Seasonal occurrence, topographical distribution and transmission of Gyrodactylus callanatis (Monogenea) infecting juvenile Atlantic cod in the Oslo Fjord, Norway

The seasonal occurrence and topographi0cal distribution of Gyrodactylus callanatis on three year-classes of juvenile Atlantic cod Gadus morhua , has been studied in the Oslo Fjord, Norway over a 2-year period. Both prevalence and abundance increased in spring and peaked in summer on the 1992 year-class and decreased in autumn. The highest intensities recorded in the study were observed on the 1992 year-class in June, with some fish harbouring more than 500 worms each. On the 1993 year-class, prevalence increased throughout the year, until it reached 100% in November; abundance, however, did not differ significantly between monthly samples, but intensities on individual fish were highest in June. The 0 + cod are probably infected when they immigrate to the nursery grounds in autumn and mingle with infected 1 + cod. About 74% of the worms infected the gill arches, oral cavity and pharynx, while 26% infected the body, head and fins. There was a significantly higher percentage of worms on the latter sites at intensities of less than 20 and more than 100 worms per fish. This is possibly related to the mode of transmission in G. callariatis . The initial infection sites are presumably the fins and head, and the most important route of transmission of G. callanatis is believed to be via the substrate. Other routes of transmission are also discussed.