A total of up to 350 individuals, representing 19 species across 19 genera and 10 families, were recorded in the Algeti River and reservoir (Table 1 and Suppl Mater 1). Of these, nine species are native to the Kura River basin (Fig. 2), while six (Cyprinus carpio, Silurus glanis, Sander lucioperca, Perca fluviatilis, Gobio caucasicus, and Neogobius fluviatilis) were introduced from Western Georgia (Black Sea drainage), and four (Pseudorasbora parva, Carassius gibelio, Hemiculter leucisculus, and Oncorhynchus mykiss) are invasive (Fig. 3) in the Caucasus region (Kuljanishvili et al. 2020, 2021). Notably, N. fluviatilis, not native to the Caspian Sea drainage, was widely introduced into Armenia in the late 20th century (Pipoyan and Tigranian 1998). Its first record in Eastern Georgia was in 2021 in the Debeda River, near the Georgia-Azerbaijan border (own unpublished data); thus, this Algeti record is the second evidence of its expanded range in the region. This species likely spread into the Algeti independently from Armenian populations or was introduced as a hitchhiker with P. fluviatilis from Western Georgia, a common, though often illegal, practice by anglers. Among the invasive species in the Algeti River and reservoir, O. mykiss appears to have escaped from a fish farm downstream of the Algeti HPP dam. The other invasives were likely introduced by anglers. The HPP dam on the Algeti River lacks a fish pass, isolating upstream populations from downstream ones. Furthermore, the reservoir's mass of water creates distinct habitats absent in the river. Consequently, the Algeti River basin can be divided into three relatively isolated parts: (1) the upstream section above the reservoir, (2) the reservoir itself, and (3) the downstream section below the reservoir (Fig. 4).

Figure 2. 

Native fish species to the Algeti River basin. A – Kura spirlin (Alburnoides eichwaldii); B – Kura barbell (Barbus cyri); C – Khramulya (Capoeta capoeta); D – Kura chub (Squalius agdamicus); E – European bitterling (Rhodeus amarus); F – Caspian trout (Salmo caspius); G – Kura bleak (Alburnus filippii); H – Kura loach (Oxynoemacheilus brandtii); I – Golden loach (Sabanejewia aurata). © Epitashvili G and Ramazashvili G.

Figure 3. 

Introduced/invasive species to the Algeti River basin. A – Gibel carp (Carassius gibelio); B – Common carp (Cyprinus carpio); C – Monkey goby (Neogobius fluviatilis); D – Sharpbelly (Hemiculter leucisculus); E – Stone moroko (Pseudorasbora parva); F – Colchic gudgeon (Gobio caucasicus); G – Rainbow trout (Oncorhynchus mykiss); H – Zander (Sander lucioperca); I – European perch (Perca fluviatilis); J – Wels catfish (Silurus glanis). © Epitashvili G and Ramazashvili G.

Figures 4. 

A – Upstream part of the Algeti River (Near vill. Algeti); B – Algeti Reservoir; C – Downstream part of the Algeti River (Near the Kura River confluence). © Epitashvili G and Ramazashvili G.

The upstream Algeti section spans from the river's headwaters in Algeti National Park, situated on the Trialeti ridge's southern slopes near Kldekari Castle, to the Algeti Reservoir. The reservoir’s variable water level marks the lower boundary, or confluence point. The Algeti river's headwaters, within the National Park, are well-protected from human impact. This upper section features a steep, narrow riverbed, largely covered by boulders and large sandstones, and with a fast flow rate. Near Manglisi village, the riverbed's slope decreases, and the speed of flow becomes more reduced. Here, the riverbed widens, and the water temperature significantly rises due to the approximately 900-metre elevational drop from the headwaters. Seasonal floods contribute to substantial downstream accumulation of debris, sludge, and household waste. This section is home to six fish species (Table 1); notably, Salmo caspius is the only species found within the National Park, with its downstream limit located a few kilometers upstream from the reservoir (N41.627245, E44.479057). Alburnoides eichwaldii and Barbus cyri are prevalent throughout the upstream section, excluding the headwaters; Capoeta capoeta ranges from the reservoir to Algeti village (N41.668247, E 44.391128), while Oxynoemacheilus brandtii is found only near the Manglisi bridge (N41.675471, E44.379027), and Squalius agdamicus is restricted to the section from the reservoir to near Abrameti village (N 41.604459, E 44.488717), at 800 metres above sea level. Notably, this marks its upper distribution limit within the river.

The Algeti Reservoir was commissioned in 1983, primarily to store water for irrigation. At maximum water level, the reservoir's surface area reaches 2.3 km², and its maximum depth is about 42 m. Locals report that prior to the dam's construction, this section of the river was known for its abundant fish, with poaching being common. The lack of a fish passage on the dam has severely disrupted fish migration between upstream and downstream areas.

A survey of the reservoir revealed ten fish species (Table 1). The most abundant species caught in the Algeti Reservoir is Perca fluviatilis (33.5% of the total catch), introduced from Western Georgia. Four other species, Carassius gibelio, Squalius agdamicus, Capoeta capoeta, and Silurus glanis, are also common, each representing 10-17% of the total catch (Fig. 5). The remaining species, Sander lucioperca, Cyprinus carpio, Alburnoides eichwaldii, Alburnus filippii, and Pseudorasbora parva, are relatively rare, comprising less than 2% of the total catch.

Figure 5 

. Percentage distribution of commercially important fish in the Algeti Reservoir during the 2020-2021 survey.

The downstream section of the river begins at the dam and extends to Kesalo II village in the Marneuli municipality, where it joins the Mtkvari (Kura) River. Water is discharged from the reservoir through three main pipes and one emergency pipe. The first main pipe connects to the lower riverbed, the second to the former HPP (no longer operational), and the third to the reclamation station. Although not studied, we assume the pipes' flow is rapid enough to impede upstream fish migration. The downstream section is characterised by numerous human-modified habitats. Anthropogenic impacts, such as wastewater, household waste pollution, and riparian forest degradation, are prevalent and intensify towards the Kura confluence.

This section, with 14 fish species (Table 1), is the most diverse. Fish species distribution is heterogeneous. Only three species — Alburnoides eichwaldii, Barbus cyri, and Oncorhynchus mykiss, are found immediately below the dam. The latter entered the river from a trout farm near Tbisi village (N41.588762, E44.538311), just hundred of metres from the dam. In the mid-downstream section, near Marabda village (N41.518376, E44.756174), eight species were caught: A. eichwaldii, B. cyri, Gobio caucasicus, Alburnus filippii, Squalius agdamicus, Capoeta capoeta, Oxynoemacheilus brandtii, and Sabanejewia aurata. The water level drops significantly from the Marneuli municipality border to the Kura confluence, due to irrigation use (“Reclamation of Georgia” LLC, 2021). Simultaneously, household waste and sewage water pollution dramatically increase, with recent detections of heavy pollution (ammonium compounds, sulphates, phenols, heavy metals, and petroleum products) (Lomsadze et al. 2016). A field survey in this area (N41.403818, E45.001482) yielded no fish species. In the lower downstream section near Kesalo II village (N41.390930, E 45.033301), seven species were observed: C. capoeta, B. cyri, Rhodeus amarus, Pseudorasbora parva, Carassius gibelio, Hemiculter leucisculus, and Neogobius fluviatilis. These species, except C. capoeta and B. cyri, are unique to this section, indicating their entry into the Algeti from the Kura River.

Human intervention has significantly impacted the species composition and distribution of fish in the Algeti River. Overall, the native fauna has been severely altered throughout the river’s length. The dam and reservoir construction created extensive habitats for native species like Capoeta capoeta, Alburnoides eichwaldii, Squalius agdamicus, and Alburnus filippii, now more abundant than expected in the corresponding river sections. Capoeta capoeta populations from the reservoir migrate upstream to spawn in the river's fast currents, a typical behaviour supported by the age distribution in our catch. Conversely, A. eichwaldii is abundant in the upstream Algeti River, suggesting it spends much of its life cycle there and reproduces in the reservoir's shallow areas. This view is supported by A. eichwaldii individuals only being observed in the reservoir during their spawning period (May-June), at maturity stages 3 and 4. Squalius agdamicus, considered a riverine species, is found in all three river sections. However, it is primarily restricted to the river mouth and reservoir, with few individuals caught in the lower upstream flow. It appears to use the reservoir as a feeding environment, due to the higher abundance of prey in this area of the catchment, an assessment confirmed by size distribution (upstream individuals were much smaller than those in the reservoir). Alburnus filippii, has a similar distribution to S. agdamicus, and does not seem to utilise the upstream river; its population is limited to the reservoir. Two other small-bodied native species, Barbus cyri and Oxynoemacheilus brandtii, which likely originally occurred throughout the river, now have isolated populations in the upstream and downstream sections. The dam and the heavily polluted river section near Marneuli act as insurmountable barriers for these species. Salmo caspius also only occurs in the isolated upstream population. In the downstream section, A. filippii, Rhodeus amarus, Gobio caucasicus, and Sabanejewia aurata are restricted to the river mouth, and do not approach the dammed area. The range of these species within the Algeti River appears heavily contracted due to anthropogenic factors (dam, pollution, and introduced species).

The introduction of non-native species poses a threat to native fauna and the environment, although quantifying this impact remains challenging due to the lack of baseline data. Nevertheless, the abundance of non-native species, including aggressive predators like Perca fluviatilis and strong bioturbators like Carassius gibelio, has a significant impact. Non-native species distribution shows at least six species in the downstream section and another six in the reservoir, while the upstream section remains free of non-native species. This pattern appears attributable to limited local species releases and the less favourable environmental conditions for non-native species upstream. Only Cyprinus carpio was introduced legally into the Algeti Reservoir; this was done for aquaculture purposes soon after the reservoir was put into operation, about 40 years ago (pers. obs.). However, its population is now endangered. Fishing efforts in the last two years yielded only one individual in 2021, and local anglers confirm (pers. comm.) the severe depletion of the carp population. The introduction of other species into the Algeti River system occurred either accidentally or intentionally and illegally by anglers. Deliberate illegal introductions include C. gibelio (an older introduction, possibly accidental with C. carpio fry), Silurus glanis (approximately 16 years ago), P. fluviatilis (2016-2017), and Sander lucioperca (2022), all represent species of interest to anglers (data from local fishermen and social networks; see also Kuljanishvili et al. (2020), Mumladze et al. (2022), Epitashvili et al. (2023b)). According to local fishermen, the populations of all species in the reservoir, important for fishing, have recently declined and are now at extremely low densities due to the introduction of P. fluviatilis. Accidental introductions include Pseudorasbora parva, Hemiculter leucisculus, Neogobius fluviatilis, Gobio caucasicus and Oncorhynchus mykiss. These species might have spread as hitchhikers with other species or entered the river independently via the Kura River. All of them, except O. mykiss, are recorded in the downstream part of the river, while O. mykiss appears to have escaped from nearby trout hatcheries.

The fish fauna and ecosystems in the Algeti River basin are under significant anthropogenic pressure. The most pronounced threats include habitat fragmentation, pollution, invasive species, and illegal activities (poaching, non-native species releases, and habitat modification), which collectively exacerbate the effect. The dam's construction, lacking a proper fish passage, severely limits native fish migration (at least towards upstream) and results in population isolation. Additionally, another barrier near Tbisi village, a steel construction built by a nearby restaurant owner (Fig. 6A) for rainbow trout releases, is illegal and violates Article 48 of the "Code of Administrative Offenses of Georgia" (https://matsne.gov.ge), further disrupting connectivity. In the mid-downstream section, there is heavy pollution from household waste and sewage peaks at the Marneuli municipality. The riverbed in this area is covered with municipal sediment (sludge, fine fraction), reaching 50-70 cm deep in some places. This section is nearly sterile, devoid of fish and even macroinvertebrates. These barriers likely cause complete isolation of the upstream segment and the reservoir. Several native species to Kura River basin, such as Luciobarbus mursa, Acanthobrama microlepis, and Alburnus chalcoides, are still present in the Kura River and would be expected to utilise the Algeti River, but do not enter this section due to the pollution. The remaining native species upstream of the reservoir are threatened with local extinction. The isolated headwater trout population is particularly vulnerable, and within Algeti National Park, a decline in the trout population is clear despite the lack of comprehensive monitoring data, even though the National Wildlife Agency of Georgia has released trout fry multiple times between 2020-2025.

Figures 6. 

A – closed section of the Algeti River near Tbisi village; B – a snapped net at the bottom of the Algeti Reservoir. © G. Ramazashvili

Non-native and invasive species pose another substantial threat, significantly impacting the river and replacing native fauna. In the reservoir, non-native species, including the invasive crayfish Pontastacus leptodactylus (Blaha et al. 2021), which was found in Silurus glanis gut contents, have been introduced; leading to the establishment of a homogenised biota with widespread non-native taxa, while the local fauna experiences significant declines. Currently, half of the fish species in the river are introduced. However, beyond speculations and opinions from local anglers, research-based understanding of non-native species' effects on local fauna and ecosystems is lacking.

Poaching remains a significant negative factor throughout the river and reservoir. For instance, during periods of lowered water levels in the Algeti Reservoir, numerous abandoned gill nets were observed (Fig. 6B). Although this poaching primarily targets introduced species, abandoned nets (and other poaching activities), coupled with limited connectivity and pollution, pose a severe threat to Algeti native fish species. In addition, habitat modification, degradation, and water abstraction have also impacted the Algeti River. In most sections, the surrounding riparian forests have been cleared, leading to excessive sediment input during rainy periods. Artificial deepening or modification of the riverbed is also common, especially near settlements. Near the Marneuli municipality, the irrigation system extracts water from the river to irrigate 2034 hectares of agricultural land (“Reclamation of Georgia” LLC, 2021), significantly affecting fish migration and water quality. Consequently, downstream of Marneuli municipality, the river flow diminishes significantly to ecological flow during dry seasons.

In conclusion, the ecological condition of the Algeti River basin is critical. Therefore, it is essential to implement necessary measures to mitigate negative pressures and preserve the habitats and biodiversity of this water body. The river's headwaters are part of a protected area system, and two species found in the river – Sabanejewia aurata and Salmo caspius – are listed as vulnerable (VU) in the Red List of Georgia. However, these conservation designations alone are insufficient to safeguard local fish fauna. Greater efforts, such as law enforcement against poaching and other illegal activities, pollution control, and habitat modification management, are needed to mitigate biodiversity loss in the Algeti River.

The authors have declared that no competing interests exist.

No ethical statement was reported.

No funding was reported.

GE, GR, BJ, LM – research conceptualisation; GE, GR – investigation and data collection; GE, GR, LM – data analysis and interpretation; GE, GR – writing – original draft in Georgian and translation; LM – writing final draft; BJ, LM – writing, review and editing.

Giorgi Epitashvili https://orcid.org/0000-0001-5921-5362

Bella Japoshvili https://orcid.org/0000-0003-0966-0622

Levan Mumladze https://orcid.org/0000-0002-2172-6973

All of the data that support the findings of this study are available in the main text or Supplementary Information.