For many years, brackish environments were avoided by researchers who have focused their studies on more homogeneous environments such as the open sea or freshwater reservoirs. Therefore, knowledge about the organisms living there and their parasites is very scanty.
The shallow coastal lakes are very dynamic ecosystems with the constant fluctuations in physical and chemical parameters. The communities of organisms living there must adapt to these fluctuations . The coastal lake Łebsko is connected with the Baltic Sea through the Łeba Channel so that freshwater and marine species, with a predominance of euryhaline species occurred.
Because of the conservation programs conducted in the Słowiński National Park, ichthyologic study has been carried out regularly, but parasites of different fish species from the Lake Łebsko has not been studied quite extensively [2-4].
Metazoan parasites of European perch from the Polish coastal zone were studied from the Pomeranian Bay and Szczecin Lagoon , Gulf of Gdańsk [6,7], Vistula Lagoon [7,8], and from the coastal lake Resko and the Baltic Sea off the Dźwirzyno . The majority of studies on parasites of perch were carried out outside of the coastal zone [e.g. 10-15].
Fish living in lacustrine environment are important to the ecosystem and to carried fishery management. They are associated with the other inhabitants of the lake by food webs and act as intermediate, definitive or paratenic hosts for many species of parasites (autogenic and allogenic). The aim of this study was to present the parasite fauna and communities of parasites of European perch from the coastal lake Łebsko, the largest lake within the Słowiński National Park, and complete knowledge on the distribution of parasites affecting perch in Poland. The parasite communities were analyzed and compared with results of studies from the Polish coastal zone.
The coastal lake Łebsko (54° 42′ 49″ N, 17° 24′ 32″ E) is located on the Central Coast of Poland. Lying in the Słowiński National Park, Łebsko with an area of 7142 ha, is the largest coastal lake in Poland. It is shallow water bodies (max. depth 6.3 m, average 1.6 m), bordered by reeds and sedges, which provide good shelter for a rich variety of migratory and breeding water birds .
This mixo-oligohaline reservoir is separated from the Baltic Sea by a narrow strip of land (Mierzeja Łebska) and is connected to the Baltic Sea by the Łeba River . This lake is situated 30 cm above the sea level, and seawater flows back into them when winds blow from the north and northwest. After inflow salinity could increased to 3‰ . As a result of this connection, fish or other organisms, can migrate between the lake and the Baltic coastal waters.
Many fish species, both freshwater and marine, are found here, and the following are commercially caught in Łebsko: carp bream Abramis brama (Linnaeus, 1758), roach Rutilus rutilus (Linnaeus, 1758), European perch Perca fluviatilis Linnaeus, 1758, European ell Anguilla anguilla (Linnaeus, 1758), pikeperch Sander lucioperca (Linnaeus, 1758), Northern pike Esox lucius Linnaeus, 1758 and various salmonids. Rarely tench Tinca tinca (Linnaeus, 1758), crucian carp Carasius carasius (Linnaeus, 1758), Prusian carp Carasius gibelio (Bloch, 1782), rudd Scardinius erythrophtalmus (Linnaeus, 1758), white bream Blicca bjoerkna (Linnaeus, 1758) and ide Leuciscus idus (Linnaeus, 1758) are fished.
European perch is very important link in the lacustrine food web and one of the most valuable commercial fish species in the coastal lakes and offshore zone of the Baltic Sea. This species has been caught also in the sea up to 4 km from the coasts . Perch fry feed on zooplankton and larger invertebrates while adults feed on fish and various invertebrates.
Materials and Methods
Throughout six years (2001-2006), a total of 556 European perch from the coastal lake Łebsko were sampled. The fish were purchased from professional fishermen. Fish were weighed (range 35.2-770.4 g, average 194.2 g) and measured (range 15.0-38.0 cm, average 23.8 cm). European perch were subsequently examined for ectoparasites and endoparasites using the standard procedures of parasitological examination and parasitic identification [e.g. 20]. The examination included the skin, fins, gills, eyes (lens and vitreous humour), body cavity and visceral organs (stomach and intestine, liver, heart, kidney, gonads and swim bladder).
The parasite communities were identified based on location in/on the hosts (e.g. community of eye, intestinal or gill parasites) and depending on where they complete their life cycles and on their dispersal strategy (autogenic and allogenic species) . Autogenic parasites utilizing fish either as intermediate or definitive hosts and completing their entire life cycle in aquatic habitats, and allogenic parasites utilizing birds, mammals or reptiles as definitive hosts (fish act only as intermediate hosts) and completing their life cycles mainly in terrestrial environments . Allogenic species can be easily transferred between reservoirs than autogenic species .
Parasitological indices (prevalence, mean and range intensity, abundance) were calculated according to Pojmańska  and Bush et al. . Prevalence (expressed as a percentage) is the number of hosts infected with a particular parasite species divided by the number of hosts examined. Mean intensity is the total number of individuals of a particular parasite species found in a sample divided by the number of hosts infected with that parasite. Range intensity is the highest and lowest number of individuals of a particular parasite species found in a single infected host in a sample. Abundance is the total number of individuals of a particular parasite species found in a sample divided by the number of examined fish, infected and uninfected.
The structure of parasite fauna was evaluated according to the abundance index: A >2, core species; 0.6-2, secondary species; 0.2-0.6, satellite species; and A < 0.2 rare species  and dominance index.
To obtain the dominance, dominance index (Di) was calculated as follow:
Di = ni x 100/N [%]
where ‘ni’ is the total number of individuals of a species i and ‘N’ is the total number of all individuals [26,27].
The following scale for species dominance was used: Di > 10%, eudominant (E); 5.01% < Di < 10%, dominant (D); 2.01% < Di < 5%, subdominant (Sd); 1.01% < Di < 2%, recedent (R); Di < 1%, subrecedent (Sr) .
Diversity and evenness were measured using ecological indices, counted with software BioDiversity Professional 2.0:
Shannon-Wiener diversity index
H’ = - Σ pi ln pi
where ‘S’ - species richness (number of species), ‘pi’ - proportion of species i;
Margalef 's diversity index
M = S-1/lnN
where ‘S’ - species richness, ‘N’ - total abundance;
Pielou index of evenness
J’ = H’/H’MAX
where H’ – Shannon-Wiener index, H’MAX = ln S (‘S’ – number of species)
Simpson's diversity index and inverse Simpson's index
D = Σ ni(ni-1)/N(N-1)
where ‘ni’ is the total number of individuals of a species i and ‘N’ is the total number of all individuals
Berger-Parker dominance index
d = Nmax/N
where ‘Nmax’ – number of individuals from the most abundant species, ‘N’ – number of individuals of all species. The reciprocal of the index, 1/d, is also used.
A total of 23 parasitic taxa were recovered from the European perch. The parasites represented Digenea (7 species, including metacercariae Diplostomum spp. identified to the generic name) - Azygia luci (Müller, 1776), Bunodera luciopercae (Müller, 1776), Apatemon gracilis (Rudolphi, 1819), Diplostomum sp., Postodiplostomum brevicaudatum (Nordmann, 1832), Ichthyocotylurus platycephalus (Creplin, 1825), and Tylodelphys clavata (Nordmann, 1832), Cestoda (4) - Bothriocephalus scorpii (Müller, 1776), Proteocephalus filicollis (Rudolphi, 1802), P. percae (Müller, 1780), Triaenophorus nodulosus (Pallas, 1781), Nematoda (5) - Camallanus lacustris (Zoega, 1776), C. truncatus (Rudolphi, 1814), Hysterotylacium aduncum (Rudolphi, 1802), Philometra obturans (Prenant, 1886), Raphidascaris acus (Bloch, 1779), Acanthocephala (5) - Acanthocephalus anguillae (Müller, 1780), Acanthocephalus lucii (Müller, 1776), Echinorhynchus gadi (Zoega, 1776), Neoechinorhynchus rutili (Müller, 1780), Pomphorhynchus laevis (Zoega, 1776), Crustacea (1) - Ergasilus sieboldi Nordmann, 1832 and Hirudinea (1) - Piscicola geometra (Linnaeus, 1758).
The most frequent taxon of parasites was the digenean, representing 86.9% of all parasites (Figure 1). Fifteen species (13.8 % of the total parasites) were present as adults and eight as larval stages. The highest number of parasites in the larval stages occurred in digeneans and tapeworms, respectively 95.7% and 64.7%. In the last group of nemathodes represented by both development stages, larval stages accounted for 25.4%. (Figure 2). Parasite fauna was dominated by endoparasites. Only two species of ectoparasites were noted, copepod E. sieboldi and leech P. geometra, both were rare species.
The most prevalent parasites were T. clavata and Acanthocephalus lucii, respectively 23.74% and 15.65%. The highest mean intensity was observed in the case of two digeneans, T. clavata, 65.39 individuals and B. luciopercae 19.24 individuals. The highest number of parasites in a single fish was found for T. clavata, 417 individuals and 166 individuals of B. luciopercae. The infection parameters of all parasites are given in Table 1.
The parasite fauna of European perch consist of three marine species: B. scorpii, H. aduncum and E. gadi. All of them are intestinal parasites. Their parasitological indices were low and dominance index located all in the subrecedent class (Tables 1, 2).
The community of intestinal parasites was taxonomically diverse, and consist of fourteen species, in these: two digeneans A. lucii, B. luciopercae; four tapeworms B. scorpii, P. filicollis, P. percae; four nematodes C. lacustris, C. truncatus, H. aduncum, R. acus; five acanthocephalans A. lucii, A. anguillae, E. gadi, N. rutili and P. laevis. One of them, P. percae is classified as a specialist species. Acanthocephalus lucii is a dominant species (Table 2). Only 13.6% parasites were in the larval stages (Figure 3). Acanthocephalans dominated quantitatively (52.1%) compared to remaining taxa (Figure4). With the digestive system was associated T. nodulosus. This tapeworm was encysted in the liver.
The community of eye parasites was composed of digeneans metacercariae, included A. gracilis, Diplostomum spp., P. brevicaudatum and T. clavata, and also nematode larvae P. obturans. The eye parasites were the most numerous group with eudominant species T. clavata (Table 2).
The community of gill parasites consist only one species of copepod E. sieboldi. This species is a member of the subrecedent class.
The most diverse parasite community were autogenic species (18 species), including two digenean (A. luci, B. luciopercae), four tapeworms (B. scorpii, P. filicollis, P. percae, T. nodulosus), five nematodes (C. lacustris, C. truncatus, H. aduncum, P. obturans, R. acus), five acanthocephalans (A. anguillae, A. lucii, E. gadi, N. rutili, P. laevis),one copepod (E. sieboldi) and one leech (P. geometra). Three species occurred as larval stages: T. nodulosus, H. aduncum, P. obturans (13.2%) and the remaining as adults (Figure 3). Dominant taxon was acanthocephalan (Figure 5).
Only five taxa belong to allogenic species (A. gracilis, Diplostomum spp., P. brevicaudatum, I. platycephalus and T. clavata). All of them have birds as a final host. Nevertheless most of parasiete individuals collected from Łebsko among to allogenic group (83.2%) (Figure 6).
Based on the abundance index, the core of the metazoan parasites is one species: T. clavata and secondary species are Acanthocephalus lucii and B. lucioperca. Satellite species included two digenean: A. gracilis and Diplostomum spp., and cestode T. nodulosus. The remaining species of parasites were classified as rare.
According to the dominance index only one species is eudominant, T. clavata (78.96%), and one dominant, Acanthocephalus lucii (6.60%). The most frequent class of dominance is subrecedent with seventeen species (Table 2).
The values of biodiversity indices indicated that the highest diversity and evenness were observed in the community of autogenic species and the next in the community of intestinal parasites. The highest dominance occurred in the community of eye parasites and in the community of allogenic species (Table 3).
Digenea was the most abundant group of parasites in freshwater [e.g. 12,13,28, 29] and brackish water localities. Flukes were detected in many species of fish from the Gulf of Gdańsk [6-8,30] and the Vistula Lagoon [7,8], in roach and carp bream from the Lake Łebsko [2,3], carp bream from the Lake Jamno  and also in perch from the Baltic Sea and coastal lake Resko  or in the estuary of the Odra River .
Five of the seven digeneans (A. gracilis, Diplostomum spp., P. brevicaudatum, I. platycephalus and T. clavata) represented allogenic species, and mostly belonged to the community of eye parasites. All of them are limnetic species, but Rolbiecki  suggests that Diplostomum spp. and T. clavata have already been classified as brackish water species. Kesting and Zander  report that Diplostomum spp. can still exist in the water at salinity of 10-12 ‰. A similar distribution is described for the common in the Baltic Sea, autogenic species, like acanthocephalans Pomphorhynchus laevis and Neoechinorhynchus rutili .
Wierzbicka et al.  compared infection of European perch from the sea and Lake Resko Przymorskie. They found, that Diplostomum spp. often infects fish from the marine position and T. clavata prefer hosts from the lake. Digenean species belonged to the community of eye parasites have a complex life cycle and reach maturity in the different species of fish feeding birds. High values of parasitological indices of infection with digenean metacercariae are a consequence of the occurrence of gastropods (intermediate hosts) and richness of water birds serving as definitive host. Because the snails can also exist in brackish water of lower salinity these digeneans are found in several fish species of the coastal zone of the Baltic Sea . A total of 59 breeding species of water birds was observed in the vicinity of Lake Łebsko . As in Łebsko T. clavata is one of the most prevalent parasite of perch in the Polish coastal zone and adjacent waters [30,8,9,5]. Subdominant species, A. gracilis is considered to be a typical parasite of perch. In brackish waters is known also from both, three-spined and nine-spined sticklebacks . Rolbiecki et al.  found that in the Gulf of Gdańsk only 1.7% of perch was infected with A. gracilis, while at the same time, up 15.4% of three-spined sticklebacks were infected. But in the Vistula Lagoon they found A. gracilis in 5.1% of perch. The last species of community of eye parasites, P. brevicaudatum belongs to a rare species (subrecedent), not only in Łebsko, but also in other reservoir [10,11,28,8].
The last allogenic species, I. platycephalus, also have a complex life cycle with snail, fish and bird hosts . Many freshwater fish species act as second intermediate host of I. platycephalus. This parasite was found in small number in the Gulf of Gdańsk , Vistula Lagoon [7,8] and Lake Resko . Wierzbicka et al.  not found this species in adjacent sea localities. Similarly perch from the estuary of the Odra River studying by Sobecka and Słomińska  was uninfected.
Among the autogenic species limnetic species occur predominantly. They are well-known from the European perch from freshwater reservoirs [10-14]. Freshwater parasites are typical also for freshwater fish species living in the coastal zone of the Baltic Sea [5,6,8,9,35].
Acanthocephalan A. lucii, the second core species occur with high parameters of infection in perch from freshwater localities [e.g. 12,13] and also from the coastal zone, where is one of the most abundant parasite [6,8,9,30]. A. lucii lake other acanthocephalans is unspecialized.
Two species of Proteocephalus tapeworms was noted in perch. P. filicollis, first of all matured in three-spined stickleback Gasterosteus aculeatus Linnaeus, 1758  but sometimes extend in percid fishes [38,39]. P. filicollis was found also in round goby Neogobius melanostomus (Pallas, 1811) an invasive species in the Polish fauna of the Vistula Lagoon . Rolbiecki  found 20.2% of perch infected with P. filicollis in the Vistula Lagoon and only 4.9% with P. percae. Sobecka and Słomińska  in the Odra River estuary found 9.09% perch infected with P. percae but in Resko prevalence is 28.8% . In Łebsko it is respectively 1.08% and 6.47%. Perch longest than 22.5 cm acquired tapeworms P. percae. Valtonen and Rintamäki  studying perch from the Gulf of Bothnia and Lake Yli Kitka. They found highest prevalence of infection with P. percae in the bay than lake, respectively 60% and 10%. The highest infection authors noted among medium-sized fishes, more than 18 cm length.
In Lake Łebsko, like in Scholz  study, T. nodulosus was recorded often than P. percae. Tapeworm T. nodulosus is monoxenous parasite of Northern pike. The first intermediate hosts are Copepoda and the main second intermediate host is European perch in which plerocercoids was encysted in the liver. Brinker and Hamers  perceived that the trophic state of water is a important factor affecting the infection level with T. nodulosus. Parasitological indices increase with decrease of trophy level and are highest in oligotrophic waters. Because Łebsko is hypereutrophic lake, parasitological indices are not very high.
P. obturans is a species widely distributed in Palearctic and occurs in fresh and brackish waters . Northern pike is a specific final host while perch, pikeperch, bream and rudd are the main paratenic hosts in which larvae located in the vitreous body of eye . Several copepods act as intermediate host [43,45]. The prevalence of P. obturans in pikeperch in Łebsko is 5.7%. The reservoir of this parasite are bream and perch, prevalence is, respectively 0.3% and 0.2%.
European perch is one of the many hosts of C. lacustris and infected by feeding on infected copepods. In Łebsko C. lacustris occurs with C. truncatus. Wierzbicka et al.  also noted both nematodes. They observed that C. truncatus more often infected perch from the sea and C. lacustris from the lake.
Because of low salinity, marine parasites are not very frequent. Only B. scorpii, H. aduncum and E. gadi are a marine species [9,38,46-48]. All of these species are widely distributed. In the Baltic Sea B. scorpii and H. aduncum are also found as far as the Gulf of Gdańsk and E. gadi is found as far as the central Baltic and the Gulf of Finland . List of marine parasites found in European perch from the Polish coastal zone is provided in Table 4.
B. scorpii and H. aduncum are accidental parasites of perch. H. aduncum parasitizing mainly marine fishes, especially cods, and migratory fishes, and with them may be brought into freshwater environments [49,50]. E. gadi is unspecialized regarding both intermediate and final hosts  and also occurred in many fishes from the Baltic and adjacent waters. These species are known from perch from the Baltic Sea, the Vistula Lagoon, the estuary of the Odra River and the coastal lake Resko [5,6,8,9].
Because of the connection between the Łebsko and the Baltic Sea, fish, as other organisms, can migrate between these environments. Some of marine fish like Clupea harengus (Linnaeus, 1758), Gadus morhua (Linnaeus, 1758), Belone belone (Linnaeus, 1758), Platichthys flesus (Linnaeus, 1758), Pleuronectes platessa (Linnaeus, 1758), Psetta maxima (Linnaeus, 1758), and in spring (for spawning) Osmerus eperlanus (Linnaeus, 1758) and also freshwater species from the coastal water of the Baltic Sea had been found in the coastal lake Łebsko . Some of them can be a reservoir of parasites and transmit them between lake and sea. Rokicki  suggested that European perch, pikeperch and European eel from the Vistula Lagoon infected with parasites by feeding on migrated from the Gulf of Gdańsk flat fish and eelpout. Abundant population of smelt and three-spined stickleback had been found in Łebsko , and also in coastal zone of the Baltic Sea. Smelt and stickleback are also a food component of perch diet  and therefore parasites can infect perch. H. aduncum occurred in smelt [6,53] and stickleback . Stickleback can be also a host of B. scorpii. H. aduncum and B. scorpii, occurred in flounder . Flounders occurred in the lake Łebsko by whole year  and could be a reservoir of these parasites and also P. laevis .
Results of studies conducted on European perch from the coastal lake Łebsko and their comparison with results of the other authors from various reservoirs (fresh and brackish waters) indicate that the dominant species are found with high parasitological indices everywhere.
The most frequent taxon was Digenea, representing 86.9% of all parasites. Most of them belong to the eye parasite community with eudominant species T. clavata. Allogenic species quantitative dominated over autogenic species like larvae stages over adults. But most diverse was community of autogenic species. Ectoparasites were represented only by two species.
The presence of marine species (B. scorpii, H. aduncum, E. gadi) in parasite fauna of perch from the coastal lakes and the coastal waters of the Baltic Sea is the only difference between freshwaters and estuaries, but their share is small. Marine species of parasites nevertheless are more often observed in marine fish species.