Middle Caspian Research Articles

Estimation of bacterio‐, picophyto‐ and nanophytoplankton abundance in the Middle Caspian Sea by flow cytometry

Aim. For the first time, flow cytometry has been used to study the structure and functional characteristics of bacterio‐, pico‐ and nano‐ phytoplankton in the Middle Caspian during the bloom period in early September 2022.Water samples were taken at different horizons (from the surface to the bottom layer) at a series of stations along the western coast of the Caspian Sea on the border between its northern and central parts to quantitatively assess heterotrophic bacterioplankton, picophyto‐ and nanophy‐ toplankton. Sampling was undertaken in early September 2022 under summer hydrological conditions with water surface temperatures ranging from 24.7°C to 26.7°C. Sample analysis was performed using flow cytometry.Extremely high concentrations of picophytoplankton (up to 1.8×105 cells/mL), probably of Synechococcus picocyanobacteria, were detected off the northwestern coast of the Middle Caspian. Very high values of nanophytoplankton abundance (up to 1.6×105 cells/mL), dominated by small flagellates, were also found. A clear spatial uncoupling of pico‐ and nanophytoplankton was revealed. Maximum levels of nanophytoplankton were confined to the area of intensive phytoplankton blooming with high concentrations of chlorophyll a, while picophytoplankton reached peak abundance further south, outside this area. The influence of the thermocline on the vertical structure of microbial communities was detected. The concentrations of picophytoplankton and the proportion of physiologically active HNA‐bacteria decreased sharply under the thermocline.The results obtained indicate the leading role of the smallest phototrophs in the Caspian pelagial and emphasise the need for further comprehensive studies of Caspian microbial communities using modern methods.

ДРЕЙФУЮЩИЕ НИТЧАТЫЕ ВОДОРОСЛИ В ПРИБРЕЖНОЙ ЗОНЕ ВОСТОЧНОГО КАСПИЯ КАК САМОСТОЯТЕЛЬНОЕ ВЕРХНЕПЕЛАГИЧЕСКОЕ СООБЩЕСТВО

The coastal zone of the Middle Caspian is currently undergoing significant changes due to falling sea levels. With shallowing, the nature of the influence of water flows on bottom landscapes changes, including the intensification of the impact of storm waves on their surfaces inhabited by bottom communities. The impact of rolling and storm waves on bottom algae is also increasing. Cenoses of bottom macrophyte algae, which was previously growing on underwater rocks, including flat rock platforms, which abound along the eastern coast of the Middle Caspian, at depths of 10-12 m, were previously affected only by particularly strong storm waves. Now their habitats are at a significantly shallower depth (about 7.5-9.5 m), which led to more frequent and massive detachments of macrophyte thalli and their transfer to the pelagic zone. Mass transfer of living thalli of green and red algae to the upper layers of the water column during different seasons of the year led to the formation of communities of a new type, differently structuring the surface layer of the phytopelagic zone. New cenoses are a set of metamorphosed thalli of filamentous algae with a microepiphytic complex abundantly growing on them, live in the surface layer of water for a long time, serve as food for pelagic fish and have a significant impact on coastal landscapes. Mass development of macrophytes on the bottom, and their further intensive development in the pelagic zone, in turn, can be caused by increased eutrophication of the coastal zone in areas of large settlements.

Feeding of the caspian seal (Pusa caspica Gmelin, 1788) during haul out periods at the Kenderli rookery (Middle Caspian)

The Kendirli Haul-Out Site of the Caspian Seal served as the sole haul-out location in the Central Caspian Sea. Consequently, data obtained regarding the seal's diet at this site holds significant importance for the study and conservation of the Caspian Seal population. This study analyzed the diet of the Caspian Seal based on fecal collections from the Kendirli Haul-Out Site spanning from 2015 to 2019. Given the Caspian Seal's endangered status, a vital in vivo method of coprological analysis was employed to assess its diet. Comparative analysis of the seal's diet was conducted across seasons and years, and fish lengths consumed by seals were determined using otoliths extracted from feces. The study revealed that the Caspian Seal's diet at the Kendirli Haul-Out Site comprises 11 fish species, including species of the Cyprinidae family and Benthophilus genus, along with unidentified species of gobies. It was observed that the seal's diet varies seasonally; during autumn, the primary food source consists of gobies, whereas in spring, Atherina caspia predominates the diet. Additionally, carp species, Clupeonella caspia, and Alosa braschnikowi are present in the spring diet but absent in the autumn diet. Reconstruction of fish length ranges for three fish species — Neogobius melanostomus affinis, N. pallasi, and Alosa braschnikowi — demonstrated that seals consume fish ranging from 30 to 260 mm in length.

Detection of upwelling events in the Caspian Sea using thermal satellite image processing

In this article, we studied the upwelling phenomena observed in the surface waters of the Caspian Sea. Studying upwelling is crucial for understanding and managing the complex interactions between the ocean, climate, and marine life, with implications for both environmental conservation and human well-being. To accomplish this, we utilized data from MODIS-Aqua satellite observations in the infrared range of 11 microns. These observations had a spatial resolution of 4 km and covered the period from 2003 to 2021. The data was accessed through the NASA Giovanni online data system databases. Our findings indicate that upwelling phenomena are primarily observed in the eastern part of the Middle and South Caspian from May to September. The most intense upwelling occurs along the eastern coast of the Middle Caspian in July and August. Based on long-term averaged data, the upwelling phenomenon is typically observed between 40-44º latitude during this period. The width of the upwelling zone increases gradually from north to south, extending approximately 60-70 km towards the Kazakh Gulf before decreasing towards the south. In the upwelling zone, the temperature gradient can at times reach 4.0ºC per 100 km. In certain years, the upwelling zone that initially forms along the eastern coasts can extend over long distances and even reach the western coasts. Generally, the upwelling phenomenon occurs alongside the advection of warm waters from the South Caspian towards the Middle Caspian.

Чужеродные моллюски в прибрежной части дагестанского района Каспийского моря

The species Corbicula fluminea and C. fluminalis (O.F. Müller, 1774), alien to the ecosystems of the Russian sector of the Middle Caspian, penetrated into the estuaries, forming new ecotones. Mollusks of the genus Corbicula are included in one hundred of the most actively spreading universes around the world. In 2018-2022, live individuals of the bivalve mollusk C. fluminalis (O.F. Müller, 1774) were found in the waters of the northern part of the Agrakhan Bay, in the area of the Chased Gate, Kubyakinsky Bank, Kuznechonok. For the first time, finds of C. fluminalis and C. fluminea specimens of different ages were recorded in the Prisulak canal and the Yuzbash-Sulak reservoir. There is an expansion of the habitat of corbicules in the study area. Possible ways of invasion are assumed: introduction of corbicule larvae into the Caspian Sea basin with ballast waters of ships plying through the Volga-Don Canal; introduction of Asian fish species into the inland waters of Dagestan; transfer by waterfowl. High adaptive abilities and different breeding strategies of Corbicula mollusks increase the possibility of successful naturalization in estuarine and freshwater ecosystems of the Dagestan region of the Caspian Sea, as well as in desalinated areas near the mouths of the rivers of the Northern Caspian and the northern part of the Middle Caspian.

Prediction of ctenophore Beroe ovata Bruguiere, 1789 habitat expansion and control of invading Mnemiopsis leidyi Agassiz, 1865 in the Ponto-Caspian basin associated with climate change

The seas of Ponto-Caspian basin (Black, Azov and Caspian) are exposed to species invasions, including harmful ctenophore Mnemiopsis leidyi A. Agassiz, 1865 and its predator Beroe ovata Bruguière, 1789. Current environmental conditions of invasive ctenophores M. leidyi and B. ovata occurrence, development and variability in the Ponto-Caspian basin have been compiled, based on own field observations and published data. Analysis of climatological data on basic environmental parameters (water temperature, salinity, chlorophyll concentration) and modeling approach were used to predict favourable for both ctenophores conditions, and changes of those conditions, associated with the climate variations. The role of B. ovata as a bio-controller of M. leidyi population has been assessed. Several climate change scenarios have been considered in this study i.e., uniform increase/decrease of SST throughout the year by 2 °C and only spring temperatures increase/decrease by 2 °C. The most sensitive to increase in SST in terms of M. leidyi reproduction duration are the western coast of the Black Sea and the Southern and Middle Caspian. On the other hand, B. ovata reproduction duration is expected to increase in several areas, including the northern coast of the Black Sea and the Southern and Middle Caspian. The coastal areas of the Black Sea and the transitional regions between the Southern and Middle Caspian are exposed to an earlier start of M. leidyi reproduction during warmer springs. Regarding B. ovata, the whole Black Sea is vulnerable to spring SST changes, but in the Caspian Sea B. ovata extends its reproduction duration only in the Middle Caspian during warmer springs. Since B. ovata consumes mostly M. leidyi, it is an important biocontrolling agent of M. leidyi, harmless for the ecosystem.

Population Status of the Ctenophores Mnemiopsis Leidyi Agassiz, 1865 and Beroe Ovata Bruguiere, 1789 in Waters of the North and Middle Caspian Western Shelf in August–September, 2022

Population Status of the Ctenophores Mnemiopsis Leidyi Agassiz, 1865 and Beroe Ovata Bruguiere, 1789 in Waters of the North and Middle Caspian Western Shelf in August–September, 2022

ВНУТРИГОДОВАЯ ИЗМЕНЧИВОСТЬ ЦИКЛОНИЧЕСКОЙ ЦИРКУЛЯЦИИ ВОД СРЕДНЕГО КАСПИЯ

Hydrophysical studies in 2004-2018 showed that the cyclonic gyre of the Middle Caspian Sea is not a year-round circulation. In the cold season, the current in the cyclonic gyre intensifies, and in the warm season, the circulation weakens and the directed current turns into a drift one, except for the western part, where the water is transferred from the Middle Caspian to the southern part of the sea due to the difference in the levels of the basins. It is shown that the current in the cyclonic circulation follows the contour above the slope of the Derbent Basin in the entire layer from the surface to the bottom. The seasonal boundaries of the annual intensification and weakening of the cyclonic circulation are revealed.

Epilithic biofilms of the Eastern Caspian (Aktau region, Kazakhstan) under conditions of falling sea level

Communities of microorganisms that form biofilms on the rocky shores of the Middle Caspian are capable of maintaining the integrity of the coastal landscape in the conditions of sea retreat associated with the ongoing drop in sea level since 1995. Algal-bacterial biofilms developing on rocky substrates of the East Caspian coast within the City of Aktau were studied in autumn (October 2022) and spring (April 2023) seasons. Communities in biotopes with different structure were investigated. It has been established that the most stable and diverse communities with a pronounced vertical structure live in the areas with high sheer cliffs on capes. On flat areas of the coast, which have undergone recent shallowing and are protected from surfing waves by ridges of large stones, less diverse cenoses, demonstrating clear spatial organization. In the shallow open areas of the shore, which are subject to intense wave action, live communities of two types were observed. Both cenoses contained a relatively small number of species, but differed in structure and potential for protecting the coast from erosion. The first of them live in biotopes outside the impact of urban domestic sewage, and are able to protect coastal marl from destruction due to the development of stable, rapidly regenerating complex structural elements. The latter were noted in the conditions of along-shore spreading of untreated domestic wastewater from new city developments located close to the coast. Such biofilms were no longer able to protect marls from being destroyed by waves, and took the form of constantly renewed patches with minimum diversity of components and unstable spatial organization.

Influence of Long-Term Wind Variability on the Storm Activity in the Caspian Sea

Wind and wave conditions are limiting factors for economic activity, and it is very important to study the long-term variability of storm activity. The main motivation of this research is to assess the impact of wind variability on the storm activity in the Caspian Sea over the past 42 years. The paper presents the analysis of a number of storms based on the results of wave model WAVEWATCH III and the Peak Over Threshold method. The mean, maximum, and 95th percentile significant wave heights were analyzed by season. The highest waves were in the Middle Caspian Sea in winter. Detailed interannual and seasonal analyses of the number and duration of storm waves were performed for the whole Caspian Sea and its separate regions. Positive significant trends were found in the whole sea. Significant positive trends in the number and duration of storms were found for the North and Middle Caspian. In the South Caspian, the trends were negative and not significant. High correlations were found between the number of storms and events with wind speed > 10–14 m/s and 95th percentile wind speed. Positive trends in the number of storms in the Middle Caspian were caused by positive trends in extreme wind situations.

Seasonal and Long-Term Phytoplankton Dynamics in the Middle Caspian According to Satellite Data and In Situ Observations in the First Decades of the 21st Century

Seasonal and long-term variability of phytoplankton in the Middle Caspian was studied based on remote sensing data of the sea by SeaWiFS and MODIS-Aqua scanners in 1998–2021 and the results of field observations in 2004–2021. The seasonal variability of chlorophyll “a” (CHL) calculated from satellite data using a regional algorithm indicated that the autumn and winter seasons were the main phytoplankton production periods of the year. In summer, a period of stagnation was observed in phytoplankton growth in the surface layer. However, according to satellite data in the first months of each year, winter blooms were observed recurrently in the Middle Caspian Sea, as confirmed by the results of field observations in 2004–2021. The phytoplankton biomass during the winter vegetation period reached 4.5–5.0 g/m3. In the first decade of the century (2004–2006), winter blooms were almost entirely (as much as 96%) formed through the mass growth of the alien diatom Cerataulina pelagica. In the modern period (2021), the winter bloom was formed both by three endemic diatom species Thalassionema nitzschioides, Cyclotella comta and Dactyliosolen fragilissimus and by two alien species Pseudo-nitzschia seriata and Cerataulina pelagica. In spring, the diatom Cyclotella caspia and the dinoflagellate Prorocentrum micans, both endemics, dominated. In summer, the phytoplankton biomass was composed of the mass growth of small flagellates and dinoflagellates in the seasonal thermocline layer, which current remote sensing methods cannot record. The diatoms’ contribution to the community’s total biomass in summer did not exceed 3%. In the autumn phytoplankton, the main role was played by a diatom component represented by alien species, mainly Chaetoceros peruvianus.

Современное состояние биологии, запасов и промысла атерины (Atherina boyeri caspia (Eichwald)) в западной части Среднего и Северного Каспия

The article considers the original results of observations and analysis of the catches of atherina (Atherina boyeri caspia (Eichwald)) by months by stationary nets in the Terek-Caspian fishery subarea (Republic of Dagestan). The size-weight and age indicators of the spawning stocks of atherina have been characterized. For the first time in recent years the conditions for atherina spawning in the western part of the Middle and Northern Caspian are described. All biostatistical indicators of atherina obtained in the recent years (2016–2021) are listed: average linear weight parameters, average fatness coefficient according to Fulton (0.944) indicate a high fattening of fish. Size and weight composition of females and males, age-related fluctuations in length and weight, comparative analysis of atherina concentrations, characteristics of atherina reproduction in the conditions of the Northern and Middle Caspian, fecundity of different age groups, quantitative distribution of roe, larvae and fry of atherina, as well as dynamics of the age composition from catches prove a satisfactory state of stocks of this fish species. Comparative analyses of autumn concentrations of atherina in the Middle Caspian in 1958–1963 and in 2018–2021 are presented, as well as spp/h of trawling, age-related fluctuations in the length and weight in the autumn period (2016–2021). It has been found that atherina in the Caspian Sea is the main object of by-catch when fishing for kilka with stationary seines (in average 56.3 tons per year); it plays an important role as a food component for many fish species (sea herring, sturgeon) and mammals (Caspian seal) living in the waters of the Caspian Sea. Prospective volumes of processing atherina for fishmeal are also considered for the first time. On the scale of the Caspian Sea, with intensive kilka fishing by stationary seines as by-catch, it is possible to catch atherina in the amount of 150–200 tons. Despite the small volumes, they would significantly cover the deficit in the fish feed. Due to the increasing population of atherina in the natural conditions it is required to actively involve atherina in the fishing processes of the organizations in order to maximize the development of the commercial stock.

Plankton Community Structure and Biomass in the Eastern Middle Caspian Sea

Despite the role of planktonic organisms in the fishery value of the Caspian Sea and in addition to the increase in the plankton population in the Caspian Sea, there are few detailed studies regarding the determination of the planktonic organisms in the Caspian Sea. In the present investigation, we aimed to study the cell abundance and diversity of the phyto- and zooplankton in the eastern part of the Middle Caspian Sea in the spring of 2016. The composition of phytoplankton consisted of 50 species belonging to 4 systematic groups which were registered in the eastern part of the Middle Caspian: Cyanophyta (7), Bacillariophyta (24), Dinoflagellata (13), and Chlorophyta (6). Zooplankton were represented by 9 taxa: Cladocera—6, Copepod—1, and other groups—2. The most important phytoplankton species were Rhizosolenia calcar-avis, Anabaena bergii, Exuviaella cordata, and Binuclearia lauterbornii. The 0–50 m layer in the eastern Middle Caspian was the most productive, where the biomass varied from 98 mg/m3 to 109 mg/m3, consisting largely of diatoms. The cladoceran Evadne anonyx typica, the copepod Acartia tonsa, and the larvae of barnacle crustaceans (Cirripedia) were widely distributed in this layer. Zooplankton abundance decreased markedly with the depth. The maximum zooplankton concentrations were confined to depths down to 200 m.

Neogobius bathybius (Kessler), как один из объектов прибрежного лова Дагестанского побережья Каспия

A brief historical reference is given on the state of bullheads and their stocks off the Dagestan coast of the Middle Caspian in the second half of the twentieth century. The data on catches and by-catch during the spawning migration of gobies in nets and seines when fishing for Vimba vimba and herring Clupeidae in the Tersko-Caspian fisheries subdistrict of the Republic of Dagestan 2018-2021 are presented. The biological parameters of the deep-sea goby ponticola bathybius, which is the most numerous in by-catches (90%) during the fishing period, are given. Data on the total and commercial stock of goby fish in this research area are presented.

QUALITATIVE STRUCTURE OF SANDER MARINUS CUVIER, 1828 IN THE MANGISTAU COAST OF THE MIDDLE CASPIAN.

Predatory fish in aquatic ecosystems regulate the abundance of other species, which weakens the tension of competitive relations and stabilizes the community. Walleye refers to large pelagic predators, which, with a high number, effectively eat small and low-value commercial fish, performing a biomeliorative function in the fishing part of the community. At the same time, pike perch is in great demand, both on the foreign and domestic market. Its meat is considered a dietary product — its fat content is minimal. The nutritional value of walleye is very high — the protein content in it exceeds 18%. Zander meat contains 20 amino acids, 8 of which are irreplaceable (that is, they are not synthesized by the human body), and, in addition, contains a lot of minerals necessary for humans (phosphorus, potassium, iodine, molybdenum, manganese and others). It should also be noted that in Kazakhstan, only ordinary pike perch is caught and exported, the reserves of which are being depleted. Therefore, as an alternative, it is supposed to use a sea walleye.The biological resources of sea pike perch and other marine fish species of the Caspian Sea are not sufficiently exploited, since marine fisheries in our country are poorly developed, and the commercial population of these fish species allows increasing the catch in water areas under the jurisdiction of Kazakhstan. This paper describes the qualitative characteristics of sea zander caught in the research catches of the Mangistau coast of the Middle Caspian, as well as the degree of study in the Caspian Sea and other water bodies. The article describes the methodology for conducting a complete biological analysis of fish and presents statistical data processing. The paper gives a description of the following indicators: size-weight, age, sex ratio. Calculated fatness and determined the rate of size and weight growth in sea zander. Based on the results of these studies, a high correlation was found between age and weight, age and size of fish, and size and weight.

Фитоперифитонные сообщества прибрежной акватории Каспийского моря

Since phytoperiphyton, along with phytoplankton, is the first trophic link and the most informative biological object, the analysis of its species structure makes it possible to get an idea of the ecological state of the reservoir. Hydrobiological monitoring of the sea area, as well as hydrochemical monitoring, makes it possible to assess the quality of water, not only at the moment, but also in the previous period, as well as in case of chronic pollution. Thus, the conducted studies of the species diversity and distribution of phytoperiphyton make it possible not only to broaden the overall understanding of the algоflora of the coastal waters of the Middle Caspian, but also to supplement and update the data of the implemented environmental monitoring programs.

INVASIVE SPECIES OF DIATOMS IN THE AUTUMN PHYTOPLANKTON OF THE CASPIAN SEA: THE ROLE IN THE FORMATION OF TOTAL BIOMASS AND DISTRIBUTION IN THE SALINITY GRADIENT

The phytoplankton structure in ecologically diverse areas of the sea and the role of marine invasive species in the formation of the community were studied on the meridional and latitudinal trans-Caspian sections in the autumn period of 2008, 2009, and 2012. It was established that the transformation of the phytoplankton community continues in the Caspian Sea, associated with the entry of the Black Sea diatoms Chaetoceros peruvianus, Cerataulina pelagica, and Pseudo-nitzschia seriata into their composition. During the autumn bloom of phytoplankton in November 2008, the number of invasive species reached 1.3-2.3 × 10 cells/L, C. peruvianus and P. seriata to the total weight biomass of the community reached 49-50%. The highest abundance of invasive species was recorded in the shelf areas of the Middle Caspian Sea in its eastern ( C. peruvianus , 2.3 × 10 cells/L), western ( P. seriata , 1.4 × 10 cells/L), and northern ( C. pelagica , 1.3 × 10 cells/L) parts. The north boundary of the distribution of these species in the Caspian Sea was the 5.0 isohaline, which coincided with the highly productive frontal zone (4.0-6.0) of the marginal filter of the Volga River. For the first time, it was shown that the C. peruvianus diatom was a part of the phytoplankton of the upper productive 25-30-meter water layer both in the Middle and in the Southern Caspian during the periods preceding the autumn bloom of phytoplankton (September-October). During that time the most considerable abundance of this species (1.6 × 10cells/L) was recorded in the shallow areas on the Apsheron Sill and the eastern part of the Middle Caspian in the zone of coastal wind upwelling.

About “Paleozoic roots” of the oils of the Eastern Ciscaucasia

The paper considers geochemical data on the biomarker composition of oils from the fields of the Terek-Caspian oil and gas basin. Samples of oils from the fields of the main oil and gas bearing regions of the Eastern Ciscaucasia – Terek-Sunzha zone, Prikumsky swell, Piedmont Dagestan and the Middle Caspian Sea were studied. To determine the “geochemical” age of oils, the ratios of regular steranes in saturated oil fractions were analyzed. It was found that the value of the biomarker parameter St28/St29, which determines the geological age of the original organic matter for oils, varies widely – 0.36–1.47. The most “ancient” age (Devonian-Carboniferous) was determined for the oils of the Prikumsky swell and the water area of the Middle Caspian; in the Terek-Sunzha zone and Piedmont Dagestan, an increase in the proportion of St28 sterane is recorded in oils, which indicates a younger generation age (up to the Paleogene). The data obtained can be used to clarify the history of the formation of hydrocarbon accumulations in this region.

Изменения в развитии фитопланктона Среднего Каспия в 2021 году

The research of taxonomic diversity was completed in the water area of the Middle Caspian as a whole and by sections (Makhachkala - Cape Sagyndyk, Derbent - Cape Peschany, Divichi settlement - Kenderli Bay) for 2021. During the research period 108 samples were collected and processed. As a result of the work performed 105 taxonomic units were found, which is more than the average long-term value (87), but less than the indicators of the previous year of the research (130). The diatoms were the dominant group and accounted for 50% of the total composition. Dinophyte algae were the subdominants. The basis of the ecological complex of 2021 was formed by the representatives of seaweeds. A representative of green algae Ankistrodesmus pseudomirabilis v.spiralis influenced the formation of the abundance, reaching the greatest development in the section of the city of Derbent - Cape Peschany. In the same section of the sea the maximum biomass was also noted with a predominance of a group of dinophytes. In general, in the Middle Caspian the layer of 0–25 m was the most productive. The maximum concentration of plant cells was noted at a horizon of 25–50 m in the section of the city of Makhachkala - Cape Sagyndyk. In the section of the city of Derbent - Cape Peschany it increased from the surface horizon (34.22 mg/m3) up to a layer of 25–50 m. On the section of Divichi village - Kenderli Bay the highest concentrations were noted in the 10–25 m water layer. 
 In 2020–2021 b-mesosaprobes T. nitzschioides, E. cordata, Fragilaria construens were the dominant indicator species. These species are not so demanding to the environment. Organisms of the α-saprobic zone were less common, which suggests that the saprobiological state of the waters of the Middle Caspian is moderately polluted

Cadmium content in aquatic organisms of the Caspian Sea

Aim . Reveal the distribution of cadmium in the components of the ecosystem of the Caspian Sea. Materials and Methods . The sampling was carried out according to generally accepted methods, the determination of heavy metals was carried out by the method of atomic absorption spectroscopy using an atomic absorption spectrometer with electrothermal atomization MGA‐915 MD. Results . As a result of the studies carried out, it was found that the bottom sediments of the Middle Caspian were distinguished by a greater accumulation of cadmium than the North Caspian. Invertebrates of the Caspian Sea accumulate cadmium from water and soil, with the exception of shrimp, for which water is the only source of metal. Among the taxonomic groups of organisms studied, molluscs of the genus Didacna are cadmium accumulators. In the body of the Russian and Persian sturgeons, cadmium is mainly accumulated in the kidneys. Conclusion . Coefficients of cadmium accumulation in the organs and tissues of Russian and Persian sturgeons, black‐backed herring and Dolginsky herring relative to their food objects indicate that cadmium is able to accumulate in food chains.