The European eel, Anguilla anguilla (Linnaeus, 1758), is a critically endangered species with a complex life cycle affected by many environmental factors. Its high flexibility in several biological parameters emphasizes the need for regional management strategies. To support management and population modeling, a study was conducted on the age, growth, and other biological characteristics of eels in a coastal lagoon—this being the first study of its kind conducted in the southwest of the Adriatic Sea for the European eel to date. A total of 435 silver eels were collected from commercial catches in the Karavasta Lagoon, Albania, at the start of their migratory journey from 2022 to 2024. The length–weight relationships revealed distinct growth patterns for both males (b = 2.8) and females (b = 2.7), indicating negative allometric growth. The ages of silver eels ranged from 4 to 11 years, with males averaging 6.2 years and females 7.0 years, the latter being significantly older. Throughout their lives in the lagoon, females also exhibited a higher mean annual growth in length (6.4 cm) compared to males (5.7 cm). Swim bladders of all eels were free from infection with Anguillicola crassus, a common and widespread nematode parasite. These results offer important insights into eel population dynamics in transitional waters and form the basis for conservation efforts in Adriatic Sea ecosystems. These data are vital for creating sustainable management strategies, mainly due to the decline in eel populations.

Anguillicola crassus, eel management, LWR, sex-related growth, silver eel, von Bertalanffy

The European eel, Anguilla anguilla (Linnaeus, 1758), is a catadromous fish species with a complex life cycle that involves migrating from freshwater and coastal waters to the Sargasso Sea for spawning (Tesch and Thorpe 2003; Miller et al. 2019; Podda et al. 2021). This species can thrive in a variety of freshwater and marine habitats, including lagoons, estuaries, and bays, as well as still waters, rivers, and creeks, up to an elevation of 1000 m above sea level (Tesch and Thorpe 2003; Adam et al. 2008).

Due to its complex life cycle, which includes migrations from freshwater to marine environments, the European eel faces numerous threats (Capoccioni et al. 2014). From 1980 to 2011, the recruitment of young eels declined rapidly (Moriarty 1990; ICES 2024). Recruitment has not increased again to date but has stabilized at a very low level. Based on ICES (2024) data, recruitment was estimated to be 0.5% of the 1960–1979 level in the North Sea and 7.4% in other European waters (ICES 2024).

The main factors that contribute to the decline of the European eel include climate change (Drouineau et al. 2018), loss and degradation of habitat (Bevacqua et al. 2015), migration barriers (Morais and Daverat 2016), overfishing (Feunteun 2002), pollution (Geeraerts and Belpaire 2010), and the introduction of various parasites such as Anguillicola crassus Kuwahara, Niimi et Itagaki, 1974 (see Kirk 2003).

As a result of this sharp decline, the species was listed as Critically Endangered by the International Union for Conservation of Nature (Pike et al. 2023). In response to these worrying trends, the European Union (EU) adopted targeted conservation measures, including the European Eel Regulation (EU 2007), which mandates EU member states to develop and implement Eel Management Plans (EMPs). These EMPs aim to support the recovery of eel populations through habitat restoration, fisheries management, and specific restocking efforts (EU 2007).

Recovery efforts are complex and slow, mainly because of the eel’s long generation time. It takes between 5 and 30 years before an eel begins to mature and migrate for spawning (Aoyama and Miller 2003).

The European eel is a panmictic species, meaning there is no population structure (Als et al. 2011). Models of population dynamics in eels usually focus on large spatial scales and are mainly based on existing research (Aprahamian et al. 2007; Lambert and Rochard 2007). However, for the European eel, population traits can differ depending on the habitat and specific systems where they complete their life cycle (Vøllestad 1992; Poole and Reynolds 1998; Melià et al. 2006; Lin et al. 2007). This variability can introduce bias into the models, which may lead to ineffective management strategies based on those models.

Most existing studies on silver eel populations focused on northern Europe (e.g., Aprahamian et al. 2007; Breteler et al. 2007; Bilotta et al. 2011; Andersson et al. 2012). In contrast, relatively few studies were conducted in southern regions (e.g., Bevacqua et al. 2007; Amilhat et al. 2008; Charrier et al. 2012). To our knowledge, no research has been carried out in the southeastern Adriatic Sea to estimate the age of migrating silver eels, except for the study of Milošević et al. (2021), which focused on the determination of otolith shape index values and microchemistry composition of European eel otoliths between riverine and lacustrine stocks from the Adriatic Basin in Croatia and Montenegro.

Therefore, the purpose of our study is to examine the European eel population in Albania’s largest—and most important—eel fishery lagoon (Fig. 1; MARD unpublished¹) during the migration of silver eels, focusing on their age, growth, condition, and infection with the parasite Anguillicola crassus. We hypothesize that the European silver eels show sexual dimorphism in some examined parameters.

Figure 1. 

The commercial landings for European eel in Karavasta Lagoon and the entire Albanian territory (MARD unpublished).

Study area. The Karavasta Lagoon (40°55′N, 019°29′E, Fig. 2) is located in the southeastern Adriatic between the Shkumbin and Seman rivers on the coast. The lagoon connects to the Adriatic Sea through three canals, with the northern canal requiring periodic dredging. It covers an area of 41 km², with a maximum length of 10.6 km, a maximum width of 43.8 km, a maximum depth of 1.3 m, and a mean depth of 0.7 m (MARD 2019). The lagoon’s mean salinity is 29.7‰ (range: 20.3‰–42.2‰; Koto et al. 2022), the annual mean water temperature is 19.7°C (range: 7–27°C), and dissolved oxygen levels are 10.6 ppm (range: 6.5–13.5 ppm; Koto et al. 2022). Eutrophication in the lagoon increases during the summer months (Marzano et al. 2010; Koto et al. 2022). Dystrophic crises accompanied by anoxic conditions were reported in certain parts of the basin following significant algal blooms (Crivelli et al. 1996; Chauvelon et al. 2006).

Figure 2. 

The location of the Karavasta Lagoon in Albania (map based on dataset of the GADM created with QGIS 2025; QGIS Development Team 2025).

The hydrogeomorphology of the Karavasta Lagoon is shaped by sediment input from the Seman and Shkumbin rivers, Adriatic erosion, and an expanding hydrographic network, with minimal freshwater inflow. The lagoon’s fish fauna includes 13 species (six marine and seven euryhaline species) (MARD 2019), with the dominant fish in shallow waters being Aphanius fasciatus (Valenciennes, 1821), followed by Atherina boyeri Risso, 1810, and Mugil spp. (Peja et al. 1996).

Sample collection. To determine the age at which eels in the Karavasta Lagoon begin their migration to the sea, a random subsample of approximately 140 eels at the silver eel stage was collected from a commercial catch in the lagoon each year at the start of their migratory journey in December from 2022 to 2024 (Table 1). The eels were caught at the primary barrier among the three barriers in the lagoon, which serves as the main migration route to the sea.

The collection process was conducted by the Fisheries Management Organization that oversees the Karavasta Lagoon. Eels were collected at the fishing location in the main fish barrier. The fish barrier is constructed from vertical plastic rods that span the entire width of the lagoon’s entrance, except for designated points that guide fish entering or exiting the lagoon. Behind this barrier, a fence made up of 30 to 40 fyke nets (5–6 m each, no leader, wing length 5 m, throat diameter 50 cm, mesh size in the cod end 14 mm; Fig. 3) is positioned within the large fish barrier, while 11 to 12 fyke nets are arranged in each of the two smaller barriers. These fyke nets extend across the full width of the canal connecting the lagoon to the sea (Fig. 4).

Figure 3. 

Fyke net used to catch European eel in the Karavasta Lagoon (MARD 2019)

Figure 4. 

Installation of the system of fyke nets along all the channel sections.

All fyke nets were emptied three times daily and checked for eels. All captured eels were transferred to a holding device, referred to as “marrota”. The sample for the study was taken no later than one week after the eel captures had accumulated in the marrota. The selection of eels for analysis was conducted randomly. For the study, only silver eels, identified by external characteristics such as large eyes and a silvering body, were selected by the research team in collaboration with the fishers.

Biometrics and dissection protocol. Some eels were analyzed in the laboratory on the day they were sampled. The remaining eels were stored in a plastic bag at −20°C for 5 to 30 days until analysis. For each eel specimen, body length (total length, L_T, ±1 mm) and body weight (total weight, W, ±0.1 g) were recorded. The L_T and W values of defrosted eels were corrected using formulas from Simon (2013). The Durif silvering index (Durif et al. 2009) was calculated by measuring the pectoral fin length (±0.01 mm) and eye diameter (±0.01 mm) with a caliper. Visceral fat level was assessed with a 0–3 scoring system, as described by Simon (unpublished²), where 0 indicates no fat, 1 low fat, 2 moderate fat, and 3 high fat. Sex was determined by examining the macroscopic gonads of sampled individuals (Tesch and Thorpe 2003). Additionally, the swim bladder of each eel was examined macroscopically to identify the presence of preadult and adult forms of the invasive parasite Anguillicola crassus. The 5-class scale developed by Hartmann (1994) and compiled by Lefebvre et al. (2011) was used to visually assess the severity of swim bladder damage caused by the parasite and to ensure comparability with previously published studies (e.g., Wysujack et al. 2014; Simon et al. 2023b; Unger et al. 2024).

Age determination. Sagittal otoliths were extracted from each eel and preserved in 90% alcohol until analysis. Age estimation was conducted using the cut and burn technique on sagittal otoliths, following the protocol described by ICES (2011). This method detects annual growth rings by identifying transparent areas, which represent summer growth, and opaque areas, which indicate winter growth. Three different readers (EH, JS, and MK) examined each set of otoliths, and any otoliths with inconsistent age estimates were considered unreadable and excluded from the dataset.

Data analysis. The length–weight relationship (LWR) was determined using the equation:

W = aL^b

where W is the whole wet body weight of the fish in grams, L is the fish length in cm, a is a constant (intercept) that reflects the body form of the fish, and b is the growth coefficient (slope). The regression parameter b is in the majority of fish species larger than 3.0, indicating positive allometric growth (Froese 2006; Verreycken et al. 2011), but in some species equal to 3.0 (isometric growth) or smaller than 3.0 (negative allometric growth). The parameters a and b are estimated via regression analysis on log-transformed data.

The Fulton condition factor (K) for each eel was calculated using the formula provided in Bagenal and Tesch (1978):

$$K=\left(\frac{W}{L_T^3}\right)\times 100$$

where K is the condition factor, W and L are variables as defined above, and 100 is a scaling factor to bring K to a manageable number range.

The statistical analyses were performed with R and RStudio (R Core Team 2023, v.4.3.1). Whether the samples and, if applicable, the residuals of the samples originate from a normally distributed population was tested at a significance level of 0.1 using the Kolmogorov–Smirnov test. The subsequent check of the values for variance homogeneity was performed at a significance level of 0.1 using the Levené test. As a result, the non-parametric Mann–Whitney (Wilcoxon) test was selected as the most suitable statistical method to compare groups. A p-value below 0.05 was considered statistically significant.

The eels were caught at the end of each year, which is why the otoliths ended with a growth zone, as the new winter ring had not yet formed, meaning they were age class “+”. For data analysis, the eels’ ages were rounded up to the next age class because the growth phase had already been completed, and growth would otherwise be overestimated. Age and growth parameters for each sex were analyzed separately due to sexual dimorphism in growth and maturation (e.g., Tesch and Thorpe 2003; Simon 2015).

The mean annual growth of each eel was determined by subtracting the mean size of glass eels from their length at the time of capture and then dividing by the eel’s age. According to Hegediš (2007), glass eels migrating into the Buna/Bojana River (Montenegro, Albania), which drains into the Adriatic Sea, had a mean L_T of 5.7 cm. Therefore, we used 5.7 cm as the mean size of glass eels for our calculation.

The von Bertalanffy growth model (Von Bertalanffy 1938) was applied to estimate eel growth, as it is widely accepted as the standard method for measuring fish growth (Lin and Tzeng 2009; Simon 2015). The mean size of glass eels after Hegediš (2007) and the mean lengths at different ages of eels were used to fit the L_∞, k, and t₀ parameters of the von Bertalanffy growth equation (Von Bertalanffy 1938):

$$L_t=L_{\mathrm{\infty }}(1-e^{(-k(t-t_0))})$$

where L_t is the length at time t, L_∞ represents the theoretical maximum length the fish can reach, k is the rate at which length approaches L_∞, t is the age of the eel in years, and t₀ is the (hypothetical) time at which the fish would have been zero size if it had always grown according to the von Bertalanffy equation, using Excel’s Solver function.

For this study, a total of 435 silver eels (235 males and 200 females) were collected from the Karavasta Lagoon (Table 1). The sex ratio of silver eels varied significantly between years, ranging from 30% to 60% for females. The L_T of silver eels ranged from 342 mm to 452 mm for males and from 418 mm to 634 mm for females (Table 2). The length frequency distribution of migrating silver eels was bimodal, and approximately 50% of the migrating silver eels were males (Fig. 5). 52% of the males had an L_T of less than 40 cm, whereas none of the females did (Fig. 5). Additionally, 59% of the females were smaller than 50 cm.

Figure 5. 

Length–frequency distribution for male (N = 235) and female (N = 200) silver eels from the Karavasta Lagoon, Albania, captured in December from 2022 to 2024. Note: the x-axis starts at 30 cm.

The LWR for the entire silver eel sample showed a strong positive correlation, with weight increasing proportionally to length, and the growth pattern was nearly isometric (b = 2.987, Table 3). The LWRs for male and female eels also displayed positive correlations, but with different growth patterns. Both male eels (b = 2.8) and female eels (b = 2.7) showed negative allometric growth, with weight increasing at a slower rate relative to length.

Female silver eels had a comparable mean K (0.18) and slightly higher (not significant) fat index (2.5) compared to males (0.18 and 2.2) (Table 2).

All otoliths were clearly readable, and the eels’ ages ranged from 4 to 10 years in males and 5 to 11 years in females. In each of the age groups five to nine, females were significantly larger than males (U-test, d.f. 1, P < 0.001, Fig. 6).

Figure 6. 

Correlation between age and length of male (grey boxplots, N = 235) and female (white boxplots, N = 200) silver eels from the Karavasta Lagoon, Albania. O = outliers, * = extreme values.

The overall mean annual increase in length over the years of life was higher in females (6.4 cm per year) compared to males (5.7 cm per year) (Table 2). The highest mean annual increase in length was shown by a six-year-old female with 9.6 cm per year (extreme value in Fig. 6). The current growth of eels from the Karavasta Lagoon was reflected by the von Bertalanffy growth formulas L_t = 41.3(1 – e^{–0.304(t + 0.517)}) for males and L_t = 52.2(1 – e^{–0.274(t + 0.444)}) for females (Fig. 7).

Figure 7. 

von Bertalanffy growth curves of male and female eels from the Karavasta Lagoon, Albania. Whiskers show one standard deviation.

Anguillicola crassus parasite was not found in any of the eels’ swim bladders examined, and all swim bladders examined had the Hartmann class 0, which represented swim bladders exhibiting no tissue alterations and characterized by a thin and transparent wall.

This study provides the first insights into biometric data and growth patterns of European silver eels from the Karavasta Lagoon and the southeastern part of the Adriatic Sea. As hypothesized, distinct differences between sexes in terms of length, weight, LWR, eye diameter, pectoral fin length, age, and growth were observed, which are crucial for understanding their biology, population dynamics, and for successful management.

Due to the random sampling across three different years, it is assumed that the length–frequency distribution of silver eels caught by fishers reflects the distribution of silver eels that migrated past the sampling site during the study period. The Karavasta Lagoon is well known, based on anecdotal information from fishers, for not producing larger-sized eels.

Eel populations with comparable or higher male proportions to those in this study have also been recorded in other estuaries and coastal lagoons. For example, Rossi and Villani (1980) observed that during sampling in Italy’s Lesina Lagoon, the number of male silver eels was nearly five times that of females. In the same study in Varano Lagoon, silver males outnumbered silver females by a ratio of over 13 to 1. More recently, Correia et al. (2021) found that in Santo André Lagoon, Portugal, male silver eels outnumbered females by more than four times. In contrast, female-dominated eel stocks were also seen in other estuaries and coastal lagoons (e.g., Fernéndez‐Delgado et al. 1989; Castaldelli et al. 2014).

Length–weight relationship and condition. Within a fish species, LWRs can vary significantly depending on sex, life stage, and gonadal development stage (Le Cren 1951; Froese 2006). In this study, differences were observed in LWR between males and females. This indicates sexual dimorphism in LWR, similar to Morato et al. (2001), who observed significant differences between males and females in two out of 15 coastal fish species from the Azores.

The observed b values of the LWRs in our study fall within the range reported for European eel (2.26–3.67, Boulenger et al. 2015; Froese and Pauly 2024). Despite the different body shapes of fish species, b is higher than 3.0 in most species, indicating positive allometric growth (an increase in relative body thickness; Froese 2006; Verreycken et al. 2011). In this study, the overall eel population demonstrated better growth, with an exponent (b) value of 2.987, when compared to the analysis of each sex separately. When examining females and males independently, both exhibited negative allometric growth (b < 3). This may be attributed to the limited size range present in the samples of both sexes. Milošević et al. (2022) reported for silver eels from Lake Skadar/Shkodra (Montenegro) positive allometric growth (b = 3.12) for females and negative allometric growth (b = 2.12) for males.

Although several studies focused on the LWRs of the European eel, most concentrated on lakes and rivers, with limited research on other habitats such as lagoons (Boulenger et al. 2015; Froese and Pauly 2024). LWR b-value data on this species in lagoon environments include the Bages-Sigean Lagoon in France (males = 2.94, females = 3.42, overall = 3.16), Comacchio Lagoon in Italy (b = 2.78), Camargue lagoons in France (males = 3.15, females = 3.22, overall = 3.36, silver = 3.15), Umm Hufayan Lagoon in Libya (b = 3.23), Homa Lagoon in Turkey (b = 3.27), and Bardawil Lagoon in Egypt (males = 3.01, females = 2.87, overall = 2.95; Melià et al. 2006; Acarli et al. 2014; Boulenger et al. 2015; Abdalhamid et al. 2018; Mahmoud et al. 2024). These values were higher than those recorded in our study. Regarding the LWR for eel in the Adriatic catchment area, Milošević and Mrdak (2016) reported a b value of 2.96 for Lake Skadar/Shkodra in Montenegro, which was lower than our findings for this species in both sexes.

K was comparable between the sexes, with males showing a mean K of 0.18 ± 0.03 and females a similar value of 0.18 ± 0.02. The correlation of −0.08 between length and K for males indicates a very weak negative relationship; as males grow longer, K tends to decrease slightly. A similar correlation of −0.16 for females also suggests a weak negative relationship, indicating that as females grow longer, K tends to decrease as well. Similar results were reported by Milošević et al. (2022) in a study conducted in Lake Skadar/Shkodra, which is part of the Montenegrin section of this body of water shared with Albania. Although this is a freshwater habitat, their study revealed comparable values for males, with a slight decrease in the correlation between length and K, while females showed a slight increase. Casalini et al. (2024) reported that when the maximum silvering index is reached, there is a reduction in K, length, and weight. Additionally, Blackwell et al. (2000) indicate that less favorable environmental conditions occur when K is low.

Comparable mean K values for wild eels in the 23.1–44.4 cm size range were observed in Umm Hufayan Lagoon in Libya, with 0.17 recorded in December (Abdalhamid et al. 2018). The mean K values estimated in this study also align with the mean values of 0.18 and 0.19 reported for both male and female eels in Bardawil Lagoon (Egypt; Mahmoud et al. 2024) and Lesina Lagoon (Italy; Rossi and Villani 1980). However, the mean K values of eels in this study were lower than those observed in eels from Ghar El Melh Lagoon (Tunisia; Dhaouadi et al. 2014). K values in eels can vary depending on environmental conditions, food availability, sexual maturity, and month, and they generally increase with body size (Abdalhamid et al. 2018; Simon 2023; Simon et al. 2023c, 2023a).

The fat index, an indicator of the eel’s energy reserves, showed that females had a slightly higher mean fat index (2.5) compared to males (2.2), suggesting that females are better prepared for migration and reproduction. This finding aligns with the silvering process, where fat reserves are essential for the eel’s long journey and survival during spawning (van Ginneken and van den Thillart 2000). Both sexes exhibit similar fat indices, but the greater fat stores in females likely reflect their reproductive strategy, which requires more energy reserves to produce eggs than sperm.

Age and growth. Overall, the growth rates and von Bertalanffy growth parameters of the studied eels fall within the ranges reported in previous research on eel growth (Tesch and Thorpe 2003; Froese and Pauly 2024). The L_T of eels when arriving on the European continent (glass eel size), as calculated with the von Bertalanffy functions from this study (60 mm; Fig. 7), also closely matches the observed length of glass eels found approximately 100 km away in the Buna/Bojana River (57 mm; Hegedis unpublished³). Additionally, the L_∞ values calculated for both sexes are consistent with those reported in earlier studies (Froese and Pauly 2024).

In the presently reported study, females exhibit a significantly higher L_∞ value and a greater mean annual length increment compared to males. However, the growth coefficient (k), which reflects the rate at which fish approach their maximum length, is higher in males than in females. This indicates that males reach their maximum size earlier than females. Our results show that females tend to age more and grow larger, likely reflecting reproductive strategies.

The sexual size dimorphism observed in the eels of this study matches research on the European eel and other fish species (e.g., Hart 1948; Imsland et al. 1997; Tesch and Thorpe 2003; Simon 2015). The tendency for larger females is well documented and is generally linked to reproductive benefits, such as higher fecundity and greater energy reserves needed for long spawning migrations (Vøllestad et al. 1986). This female-biased growth pattern is not unique to eels and can also be seen in some Mediterranean species, including European hake, Merluccius merluccius (Linnaeus, 1758) (see Hart 1948), sea bass, Dicentrarchus labrax (Linnaeus, 1758) (see Pavlidis et al. 2000), gilthead sea bream, Sparus aurata Linnaeus, 1758 (see Papadaki et al. 2024), turbot, Scophthalmus maximus (Linnaeus, 1758) (see Imsland et al. 1997), and dusky grouper, Epinephelus marginatus (Lowe, 1834) (see Marino et al. 2000). These broad patterns suggest a common evolutionary strategy among migratory or catadromous species, where larger female size improves reproductive success (Helfman et al. 1984; Davey and Jellyman 2005). The findings of this study support this broader trend and strengthen the idea that sexual dimorphism in growth is a reliable indicator of ecological or reproductive differences between males and females in fish populations.

The age of eels in the study ranged from 4 to 10 years, with males averaging 6.2 years and females averaging 7.0 years. The observed age range aligns with the general understanding that males typically mature earlier than females (Denis et al. 2022; Tahri and Panfili 2023; Rasmussen et al. 2024). The slightly higher mean age of females may indicate that they need more time to build the fat reserves required for migration and spawning. In the northern Adriatic lagoons of the Italian region, the mean age of migrating silver eels was between 8 and 9+ years (range: 7–9+ years; Mordenti et al. 2023; Casalini et al. 2024). Literature does not provide data on the age of migrating silver eels in lagoons and estuaries across Croatia, Montenegro, Greece, and Turkey. According to unpublished data provided by Papadopoulou and Sapounidis female silver eels in Greek lagoons are generally between 8 and 9 years old. These findings suggest that eels in the Mediterranean mature and become capable of initiating migration much earlier than those in the Atlantic and northern Europe, where silver eels range from 7 to 57 years old (e.g., Poole and Reynolds 1996b; Svedaung et al. 1996; Van Den Thillart et al. 2009; Simon 2015).

According to the calculated von Bertalanffy growth formulas, eels in the Karavasta Lagoon generally reach the legal minimum catch size of 40 cm after six years for males and after three years for females. The L_∞ values for about 15% of males and all females in the European studies exceeded the maximum lengths observed in this study (Froese and Pauly 2024). Poole and Reynolds (1996a) found that faster growth tends to result in lower L_∞ values compared to slower-growing eels. Additionally, the results support previous research indicating that eels from coastal areas grow faster than those from freshwater habitats (e.g., Fernéndez‐Delgado et al. 1989; Melià et al. 2006; Shiao et al. 2006; Lin et al. 2007; Simon 2013).

Anguillicola crassus. No case of the infection was found in the silver eel samples, and the swim bladder’s condition was intact across all eels. The swim bladder was completely clear, with elastic walls that lacked inflamed blood vessels. Several factors could explain the absence of this nematode. One possible reason is the lack of direct river inflows into the lagoon’s aquatic system. Although the Shkumbin River flows into the lagoon from the north and the Seman River from the south, neither river discharges directly into the lagoon, preventing the introduction of eels from other watersheds.

Furthermore, it is difficult for eels entering Karavasta Lagoon to migrate to nearby freshwater systems. Additionally, there are no eel restocking, trade, or aquaculture activities in any part of Albanian territory, including the lagoon. Lastly, the lagoon’s salinity, which ranges from 20.3‰ to 42.2‰ (Koto et al. 2022), fluctuates seasonally and may influence the absence of infection by A. crassus (see Kirk et al. 2000a; Kirk 2003; Sauvaget et al. 2003; Jakob et al. 2009). While this parasite generally thrives in freshwater environments (Sauvaget et al. 2003; Jakob et al. 2009), it can also be spread and transmitted in brackish waters (Kirk et al. 2000a, 2000b). However, higher salinity levels can negatively affect A. crassus during its free-living stages, reducing egg hatching, larval survival, and infectivity and limiting the availability of intermediate hosts (Kirk et al. 2000a; Lefebvre and Crivelli 2012). Despite these adverse conditions, the parasite has been recorded in eels from estuarine and marine environments, demonstrating its ability to adapt to different salinity levels (Pilcher and Moore 1993; Kirk et al. 2000a; Lefebvre and Crivelli 2012).

Our observations, along with the findings of Órfão et al. (2024) in Madeira (Portugal), suggest that another local population in the Karavasta Lagoon in the southeastern Adriatic Sea remains uninfected with A. crassus. The absence of this parasite, combined with the optimal condition of the swim bladder, suggests that these migratory eels are well prepared for their journey to the Sargasso Sea.

Study limitations. As noted by Shiao et al. (2006), some eels go through multiple migrations between freshwater and marine environments during their growth phases. In this study, we have not determined whether the sampled eels had previously moved between freshwater and saltwater habitats.

The study offers valuable insights into the population dynamics of European eels, emphasizing their silvering and migration. It presents data on length, weight, growth, and age, which are essential for developing accurate growth models necessary for effective eel population management and addressing their decline. The research highlights the importance of understanding sexual dimorphism and growth variations between male and female eels, which can help guide targeted conservation efforts to protect both sexes throughout their life cycle.

Furthermore, the study emphasizes the need for additional research that considers environmental factors such as water temperature, salinity, and food availability, since these elements are vital for a complete understanding of eel growth and maturation. Ongoing monitoring of silver eels in Albanian waters is also recommended to track population changes and support broader conservation efforts for European eels across the Mediterranean region and beyond.

We thank the Fishery Management Organization of Divjaka for their invaluable assistance in sampling the European eels used in this study. Their expertise and resources were crucial in collecting the data that formed the foundation of this research.

This project had no external financial support. The first author personally funded the research, covering all associated costs. The absence of external funding preserved the author’s independence in designing the study, analyzing results, and reaching conclusions without potential biases or influence from funding sources.