Ladislav Pekárik

During the three decades following the publication of the first comprehensive monograph on the European mudminnow ( Umbra krameri Walbaum, 1792), knowledge of the species’ biology, distribution, genetics, ecology, and conservation has expanded substantially. In several countries, a dynamic distribution pattern has become apparent, with many local populations seemingly disappearing while new populations continue to be documented. However, many of these newly recorded occurrences likely represent previously overlooked remnant populations rather than genuine range expansions. Despite the uncertainty associated with detectability, the overall distribution pattern of U. krameri clearly reflects a large-scale process of range contraction and habitat fragmentation across most of its native range. Recent phylogeographic and population genetic studies have fundamentally improved understanding of the species by revealing pronounced drainage-based structuring, limited contemporary gene flow, and the existence of distinct evolutionarily significant units (ESUs). At the same time, the development of genomic resources, complete mitogenome assemblies, and environmental DNA (eDNA) approaches has created entirely new opportunities for monitoring and conservation. Numerous studies have also advanced knowledge of habitat use, trophic ecology, population dynamics, and conservation management, while conservation breeding and translocation programs have been initiated in several countries across the species’ range. Considering the threats affecting the species, together with the documented population declines and numerous local extinctions, its current IUCN Red List status should be reassessed and upgraded from Vulnerable to Endangered, or possibly even Critically Endangered.

Great Rye Island (Veľký žitný ostrov, Csallóköz) hosts the most significant population of the European mudminnow ( Umbra krameri ) in Slovakia. While this population exhibited relative stability during the first decade of the 21st century, it has experienced a continuous decline, with the core population now primarily confined to the Čilizský potok. The primary driver of this decline is climate change, characterised by increasingly dry and hot summers that lead to the complete desiccation of the mudminnow's habitats. Notably, similar drought conditions were observed in the 1970s and 1980s, suggesting a historical precedent for these fluctuations. In addition to climate change, the proliferation of stagnophilic invasive species poses a secondary threat to the mudminnow population. While the Amur sleeper has not yet been recorded in the area, other invasive species have begun to encroach upon the mudminnow's habitat. Furthermore, human activities represent a tertiary factor contributing to habitat degradation, rendering these environments unsuitable for the survival of the species. For the European mudminnow population to recover, it is essential to address both external environmental conditions, such as the occurrence of wet years, and the restoration of habitats to ensure long-term viability. This includes the elimination of invasive species and mitigation of direct human impacts on local ecosystems. Effective conservation strategies must prioritise these actions to support the resilience and sustainability of the European mudminnow population in Great Rye Island.

Sturgeons are a group of iconic rheophilic fish whose populations worldwide are currently undergoing significant declines. The study investigates the impact of climate change and anthropogenic activities, particularly the Gabčíkovo barrage system, on the survival and distribution of the sterlet, the last surviving sturgeon species in the Middle and Upper Danube River, specifically in the river part rkm 1708–1920 divided into three river sections: PT1 (Danube river part 1: rkm 1708–Gabčíkovo impoundment), PT2 (Danube river part 2: rkm 1854–1920), and PT3 (Old Danube part 3: rkm 1850–1810). Between years 1996 and 2020, water temperatures in the Danube River (PT1, PT2) rose by over 1°C on average, with the Old Danube (PT3) experiencing an alarming average increase of 3.5°C (df = 2, F = 145.03, p = 3 s ⁻¹ ) due to altered flow regimes, water levels, and siltation. Sterlets are increasingly caught in an artificial channel below to the Gabčíkovo Hydroelectric Power Plant (80%–90% of annual total catch), suggesting a shift in their distribution patterns. Conversely, there has been a notable decline in sterlet populations in other river sections, including a decrease in a section with moderate ecological status according to Water Frame Directive criteria. In light of these findings, the study proposes several mitigation measures to improve the status of the sterlet population in the area.

This study confirms the extended distribution of two invasive species of the genus Ameiurus in Ukraine. Specifically, A. melas is recorded for the first time in the Southern Buh basin and A. nebulosus has expanded further eastward within the Dnipro basin. Material collected in 2019 and 2022 was identified by morphological features and confirmed by molecular genetic analysis. The most reliable morphological characters for distinguishing these two species include anal‐fin membrane pigmentation (light or black), gill raker count (fewer or more than 16), and serrations on the pectoral‐fin spine (well‐developed along the full length or small, absent near the tip). The analysis of the cytochrome oxidase subunit I barcoding marker identified all samples from the Dnipro Basin (Tnia and Velykyi Luh localities) as A. nebulosus, while all specimens from the Vinnytsia region within the Southern Buh basin (Sotskoho and Vyshenske lakes) were attributed to A. melas. The maximum‐likelihood analysis revealed clearly separated clades with high bootstrap support (>75%), strongly supporting the presence of the two separate species. This study suggests the potential for further eastward expansion of both species within Ukraine: A. nebulosus in the northern direction and A. melas in the southern direction.

In complement to the JDS4 traditional fish survey (TFS, mainly by electrofishing), a fish eDNA metabarcoding-based survey has been implemented within the framework of the monitoring program organized by DNAqua-Net and in collaboration with the INTERREG "MEASURES" program. Water samples were collected at 29 sites from the source to the mouth of the Danube River, and at 18 tributaries. Two water samples (mean volume 29 L per sample) were collected at each site and the water filtered in situ. Twelve PCR replicates were performed per sample using teleo primers. In total, 80 taxa were detected, of which 19 corresponded mainly to farmed fish or food fish due to eDNA release in waste waters, most often downstream of large cities. Of the remaining 61 taxa, 50 taxa were identified at the species level, six taxa at the genus level, and five taxa at a higher taxonomic level. Concerning the Danube river itself, 69 and 57 species/markers were detected along the Danube River by TFS and eDNA surveys, respectively. 50 of these taxa were detected by both methods. Nine species were captured by TFS alone, mainly due to gaps in reference librairies and too colosely related barcodes. Eight species were only detected by eDNA. Except for the Salvelinus group, these were all benthic species, which are difficult to catch by electrofishing in large rivers (Acipenser ruthenus, Acipenser stellatus, Benthophilus sp., Romanogobio uranoscopus, Sabanejewia balcanica, Umbra krameri). The species richness tended to increase from upstream to downstream and the relative number of DNA sequences per taxa showed a clear succession of species along the river. Barbatula barbatula, Cottus gobio, Hucho hucho, Lampletra planeri, Phoxinus phoxinus and Thymallus thymallus were restricted to the Upper Danube whereas A. ruthenus, Neogobius fluviatilis, S. balcanica and Scardinius erythrophtalmus were detected from Vienna to the Danube river mouth. Abramis brama, Alburnus alburnus, Cyprinus carpio, Silurus glanis, Sander spp, Zingel streber were detected all along the river course; Alosa spp. and Syngnathus abaster downstream from the Iron Gate; A. stellatus and U. krameri only on the most downstream site (Danube delta). The calculation of a fish index, based on the common metrics used to intercalibrate national fish assessment methods on a European scale, classified most of the sites as being in moderate ecological status. Comparison of the indicative ecological status calculated using the same assessment method based on TSF and eDNA data at the common Danube sites showed a similar classification for six of the 13 sites and a difference of one class for the remaining seven sites.

In the past, sturgeons played an important role in commercial and recreational fisheries in the Danube River and its tributaries. Human impacts in the Danube River Basin coupled with exploitation of sturgeon stocks led to all species being either locally extinct, critically endangered or of unknown status. Sterlet (Acipenser ruthenus, Linnaeus 1758) is the last known sturgeon species occurring in the upper and middle Danube; however, the population of this species is considered unbalanced and decreasing since the beginning of the twenty‐first century. The decline of sturgeon stocks has been noted before owing to their economic importance. With commercial fisheries being forbidden in the Slovak section of the Danube River, there is generally no information available about the status of what is considered a local population. Databases containing recreational catch of sterlet (2003–2018) and historical records of commercial harvest (1961–1990) were used to describe the trend in the weight and number of sterlet caught over the following years. Modelling indicated that the number of fish caught each year appears to be lower, while the average weight of each individual is increasing. This might suggest that the population is ageing. Although older individuals can contribute a great deal more to spawning because they produce a greater number of eggs, several problems are apparent. The number of spawners might be decreasing as a result of bycatch or fishing, their ability to spawn might be obstructed, or annual recruitment may fail owing to unpredictable events. Although restocking programmes are in place to help maintain the sterlet population in the Danube River, their efficiency seems to be drastically low. In fact, restocking could be of little value unless studies on the availability of key habitats are conducted and their protection and restoration are ensured.

Hybridization and polyploidization are important evolutionary processes whose impacts range from the alteration of gene expression and phenotypic variation to the triggering of asexual reproduction. We investigated fishes of the Cobitis taenia-elongatoides hybrid complex, which allowed us to disentangle the direct effects of both processes, due to the co-occurrence of parental species with their diploid and triploid hybrids. Employing morphological, ecological, and RNAseq approaches, we investigated the molecular determinants of hybrid and polyploid forms. In contrast with other studies, hybridization and polyploidy induced relatively very little transgressivity. Instead, Cobitis hybrids appeared intermediate with a clear effect of genomic dosing when triploids expressed higher similarity to the parent contributing two genome sets. This dosage effect was symmetric in the germline (oocyte gene expression), interestingly though, we observed an overall bias toward C. taenia in somatic tissues and traits. At the level of individual genes, expression-level dominance vastly prevailed over additivity or transgressivity. Also, trans-regulation of gene expression was less efficient in diploid hybrids than in triploids, where the expression modulation of homoeologs derived from the “haploid” parent was stronger than those derived from the “diploid” parent. Our findings suggest that the apparent intermediacy of hybrid phenotypes results from the combination of individual genes with dominant expression rather than from simple additivity. The efficiency of cross-talk between trans-regulatory elements further appears dosage dependent. Important effects of polyploidization may thus stem from changes in relative concentrations of trans-regulatory elements and their binding sites between hybridizing genomes. Links between gene regulation and asexuality are discussed.