The present research was conducted within the framework of a broader investigation with the objective of identifying freshwater parasites and bacteria on national territory, with the aim of gaining a deeper understanding of the relationships between parasites, hosts, and the environment, and to assess the potential detrimental effects of parasitic infestation on the conservation status of fish populations. Within this study, it was documented the first recordings of parasites Vorticella globularia and Epistylis sp. in the Romanian Petro-leuciscus borysthenicus celensis fish species. A total of 42 specimens were collected from the Gurban River to study their infestation status, focusing on the skin, gills, and fins. From the samples analyzed, 16 specimens presented signs of infestation with five distinct parasite species (Ichthyophthirius multifiliis, Dactylogyrus vastator, Trichodina acuta, Vorticella globularia and Epistylis sp) and four bacterial strains (Aeromonas veronii, Shewanella putrefaciens, Aeromona hydrophila, and Citrobacter freundii). The results indicate that the skin is the organ most severely impacted by parasites and bacteria, followed by the gills. The fins, on the other hand, are the least susceptible to infection. Furthermore, the significant amount of parasitic infestation coupled with the high bacterial load of Aeromonas veronii indicates a plausible link between the two.

The authors provide a list of the alien fish species that have entered the Romanian ichthyofauna fauna and specify the year, place of signaling and the ecological impact of the respective species. Further on, the authors describe some behavioral aspects of the following species: Lepomis gibbosus (pumpkin seed sunfish), Carassius auratus gibelio (silver crucian carp) and Pseudorasbora parva (topmouth gudgeon).

Anadromous sturgeon populations in the Lower Danube are in severe decline, and this is due to a combination of anthropogenic and natural factors that have a major impact on the life cycle of the species. Stationary swimming parameters, namely water temperature and depth, represent the starting point for developing the best procedures to identify breeding habitats and subsequently develop the best conservation measures in the context of anthropogenic impacts and climate change. According to current data, Acipenser gueldenstaedtii is the most endangered species, as it is the rarest species captured through scientific fishing and has highest temperature sensitivity of the three taxa studied.

The present study aimed to evaluate whether commonly used condition indices reflect parasite load and bacterial colonization in juvenile Cyprinus carpio under natural environmental conditions in the Lower Danube River. A total of 260 specimens were examined for parasitological, microbiological, and biometric parameters, including 20 individuals analyzed for bacterial communities. Twenty-three parasite taxa belonging to eight major taxonomic groups were identified. Ectoparasites were found on the gills, skin, and fins, with monogeneans and ciliates, notably Dactylogyrus ssp. and Trichodina ssp., representing the dominant groups. Infection intensity was generally low to moderate, and histopathological examination revealed only mild epithelial alterations, including focal hemorrhage and mucus hypersecretion in more heavily infected individuals. Microbiological analysis identified six bacterial taxa associated with the skin, with Aeromonas hydrophila being the most frequently detected species. Correlation analyses showed no significant relationships between parasite abundance and condition indices (Fulton’s K, Le Cren’s Kn, scaled mass index, and BMI), although a slight reduction in Fulton’s K was observed in infected individuals. These findings indicate a stable host–parasite–microbiota equilibrium under natural environmental conditions. The results provide baseline ecological data that contribute to understanding fish health dynamics in the Lower Danube River and may support future monitoring and management strategies.

Aquaculture water was exposed to very low dosages of copper sulfate and chloroquine sulphate in order to observe their purgative effects at minimal concentrations, where both substances are commonly employed for their antimicrobial and antiparasitic properties in fish farming. It was observed that the well-known treatment using cooper sulphate is effective even at a low dosage, while the proposed novel treatment using chloroquine was not. The present study highlights the need of effective treatment while avoiding compromising medicine misusage which greatly leads to host resistance.

The investigation of sediment morphology and sediment regime is crucial for the initial stages of the hydromorphodynamic modeling of the Lower Danube basin. This helps in identifying significant habitats and potential obstacles that may disrupt the migration patterns of anadromous sturgeon species in the Lower Danube. This investigation involved the analysis of 10 samples, which were collected in equal quantities from specific places where hydrotechnical activities were conducted, specifically the Bala and Caleia branches. The sediment samples were analyzed to determine their morphological and structural characteristics through granulometric assessments. Additionally, three significant parameters, namely, the standard deviation, symmetry index (skewness index), and flattening index (kurtosis index), were used for further characterization.