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The three-spined stickleback Gasterosteus aculeatus (L.) has been a very frequent fish in the Baltic coastal waters. In the Baltic Sea occurred three morphological forms of the three-spined stickleback: trachurus, semiarmatus and leiurus. The distribution and frequency of all forms of the stickleback in the Baltic have been different. The examined sticklebacks were collected from the Gulf of Gdańsk (Gdynia, Puck, Hel) and the mouth of the Dead Vistula (Górki Wschodnie).
The studies on the correlation between the morphological forms of the stickleback and parasites’ infestation were carried out. Some species of parasites infected first of all form trachurus or leiurus of the stickleback. Correlation was observed in some of marine parasites like Thersitina gasterostei (Pagenstecher, 1861) (Copepoda) and Gyrodactylus arcuatus (Bychowsky, 1933) (Mo-nogenea), infected generally trachurus form. Freshwater ciliates from the family Trichodina or plerocercoids of Schistocephalus solidus (Müller, 1776) (Cestoda) were found generally on/in form leiurus.
Key words: three-spined stickleback Gasterosteus aculeatus (L.), morphological forms: trachurus, semiarmatus, leiurus, parasites, prevalence of infestation