Trichodina sp. (Ciliophora: Peritrichida) in eel Anguilla anguilla in recirculation systems in Denmark: host-parasite relations.

Farmed eel cultured in recirculation systems in Denmark were found infected by Trichodina jadranica Raabe, 1958. Associations between parasite abundance and fish size was examined in 2 different production systems. In one sys- tem, stocked with relatively well-nourished eels (3 to 31 g), most of the parasites (66%) were found on the dorsal part on the skin and relatively few were found on the gills (approx. 8%). The infection level was significantly positively corre- lated both with fish weight and length. In the other system, stocked with relatively malnourished small eels (0.5 to 4 g), significantly more parasites (0.06 ± 0.02 (SD)) were present on fish with a low condition factor than on fish with a higher condition factor (0.13 ± 0.01 (SD)). In this eel stock the vast majority of the trichodines were found on the gills.