The endangered lake minnow, Rhynchocypris percnurus (Pallas, 1814), has been known from Lithuanian inland waters for over 20 years, but we have a very limited understanding of its current population size. The main purpose of this study was to provide a concise account of the species in Lithuania, including a rough assessment of threats to its populations and habitats. In 2018–2019, we investigated 360 small water bodies across the country, all of which were potentially suitable for lake minnows. Results revealed that lake minnows were present in only 12 water bodies, all concentrated in one regional park in the southern part of the country. However, by 2024 the species had gone extinct at nine of these sites due to habitat loss. At present (2025), only one viable Lithuanian population of this species can be considered to exist. Our results conclusively show that the species is on the very edge of extinction. Urgent action is needed to protect this species in Lithuanian waters, with special emphasis on revitalizing its most suitable habitats and translocating fish from the only currently known population in Lithuania.

endangered species, extinction risk, habitat loss, Natura 2000, occurrence

The lake minnow, Rhynchocypris percnurus (Pallas, 1814), is a tiny, short-lived fish representing the family Leuciscidae in the order Cypriniformes (Fig. 1). The systematic position of the species remains a subject of debate. Rhynchocypris percnurus (Pallas, 1814) was originally described in the literature as Cyprinus percnurus Pallas, 1814 (Pallas 1814). Subsequent taxonomic classification of this species was either as Phoxinus or Eupallasella (see Berg 1949; Kottelat 1997; 2006). According to the findings of the morphological studies (Howes 1985), the species was assigned to the genus Eupallasella, from which it had previously been moved from the genus Phoxinus. However, molecular phylogenetic studies (Ito et al. 2002; Sakai et al. 2006; Sasaki et al. 2007) have confirmed the decision to exclude the lake minnow from the genus Phoxinus, but also convincingly demonstrated that it should rather be included in the genus Rhynchocypris. Nevertheless, the nomenclature of the species had been the subject of considerable controversy until Bogutskaya et al. (2005) convincingly demonstrated that the correct form is percnurus.

Figure 1. 

Live specimen of Rhynchocypris percnurus collected in the studied sites located in Lithuania.

The species has a widespread but highly disjunct distribution in the Northern Hemisphere, extending from the Odra River basin in Poland in the west to the Pacific Ocean in the Far East (Kusznierz et al. 2017). This species is not considered endangered globally (Kottelat and Freyhof 2007); its present status in Europe is Least Concern. However, in countries located near the western edge of its range, such as Lithuania and Poland, R. percnurus is one of the rarest and most threatened freshwater fish species. This is primarily due to the specificity of its habitats, which are small, shallow water bodies often of anthropogenic origin, such as former peat or clay excavations dug in the previous century (Wolnicki and Sikorska 2020; Wolnicki et al. 2022). These habitats are highly vulnerable to destruction resulting from natural processes (e.g., shallowing, vegetation encroachment, desiccation) or human activity (land reclamation, landfilling). There is no doubt that their existence is limited to just a few decades. In recent years, their rate of disappearance in Poland has increased significantly due to the effects of climate warming (Sowińska-Świerkosz and Kolejko 2019; Wolnicki and Sikorska 2020; Wolnicki et al. 2022).

At present, R. percnurus sites are widely recognized in five European countries: Belarus, Lithuania, Poland, the Russian Federation, and Ukraine. However, knowledge of the actual status of this species in these countries, except for Poland, remains highly deficient (Ford 2024). The first official record of R. percnurus in Lithuanian inland waters is from 2006 (Kaupinis 2006), when several individuals were caught accidentally while fishing in a small, nameless shallow lake near the town of Alytus in southern Lithuania. In 2007, R. percnurus was placed under strict legal protection in Lithuania and included in the Lithuanian Red Data Book of Animals as a species critically endangered with extinction (Kaupinis 2007). A second lake minnow site was also accidentally discovered in 2010 in Meteliai Regional Park (V. Kesminas, pers. com.). Reports from local anglers clearly suggest that the species was widespread in southern Lithuania during the previous century. However, until the present study, no comprehensive inventory of R. percnurus in Lithuania had been performed, and only two sites had been reported for this species in the country (Fig. 2A).

Figure 2. 

Distribution of Rhynchocypris percnurus sites in Lithuania, based on records from 2010 to 2024. (A) previously known (until 2018) sites of R. percnurus; (B) sites investigated for the presence of R. percnurus within the country during 2018–2024; (C) all known sites of R. percnurus in 2019; (D) all known sites of R. percnurus in 2024. Different river basins are marked by different shading. Site numbers in C–D parts are coincident with the numbers in Tables 3–5.

The primary objective of this study was straightforward: to provide a precise overview of the past and present status of R. percnurus in Lithuanian inland waters. This included a rapid evaluation of threats to its habitats, populations, and sites, along with clear recommendations for the implementation of measures vital to the protection of this species in Lithuania.

Study area. The R. percnurus inventory was conducted across Lithuania, which is divided into seven main river basins. The largest catchment area, comprising 72% of Lithuanian territory, is the Nemunas basin (Fig. 2A). The country has 2850 larger lakes with a surface area exceeding 0.005 km² and 3150 smaller lakes, covering a total area of 913.6 km². In addition, there are more than 4000 other small water bodies of natural or artificial origin in Lithuania (Kažys 2013).

Inventory of lake minnow sites. The R. percnurus inventory survey was conducted within 2018–2019. In Poland, near the western border of the European range, R. percnurus inhabits small (<2 ha), very shallow water bodies of natural or anthropogenic origin, overgrown with submerged and emergent vegetation (Wolnicki and Radtke 2009). In total, 360 water bodies potentially suitable for R. percnurus were investigated within the entire territory of Lithuania (Fig. 2B). Water bodies were identified using orthophoto maps (scale 1:1000; 2018–2019, https://maps.lt), and reliable suggestions from local anglers were also checked in the field. During this study, we reinvestigated two previously known sites. Figure 2 shows all sampled locations.

We assessed the presence of R. percnurus in each location from July to the end of August, following the Polish methodology for R. percnurus inventory and monitoring (Wolnicki and Sikorska 2019). Fish were caught using specialized folding traps with two openings. These traps were baited with bread and measured 25 × 25 × 40 cm. The mesh size was 5 mm, with an opening diameter of 60 mm (Wolnicki et al. 2008). We set five to ten baited traps for one hour, depending on the size of the water body.

Assessment of R. percnurus population and habitat state. The presence of R. percnurus was confirmed at several sites. The population status and habitat of the species at these sites were assessed in June 2019 and again in 2024. The assessments were based on the approved criteria for the favorable conservation status of R. percnurus and expert knowledge (Wolnicki and Sikorska 2019). The abundance, sex ratio, and age structure of R. percnurus were assessed when considering the population state (Table 1). All catches were made at the end of the R. percnurus spawning period (June), when it is possible to identify mature females, mature males, and immature individuals with absolute certainty (Wolnicki 2004). All captured individuals of R. percnurus were counted and divided into three groups: mature males, mature females, and juveniles. We determined the sex of the fish by examining both their primary and secondary sex characteristics. Males were identified by gently pressing their belly, which caused them to release profuse milt. This could be confirmed by the presence of a protruding sex papilla. Fish shorter than 50 mm in total length and exhibiting no distinct male or female morphological features were classified as juveniles. Sexual structure was evaluated only when at least 10 sexually mature individuals were captured from a population, and age structure when at least 20 individuals aged at least 1+ were captured in a population. We assessed water body size, maximum depth, and percentage cover by helophytes of the water body when considering the habitat state (Table 2). Habitat measurements were taken in August, when the dry season is at its peak and thus the lowest depths and maximum helophyte cover are expected. We determined the approximate water surface area of all these water bodies using Google Maps tools. Furthermore, we assessed various hydrochemical parameters (pH, electrolytic conductivity, total nitrogen, and total phosphorus) of confirmed R. percnurus habitats (Table 3).

Distribution. Rhynchocypris percnurus was identified in 12 (3.3%) of the water bodies investigated (Fig. 2C). All identified R. percnurus sites were found in southern Lithuania, within the territory of Meteliai Regional Park located near one of the previously known R. percnurus sites. The comprehensive species inventory revealed that the total range of R. percnurus extends to approximately 177 km², constituting about 0.3% of Lithuanian territory. An investigation was conducted into the two previously documented (until 2018) R. percnurus sites (see Fig. 2A). The investigation revealed that the species had disappeared from one site, while the other remained abundant (see Site 1, Fig. 2C; Table 4). A total of 12 R. percnurus sites were identified during the inventory, of which 11 were previously unknown (Fig. 2C). However, within the last 5 years (2019–2024), nine sites initially identified as R. percnurus habitats have become extinct, leaving only three sites distributed over a relatively small area (Fig. 2C–D).

Habitats. As demonstrated in Table 3, the R. percnurus sites were characterized by their diminutive size, with individual water surface areas ranging from 0.01 to 0.60 ha and maximum water depths of 0.3 to 1.6 m. A total of three water bodies were determined to be of natural origin, specifically classified as small forest lakes. The remaining water bodies were dug many years ago either for peat extraction (four sites) or as reservoirs for cattle drinking (two sites). All sites were characterized by neutral pH values (6.9–7.6). Electrolytic conductivity exhibited a broad spectrum (45–562 µS · cm⁻¹). Measurements of total nitrogen and phosphorus in water samples indicated that all R. percnurus sites were eutrophic (Table 3). All water bodies inhabited by R. percnurus populations were densely overgrown with submerged and emergent macrophytes (Table 3). Of the R. percnurus sites, 50% were isolated closed water bodies, while the remainder were artificially reclaimed waters connected by small seasonal streams.

In 2019, only four water bodies were classified as having a favorable habitat state according to the approved criteria for R. percnurus habitat requirements (Table 3). Five sites were found to be in an unfavorable-inadequate state, and three in an unfavorable-bad state. However, in the last 5 years, most of the known habitats of R. percnurus have undergone a significant decline. As illustrated in Fig. 3, certain water bodies underwent complete desiccation, while others exhibited a substantial drop in water level. In summary, in 2024, only one habitat was found to be in a favorable state, while the remaining known habitats were classified as unfavorable (Table 5).

Figure 3. 

Disappearing Rhynchocypris percnurus habitat in southern Lithuania: (A) the first site view in 2019; (B) the first site view in 2024.

Population state. A comprehensive analysis of the total fish catch revealed that R. percnurus had become well established and dominant within the fish communities of five water bodies in 2019 (Table 4). A substantial age range of specimens (from 0+ to 4+) was identified in these sites, indicating well-developed populations and suggesting the further survival of the species. In one particular body of water, the species formed a monospecific fish assemblage (Site 12, Table 4). However, at certain locations, the population density of R. percnurus was minimal. The overall abundance of individuals captured in the studied water bodies ranged from 5 to 244 per 10 traps. As indicated in Table 4, the majority of populations exhibited a preponderance of females; however, in some water bodies, males demonstrated higher frequency. The proportion of juveniles exceeded 10% only in well-established and vulnerable populations; in all other cases, it was negligible or absent. In conclusion, a total of five sites were classified as being in a favorable state, three in an unfavorable-inadequate state, and four in an unfavorable-bad state. In the context of population state assessment, abundance was identified as the paramount consideration.

Nevertheless, a considerable shift occurred in the situation over the past 5 years. In 2024, R. percnurus was found to be well established and dominant within the fish community of a single water body (Table 5). During the preceding 5-year period, the species became extinct in nine water bodies (Fig. 2C–D). In two water bodies, the species appeared to be on the brink of extinction.

Accompanying fish species. In all cases, populations of R. percnurus were recorded from small, eutrophic water bodies exhibiting very low fish diversity. Such fish assemblages were characterized by the presence of one to three species, with no piscivorous species recorded. During sampling, only two additional fish species were documented: Carassius gibelio (Bloch, 1782) and Leucaspius delineatus (Heckel, 1843). Each species was present in eight (67%) of the R. percnurus sites. In 2019, no overlap between R. percnurus and predatory fishes was observed. Nevertheless, in 2024, individuals of Perca fluviatilis Linnaeus, 1758 and Esox lucius Linnaeus, 1758 were recorded at four sites where R. percnurus had previously been observed. These findings indicated a possible decline in the population of R. percnurus. It was revealed that predatory fish had been stocked in the lake by local anglers, albeit illegally, with the intention of creating a competitive environment for fish populations.

The paucity of data concerning the occurrence of R. percnurus in the Nemunas River basin during the preceding century is striking. Nevertheless, local anglers have attested to the species’ former prevalence in the area. The initial documentation of the occurrence of R. percnurus within the geographical confines of Lithuania was furnished by Kaupinis (2006), who reported this species in a water body near the town of Alytus in southern Lithuania. In 2010, a further location in southern Lithuania was documented as harboring R. percnurus. It is noteworthy that these two sites were the only reported occurrences of R. percnurus until the presently reported study (Fig. 2A). Importantly, this study constitutes the first inventory of this species to be conducted in Lithuania. Several mentions of R. percnurus captures from other Lithuanian regions many years ago were also recorded during communication with local anglers (V. Rakauskas, pers. com.).

It is hypothesized that, in the past, southern Lithuania functioned as a significant refuge for the Lithuanian R. percnurus population. This is attributable to the region’s topography, which is highly diverse, and to the presence of numerous small, shallow water bodies, spring-fed wetlands, and swamps. The excavation of ponds for household use, a prevalent local custom, is also a contributing factor. Populations of R. percnurus have also been documented in adjacent countries such as Poland, Belarus, and the Russian Federation. This observation suggests the potential for the species to have previously had a wide distribution within the Nemunas River basin. The potential habitats for this species should be comparable to those found in the northeastern part of the Russian Federation and Belarus, near Lithuania. These habitats are characterized by small, shallow water bodies, spring-fed wetlands, and swamps that exhibit significant seasonal fluctuations in water level. However, they are seasonally connected by springs (Ford 2024).

In 2019, a total of 12 populations of R. percnurus were documented in Lithuania (Fig. 2C). All of the sites were situated within the territory of Meteliai Regional Park, located exclusively in southern Lithuania. The species was discovered in only 3.3% of its suitable habitats during the intensive inventory. Similarly, the distribution range of R. percnurus is scattered non-uniformly in neighboring Poland. As demonstrated by Sikorska and Wolnicki (2025), there are about 80 sites of R. percnurus, distributed non-uniformly in the Vistula River drainage area. The presence of only one site in the Odra River drainage area was also noted. The distribution range of R. percnurus in Belarus and Russia remains to be fully elucidated.

However, within the last 5 years (2019–2024), most of the initially known R. percnurus sites in Lithuania have become extinct (Fig. 2C–D). The results strongly indicate that the process of species disappearance is accelerating. The primary cause of this decline was attributed to prolonged periods of hot and dry weather during the summer months, which resulted in a significant decrease in the water levels of R. percnurus habitats. In certain locations, the introduction of predatory fish species has been identified as another pivotal factor. A parallel decline in the population of R. percnurus has been observed in neighboring Poland for similar reasons (Sowińska-Świerkosz and Kolejko 2019; Wolnicki and Sikorska 2020; Wolnicki et al. 2022; Sikorska and Wolnicki 2025).

In Lithuania, known R. percnurus populations inhabit small (<0.7 ha), very shallow (0.3–1.7 m), eutrophic water bodies overgrown with submerged and emergent vegetation. This renders them highly vulnerable to complete destruction due to drying. Consequently, the majority of the known habitats of R. percnurus in Lithuania were found to be in an unfavorable, inadequate, or poor state. Such habitats are also typical for most R. percnurus populations in Poland (Wolnicki et al. 2022). All known Lithuanian sites contained only one to three fish species, with no piscivorous species present. Presently, the distribution of R. percnurus is confined to just three water bodies in Lithuania, constituting a single population. The assessment of species’ populations revealed that only one site could be distinguished as viable (in a favorable state).

In the past, R. percnurus is believed to have inhabited a variety of water bodies, including small, shallow forest lakes, spring-fed wetlands, and swamps characterized by significant seasonal fluctuations in water level. These bodies of water are seasonally connected by springs, enabling individuals to disperse short distances during spring floods. This strategy may have contributed to the species’ regional survival. However, extensive deforestation and the reclamation of wetlands and swamps have forced the species to occupy anthropogenically influenced habitats, such as flooded quarries and water retention ponds. The majority of these water bodies are isolated and contracting, thereby preventing dispersal and condemning small subpopulations to extinction.

Currently, R. percnurus faces additional threats, including climate change, the introduction of non-indigenous species, domestic and industrial pollution, and land-use changes, all of which contribute to accelerated habitat and subpopulation loss (Ford 2024). These threats are especially concerning, given that they all occur within the geographical area where Lithuanian R. percnurus populations have been identified. The most significant threats to the species are the ongoing shallowing of water bodies and low groundwater levels. Prolonged periods of hot, dry weather across several subsequent years have resulted in severe drying and shallowing of small water bodies in the region. Climate change, with protracted droughts and elevated summer temperatures, constitutes a principal threat to R. percnurus habitats in Poland as well (Wolnicki and Sikorska 2020). Another potential threat to Lithuanian populations is the recent alien invasive species Perccottus glenii Dybowski, 1877, which has spread to small water bodies in southern Lithuania (Rakauskas et al. 2021). Similarly, invasive species such as P. glenii and Ameiurus nebulosus (Lesueur, 1819) have been shown to cause the loss of R. percnurus populations in Poland (Wolnicki et al. 2022). Furthermore, the introduction of native predatory fishes into R. percnurus habitats is a growing concern, as recently highlighted in Poland (Wolnicki et al. 2022). This phenomenon was also observed in Lithuania, where several specimens of E. lucius and P. fluviatilis were captured in a habitat where the R. percnurus population was extirpated, suggesting a potential link. The most probable explanation is intentional introduction of these predators. It has recently been demonstrated that the deterioration of certain known habitats of lake minnows in Lithuania may be attributed to the combined effect of these factors.

At present, R. percnurus is considered one of the most endangered fish species in Lithuania’s inland waters. This poses a significant conservation challenge, raising the question of what measures can be taken to address the situation. Firstly, it is imperative to safeguard the extant genetic variability of R. percnurus within the Meteliai Regional Park by establishing an artificial subpopulation under strict protection within a controlled environment. This population would serve as a reserve for future use as breeding material. Secondly, the restoration of known habitats, including partial deepening of water bodies, is necessary for R. percnurus survival in Lithuania. However, the deepening of some of the most prospective water bodies has proven insufficient. Therefore, stocking with artificially cultivated R. percnurus juveniles is essential. Moreover, the establishment of new populations through the translocation of wild or cultivated individuals into suitable habitats would significantly increase the chances of species survival at the regional scale. New populations should be established in sites of the highest natural value, free from threats of illegal human activity such as the introduction of predatory fish. This would ensure the best prospects for long-term success. This approach has met with considerable success in Poland (Wolnicki and Sikorska 2020; Sikorska and Wolnicki 2025). It is evident that the conservation of R. percnurus populations in Lithuania’s inland waters necessitates the urgent implementation of comprehensive measures. These should encompass habitat revitalization, artificial breeding programs, and the translocation of fish populations.

This study revealed that R. percnurus is currently at risk of extinction in Lithuania. At present, a single population dispersed across three diminutive water bodies in a relatively compact geographical area has been documented within the country. The extant population is insufficient in size to ensure its long-term survival and success. Consequently, the conservation of R. percnurus in Lithuanian inland waters necessitates the urgent implementation of comprehensive active protection measures, with particular emphasis on the revitalization of previously documented habitats and the initiation of new ones.

Financial support for this study was provided by the State Service for Protected Areas, part of the Ministry of Environment of Lithuania. Funding was allocated through the European Community’s Important Fish Species Inventory Project (Project No. F4-2019-123). This study was also supported by the Ministry of Environment of Lithuania through the National Monitoring Programme of Invasive and Non-indigenous Species in Lithuania (Project No. 05.5.1-APVA-V-018-01-0012).

We express our sincere gratitude to Linas Juzumas for his invaluable assistance during fieldwork.