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Article

Fish Fauna, Ecological Quality and Conservation Challenges in the Balkan Transboundary Lake Dojran

by
Spase Shumka
1,
Olga Petriki
2,*,
Laura Shumka
3 and
Enkeleda Berberi
1
1
Faculty of Biotechnology and Food, Agricultural University of Tirana, Rruga Paisi Vodica, AL1025 Tirana, Albania
2
Laboratory of Ichthyology, Department of Zoology, School of Biology, Aristotle University of Thessaloniki, Box 134, GR54124 Thessaloniki, Greece
3
Faculty of Applied Sciences and Creative Industries, Barleti University; Rruga Frang Bardhi, Selitë, AL1060 Tirana, Albania
*
Author to whom correspondence should be addressed.
Fishes 2025, 10(6), 272; https://doi.org/10.3390/fishes10060272
Submission received: 9 May 2025 / Revised: 29 May 2025 / Accepted: 31 May 2025 / Published: 4 June 2025
(This article belongs to the Section Biology and Ecology)
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Abstract

The European Water Framework Directive (WFD) 2000/60/EC emphasizes the use of fish communities as key indicators for assessing the ecological quality of freshwater ecosystems. Despite over two decades of WFD implementation, many Balkan countries lack standardized ecological assessment indices, particularly for fish fauna. This situation complicates efforts to monitor and manage aquatic ecosystems, especially transboundary waters facing significant environmental pressures. In this context, our study assesses fish communities and ecological quality in Lake Dojran, a transboundary lake shared by Greece and North Macedonia. Fish sampling was independently conducted by each country (North Macedonia in 2021 and Greece in 2023), using benthic multi-mesh gillnets following standardized European methodologies (CEN 2005). A total of 12 out of 16 historically recorded fish species were confirmed. Higher catch per unit effort (CPUE) values were observed in 2021 (282.50 specimens/gillnet, biomass 6321.81 g/gillnet) compared to 2023 (207.83 specimens/gillnet, 2378.67 g/gillnet). Dominant species included Alburnus macedonicus and Perca fluviatilis. No significant differences were found in CPUE values based on either number of specimens (NPUE) or biomass (BPUE) across the different depth zones. Using the Greek Lake Fish Index (GLFI), ecological quality based on fish fauna was classified as “good” in 2021 and “high” in 2023, reflecting the low relative contribution of both introduced numerical abundance and omnivorous species biomass in total catches. This study contributes valuable baseline data for transboundary ecological management and conservation strategies, supporting efforts aligned with WFD objectives.
Keywords:
Lake Dojran; fish; ecological assessment; Water Framework Directive; Balkan Peninsula
Key Contribution: This study provides the first harmonized fish-based ecological assessment of Lake Dojran across its transboundary regions, revealing good/high ecological quality and offering a valuable baseline for coordinated conservation and monitoring efforts in line with the EU Water Framework Directive.

Graphical Abstract

1. Introduction

The European Water Framework Directive (WFD) 2000/60/EC was established to provide a comprehensive framework for the protection and sustainable management of European water bodies [1]. It mandates the assessment of ecological quality into one category, such as ‘high’, ‘good’, ‘moderate’, ‘poor’, or ‘bad’, based on biological, chemical, and hydromorphological quality elements. Among these, fish fauna serves as a key biological indicator as fish communities respond sensitively to both natural and anthropogenic pressures, offering valuable insights into ecosystem health [2,3,4]. Specifically, fish communities can reflect changes in water quality, habitat integrity, and ecological stability [4,5]. Anthropogenic pressures such as pollution, habitat degradation, overfishing, and climate change directly or indirectly impact fish assemblages, altering species composition, abundance, and trophic interactions [3]. Sensitive species, particularly endemic and specialist fish, often decline in response to increased pollution and habitat fragmentation, while more tolerant, opportunistic species may proliferate, leading to shifts in community structure [5]. Metrics such as species richness, diversity indices, species abundance, and the presence of indicator taxa are commonly used to assess ecological health and human impact on aquatic ecosystems [5,6,7]. By integrating fish community assessments into environmental monitoring programs, policymakers and conservationists can better understand ecosystem responses to stressors and develop targeted management strategies to mitigate negative impacts on freshwater biodiversity.
Despite over two decades of WFD implementation, many Balkan countries still lack standardized ecological quality assessment indices for various biological elements, including fish. As a result, the classification and monitoring of numerous aquatic ecosystems remain incomplete, impeding effective conservation and management efforts. These challenges are particularly pronounced in transboundary water bodies, where discrepancies in national methodologies, data availability, and policy priorities hinder the development of a unified assessment framework [8,9].
The inland waters of the Balkan Peninsula face increasing environmental pressures, including pollution, habitat degradation, resource overexploitation, species introductions [10], and climate change. Many lakes, rivers, and wetlands in the region exhibit significant ecological decline, highlighting the urgent need for robust monitoring programs and effective mitigation strategies. However, the absence of standardized methodologies for fish-based assessments further complicates efforts to evaluate and manage these freshwater systems.
In this study, we assess the fish fauna and ecological quality of Lake Dojran, a transboundary lake between Greece and North Macedonia, to provide insights into its current ecological quality. Although fish sampling was conducted on both sides of the lake, it was not initially part of a coordinated transboundary monitoring strategy. Instead, sampling efforts were carried out separately within national frameworks, with formal scientific collaboration between the two countries emerging at a later stage. Despite this, the available data allows for an evaluation of the lake’s fish community structure and ecological quality, contributing to broader conservation efforts in Balkan freshwater ecosystems. By aligning our analysis with WFD guidelines, we aim to enhance understanding of fish community structure in relation to habitat conditions and trophic interactions. By applying a fish-based ecological assessment approach, this study contributes to the ongoing effort to classify and monitor the ecological quality of Lake Dojran. Our findings provide a foundation for the development of regionally adapted assessment indices and support future conservation and management strategies in line with WFD objectives.

2. Material and Method

2.1. Study Area

2.1.1. Geomorphological Characteristics

Lake Dojran (Figure 1) is a shallow transboundary lake shared between North Macedonia and Greece, with a bottom elevation of 138 m above sea level (masl) and a maximum recorded altitude of 148 masl [11]. The lake has a maximum depth of 10 m and a total water volume of 262 million cubic meters. It has a maximum length of 8.9 km and a maximum width of 7.1 km, forming a relatively regular shape. Evaporation is the lake’s only natural outflow. The total catchment area is 271.8 km2, of which 32% is in North Macedonia. At normal water levels, North Macedonia accounts for 63.6% of the lake’s 42.2 km2 water surface area [12,13].
The lake basin was formed through a combination of tectonic and Tertiary geological processes, creating a karstified depression. The watershed sediments consist primarily of mineral-rich alluvial and limestone deposits, while a small portion of the North Macedonian side contains diluvial clay sediments [12,13,14]. The eastern and northern areas of the watershed are characterized by rocky terrain, covered with low vegetation and shrubs, while the majority of the basin is occupied by forests, semi-natural areas, and agricultural land.
The lake has no surface outflow and is mainly replenished by groundwater, small rivers, and direct runoff [15]. Between 1990 and 2010, Lake Dojran experienced a significant decline in water level, mainly between 1987 and 2003 [12], leading to reduced plankton populations, biodiversity loss, and water quality deterioration [13].

2.1.2. Ecological Importance and Conservation Status

Despite ongoing environmental challenges, Lake Dojran remains a biodiversity hotspot and has been designated as a key conservation area for European species and their habitats. The lake is part of several global and regional conservation initiatives, including Emerald Network of Areas of Special Conservation Interest [16,17], Balkan Green Belt [18], Ramsar Site—Wetlands of International Importance [19], Important Bird Area (IBA) [20], and Candidate Natura 2000 Site. Furthermore, Lake Dojran has been legally protected as a “Monument of Nature” under North Macedonian national law since 1977 [21].

2.1.3. Fish and Fisheries in Lake Dojran

A total of 16 fish species have been confirmed in the lake, 8 of which are endemic to the lake or the Balkan Peninsula, highlighting its conservation significance (Table 1). Species Carassius gibelio (Prussian carp) and Tinca tinca (tench) have established populations in the lake. Additionally, there are unconfirmed reports of several other species, such as Esox lucius (Northern pike) and Gambusia holbrooki (Eastern mosquitofish); however, many of these species do not maintain self-sustaining populations [22].
Historically, Lake Dojran has been one of Europe’s most productive natural lakes. During the 1950s, the lake’s average annual fish production was 180 kg/ha, with total annual catches ranging from 500 to 817 tons [11,23,24,25]. However, by 2002, the annual catch had declined to 25 tons. From 2010 to 2014, fish harvests remained stable at approximately 1500 tons annually [26]. However, overfishing and illegal fishing have put pressure on target species [27]. Recent fisheries data show that more than 90% of fish catches consist of Alburnus macedonicus, Rutilus rutilus, Carassius gibelio and Cyprinus carpio. Carassius gibelio populations have increased, comprising 46% of total fish captures between 2005–2006 [24].
Threats to the lake’s fish populations include overfishing and unregulated commercial fishing, agrochemical pollution from agriculture, which enhances eutrophication, sediment disturbance and habitat degradation, waste site leaks and nutrient loading, and water regime modifications (drainage, reservoirs, dams).

2.2. Samplings

Fish samplings were conducted in the North Macedonian part of the lake in March 2021 and in the Greek part in March 2023. The samplings followed reference [28] and were adapted to the specific characteristics of Lake Dojran, ensuring a standardized methodology across both national territories.
Benthic multi-mesh gillnets were used, each measuring 30 m in length and 1.5 m in depth, consisting of 12 panels with mesh sizes ranging from 5 mm to 55 mm (knot to knot) in the following sequence: 43 mm, 19.5 mm, 6.25 mm, 10 mm, 55 mm, 8 mm, 12.5 mm, 24 mm, 15.5 mm, 5 mm, 35 mm, and 29 mm. The mesh sizes followed a geometric series with a ratio of approximately 1.25 to ensure effective fish capture across different size classes, and each gillnet covered a surface area of 1.5 m × 30 m (45 m2).
The sampling effort included 32 gillnets in the North Macedonian part and 6 gillnets in the Greek part (Figure 1). The smaller sampling effort in the Greek part was a result of seasonal samplings (only the spring results are presented here). The nets were set at three depth zones: less than 3 m (13 gillnets), between 3 and 6 m (13 gillnets), and deeper than 6 m (12 gillnets). The gillnets were deployed before dusk, left overnight, and retrieved after sunrise, resulting in a 12 h sampling period.

2.3. Data Analysis

Captured fish were counted and identified to the species level. For some subsamples, total length (mm) and weight (g, ±0.1 g) were recorded. Catch per unit effort (CPUE) was calculated using two metrics: biomass per unit effort (BPUE), expressed as species biomass (g per gillnet), and number per unit effort (NPUE), expressed as species specimens (n per gillnet).
An Independent-Samples Kruskal–Wallis test was conducted in SPSS (version 29.00) to assess differences in CPUE among depth zones. Due to the limited number of gillnets deployed per depth zone in the Greek part of the lake (n = 2), data from both sampling events were pooled for conducting this analysis.
Olmstead–Tukey plots were employed to classify fish species into four categories: dominant (high frequency and high abundance), constant (high frequency, low abundance), occasional (low frequency, high abundance), and rare (low frequency and low abundance). Classification was based on frequency of occurrence, defined as the percentage of gillnet stations in which a species was present, and relative abundance and biomass, calculated as the percentage of the total number of individuals or total biomass, respectively. Thresholds for classification were set at 50% frequency and 5% abundance or biomass.
The ecological quality of the lake was evaluated using the Greek Lake Fish Index (GLFI), in accordance with the guidelines of the Water Framework Directive (WFD) [7]. The GLFI incorporates two key metrics: the relative numerical abundance of introduced species (Introduceda) and the relative biomass of omnivorous species (OMNIb). These metrics correspond to the Lake Habitat Modification Score (LHMS) and total phosphorus (TP) concentrations, serving as indicators of ecosystem degradation and eutrophication. Additionally, the GLFI takes into account specific environmental parameters such as altitude, maximum depth, and water alkalinity. It also considers the extent of anthropogenic land use within the watershed. The GLFI assesses the ecological quality of lakes by measuring the deviation from reference conditions—defined as those found in the absence of human activities—and expresses it on a five-tier color-coded scale. The index ranges from 0 to 1, where 0 represents highly degraded ecosystems and 1 corresponds to pristine conditions. Specifically, the quality class thresholds based on GLFI scores are as follows: high: (0.8–1], good: (0.6–0.8], moderate: (0.4–0.6], poor: (0.2–0.4], bad: [0–0.2]. The deviation from reference conditions is quantified using the Ecological Quality Ratio (EQR), which provides a standardized measure of ecosystem health in alignment with WFD directives.
For this study, the environmental parameters were set as follows: alkalinity: 4.84 meq/L, lake altitude: 142 masl, maximum depth: 9 m, NNLC: 95.10 km2. The fish species classified as omnivorous in this assessment included Rhodeus meridionalis, Cyprinus carpio, Carassius gibelio, Rutilus rutilus, Scardinius erythrophthalmus, and Tinca tinca.

3. Results

3.1. Fish Fauna Composition

In 2021, a total of 9040 fish specimens were recorded in the North Macedonian part of Lake Dojran, with a total biomass of approximately 202 kg (Table 2). In contrast, in 2023, 1247 fish specimens were captured in the lake’s Greek part, weighing approximately 14 kg (Table 2). The total CPUE was higher in 2021 concerning both specimen number and biomass (Table 2).
The most abundant species in 2021, based on numerical abundance (NPUE) and biomass (BPUE), were Perca fluviatilis, Alburnus macedonicus, and Rutilus rutilus. Perca fluviatilis exhibited the highest NPUE, indicating its numerical dominance, while Alburnus macedonicus followed closely in abundance. Regarding biomass, Perca fluviatilis and Rutilus rutilus had the highest BPUE values, reflecting their significant contribution to the overall fish biomass.
In 2023, the most abundant species were Alburnus macedonicus, Rhodeus meridionalis, and Perca fluviatilis. Alburnus macedonicus showed the highest NPUE, making it the numerically dominant species, followed by Rhodeus meridionalis. Regarding biomass, Alburnus macedonicus and Perca fluviatilis were the most abundant species, although their BPUE values were lower than in 2021.
The resulting Olmstead–Tukey plots are shown in Figure 2. Perca fluviatilis and Alburnus macedonicus were consistently identified as dominant species in terms of both abundance and biomass across both sampling years. In contrast, species such as Silurus glanis and Cobitis vardarensis were recorded at only a few sampling stations and contributed minimally to the overall abundance and biomass, thereby being categorized as rare.
No significant differences were found in NPUE (p = 0.411) or BPUE (p = 0.562) values across the different depth zones.
The mean NPUE in 2023 was 207.83 (SD = 263.79) specimens/gillnet, lower than the 2021 mean of 282.5 (SD = 135.92) specimens/gillnet (Figure 3). The NPUE range increased from 532 specimens in 2021 to 647 in 2023, with the minimum decreasing from 121 to 19 specimens/gillnet. The mean BPUE in 2023 was 2378.67 (SD = 2499.91) g/gillnet, compared to 6321.81 (SD = 2848.58) g/gillnet in 2021 (Figure 2). The BPUE range declined from 11,053.9 g in 2021 to 6391 g in 2023, with the minimum dropping from 2121.6 g/gillnet to 384 g/gillnet.

3.2. Ecological Quality Assessment

The values of the two metrics of the GLFI index, derived from the 2021 and 2023 samplings, are presented in Table 3. In 2023, the contribution of omnivorous species to catch biomass was nearly ten times lower compared to the earlier sampling. Likewise, the relative abundance of introduced species in the total catch was also lower. These lower values resulted in higher GLFI index metrics for 2023. Consequently, based on fish fauna, the ecological quality of the lake was classified as good in 2021 and high in 2023.

4. Discussion

4.1. Species Composition and Abundance

Lake Dojran was initially classified as part of the Vardar ecoregion [29], later adapted as part of Makedonia–Thessali on the Greek side [30]. It is considered a relic of the Plio-Pleistocene Peonic Lake [31,32], with its fish assemblage significantly shaped by historic river connections and colonization events. According to the hypotheses of [33], Euro-Siberian taxa spread southward into the Balkans during the Oligocene and Miocene through temporary river connections. A second wave of colonization occurred at the Pliocene-Pleistocene boundary via river capture between the Danube and Vardar, linking the Black Sea with the Aegean drainage system [34]. The former outlet of Lake Dojran, the River Doiranitis, likely facilitated the exchange of fish species, enriching the lake’s ichthyofauna with northern lineages while possibly eliminating former colonies.
Of the 16 previously recorded fish species in Lake Dojran, 12 were confirmed during this study. Among these, only the species Alburnus macedonicus is classified as Endangered (EN) [35], emphasizing its high conservation significance and the need for continued monitoring to ensure its population stability. Several other species, including Cobitis vardarensis, Pachychilon macedonicum, Rhodeus meridionalis, Sabanejewia balcanica, and Squalius vardarensis, are endemic to the Balkan Peninsula [36] and are categorized as Least Concern (LC), although their restricted distribution makes them vulnerable to environmental changes.
The absence of Barbus macedonicus (Macedonian barbel) and Gobio bulgaricus (Aegean gudgeon) was expected, given their rheophilic nature and preference for adjacent streams with only sporadic occurrences in the lake. Additionally, previously unverified species like Anguilla anguilla (European eel), Esox lucius (pikeperch), and Gambusia holbrooki (mosquitofish) were also not recorded in our sampling, supporting the assumption that these species lack established populations in Lake Dojran [22]. The higher species richness recorded in 2021 may reflect the higher sampling effort undertaken during that year, as increased sampling effort typically enhances the likelihood of detecting species with smaller or less stable populations [37].
Comparison of our findings with a previous study conducted by [38], who also used multi-mesh gillnets (although of a different type) during 2006–2007, generally reveals agreement. Ref. [38] reported that during spring, Rhodeus meridionalis dominated numerically (NPUE), while Pachychilon macedonicum dominated biomass (BPUE). Considering total annual catches, Alburnus macedonicus was the most abundant species, followed by Perca fluviatilis. Similarly, our study identified Alburnus macedonicus and Perca fluviatilis among the most abundant species in both 2021 and 2023, with Rhodeus meridionalis also numerically significant in 2023. Despite its numerical abundance, Rhodeus meridionalis contributed little to overall biomass due to its small body size, contrasting with other larger-bodied species. These findings align with previous results, reflecting that lakes in this region typically have fish communities dominated by a few species [39,40].
Differences observed in species ranking and abundances between the two surveys—considering that they were not conducted simultaneously—represent only approximations of relative abundance rather than precise quantitative comparisons. These differences may be attributed to variations in sampling methodology and effort, temporal fluctuations in fish populations, and differences in species mobility and catchability influenced by factors such as reproductive activity or seasonal behavior [41,42].
In our study, NPUE values ranged from 19 to 666 specimens per gillnet, with mean values of 207.8 in 2023 and 282.5 in 2021. BPUE values ranged from 384 to 6775 g/gillnet in 2023 and from 2121.6 to 13,175.5 g/gillnet in 2021, with corresponding mean values of 2378.7 and 6321.8 g/gillnet. Notably, BPUE values exceeding 3 kg/gillnet, such as those recorded in 2021, may indicate gillnet saturation—a condition in which the gillnet’s capacity to retain additional fish is limited due to crowding or entanglement—potentially leading to underestimation of actual fish abundance and biomass [43]. According to the same study, the maximum biomass retention capacity of this gillnet type is estimated at approximately 11 kg.
Zero catch values were not recorded in our dataset, contrasting with observations from other Mediterranean lakes, where oxygen depletion at greater depths often results in zero catches [37]. The absence of significant differences in CPUE across depth zones in our study may suggest a relatively homogenized fish distribution throughout the lake. This uniformity may have been facilitated by favorable environmental conditions during the sampling period, such as elevated oxygen concentrations throughout the water column.
Overall, our samplings revealed relatively high CPUE values compared to other shallow and deep lakes in Greece using the same methodology and gillnet type (45 m2 multimesh gillnets; [7]). By comparison, mean CPUE values reported by the authors of reference [7] per studied lake ranged from 35 to 379 specimens per gillnet, with a mean NPUE of 143,3 specimens/gillnet, and 595 to 4995 g/gillnet (BPUE), with a mean BPUE of 2029 g/gillnet. The BPUE values observed in our study were also among the highest reported for lakes in the wider region, exceeding those from Lake Prespa (1.7–3.2 kg/100 m2; [44]), Lake Skadar (2.5–3.5 kg/100 m2; [45]), and Lake Ohrid (1.1–1.9 kg/100 m2; [46]).
The above highlights Lake Dojran’s historically high fish productivity despite significant environmental stressors. Notably, substantial water-level reductions between 1987 and 2003 [12] led to deteriorating water quality, declines in fish abundance, and loss of critical spawning habitats [11]. Although water levels have since stabilized, fish populations and catch volumes have not shown significant recovery—possibly due to long-lasting ecological impacts from these disturbances. This limited recovery has occurred even under relatively low commercial fishing pressure; currently, only seven commercial fishers are registered on the Greek side of the lake. However, a lack of data on illegal and recreational fishing limits our understanding of the total fishing pressure on key species. Recent fisheries data from North Macedonia (2013–2018) show that recreational fishing now exceeds commercial fishing, reflecting a broader trend toward increasing popularity of recreational fishing in inland waters of Europe [47,48].

4.2. The Lake’s Ecological Quality

The ecological quality of Lake Dojran was classified as “Good” in 2021 and “High” in 2023, making it one of the few lakes in the region assessed with such favorable ecological quality based on fish fauna [7,39,40,49]. This assessment is mainly attributed to the low numerical presence of introduced species; specifically, only Tinca tinca and Carassius gibelio were recorded with both being targeted by commercial fishers. This indicates that the lake remains largely unaffected by major invasive species known to effectively establish populations in many freshwater ecosystems, thereby continuously expanding their distribution and contributing to the homogenization of fish fauna [50]. Notably, Lake Dojran is still free from highly invasive species such as Gambusia holbrooki, Lepomis gibbosus, and Pseudorasbora parva, which have widely established populations in other freshwater bodies [10,40,49].
The abundance of omnivorous species was low, which is generally considered an important factor in maintaining ecological stability. Omnivorous fish can significantly influence both direct and indirect lake trophic dynamics by altering the balance between pelagic and benthic-littoral food webs [51,52]. Initially, they can increase phytoplankton biomass by resuspending nutrients from sediments, thereby enhancing nutrient concentrations in the water [53,54]. In shallow lakes, eutrophication and climate-induced warming can lead to the increased dominance of omnivorous species. Some of these species, through selective feeding, suppress large-bodied zooplankton, weakening top-down control on phytoplankton and ultimately increasing phytoplankton biomass [52,55]. Additionally, omnivorous fish can influence not only the biomass but also the composition of both phytoplankton and periphyton [52]. Omnivorous fish may also indirectly affect benthic communities. They do so by preying on plant-associated macroinvertebrates, which can increase periphyton biomass, or through herbivory, which under eutrophic conditions may reduce periphyton biomass [52,56]. The relative scarcity of omnivorous species in Lake Dojran suggests a more resilient food web structure. This resilience may contribute to the lake’s ability to sustain high ecological quality despite external pressures.
Our assessment of Lake Dojran’s ecological quality based on fish aligns with previous evaluations using other biological quality elements, such as macrophytes [57] and benthic macroinvertebrates [58], both of which classified the lake’s ecological status as “Good”. However, this contrasts with the “Moderate” classification derived from phytoplankton assessments [59], resulting in an overall ecological quality rating of “Moderate” under the “one out, all out” principle of the Water Framework Directive [60]. This discrepancy suggests potential bottom-up disturbances, where primary producers reflect eutrophication-driven stressors affecting the lake’s ecological balance.

4.3. The Way Forward

Despite achieving a “Good” ecological classification based on fish fauna, Lake Dojran continues to face significant conservation challenges. The lack of standardized transboundary monitoring, limited data on commercial and recreational fishing impacts, and fragmented fishery management strategies hinder effective conservation efforts. Additionally, persistent threats such as overfishing, habitat degradation, eutrophication, invasive species risks, and climate-induced water-level fluctuations further jeopardize the lake’s ecological stability.
Addressing these challenges requires prioritizing strategic conservation actions. Essential measures include establishing coordinated transboundary fish monitoring programs, implementing standardized fish-based ecological assessments, and harmonizing fisheries management regulations between Greece and North Macedonia. Strengthening collaborative research initiatives will also be crucial in mitigating key threats, including invasive species introductions, nutrient pollution, and climate change impacts. This approach will facilitate data-driven decision-making, promote sustainable ecological management, and support the long-term conservation of Lake Dojran’s biodiversity.

5. Conclusions

Lake Dojran, a shallow transboundary lake between North Macedonia and Greece, remains one of the most ecologically significant freshwater ecosystems in the Balkans, supporting a species-rich ichthyofauna that includes one endemic and several conservation-priority species. This study provides an updated assessment of its fish community composition and ecological quality using standardized methodologies aligned with the Water Framework Directive (WFD). Twelve of the sixteen historically recorded fish species were confirmed, with Alburnus macedonicus—the endangered endemic species—being consistently among the dominant species in both years of sampling. High CPUE values were observed across both sampling periods, particularly in 2021, indicating the lake’s continued biological productivity. The ecological quality of the lake, as assessed using the Greek Lake Fish Index (GLFI), was classified as “Good” in 2021 and “High” in 2023. This was primarily driven by the low contribution of introduced and omnivorous species to total fish abundance and biomass. These results underscore the resilience of Lake Dojran’s food web and its relatively intact ecological status, despite persistent environmental pressures such as overfishing, pollution, and habitat alteration. Nonetheless, several challenges persist. The lack of coordinated transboundary monitoring, incomplete data on fishing pressures, and the risk of invasive species introductions represent ongoing threats to the lake’s ecological integrity. Moving forward, transboundary collaboration is essential. Harmonized monitoring protocols, joint conservation strategies, and unified fisheries regulations will be critical to safeguarding Lake Dojran’s ecological health. Establishing a shared data platform and promoting long-term ecological research can support adaptive management and ensure the sustainability of this valuable aquatic ecosystem in line with WFD objectives.

Author Contributions

Conceptualization: S.S. and O.P.; developing methods: S.S., L.S., and O.P.; conducting the research: S.S., O.P., and L.S.; data analysis: O.P. and E.B.; data interpretation and preparation of figures and tables: S.S. and O.P.; writing: S.S. and O.P.; All authors have read and agreed to the published version of the manuscript.

Funding

The research activities conducted in the North Macedonian part of the Lake has been supported by CEPF (Critically Ecosystem Partnership Fund) Project “Enhancing Dojran Lake Unique Biodiversity through Engagement of all Stakeholders and Implementation of Ecosystem-Based Approaches” implemented by Milieukontakt Macedonia (CEPF-109162). Part of this research was conducted in the framework of the project: “Monitoring/assessment of fish fauna in Kerkini and Doirani lakes according to the Directive 2000/60/EC” of the action “Management Actions for Protected Areas, Species, and Habitats in the jurisdiction area of the former Kerkini Lake Management Body of the Organization for the Management of the Natural Environment and Climate Change (NECCA)”.

Institutional Review Board Statement

The fieldwork was conducted with the appropriate authorizations from the competent authorities in both Greece and North Macedonia. Specifically, permits were secured through the Fishery Inspectorate in North Macedonia and the corresponding agency in Greece. Although there was no centralized Institutional Review Board involved, all procedures followed national regulations and ethical standards for field research involving fish fauna. The research received approval from the relevant committee in Greece for the conducted fish samplings.

Data Availability Statement

The data supporting the findings of this study are available within the paper. For additional information, please contact the corresponding author.

Acknowledgments

Special thanks to Kostas Papadopoulos, Antonis Iliadis, and Sotiris Mountzelos, members of the Natural Environment and Climate Change Agency (NECCA), Central Macedonia Management Unit of Protected Areas, Kerkini, 620 55 Kato Poroia, Serres, Greece, for their participation in fish samplings.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fishes 10 00272 g001
Figure 1. The wider area of Dojran Lake and the sampling stations per depth zone. The black line indicates the portion of the lake that belongs to North Macedonia.
Figure 1. The wider area of Dojran Lake and the sampling stations per depth zone. The black line indicates the portion of the lake that belongs to North Macedonia.
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Fishes 10 00272 g002
Figure 2. Olmstead–Tukey plots illustrating the distribution of fish species in Lake Dojran based on their frequency of occurrence (%) across sampling stations (gillnets) and their relative abundance or biomass. Specifically, (a,b) show relative abundance for the years 2021 and 2023, respectively, while (c,d) show relative biomass for the same years. Dashed lines at 50% frequency and 5% abundance divide the plot into four ecological quadrants: dominant species (upper right), constant (lower right), occasional (upper left), and rare (lower left).
Figure 2. Olmstead–Tukey plots illustrating the distribution of fish species in Lake Dojran based on their frequency of occurrence (%) across sampling stations (gillnets) and their relative abundance or biomass. Specifically, (a,b) show relative abundance for the years 2021 and 2023, respectively, while (c,d) show relative biomass for the same years. Dashed lines at 50% frequency and 5% abundance divide the plot into four ecological quadrants: dominant species (upper right), constant (lower right), occasional (upper left), and rare (lower left).
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Fishes 10 00272 g003
Figure 3. Violin plots showing the variation in CPUE across sampling stations in Lake Dojran for both sampling periods. The upper plots represent NPUE (Number per Unit Effort: specimens per gillnet), while the lower plots represent BPUE (Biomass per Unit Effort: grams per gillnet). A total of 6 gillnets were used in 2021 and 32 gillnets in 2023. The horizontal line within each box indicates the median CPUE, while the red diamond symbol (◊) represents the mean. Whiskers display the data range, excluding outliers.
Figure 3. Violin plots showing the variation in CPUE across sampling stations in Lake Dojran for both sampling periods. The upper plots represent NPUE (Number per Unit Effort: specimens per gillnet), while the lower plots represent BPUE (Biomass per Unit Effort: grams per gillnet). A total of 6 gillnets were used in 2021 and 32 gillnets in 2023. The horizontal line within each box indicates the median CPUE, while the red diamond symbol (◊) represents the mean. Whiskers display the data range, excluding outliers.
Fishes 10 00272 g003
Table 1. Fish species of Lake Dojran. Species origin and the most recent IUCN status are also provided (IUCN accessed 2/2025).
Table 1. Fish species of Lake Dojran. Species origin and the most recent IUCN status are also provided (IUCN accessed 2/2025).
Family Species Scientific Name Common NameOriginIUCN Global
Red List
AcheilognathidaeRhodeus meridionalisVardar bitterlingBELC
BlenniidaeSalariopsis fluviatilisFreshwater blennyNLC
CobitidaeCobitis vardarensisVardar spined loachBELC
Sabanejewia balcanicaBalkan golden loachBELC
CyprinidaeBarbus macedonicusMacedonian barbelBELC
Carassius gibelioPrussian carpILC
Cyprinus carpioCommon carpNLC
GobionidaeGobio bulgaricusAegean gudgeonBELC
LeuciscidaeAlburnus macedonicusDoiran bleakEEN
Pachychilon macedonicumMacedonian moranecBELC
Rutilus rutilusRoachNLC
Scardinius erythrophthalmusEuropean ruddNLC
Squalius vardarensisVardar chubBELC
PercidaePerca fluviatilisEuropean perchNLC
SiluridaeSilurus glanisWels catfishNLC
TincidaeTinca tincaTenchILC
E: endemic to the lake, BE: endemic to the Balkan Peninsula, N: native, I: introduced.
Table 2. Catch Per Unit Effort (CPUE) values for fish species in Lake Dojran, estimated for 2021 and 2023. CPUE is expressed as NPUE (Number Per Unit Effort, n/gillnet) and BPUE (Biomass Per Unit Effort, g/gillnet).
Table 2. Catch Per Unit Effort (CPUE) values for fish species in Lake Dojran, estimated for 2021 and 2023. CPUE is expressed as NPUE (Number Per Unit Effort, n/gillnet) and BPUE (Biomass Per Unit Effort, g/gillnet).
Species20212023
NPUE BPUE NPUE BPUE
A. macedonicus94.161327.51112.171503.33
C. gibelio2.66560.580.1729.50
C. vardarensis0.190.637.8325.17
C. carpio0.63139.72
P. macedonicum10.94186.530.172.67
P. fluviatilis105.381891.5724.67622.00
R. meridionalis7.1947.8259.67134.00
R. rutilus52.781661.343.1762.00
S. balcanica0.090.24
S. fluviatilis0.131.71
S. erythrophthalmus7.47237.69
S. glanis0.09213.59
T. tinca0.8152.89
Total282.506321.81207.832378.67
Table 3. Values of the two metrics OMNIb (relative biomass of omnivorous species in the benthic gillnet catches) and Introduceda (relative abundance of introduced species in the benthic gillnet catches) estimated for the years 2021 and 2023 in Lake Dojran, along with their respective Ecological Quality Ratios (EQR) and the total value of the GLFI, indicating the ecological quality class for each year.
Table 3. Values of the two metrics OMNIb (relative biomass of omnivorous species in the benthic gillnet catches) and Introduceda (relative abundance of introduced species in the benthic gillnet catches) estimated for the years 2021 and 2023 in Lake Dojran, along with their respective Ecological Quality Ratios (EQR) and the total value of the GLFI, indicating the ecological quality class for each year.
Parameter20212023
OΜΝΙb38.193.85
Introduceda1.230.08
EQR-OMNIb0.710.96
EQR-Introduceda0.690.73
GLFI0.700.85
Color coding follows the ecological quality classification of the EU Water Framework Directive (2000/60/EC).
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Shumka, S.; Petriki, O.; Shumka, L.; Berberi, E. Fish Fauna, Ecological Quality and Conservation Challenges in the Balkan Transboundary Lake Dojran. Fishes 2025, 10, 272. https://doi.org/10.3390/fishes10060272

AMA Style

Shumka S, Petriki O, Shumka L, Berberi E. Fish Fauna, Ecological Quality and Conservation Challenges in the Balkan Transboundary Lake Dojran. Fishes. 2025; 10(6):272. https://doi.org/10.3390/fishes10060272

Chicago/Turabian Style

Shumka, Spase, Olga Petriki, Laura Shumka, and Enkeleda Berberi. 2025. "Fish Fauna, Ecological Quality and Conservation Challenges in the Balkan Transboundary Lake Dojran" Fishes 10, no. 6: 272. https://doi.org/10.3390/fishes10060272

APA Style

Shumka, S., Petriki, O., Shumka, L., & Berberi, E. (2025). Fish Fauna, Ecological Quality and Conservation Challenges in the Balkan Transboundary Lake Dojran. Fishes, 10(6), 272. https://doi.org/10.3390/fishes10060272

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