their food supply and fish catches in the Small Aral Sea from the beginning of the 20th century to the late 1980s. The purpose of our work is a comparative study of changes in the fishery value of the water reservoir at different stages of its development, including during the last anthropogenic regression. Both literature data and those obtained by the authors in the period 1991–2015 were used. Not only data related to the fauna and fish catches proper, but also data on their food supply, represented mainly by invertebrates, were taken into account. The native ichthyofauna consisted of 20 species of fish, mainly related to benthophages. The biomass of benthos was small — 20 g/m2 due to the significant pressure of fish. The construction of the Tashkent railway was the reason for the emergence of the village of Aralsk in 1905, and the beginning of industrial fishing at sea, as it ensured the export of fish products. The catch of fish in the pre-revolutionary period reached a maximum of 48,300 tons. Planned acclimatization of fish and benthic organisms began to increase the fishery value of the reservoir. Some of them were unsuccessful. The introduction of plankton-eating fish affected not only the zooplankton of the sea, but also its benthic fauna, the abundance and biomass of which decreased. The increase in salinity because of the regression of the water reservoir led to a sharp depletion of the fauna. In the absence of pressure from fish, the biomass of benthic organisms increased by more than 10 times.

The origins of the study of the Caspian Sea date back to the 18th century, when the St. Petersburg Academy of Sciences was founded. The first explorers of the Caspian were academicians P.S. Pallas and S.G. Gmelin. In the 19th century, the study of Caspian fish was continued by K.E. von Baer and O.A. Grimm. Karl von Baer from 1853 to 1857 made four scientific trips to the shores of the Caspian Sea. He pioneered the fundamentals of sustainable fisheries. The main result of the expedition of K.E. von Baer — N.Ya. Danilevsky was the Charter of the Caspian fish and seal fisheries, approved by the government in 1865. According to the results of O.A. Grimm expedition 1874–1876 many new species of worms and crustaceans were discovered and it was found that from 278 species of fish, 150 are found nowhere else. The study of the biodiversity of fish and their parasite fauna in the first half of the last century was continued by scientists of the Zoological Institute N.M. Knipovich, A.N. Svetovidov, A.L. Behning, V.A. Dogel and B.E. Bykhovsky. In 2004, sciemtists of ZIN RAS published the “Catalogue of Agnathans and Fishes of Fresh and Brackish Waters of Russia with comments on nomenclature and taxonomy”, which includes valid names of taxa of agnathans and fish ranging from type to subspecies inhabiting fresh and brackish waters (up to 13 g/l) of the Azov and Caspian Seas and the freshened estuaries of the rivers of the northern and Far Eastern seas (18 orders, 43 families, 175 genera and 486 species). At present, scientists of ZIN RAS continue to study the fish resources of the Caspian Sea.

The regression and salinization of the Aral Sea, largely caused by water diversion for irrigation, is among the most severe ecological disasters of the 20th century, and has had severe health and economic consequences for the local population. Introductions of alien species to enhance commercial fisheries before the regression had already impacted the ecology of this system. Crustaceans made up about one-quarter of the original metazoan species and constituted the principal food for native and introduced fish. From 1960 on, crustaceans were recorded at numerous fixed sampling stations, including thanatocoenoses (dead animals from sediment cores). We use this previously unpublished information to document changes in species abundance and discuss their causes in the context of species interactions and changes to physical and chemical parameters. Competition from alien crustaceans led to declines in or even extinction of some native species, but eventually severe salinization became the main detriment, and resulted in the complete collapse of commercial fisheries. This seriously hurt a critical trade, which provided the principal protein source for the local population. We document how comparatively modest conservation efforts enabled the northern Small Aral Sea to partially recover and commercial fishing to resume.

Based on the literature and studies carried out by the authors over more than 30 years, an analysis of the fauna of crustaceans of the Aral Sea is presented. The faunal composition, the changes that have taken place since the 1950s, and the relations to the salinity factor are considered. Crustaceans in the Aral Sea make up about one-quarter of the species diversity of Metazoa. They are represented by freshwater, brackish water, and marine species, as well as halophiles from saline continental waters of the arid zone. The first changes in the composition of the crustacean fauna of the Aral Sea started even before its modern regression. They were the result of deliberate, as well as accidental, introductions of a number of initially absent species of invertebrates and fish. Since the 1960s, the main cause has become the rapid change in salinity. As the salinity has increased, the species diversity of the crustaceans has sharply decreased due to extinction, first of freshwater and then of brackish water species. After the level of the Aral Sea fell and it was divided into two parts, the Small Aral and the Large Aral, further evolution of these residual water bodies proceeded in different directions. As a result of the construction of the dam in the former Berg Strait, it became possible to decrease the salinity of the Small Aral Sea and gradually restore the crustacean fauna. The Large Aral has now turned into a hyperhaline water body inhabited by only a few species of crustaceans.

In 2018, in the bottom fauna of the Caspian Sea, single specimens of a previously unknown species of polychaetes were discovered. Since 2019, pelagic larvae of this species have been recorded in zooplankton samples. These worms are also found in the nutrition of migratory and semimigratory fish species. According to morphological features, this polychaete species is identified as Marenzelleria arctia , an Arctic species that dominates in the Gulf of Finland and probably invaded the Caspian along the Volga-Caspian invasion corridor.

Lake Balkhash is a large endorheic water body, the third largest by size in Eurasia and the second largest salt lake of the world. With its half-moon elongated morphology and 78% of inflows provided by the Ili River from the West, the lake has a freshwater basin in the West and saline water basin in the East, separated by the 4-km narrow and 6-m deep Uzunaral Strait. The average bathymetry is shallow, with a maximum water depth of 11 m in the West and 26 m in the East. According to investigations of the geological history of the lake in Soviet times and in international projects during the last 15 years, a large lake was formed by the Ili River in the Balkhash region encompassing the present area of the Kapchagai Reservoir during the Middle Pleistocene. The large lake basin was subsequently transformed by a series of tectonic deformations. Around 300 kiloyears before present (ka BP) the Ili River was diverted to the North where it formed a large megalake, the Ancient Balkhash, in the Balkhash-Alakol Depression. Around 110 ka BP, the lake became divided into two basins forming the Alakol Lake in the East and the modern Lake Balkhash in the West. Hydrological conditions mostly controlled by precipitation, evaporation and meltwater discharge caused three different lake-level stages in the Late Pleistocene and Holocene: lake levels between 349 and 355 m above sea level (asl) prevailed during glacial periods, between 341 and 348 m asl during the pluvial early and middle Holocene, and between 335 and 348 m asl during the arid late Holocene when extreme regressions at ca. 5.0, 1.2 and 0.8 ka BP divided the lake into more than one basin. The present lake water balance results from a major regression due to a recent phase of aridization and the filling of the Kapchagai Reservoir in the 1970s and 1980s and the compensation of lower precipitation by increased meltwater discharge from glaciers. However, meltwater runoff will diminish with rapidly shrinking glaciers in the next 50 years. This alarming perspective requires careful water-basin management which was not yet implemented. Lake Balkhash is exposed to the threat of exaggerated anthropogenic water subtraction due to an accelerated infrastructural and demographic boost that doubled the irrigated farmlands in the Chinese part of the catchment in less than 20 years. Due to the lake’s hydrology, catchment rock and hydrochemical conditions, the water of the Eastern Balkhash has high concentrations of potassium and magnesium, unfavorable for hydrobionts. Any further increase in salinity will soon cause a considerable diminution of the lake’s biomass. The ichthyofauna of the lake has been intensively manipulated during the twentieth century, with the introduction of new species and the decline of the original ones. The substitution of the native fish fauna by introduced species caused a decrease of valuable commercial fish in the lake and a decrease of the total fish catch. Thus, Lake Balkhash faces serious environmental risks today and its near future depends on the collective will and decisions of the responsible agencies.

The Caspian Sea (CS), located between Europe and Asia, is the largest lake in the world; however, its physical environment and its floor have oceanic characteristics. The CS is composed of a very shallow north sub-basin with a very low salinity mostly below 5 psu. The middle and southern sub-basins are deep and have a salinity of c. 13 psu. To the east, the Kara-Bogaz-Gol, a hypersaline lagoon, is connected to the middle sub-basin. The CS is endorheic and therefore very sensitive to changes in hydrography and climate. Because of its long history of isolation following the disconnection of the Caspian Sea from the Paratethys c. 6 million years ago, this ancient lake has many endemic species. The harsh environment of its brackish waters and the repeated salinity changes over the millennia, however, do not allow for a high biodiversity. The benthos is more varied than the plankton. The history of water-level changes remains poorly known even for the last centuries. Nevertheless, the amplitude was of >150 m in the Quaternary, several tens of meters in the Holocene and several meters in the last century. Many factors affect its natural state, such as petroleum pollution (an industry dating back to Antiquity), nutrient increase S. A. G. Leroy (B) Aix Marseille Univ, CNRS, Minist Culture, LAMPEA, UMR 7269, 5 rue du Château de l’Horloge, 13094 Aix-en-Provence, France e-mail: suzleroy@hotmail.com; leroy@mmsh.univ-aix.fr H. A. K. Lahijani Iranian National Institute for Oceanography and Atmospheric Science, #3, Etamadzadeh St., Fatemi Ave, 1411813389 Tehran, Iran e-mail: lahijani@inio.ac.ir J.-F. Crétaux Laboratoire d’Etudes en Géophysique et Océanographie Spatiale, UMR5566, Université de Toulouse, LEGOS/CNES, 14 Ave. Edouard Belin, 31400 Toulouse, France e-mail: jean-francois.cretaux@legos.obs-mip.fr N. V. Aladin · I. S. Plotnikov Laboratory of Brackish Water Hydrobiology, Zoological Institute, Russian Academy of Sciences, Universitetskaya nab. 1, 199034 St. Petersburg, Russia e-mail: Nikolai.Aladin@zin.ru I. S. Plotnikov e-mail: Igor.Plotnikov@zin.ru © Springer Nature Switzerland AG 2020 S. Mischke (ed.), Large Asian Lakes in a Changing World, Springer Water, https://doi.org/10.1007/978-3-030-42254-7_3 65 66 S. A. G. Leroy et al. (alongside >14 million inhabitants along the coast), invasive species (e.g. the comb jelly Mnemiopsis leidyi), overfishing (including sturgeon) and modifications of its coastline (e.g. sand extraction). In comparison to other ancient lakes, the CS surface temperature has suffered from the fastest increase on record. Owing to the complex natural state of the CS, it is not easy to identify the Holocene-Anthropocene transition, although it may be suggested that is was approximately AD1950 when intense human activity started to modify the lake.

The Caspian Sea (CS), located between Europe and Asia, is the largest lake in the world; however, its physical environment and its floor have oceanic characteristics. The CS is composed of a very shallow north sub-basin with a very low salinity mostly below 5 psu. The middle and southern sub-basins are deep and have a salinity of c. 13 psu. To the east, the Kara-Bogaz-Gol, a hypersaline lagoon, is connected to the middle sub-basin. The CS is endorheic and therefore very sensitive to changes in hydrography and climate. Because of its long history of isolation following the disconnection of the Caspian Sea from the Paratethys c. 6 million years ago, this ancient lake has many endemic species. The harsh environment of its brackish waters and the repeated salinity changes over the millennia, however, do not allow for a high biodiversity. The benthos is more varied than the plankton. The history of water-level changes remains poorly known even for the last centuries. Nevertheless, the amplitude was of >150 m in the Quaternary, several tens of meters in the Holocene and several meters in the last century. Many factors affect its natural state, such as petroleum pollution (an industry dating back to Antiquity), nutrient increase (alongside >14 million inhabitants along the coast), invasive species (e.g. the comb jelly Mnemiopsis leidyi), overfishing (including sturgeon) and modifications of its coastline (e.g. sand extraction). In comparison to other ancient lakes, the CS surface temperature has suffered from the fastest increase on record. Owing to the complex natural state of the CS, it is not easy to identify the Holocene-Anthropocene transition, although it may be suggested that is was approximately AD1950 when intense human activity started to modify the lake.

The history of the evolution of the ecosystem of the Aral Sea and the South Aral Sea is divided intotwo periods: the Late Holocene and the Anthropocene. The morphological indicators of the Aral Searemained relatively stable up to the early anthropocene, however, started degradation of its ecosystemin the 1960s. had very fast and irreversible character. Above all the increase in salinity of the Aral Seasince the 1960s had a catastrophic effect on its biodiversity, since in the early 1980s 70% of its watersurface and 50% of its water volume were still preserved. Consequently, the sea still possessed sufficientecological carrying capacity for the existence of aquatic life populations. At the same time, the mainproblem of preserving the biodiversity of aquatic ecosystems and developing fisheries in the SouthernAral Sea region in the Anthropocene is an extremely unstable and scarce water regime, which is aggravated by natural droughts, climate change, and deterioration in water quality. Especially the salinizationof terminal lakes and reservoirs leads to a decrease in biodiversity and biological productivity. The possible future of the Aral Sea depends entirely on possible human intervention, however, the future of theLarge Sea looks problematic, which can turn into a hypersaline reservoir. To improve the sustainabilityof hydroecosystems, it is first of all necessary: improving water management taking into account envi ronmental requirements for the conservation of biodiversity and developing fisheries, accelerated implementation of water- saving irrigation technologies, introduction of intensive aquaculture and culturebased fisheries, diversification of aquaculture by introducing salt tolerant species, strengthening researchactivity in research institutes and international cooperation.Key words: Aral Sea, Priaralye, anthropocene, salinization, fish economy

sciencemag.org SCIENCE P H O T O : © JO S É A N T O N IO M A R T ÍN E Z mostly affect declining species of steppe birds, which are poorly represented in the Spanish Natura 2000 network (3). Globally threatened large scavengers and other unique and scarce soaring birds are already paying a heavy toll due to the approximately 20,000 existing turbines, with demographic consequences for some threatened populations (4, 5). Bat-killing figures are even higher, with a minimum of 200,000 deaths per year according to estimated mortality rates (6). Studies forecasting mortality have shown scarce predictive power (7), and when mortality hotspots are detected, conflictive turbines are virtually never stopped to reduce bat and bird casualties as recommended (8). As a large-scale approach, the best way to reduce impacts is by choosing adequate locations (9). However, updated field information necessary to achieve this goal is often not available, and projects are authorized in areas with under-protected species because their status within regional and state listings is frequently outdated. Studies designed to predict and monitor the incidence of renewable infrastructures are funded by energy companies (10), often with little supervision by governments, which precludes independence. The problem is exacerbated by the fragmentation of large projects (yielding reports of the smaller impacts associated LETTERS Birds such as this red kite (Milvus milvus) are put at risk by the proliferation of Spanish wind farms.

The Aral Sea was a huge brackish-water lake lying in a tectonic depression amidst the deserts of Central Asia. Water bodies of various dimensions have repeatedly filled this depression over the past several million years. Its modern incarnation is thought to be somewhat more than 20,000 years in age. In modern times, the sea supported a major fishery and functioned as a key regional transportation route. But since the 1960s, the Aral has undergone rapid desiccation and salinization, overwhelmingly the result of unsustainable expansion of irrigation that largely dried up its two tributary rivers, the Amu Dar’ya and Syr Dar’ya (dar’ya in the Turkic languages of Central Asia means river) before they reached the Aral Sea. The desiccation of the Aral Sea has had severe negative impacts, including, among others, the demise of commercial fishing, devastation of the floral and faunal biodiversity of the native ecosystems of the Syr and Amu Deltas, and increased frequency and strength of salt/dust storms. However, efforts have been and are being made to partially restore the sea’s hydrology along with its biodiversity, and economic value. The northern part of the Aral has been separated from the southern part by a dike and dam, leading to a level rise and lower salinity. This has allowed native fishes to return from the rivers and revitalized the fishing industry. Partial preservation of the Western Basin of the southern Aral Sea may be possible, but these plans need much further environmental and economic analysis.