Abstract
Studies were performed on variations of prevalence and abundance in postlarvae and adults of the monogeneans Pseudodactylogyrus anguillae and P. bini from European eels in an intensive culture system. It appeared that a surge of settling parasite larvae always occurred after formaldehyde treatment of fish tanks, or after removal of bio-film from fish tank walls. Formaldehyde did not increase the susceptibility of eels to infection. The absence of postlarvae for certain periods was not caused by resistance to eels, which could be affected by dexamethasone-treatment. Settling of oncomiracidia was negatively correlated to activity of free-living organisms in the biofilm on the fish tank wall. Laboratory tests showed that the biofilm-organism Stenostomum sp. (Turbellaria) ingested Pseudodactylogyrus eggs. It is suggested that predatory activity of biofilm organisms influences the inflection level, although other factors may be of importance.

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