The aim of this study was to evaluate the population characteristics of spirlin, Alburnoides bipunctatus, in Serbia, since this small fish species is facing a severe decline in its abundance and its natural habitats in Europe. We investigated the spirlin population dynamics, including size, age structure, growth pattern, mortality, and exploitation rate. Additionally, we used the Uniform Manifold Approximation and Projection approach with the Decision Tree algorithm to investigate the influence of different environmental parameters on the population parameters to unveil which factors shape the abundance and distribution of spirlin. The results showed that the highest values of production, abundance, and biomass were estimated in sites with low temperature, optimal pH, and well-oxygenated water, even though we found them in heavily polluted waters with extremely high values of conductivity. Moreover, we observed a pattern of migratory behavior, in which spirlin migrate upstream to sites at a higher altitude in early summer and autumn. Despite the putative vulnerability and high sensitivity of spirlin populations, our results showed that the species was abundant, occurring in altered habitats (due to pollution, climate change, anthropogenic pressure, etc.).

The presence of microplastics (MPs) in the gastrointestinal tract, muscle, and whole-body samples of common bleak Alburnus alburnus L. from Gruža Reservoir (Central Serbia) was studied for the first time. Different protocols for MPs extraction were applied to determine the most efficient one. The study aimed to modify existing protocols to be cost-effective, efficient in digestion, and with no detrimental effect on potentially present MPs polymers. In this study, the digestion with 10% KOH during 48 h at 40°C was efficient for the gastrointestinal tract and muscle. Digestion with 10% KOH during 72 h at 40°C was the most efficient for whole-body samples. The usage of NaClO proved successful in digestion of the gastrointestinal tract overnight at room temperature. Fibers detected in the samples are assumed to be of plastic origin. The general goal was to establish a protocol for extracting MPs from fish tissue in wild populations to obtain results and determine the degree of pollution.

Microplastics (MPs) are ubiquitous freshwater contaminants, but their associations with biological characteristics of wild fish remain insufficiently understood. This study investigated MP ingestion by the common bleak (Alburnus alburnus) across three Serbian reservoirs with contrasting anthropogenic influences, assessing spatial patterns of MP occurrence and abundance and their associations with fish characteristics. Microplastics were detected in 37.2% of individuals (n = 183), with significant differences among localities. Both occurrence and abundance were higher in reservoirs characterized by greater anthropogenic influence, while the comparatively lower-impact reservoir exhibited substantially lower values. Generalized linear models identified locality as a significant predictor of MP occurrence and abundance, revealing several-fold differences among reservoirs. Fulton’s condition factor (K) was significantly negatively associated with both MP occurrence and abundance, independently of locality, age class, and sex. Polymer analysis identified polyethylene (PE), polypropylene (PP), and ethylene-vinyl acetate (EVA), indicating variation in the composition of the ingested particles. These findings demonstrate spatial differences in MP ingestion and an association between MP contamination and fish body condition. Although causality cannot be established, the results support the potential of A. alburnus as a bioindicator of MP contamination in freshwater reservoirs.

Freshwater ecosystems in Serbia (Central Balkans) are increasingly shaped by the cumulative demographic footprint of human activity, reflecting the ecological consequences of population growth, urban expansion, land-use intensification, and associated environmental pressures. Rivers, as ecologically sensitive and spatially confined systems, are particularly vulnerable to anthropogenic stressors such as pollution, habitat fragmentation, hydrological alterations, and biological invasions. These local pressures interact with climate-driven changes, such as rising temperatures, altered flow regimes, and seasonal mismatches, leading to complex and often synergistic ecological disruptions. Together, these stresors cause structural and functional shifts in aquatic biota, with fish populations being especially affected. This study aimed to disentangle and quantify the spatial manifestation of interacting anthropogenic and climate-related pressures using the spirlin (Alburnoides bipunctatus, Bloch 1782) as a model species. Typically associated with oxygen-rich and structurally complex habitats, the spirlin has demonstrated notable persistence in ecologically disturbed river systems, suggesting its potential as both a resilient species and an early-warning indicator of environmental stress. To explore this potential, we applied the ESHIPPOClim model, a composite ecological assessment framework integrating four main components: ES + ILSFP (biological components), HIPPO (anthropogenic stressors), and Clim (climate-related parameters). The model was calibrated using long-term ichthyological and environmental data collected from 2003 to 2021 across a wide range of river sectors in Serbia. Visual analytics, such as facet plots, boxplots, and min-max scaled heatmaps, revealed a consistent upward trend in cumulative ecological pressure. The spirlin's adaptability was particularly evident in sectors with elevated pressure, where high ES + ILSFP scores co-occurred with increased HIPPO and Clim values. This suggests that resilient populations can persist even under substantial and overlapping environmental stress, indicating that such areas may represent spatial thresholds where early biological responses to ecosystem degradation become measurable. These findings underscore the importance of recognizing demographic change not only as a socioeconomic factor but also as a significant ecological driver. The effective management of freshwater ecosystems requires an integrative monitoring framework that encompasses biological, hydrological, demographic, and climatic dimensions to detect ecological tipping points and guide adaptive, evidence-based conservation strategies.

Occupying a mere fraction of the Earth’s land surface, freshwater ecosystems are disproportionately rich in biodiversity and indispensable for human well-being and sustainable development. Nevertheless, these systems are undergoing accelerated biodiversity loss, driven by intensifying anthropogenic pressures that often surpass the impacts observed in terrestrial and marine environments. Protected areas are central to biodiversity conservation, but their effectiveness in freshwater systems is limited by hydrological connectivity and the mismatch between fixed boundaries and dynamic ecological processes. In this study, we assessed the ecological and fisheries performance of Special Fish Habitats (SFHs) in Serbia by comparing 25 SFH and 19 non-SFH localities across the Ibar and Nišava River basins, using field surveys conducted through standardized electrofishing from 2022 to 2025 and fish stock monitoring records covering 2016–2021. Fish production, biodiversity, ecological status, and population sustainability were evaluated through the ES-HIPPOfishing model, complemented by multivariate statistical and machine learning analyses. SFHs showed generally higher productivity and better water quality, but none of the direct comparisons were statistically significant. To our knowledge, this is the first study to systematically evaluate SFHs in Serbia, providing a robust scientific basis for refining freshwater protected area networks and offering actionable insights for biodiversity conservation in pressured river systems.