Jandari Lake (41.429045, 45.217235; 290 m a.s.l.) is located on the Kvemo Kartli lowland, in a temperate dry climate zone (Csa according to Köppen-Geiger climate classification (Beck et al. 2018)). The lake is known in literature also as Karaia and Gardabani Lake (Fig. 1A,B). It is one of the most important eutrophic lakes in the Mtkvari River Basin in eastern Georgia. The river Mtkvari is the primary source of water supply for the lake. The surface area of the lake is 10.6 km², and its maximum depth approaches to 7.2 m. The Jandari Lake is a transboundary lake, half of which is located on the territory of Azerbaijan in the south-eastern direction. Lake sides are mostly swamped with reed and bulrush. The process of eutrophication continues, and the content of dissolved oxygen in the water is limited by the metabolism of phytoplankton (Tsiskarishvili et al. 2004). Jandari Lake is characterized by high productivity (296.8 kg/g), as suggested by a study of zoobenthos biomass (Pataridze 2004).

The young individuals (up to 4 years old) of 423 specimens of gibel carp were collected by fishing net from Jandari Lake between July 2014 and May 2018. Materials were collected in all seasons during the period examined: in 2014 (July and October), 2015 (April, May, November, and December), 2016 (January and October), 2017 (June, July, and November), and 2018 (May). In each month, at least 30 specimens were investigated. Fish were immobilized, and individuals were measured and weighed before the examination. Scales were used for age determination. Three age groups (1+, 2+, and 3+) with lengths of 8–15 cm have been studied. The parasites in the specimens were investigated by parasitological dissection of the fish. For further processing, standard parasitological methods were used (Bykhovskaya-Pavlovskaya 1969; Gussev 1983; Gerasev et al. 2010; Kvach et al. 2016). The skin, fins, and gills of each specimen were examined for ectoparasites. These organs were carefully scraped with a scalpel, and a mucus smear was made, which was evenly distributed on a glass slide in a drop of water, covered with a coverslip, gently pressed, and examined with a microscope. Eyes and internal organs were also investigated for endoparasites. Parasites were fixed, and temporary or permanent plates were prepared according to the taxonomy groups. Identification of parasite specimens was performed based on Bykhovskaya-Pavlovskaya et al. (1962) and Gussev (1978). Not all parasites were identified at the species level, though. The infection level has been expressed by prevalence and intensity and was calculated according to Bush et al. (1997).

Protozoa

Ciliata

Ichthyophthirius multifiliis (Ichthyophthiriidae) is a nonspecific, single-celled fish parasite that is widely distributed around the world. In Georgia, I. multifiliis was found in many species of fish in the water bodies of the river Mtkvari basin (Gogebashvili 2000), as well as in Paliastomi and Japana Lakes (Chernova 1973). In Jandari Lake, Ichthyophthirius was found in one (0.2%) individual of gibel carp in the autumn, with a very low intensity (3 parasites in a single individual).

Trichodina sp. (Trichodinidae) – this single-celled parasite is also nonspecific towards the host and is very common in fish. Sixteen species of Trichodina have been registered in freshwater fishes of Georgia, mostly in Cyprinidae (Chernova 1973; Gogebashvili 2000). In Jandari Lake, this protozoans was detected in 25 (5.9%) individuals of gibel carp. Most of them, 22 (5.2%), were registered in the spring, and in all cases, Trichodina occurred together with monogeneans.

Mesomycetozoea

Dermocystidium sp. (Dermocystidae) – has been found in Cyprinidae on the skin, subcutaneous tissue, and gills (Cervinka et al. 1974; Kurashvili et al. 1980; Kovacs-Gayer et al. 1986; Beretar and Shevkoplyas 2011). In Carassius fish, it has been registered in the kidneys and spleen (Landsberg and Paperna 1992), as well as in the eyes (Molnar et al. 2008).

In Jandari Lake, cysts of Dermocystidium (Fig. 2A,B) were found on the gills of 5 (1.1%) individuals of gibel carp (age 3+). In three cases, cysts were spherical, 1.5–1.8 mm in diameter, fringed with a double-layer membrane, and zoospores spherical, refracting, with the white vacuole. In two fish, the membrane of some cysts was disrupted, and spores occurred as heaps that connected with each other. In both cases, the shape and size of the zoospores were identical.

Helminths

Monogenea

Monogenea is a widespread group of parasites among fish, including crucian carps. It was found by Pakosta et al. (2018) that both in the sexual and gynogenetic forms of gibel carp, monogeneans were the most frequently observed group of parasites. The majority of Monogenea species are specific to the host fish (Dogiel 1958; Bykhovskaya-Pavlovskaya et al. 1962), though nonspecific species are detected as well. For example, 29 species of fish inhabiting freshwater bodies in Georgia turned out to be invaded by monogeneans that were nonspecific for them (Matsaberidze 1993, 1996); furthermore, parasite species that were systematically close to each other were often detected in taxonomically remote but ecologically close hosts, which was explained by the author due to the ecological specificity. Fifty-five Monogenea species have been identified in fishes of Georgian freshwaters (except Jandari Lake), and 7 species (D. vastator, D. anchoratus, D. crassus, G. katharineri, G. sprostonae, G. longoacuminatus, and G. shulmani) were registered in Carassius fish (Matsaberidze 1993). Only four species of Monogenea were found in 10.1% (n = 43) of the gibel carp examined in Jandari Lake during our investigation.

Dactylogyrus vastator (Dactylogyridae) is a typical species parasitizing Carassius spp. and Cyprinus carpio (Benovics et al. 2017). This monogenea is one of the most common parasites of freshwater fish in Georgia (Kurashvili and Petriashvili 1978). In Carassius fishes, D. vastator was registered in West Georgia (Matsaberidze 1993). In the present study, this species was recorded on the gills of 3% (n = 13) of young (1+ and 2+) gibel carp. It was also registered in the gills of Cyprinus carpio (3+) and the oral cavity of Hypophthalmichthys molitrix (3+) (Murvanidze et al. 2015).

Gyrodactylus elegans (Gyrodactylidae) has been reported in Cyprinidae fish inhabiting rivers and water reservoirs in Georgia (Chiaberashvili 1962, 1967; Petriashvili 1971; Kurashvili and Petriashvili 1978). The 4 species of Gyrodactylus (G. katharineri, G. shulmani, G. sprostonae, and G. longoacuminatus) have been detected in Carassius fish; however, the intensity of invasion was exhibited by single individuals (Matsaberidze 1993). The present specimen in Jandari Lake has been found on the gills of Cyprinus carpio (Chiaberashvili 1959). G. elegans was reported by us for the first time in 2015 from the gibel carp of Jandari Lake (Murvanidze et al. 2015). In the following years (2015–18), this species was fixed with low intensity (2–7 individuals per fish) in 1.8% (n = 8) of young gibel carp (1+).

Ancyrocephalus sp. (Ancyrocephalidae) – In Georgia, five species of Ancyrocephalus have been identified in freshwater fishes (Chernova 1973; Matsaberidze 1993; Kurashvili and Petriashvili 1978), from which only A. vistulensis (= Thaparocleidus vistulensis) has been recorded in Jandari Lake parasitizing Silurus glanis (Chiaberashvili 1959). At present, Ancyrocephalus sp. is a dominant monogenean parasite of the gibel carp of Jandari Lake; it occurs in 4.9% (n = 21) of fishes.

Diplozoon paradoxum (Diplozoidae) has been recorded in Georgian Cyprinidae fishes, including Barbus lacerta, Capoeta capoeta, and Rutilus rutilus (Matsaberidze 1993). Only a single case of low-intensity invasion with D. paradoxum has been revealed in gibel carp in Jandari Lake. On the other hand, other cyprinid fishes such as Ballerus sapa, Leuciscus leuciscus, Chondrostoma cyri, Alburnus filippi, and Aspius aspius were intensely infested by this species (own unpublished data).

Trematoda

Thirty-five species of digenetic trematodes are so far registered for the Georgian ichthyofauna (Murvanidze et al. 2018). Twelve of them are present in the larval stage, and twenty-three are adults. Diplostomum species are one of the most common helminths in freshwater fish in Georgia. They have been recorded in 16 species of fish (Murvanidze et al. 2018). Diplostomum clavatum (= Tylodelphys clavata) was recorded in Rhodeus sericeus in Jandari Lake in the 1950s (Chiaberashvili 1959). Between 2007 and 2015, five types of trematodes were identified in 17 specimens of gibel carp examined from the Azerbaijan part of Jandari Lake (Shakaralieva 2016). Two of them belong to the family Diplostomidae (Diplostomum mergi and D. nordmanni). Iranian researchers (Daghigh Roohi et al. 2014) reported D. spathaceum in the eyes of gibel carp with a fairly high percentage of invasion (58.8%).

Diplostomum spathaceum (Diplostomidae) is a widespread trematode among fishes (Heckmann 1983). In recent years, the parasite was found with great intensity (22 metacercariae) in 38% of the studied gibel carps of Madatapa Lake (Japoshvili et al. 2017; Murvanidze et al. 2019). Only several metacercariae were found in the eye lenses of the gibel carp of Jandari Lake. This species was registered in 1.4% of the investigated and 6.9% (n = 6) of infested fish (intensity: min. 1-2, max. 9–10 per fish).

Nematoda

Larvae of two taxonomic groups of nematodes (Philometridae and Anisakidae) were detected in the gibel carp population of Jandari Lake. Philometra and Contracaecum are widely distributed nematodes in freshwater fish in Georgia. Decades ago, 3 species of Philometra (Ph. abdominalis=Ph. ovata, Ph. rischta, and Ph. intestinalis=Kalmanmolnaria intestinalis) and 3 species of Contracaecum (C. siluriglandis, C. squalii, and C. aduncum =Hysterothylacium aduncum) were registered in freshwater fishes in Georgia (Chiaberashvili 1959; Chernova 1973; Kurashvili and Petriashvili 1978) There have been cases of intensive infestations with the Contracaecum in gibel carp – 27 parasites per fish (Innal et al. 2020), from 7 to 1078 parasites per fish (Turković et al. 2020).

Philometra sp. in Jandari Lake was found in 2.8% (n = 12) of the examined gibel carp. Larvae were localized in the swimming bladder (9 cases) and intestine (3 cases). The intensity of the invasion was low in all cases and affected 1-2 or 3 individuals per fish.

Contracaecum sp. – Larvae of this anisakid nematode were found in 6 cases (1.4% of the total) in Jandari Lake. They have been found in 3+ gibel carp and were localized in the swimming bladder (2 cases) and the body cavity (4 cases). The intensity was 1-3 individuals per fish.

Copepoda

Ergasilus sp. (Ergasilidae) are typical representatives of parasitic arthropods. The parasite is less specific to the hosts and is widely distributed among the fish. They are found in greater quantities in adult fish, where they accumulate as the fish's age increases (Poliansky and Shulman, 1956). In the young gibel carp of Jandari Lake, Ergasilus sp. was detected in the gills of 0.5% (n = 2) of the fish investigated. The infestation of gibel carp by this parasite was limited to one individual per fish.

There is no convincing evidence that fish parasite populations are stable over time (Kennedy 2009). Parasite communities in fish hosts are also not uniform. Their diversity, composition, and abundance vary across the geographical range of a host species (Poulin 2007). Each parasite’s host and parasites at different life history stages usually have their own unique direct and indirect responses to a stressor (Lafferty and Kuris 1999). The formation of parasitic fauna in freshwater fishes is determined by a complex of biotic and abiotic factors. The feeding behavior and habitat selection of the host fish, environmental changes, including those of an anthropogenic nature, and seasonal changes in radiation level are the key factors in causing differences in the parasitic fauna of fishes (Dogiel 1958; Poliansky 1958; Chubb 1977, 1979; Thomas 2002; Kennedy 2009; Garrido-Olvera 2012).

The fish parasite community shows a tendency for change during the eutrophication of lake ecosystems. The diversity of parasitic fauna of fish in eutrophic water reservoirs is much lower with the dominance of monoxenous parasites with a direct life cycle (Ieshko and Novokhatskaya 2008; Zargar et al. 2012). This tendency is well illustrated by the chronology of the fish parasite fauna study in the eutrophic Jandari Lake. It is clear that, over the decades, the composition of the parasitic fauna of fish changed in tandem with changes in the ecosystem of the water reservoir and ichthyofauna. In the 1950s (Chiaberashvili 1959), 20 parasitic species were discovered in the fish of Jandari Lake. Among them, the polyxenic Ligula intestinalis was the most common in Cyprinidae fishes. In the 1970s, the lake's dominant fishes (Silurus glanis, Abramis brama, and Aspius aspius) were infested by the helminths of the complex life cycle (Proteocephalus osculatus, Ligula intestinalis, and Neoechinorhynchus rutilli) (Kurashvili et al. 1975). Presently, in conditions of strong eutrophication of the water body, the parasitological situation is completely modified. According to our data, the peculiarity of the parasitic fauna of young gibel carp and several adult Cyprinid fishes (Cyprinus carpio, Hypophthalmichthys molitrix, and A. brama) is characterized by the dominance of a taxonomic group of parasites with a simple life cycle (Murvanidze et al. 2015). The parasite fauna of the gibel carp is predominated by ectoparasites localized on the gills and skin, namely representatives of Protozoa (Trichodina) and helminths (Monogenea), which spread without an intermediate host. Even though during the migration and wintering the Jandari Lake is used by up to 20 000 swimming birds (Paposhvili et al., 2016), in gibel carp and other fish of this lake, the ligulosis, which was extremely prevalent in the 50th–70th years of the last century, was not revealed at all. No cases of invasion with Acanthocephala were fixed. The impact of overgrown aquatic vegetation on intermediate hosts of heteroxenous fish parasites was recorded for Japana Lake (Chernova 1973). Our results showed that the current ecological processes in the eutrophic Jandari Lake have a negative impact on the intermediate hosts of helminth parasites, inhibiting the development of the first intermediate hosts of heteroxenous cestodes and acanthocephals (Copepoda: Cyclopidae; Amphipoda: Gammaridae), as well as the development of parasitic crustaceans. This opinion is supported by the hydrobiological studies conducted in the reservoir in the 2000s (Macharashvili and Tsiskarishvili 2004; Pataridze 2004), when the tendency of decreasing quality and quantity of zooplankton and zoobenthos was detected due to a burden of organic pollutants. At this stage, the absence of polyxenic helminths in the fish of Jandari Lake indicates that the number of intermediate hosts in the reservoir has further decreased. To this end, the view of Strelkov and Schulman (1971) that the sharp abundance of parasites in the fish parasite fauna with a direct development cycle can be explained by their resilience and easy adaptability to reservoir regression and adverse climate change could also be considered.

Fish parasite fauna is strongly affected by the seasonal changes in water temperature regime and feeding intensity (Chubb 1977, 1979). The parasitic fauna of freshwater fishes is much more rich and diverse during the spring and summer seasons—during the period of active feeding of fish (Dogiel 1958). Though in each particular case the local conditions restrict or favor the development of the parasite (Chubb 1977; Kennedy 1978, 2009). Analysis of the obtained results has shown that under local climate conditions the degree of invasion of gibel carp in Jandari Lake, in general, is low (P = 20%), though it is comparatively high in the spring (P = 36.8%).

Different groups and species of parasites respond in different ways to the changes in the seasonal temperature regime of the water. For instance, the Trichodina invasion was highest in the spring (16.5%), two cases were discovered in the summer, and no cases were discovered in the autumn. The highest level of monogenean invasion was observed in spring (Dactylogyrus, Gyrodactylus, and Ancyrocephalus: 5.2%, 6.0%, and 19%, respectively).

Other species such as Ichthyophthirius (0.5%) were recorded in autumn; Dermocystidium – in winter (2.9%); D. spathaceum – with very low extensity and intensity in spring (0.75%), autumn (3.25%), and winter (1.6%); Contracaecum sp. has been revealed only during the winter period (3.2%); and Philometra sp. – in spring, summer, and autumn. Based on these data, the parasites of gibel carp in Jandari Lake can be conventionally divided into two groups according to their seasonal distribution. More sensitive to seasonal temperature changes are Trichodina, Dactylogyrus, Gyrodactylus, Ancyrocephalus, and Contracaecum, while Dermocystidium, Diplozoon, Philometra, and Copepoda are more uniform throughout the year.

The distribution of parasites among age groups (1+, 2+, and 3+) of gibel carp (Table 3) has shown that the species Trichodina sp., Gyrodactylus elegans, Ancyrocephalus sp., and Phylometra sp. were detected in all three age categories of gibel carp, with maximum invasion in the 1+ age group. Dactylogyrus vastator was registered only in 1+ and 2+-aged gibel carps. Diplostomum spathaceum was found in a minimum quantity in 1+ age fishes in the spring. Though in two- and three-year-old individuals, the metacercariae of Diplostomum were detected in autumn and winter (Table 2).

Our investigations have shown that 1+ and 2+ age individuals of gibel carp were mostly invaded by species of the direct life cycle (Trichodina sp., Dactylogyrus vastator, Gyrodactylus elegans, and Ancyrocephalus sp.). These findings are in agreement with Dogiel’s (1958) postulates on the age dynamics of fish parasitic fauna, which state that young fish individuals are primarily infested with parasites, which develop without an intermediate host. The difference in invasion between female and male individuals of gibel carp in Jandari Lake was negligible (females, 37.5%, and males, 35.8%). Similar results were shown for the population of prussian carp in Lake Sakadaš, Croatia (Vuić et al. 2022).

In the parasitic fauna of fish, the coexistence of different species is often noted among the various forms of parasitic relationships (Kennedy 2009; Dallas et al. 2019; Giari et al. 2020). A parasitological examination of gibel carp in Jandari Lake revealed the simultaneous invasion of two different taxonomic groups into the fish gills—protozoans and monogeneans. In particular, during the spring and summer seasons, Trichodina was registered in all cases together with Monogenea: Trichodina+Ancyrocephalus – 14 cases (16.2% from infected fish); Trichodina+Dactylogyrus+Gyrodactylus – 8 cases (9.3% from infected fish). A similar synergistic relationship between these taxa in Georgia was registered in cyprinid fishes of the Aragvi river basin (Kurashvili 1975). Trichodina and Monogenea are major members of the parasitocenosis of fish gills. They are particularly well adapted to freshwater regressive conditions, and their aggregation is common in gill habitat.

The results of the present study showed that the parasitic fauna of gibel carp in Jandari Lake, in general, is poor. Eleven protozoan and metazoan parasites were discovered in young Carassius gibelio. Protozoa and Monogenea are emerging as the dominant taxonomic groups. Ten species were the first recorded in the fishes of Jandari Lake. Among them, the eight species (Ichthyophthirius multifiliis, Trichodina sp., Dermocystidium sp., Gyrodactylus elegans, Ancyrocephalus sp., Diplozoon paradoxum, Philometra sp., Contracaecum sp.) were registered for the first time in gibel carp of freshwater bodies in Georgia.

The process of eutrophication negatively affects the intermediate hosts of some helminthic parasites and can be singled out as the main ecological factor determining the parasitic fauna of gibel carp in Jandari Lake. The absence of polyxene helminths in the fish of Lake Jandari at the present stage indicates a sharp decrease in the number of planktonic intermediate hosts in the reservoir. On the other hand, the same factor contributed to the development of monoxenic Trichodina and Monogenea, which were found to be more resistant to regressive changes in the reservoir.

A synergetic relationship between Trichodina from protozoans and monogeneans of gibel carps was evident. In all cases, Trichodina appeared alongside monogeneans, resulting in the formation of a long-term association. We think that the invasive gibel carp is well adapted to the Jandari Lake ecosystem and contributes to the spread of parasitic protozoa and helminths in commercial fish. None of the revealed parasites represented a threat to human health.