Marian Iani

Critically endangered Danube sturgeons rely on specific hydro-geomorphological conditions—such as deep riverbed pools and exposed hard substrates—for spawning, overwintering, and resting. However, continuous high-resolution riverbed maps across the Lower Danube remain scarce. This study provides a high-resolution bathymetric and substrate assessment of three key sturgeon habitats within the Danube Delta Biosphere Reserve: Balteni, Nufaru, and Isaccea (Noviodunum) in the Danube Delta Biosphere Reserve. Using a 500 kHz interferometric bathymetric sonar coupled with RTK-GNSS positioning and ADCP velocity profiling, we generated 1 m resolution Digital Terrain Models (DTMs) and mapped benthic substrates via side-scan sonar backscatter segmentation. Deep scour pools driven by bedrock steps were mapped at Balteni (35 m depth) and Nufaru (30 m depth), both hosting exposed hard-bottom substrates that function primarily as overwintering and resting refugia. In contrast, the Isaccea sector features a 17 m deep pool maintained by high flow turbulence over the submerged masonry of the ancient Roman fortress of Noviodunum, exposing a localized gravel-cobble substrate that acts as an active spawning ground. Our findings demonstrate that critical sturgeon habitats in the Lower Danube are highly localized micro-topographic anomalies, emphasizing the necessity of high-definition hydroacoustic surveys for targeted conservation and environmental impact assessments.

Monitoring fish species in large rivers has progressed from traditional fishing methods to modern electronic telemetry, such as acoustic and satellite tagging. These technologies provide long-term and high-precision data on fish migration in both riverine and marine environments. In the Danube River, sturgeon migration was first monitored using telemetry in 1998-1999, with a focus on anadromous species. Since 2009, several telemetry projects have been implemented, during which an increasing number of adult sturgeons have been tagged with acoustic transmitters. With a battery life of up to seven years, these tags enable continuous monitoring of sturgeon migration over extended periods. In this study, 11 adult Stellate sturgeons (Acipenser stellatus) and 5 Russian sturgeons (Acipenser gueldenstaedtii) were tagged and tracked between 2022 and 2023. Results revealed repeated autumn upstream migrations by certain individuals that reached the Iron Gates II area in both 2019-2020 and 2022-2023 under similar environmental conditions (water temperature and water level). A notable shift in migration routes across the Danube Delta branches was also observed in 2023 compared to 2022 and previous years (2012-2015). This redistribution may be linked to recent environmental and anthropogenic changes in the area during 2022-2023, including increased navigation and dredging activities. These findings highlight the influence of human-induced factors on sturgeon migration behavior and emphasize the importance of continuous acoustic monitoring for the conservation and management of endangered sturgeon populations in the Danube River.

Stellate sturgeon (Acipenser stellatus) is a species of great conservation concern throughout its range. Over the past century, it has experienced a dramatic decline in abundance and distribution in the Black Sea basin. Information regarding the genetic structure of the species is very limited in the region, despite its crucial importance for developing adequate management plans for the species’ conservation and recovery. Samples from 163 A. stellatus were analysed using mitochondrial and microsatellite DNA markers to investigate the species’ genetic characteristics in the north‐western and southern Black Sea. Both genetic markers revealed high genetic diversity (153 haplotypes and 264 alleles) and low genetic differentiation of marine and freshwater catches. The north‐western Black Sea population is undergoing a slight demographic expansion after a bottleneck event induced by overfishing and dam constructions. The genetic population structure indicates a single genetic cluster in the north‐western Black Sea region and signs of admixture of two genetic clusters in the southern region. It also provides the first evidence for a remnant reproductive stellate sturgeon micro‐population in rivers of Turkey and Georgia. A sturgeon fishing ban and stocking activities carried out during the last decades, largely without any prior genetic assessment of the sturgeon populations, added little conservation value. Outreach initiatives and urgent actions are required to foster the recovery of this Critically Endangered species in the region. Reconstruction of the connectivity of the rivers and the enhancement of habitats and spawning grounds are beneficial to all sturgeon species. An ex‐situ conservation programme for Turkish rivers which flow into the Black Sea would be beneficial to help conserve the genetic resources until the habitats and migration routes become functional. Moreover, genetic screening and recruitment programmes must be functional for all rivers flowing into the Black Sea.

The aim of this study was to determine the ecological status of fish fauna of Razim Lake under the conditions of the water salinity changing from brackish, almost 70 years ago, into freshwater nowadays. The natural processes of siltation and organic deposits, characteristic of Danube Delta lake complexes, intensified in the last decades and included also Razim Lake. The presently reported study of Razim Lake and the adjacent area was undertaken in 2020 with intention to cover fish fauna collected with three different sampling methods (electrofishing, gillnetting, and seining). For each sampling method, Catch per Unit Effort (CPUE), relative abundance, and biomass were determined, as well as selected ecological parameters to determine ecological status of richness species in the area. Published data included 55 fish species, mainly marine and euryhaline, but in 2020 only 43 species were reported. Also, the species composition shifted from marine ones to freshwater or euryhaline ones. Of those 43 species captured in 2020 from Razim Lake and neighboring areas, 39 were native and four were non-native, including a newcomer, the Chinese sleeper, Perccottus glenii Dybowski, 1877. Few species were migratory, reophilous, or reophilous-stagnophilous which rarely enter Razim Lake, but the majority were limnophilous or stagnophilous-reophilous species. Four species were dominant in terms of the abundance; Blicca bjoerkna (Linnaeus, 1758); Rutilus rutilus (Linnaeus, 1758); Alburnus alburnus (Linnaeus, 1758); and Carassius gibelio (Bloch, 1782). In terms of the biomass the dominants were: Cyprinus carpio Linnaeus, 1758; Carassius gibelio; Sander lucioperca (Linnaeus, 1758); Pelecus cultratus (Linnaeus, 1758); and Blicca bjoerkna. Some differences between sampling methods used were observed. Eudominant, euconstant, and main species were Blicca bjoerkna and the majority of fish species were accessories, with differences amongst sampling methods used. Fish diversity parameters indicate a stable ichthyocoenosis, more stable along the lake shoreline. Ecological indicators of fish fauna from Razim Lake in 2020 grade the water lake quality as a moderate ecological class according to the Water Framework Directive of the European Union.

One of the last wild populations of the critically endangered stellate sturgeon (Acipenser stellatus) survives in the Danube River. Limited knowledge about the genetic structure, ecology, and evolution of this species led to poor and inconsistent management decisions with an increased risk for species extinction in the wild. Here we show the results of genetic structure screening of the Danube River wild population over 12 years timespan. Our research does not bring evidence of population recovery. No genetic structuring was identified at the mitochondrial level concerning spawning migration timing, sampling locations, and developmental stages. Eleven maternal lineages were revealed based on restriction fragment lengths analysis of the D-loop region, with one haplotype as the most frequent. While this could be the result of a massive restocking activity using a reduced number of spawners, our data does not support it. The selection of mitochondrial haplotypes under the pressure of habitat contraction and the narrower range of temperature variation since dams’ construction on the river could explain the observed distribution. Several factors of managerial concern are discussed. Our results provide baseline data on the mtDNA diversity in a critically endangered species of exceptionally high socioeconomic and conservation interest.

The giant European catfish, Silurus glanis (total length = 200 cm; total weight ≈ 80 kg) was caught downstream of Iron Gate II hydropower dam (Danube River, 863 rkm) and tagged with an ultrasonic transmitter (V16TP; Vemco Ltd.) equipped with depth and temperature sensors. Changes in catfish diving behavior and temperature exposure were monitored over a period of roughly 2 years. Transmitter detections were recorded by nine autonomous receivers (VR2W, installed in 2015 between Serbia and Romania, as well as near Romanian shiplock and upstream Romanian turbines). The first signals were recorded on April 28, 2015 and the last on February 13, 2017. Altogether 59,355 and 59,175 detections of the catfish depth and water temperature were recorded, respectively. The greatest number of signals were recorded by the two receivers closest to the location where the catfish was caught, 72.3% and 27.1%, while only 0.6% of signals were recorded by other receivers. The mean catfish depth was 8.4 m, while minimum and maximum depths were 1.2 and 16.2 m. Results obtained showed that this catfish exhibited high site fidelity, while changes in depth at certain periods are possibly related to its search for prey and upstream migration during the spawning period. Hydropower dam and shiplock were obstacles on its migration upstream and telemetry studies could ensure habitat requirements and meet the development of restoration and conservation strategies for the fish resources in the future.

Construction of the dams in Lower Danube River (LDR- rkm 0 Sulina – rkm 1075 Baziasi) without fish passages, as well as illegal fishery and river bottom modification due to navigation had negative impact on sturgeon’s populations. Beluga Sturgeon, Stellate Sturgeon and Russian Sturgeon still migrate to spawn in LDR but there is lack of information relating to sturgeons long distance migration in this area. Tagging with Vemco V16 TP acoustic tags, equipped with temperature and depth sensors involved 36 adult Beluga and 44 Stellate Sturgeons provide new data about sturgeon migration. Their migrations are monitored by VR2W receivers, deployed on between rkm 71 and rkm 864. Analysis of data showed that 17% of Beluga and 30% of Stellate Sturgeon never recorded and 30% and 11% of Beluga and Stellate Sturgeon recorded in period more than 30 days after tagging. Only 5% of Beluga and 7% of Stellate Sturgeon have been recorded after a period of more than 1 year. During 2013-2016 4 adult Beluga and 1 adult Stellate Sturgeon were recorded in the IGII Dam area. Results of this investigation showed that sturgeons still approached the IG2 dam and construction of fish passage on this dam is needed.