Hysterothylacium aduncum (Rudolphi, 1862) infection in cod from the Oslofjord: seasonal occurrence of third- and fourth-stage larvae as well as adult worms.

Hysterothylacium aduncum infections in cod from the Oslofjord were followed for 1 year. The recovered worms were grouped according to developmental stage [third-stage larvae (L3), fourth-stage larvae (L4) and adult worms] and according to location (in the gut or encapsulated on the viscera). The cod were found to be infected with H. aduncum (all stages lumped) throughout the whole year, with the prevalence approaching 100%. The abundance of the parasites (mean number per fish studied), however, was clearly lowest during the summer months. L3 and L4 recovered from the intestine dominated most of the time but showed a very clear peak during late winter and early spring. L3 recovered from the body cavity was the only stage that showed a clear peak during the summer months. Adult worms, although adult worms were present in very small numbers throughout the year, they tended to peak in May and then again in December and January. Since there did not seem to be a correlation between the accumulation of L3 in the body cavity and the length or age of the fish, the possibility of self-infection whereby the larvae reenter the gut and continue to develop into L4 and then into adults, is discussed.