PARASITIC INFECTIONS IN POND-REARED RAINBOW-TROUT ONCORHYNCHUS-MYKISSIN DENMARK

From October 1993 to December 1995, 5 Danish freshwater rainbow trout Oncorhynchus mykiss farms were surveyed for parasites; the most compre hensive survey to date in Denmark. A total of 805 fish, aged 0+ and 1, were examined, and the parasites were identified by light microscopy , scanning electron microscopy and histochemistry. Prevalence (protozo ans and metazoans) and abundance (metazoans) were recorded. The 12 pro tozoans found were: an intestinal diplomonadid flagellate, the ectopar asitic kinetoplastid Ichthyobodo necator, and the ciliates Ichthyophth irius multifiliis, Chilodonella piscicola, Capriniana (Trichophrya) sp ., Trichodina nigra, T. mutabilis, T. fultoni, Trichodinella epizootic a, Apiosoma sp., Ambiphrya sp, and Epistylis sp. The 10 metazoan speci es found were: the myxosporean organism PKX, the monogeneans Gyrodacty lus derjavini and G. salaris, the digeneans Diplostomum spathaceum and Tylodelphys clavata, the cestodes Proteocephalus exiguus, P. longicol lis, Eubothrium crassum, and Triaenophorus nodulosus, and the crustace an Argulus foliaceus. The diplomonad occurred predominantly in anorect ic undersized fish, and was associated with mortality. I. necator prod uced discoloration and epidermal hyperplasia and I. multifiliis infect ions were associated with mortalities. G. salaris and G. derjavini pro duced alterations (lesions and hyperplasia) of the host epidermis visi ble in the scanning electron microscope. Some D, spathaceum infections caused cataract. Prevalence of I. multifiliis increased with temperat ure (maximum at 16 to 20 degrees C), whereas the diplomonad had highes t prevalence at 1 to 5 degrees C. Likewise, the gyrodactylids occurred more abundantly at lower temperatures.