Levels of infection and seasonal occurrence of Gyrodactylus alviga (Monogenea: Gyrodactylidae) on the whiting Merlangius merlangus off the Turkish and Russian coasts of the Black Sea A. O ¨ zer 1 *, Y. Kornyychuk 2 , T. O ¨ ztu ¨ rk 1 and V. Yurakhno 2 1 Sinop University, Faculty of Fisheries and Aquatic Sciences, 57000 Sinop, Turkey: 2 O.O. Kovalevsky Institute of Biology of the Southern Seas, 2 Nakhimov av., 299011 Sevastopol, Russia (Received 17 November 2014; Accepted 14 April 2015; First Published Online 8 June 2015) Abstract Gyrodactylus alviga is a generalist species reported in many Black Sea fish species, but whiting is known to be its main host. It is the only monogenean parasite that has been reported so far on the skin, fins and gills of whiting Merlangius merlangus in the Black Sea. A total of 690 fish from Turkey and 423 fish from Russia were examined to detect parasites. Infection indices of prevalence, mean intensity and mean abundance values were calculated for length classes and sex of fish, as well as for the seasons. There is a gradual increase in infection indices in relation with increasing host length classes and there are higher infection indices on female fish. Furthermore, despite lower prevalence indices in spring and winter, higher parasite intensity values were determined in these seasons. It is concluded that larger fish provided more space and food for G. alviga, and female fish were more parasitized as they were more active in searching for food, which enables the parasite to switch between hosts. In addition, winter and spring were more appropriate for reproduction of G. alviga. Introduction Monogeneans are among the most common parasites of marine, brackish and freshwater fish. They have a simple direct life cycle without any requirements for inter- mediate hosts, and new hosts can be infected by tiny, free-swimming, ciliated larvae, or oncomiracidia (Kearn, 2014). Gyrodactylids, however, are the exception to this, by giving birth to unciliated individuals similar in size to the parent (Kearn, 1998). The simplicity of the life cycle allows gyrodactylids to invade new hosts and habitats with their hosts (Kennedy, 1994; Mo, 1994). Gyrodactylus alviga Dmitrieva et Gerasev, 2000 is a generalist species registered from many Black Sea fishes, but whiting is known to be its main host (Dmitrieva & Gerasev, 1997). It is the only monogenean parasite reported so far on the gills of whiting Merlangius merlangus in the Black Sea. This fish species is widely distributed in the eastern North Atlantic Ocean, the northern Mediterranean, western Baltic and the Black Sea (International Council for the Exploration of the Sea, 2006). It is particularly distributed along Romanian, Bulgarian, Turkish, Ukrainian and Caucasus coasts in the Black Sea (Burdak, 1960; Svetovidov, 1964; Go ¨ nener & Bilgin, 2010; O ¨ zer et al., 2014). The objective of the present study was to investigate the infection levels of G. alviga on whiting, M. merlangus (L., 1758) from the Turkish and Russian Black Sea coasts, to provide an ecological analysis of G. alviga in both locations for the first time. *E-mail: aozer@sinop.edu.tr Journal of Helminthology (2016) 90, 347–352 doi:10.1017/S0022149X1500036X q Cambridge University Press 2015