The Danube Delta is one of Earth’s biodiversity hotspots and includes many endemic, rare, and important species of both major conservation and economic value. This unique complex of ecosystems also plays a key role for Danube River and Black Sea fish fauna through its role as a natural safe buffer, shelter, feeding, reproduction, and smooth transitional area for a large number of fish species. Climate change is inducing a progressive sea level rise in the Black Sea, a fact that is expected to impact the delta’s key complex and dynamic habitats, biocoenoses, and associated biota, and last but not least the key taxonomic group, namely, fish. Around one-third of the fish species of this delta will be greatly affected, sometimes negatively, by this climate change scenario, another one-third to a lesser extent, and the final one-third not at all. The ecological positive feedback of fish can stimulate environmental change and is expected to be responsible for changes within Danube Delta ecosystems, and also for the near Danube River and Black Sea diverse matrix of aquatic and semi-aquatic ecosystems. Sea level rise in the Black Sea is considered to have been one of the main stress factors of the Danube Delta fish fauna in the past, and is likely to be the case in the future. In this spatio-temporal dynamic context, for the fish species under threat and risk, in situ-adapted management measures are highly required. The current work brings for the first time such a prospective knowledge about the potential impact on Danube River–Danube Delta–Black Sea coast fish diversity in the potential climate change–sea level rise scenario.

This paper presents the 2024 research results from the Danube Delta Biosphere Reserve (DDBR), including reed bed mapping and typology for fire management, assessment of sustainable use of vegetal resources, conservation status of invertebrates, fish, reptiles, amphibians, colonial waterbirds, and mammals of Community interest, evaluation of species invasiveness under climate change, and studies on human well-being and ecotourism within a sustainable development framework.

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.

In autumn 2015 the fish fauna of river watersheds from six sites of community interest (SCI) Natura 2000 were investigated as follows: river Mureș (two SCIs) and Someș River in four SCIs from Someșul Mic, Someșul Mare and Someș. The researches were focused on the fish species of community interest (Annex II of Habitats Directive), its position in the ichthyocoenose (fish species richness, abundance and ecological parameters) and anthropogenic pressure. In investigated SCIs rivers researchers found 27 fish species including 11 species of community interest. IBI index shows moderate to good, or excellent levels of the evaluation integrity degree in fish fauna ecosystems.

In the period 2018–2024, new species of fish belonging to the order Siluriformes (silurids) were reported in the freshwater habitats of the Romanian part of the Danube Delta Biosphere Reserve (DDBR). These species were: Ameiurus melas (black bullhead) and Ictalurus punctatus (channel catfish), which are both American catfish species belonging to the Ictaluridae family, as well as Clarias gariepinus and its hybrid, Heteroclarias (North African catfish, a cross between Clarias gariepinus and Heterobranchus longifilis, belonging to the Clariidae family). These two American catfish species have been present in the DDBR for several years, following the first recorded appearance of the species in Romanian freshwater upstream of the Danube Delta. However, the North African catfish (Clarias gariepinus and the C. gariepinus × H. longifilis hybrid) should not be included in the list of alien species in the DDBR because they appeared accidentally in the DDBR and cannot withstand the low water temperatures in temperate zones during winter. The DDBR’s list of alien freshwater fish already includes seven species, such as Hypophthalmichthys molitrix, H. nobilis, Ctenopharyngodon idella, Pseudorasbora parva, Perccottus glenii (all five fish species are native to Asia) and Lepomis gibbosus (native to North America), as well as an extension of the range of Percarina demidoffi. Records of alien silurids confirm that two other alien fish species on the DDBR list have spread by natural dispersal from Romanian waters to the Danube Delta. Monitoring activities in the area should continue in future studies to observe possible numerical expansion and assess the impact of alien silurids on native species. For now, only Ameiurus melas raises concerns about its negative impact on native ichthyofauna.