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.).

In the inland waters of the Balkans, many brown trout populations have been severely depleted. Therefore, identifying potential threats to their continued survival and ranking populations based on their biological and evolutionary importance enables setting conservation priorities. To assess the sustainability of the brown trout populations in the territory of Serbia (central Balkans), a modification of the ESHIPPO model was performed. The main modification involves incorporating the investigated populations’ genetic structure into the model. Therefore, the new ESHIPPOsalmo model includes an analysis of biological parameters and the impact of multiple factors, including habitat alterations, invasive species, pollution, human population growth, and over-exploitation. In order to investigate individual levels of influence of the model’s analyzed parameters, a combination of supervised and unsupervised machine learning methods was used. The structure of the model is based on general and easily measurable indicators, which enables its application in any salmonid river in the world. By evaluating the parameters of the ESHIPPOsalmo model, we were able to establish that, of the analyzed populations from 46 localities, 37% have a moderate level of sustainability, 43% low, and 20% critically low.

The roach, Rutilus rutilus (Linnaeus, 1758), is one of the most common fish species in mesotrophic and eutrophic lakes throughout Europe. In the Serbian reservoirs selected for this study, this species accounts for the majority of juvenile fish biomass. The aim of this study was to investigate the diet composition of juvenile roach to assess their niche based on resource availability in five Serbian reservoirs with different trophic statuses. A modified Costello graph and Kohonen artificial neural network (i.e., a self-organizing map, SOM) were employed to examine the feeding habits of 142 specimens of roach caught in five reservoirs. Our results show that juvenile roach use zooplankton, benthic macroinvertebrates, algae and detritus in their diet. In addition, five neuron clusters (A, B, C, D and E) were isolated in the SOM output network. The SOM identifies specimens that share similar feeding patterns and categorizes them onto the same or adjacent neurons, determined by dominant prey. In terms of the number of specimens, cluster B was the most numerous, and the predominant prey of these specimens were Daphnia sp., Bosmina sp. and calanoid and cyclopoid copepods. The cluster with the lowest number of specimens is cluster C, and the specimens in it benefited from Chironomidae and Insecta. Due to the different trophic statuses of the reservoirs selected for this study, knowledge of fish feeding habits is essential for the formulation of effective conservation and management strategies for both the species and the reservoirs.

The objective of this study was to develop a successful freezing protocol for cryopreservation of Danube barbel sperm, and to identify the presence of different spermatozoa subpopulations. By testing different concentrations of different cryoprotectants, we determined that the use of 5% dimethyl sulfoxide (DMSO) yielded the highest total motility of ~25%. Cooling rates influenced by frame height and cooling time in liquid nitrogen vapor showed that a frame height of 3 cm and cooling time of 2 min yielded the highest post-thaw motility. Supplementation of cryomedia with 0.1 M of sugars led to an increase in the total post-thaw motility by ~50%, while protein supplementation lowered post-thaw motility. Motile spermatozoa hierarchically clustered according to their motility parameters, displaying a four-subpopulation (SP1-SP4) structure. SP1 was defined by low values of velocity but high overall linearity; SP2 was comprised of fast non-linear spermatozoa, that had high velocity values but low linearity; SP3 was characterized by fast linear spermatozoa, and SP4 by slow non-linear spermatozoa. Protocols developed in this study will lead to the creation of new and enhanced conservation strategies for this species.

Artificial propagation and stocking of brown trout is a standard practice in recreational fishery management. In recent decades, the importance of maintaining intraspecific diversity and protecting locally adapted lineages has been recognized for the species' long-term survival. The first step in selecting donors for stocking involves distinguishing native trout from non-native and introgressed individuals. The established method for discerning Atlantic hatchery strains from the wild populations involves genetic screening of individual diagnostic SNPs and microsatellite assignment tests. This study, using Serbia's Panjica hatchery as an example, illustrates the proper conduct of routine genetic screening for identifying suitable donors for supportive stocking. The broodstock and reference populations were screened using mtDNA control region, LDH nuclear gene, and 12 microsatellite loci to assess the origin, diversity, and inbreeding levels. The analysis revealed only moderate contamination with Atlantic trout and showed the regional origin of the Danubian genes – over 50% of the broodstock was composed of non-introgressed Danubian individuals tracing their origin to the Zapadna Morava River system. Additionally, the study highlighted a considerable discordance between LDH locus and microsatellites in identifying introgressed individuals, raising concerns about the sole reliance on LDH locus for the identification of Atlantic genetic origin in nuclear DNA.

Length–weight relationship give information on the condition and growth patterns of fish. This study reports length– weight relationships for Abramis brama (Linnaeus, 1758), Alburnus alburnus (Linnaeus, 1758), Carassius auratus (Linnaeus, 1758), Cyprinus carpio (Linnaeus, 1758), Rutilus rutilus (Linnaeus, 1758), Silurus glanis (Linnaeus, 1758), Ameiurus nebulosus (Le Sueur, 1819), Perca fluviatlis (Linnaeus, 1758) and Sander lucioperca (Linnaeus, 1758). Specimens were collected from 2007 to 2013 in Gruza Reservoir (Central Serbia). The b values in the LWRs of analyzyed fish varied between 2.274 and 3.213.