Species richness and the diversity of parasite communities of eelpout Zoarces viviparus (Linnaeus, 1758) in the Oder River estuary, Poland

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.