Abstract:The global trade in live ornamental fish carries with it the potential threat of unwanted movement of pathogens. The live ornamental fish, when released into the natural environment of their destination countries, may disseminate their parasites, threatening local fish and causing economic loss in fish aquaculture. This study was aimed to explain the reasons for deterioration of health and premature death of ornamental fish: Nannostomus spp. from Colombia and Symphysodon spp. Some specimens of Symphysodon aequifasciatus Pellegrin, 1904 were captured from their natural habitats in the Amazon River basin and shipped to Poland by air mail. The hybrids of Symphysodon spp. originated from an aquarium in Poland (shared with S. aequifasciatus). The third discus fish species, Symphysodon discus Heckel, 1840 came from a fish farm in Thailand and represents an Asian breeding line. All the studied fish were infected with pathogens. Nannostomus spp. housed a single parasite species on its skin, namely Artystone minima. This isopod species was not recorded in Poland and probably in Europe before. The examined fish of the genus Symphysodon yielded 13 pathogens. Five of them Philodina sp., Ichthyobodo necator, Hexamita symphysodonis, Sciadicleithrum variabilum and Eustrongylides sp. were not earlier recorded in this host species in Poland. The findings of Ichthyophonus hoferi and Myxobolus sp. are new host records for Symphysodon spp. fish. Hybrid fish and S. aequifasciatus are reported here as hosts of Entamoeba sp. for the first time.

A total of 142 cods: 60 from the South−East Ground of Bear Island and 82 from the Pomeranian Bay (Baltic Sea) were examined for their ecto− and endoparasites. Twenty different parasite species, comprising one Myxosporea, three Cestoda, four Digenea, seven Nematoda, three Acanthocephala and two Crustacea were found. The parasite component communities comprised 1446 individuals (17 species, six higher taxa) from the Bear Island and 6588 individuals (nine species, three higher taxa) from Pomeranian Bay. The observed parasite host specificity was low, and the intensity in a single fish ranged from one to 279 specimens. The eudominant parasite species were Echinorhynchus gadi, Hemiurus levin− seni and Contracaecum osculatum. The dominant parasite communities from the Bear Is− land were nematodes, but acanthocephalans dominated in cod from the Baltic Sea. It ap− pears that one group of parasites, better adapted for the specific conditions of the macro− habitat, has replaced another. The most prevalent parasites were E. gadi, Anisakis simplex, C. osculatum and Hysterothylacium aduncum, and the mean values of crowding were the highest for E. gadi and Pomphorhynchus laevis. The nematode Camallanus lacustris was noted in this host species for the first time. Only six species of parasites were common to cod from both fishing grounds.

A parasitological study was carried out on 330 eelpouts from two fishing grounds of the Polish fishing zone off the Oder estuary (Pomeranian Bay and Dziwna Mouth). A total of 11 species and genera of parasites from six higher taxa were recorded: 1 monogenean, 1 digenean, 5 nematodes (eelpout was a new host for Ascarophis morhuae and Capillaria gracilis), 2 acanthocephalans, 1 mollusk and 1 protist (a fungus-related pathogen). A total of 4284 autogenic countable parasites were identified. These included parasites of eight species and two genera (six higher taxa) from Pomeranian Bay, and five species (two higher taxa) from the Dziwna Mouth. The abundance of parasites per host was higher in fish from the Dziwna Mouth, while the parasitic biodiversity index was almost two times lower than in Pomeranian Bay. Pomphorhynchus laevis dominated among the eelpout parasites in both fishing grounds, but the parasite communities from the Dziwna Mouth consisted of a larger number of dominating classes. The high intensity of infection of Hysterothylacium auctum and the relative density affect more the dimensions of the dominant parasite (P. laevis) in the Dziwna Mouth than the crowding of parasites. Infection by more abundant large parasites have an important influence on the value of Fulton’s coefficient.

The research was conducted on the parasite fauna and food composition of Eutrigla gurnardus caught as by-catch in commercial catches of the Atlantic cod Gadus morhua near the Shetland Islands. Thirteen species and two genera of pathogens were identified, including six species and one genus recorded for the first time in this host. Copepoda -Euphausiacea dominated in the stomach contents (they are also the intermediate hosts for most of the parasites found), while Gadidae dominated among the fish. A checklist of E. gurnardus parasites is included.