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.
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 swimming ground speed and swimming depths of beluga sturgeon (Huso huso) and stellate sturgeon (Acipenser stellatus) were investigated using ultrasonic telemetry between 2011 and 2017 in the Lower Danube River (river kilometer (rkm) 0 Sulina-rkm 1075 Baziaș). Acoustic telemetry receivers VR2W and V16TP tags (Vemco, Canada) were used to collect data about sturgeon migration behavior in the Lower Danube River between the Black Sea and the Iron Gate II dam. The tags, equipped with depth and temperature sensors, provided data about beluga and stellate sturgeon migration periods, swimming speeds, and depths. Twenty-three beluga sturgeon (184-245 cm TL) and twenty-one stellate sturgeon (92-135 cm TL) were ultrasonically tagged and passively tracked using hydrophone receivers installed in the river between rkm 71 and rkm 860. Three tagged beluga sturgeon returned after 2 and 5 years and two stellate sturgeon returned after 2 years since the time of their initial release in the river. This study provides, for the first time, further details concerning beluga sturgeon and stellate sturgeon migratory behavior patterns, traveled distances, swimming depths, and their swimming ground speeds during spawning migration.
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.
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