RECURRENT AMEBIC GILL INFESTATION IN RAINBOW-TROUT CULTURED IN A SEMICLOSED WATER RECIRCULATION SYSTEM

Five lots of commercially purchased juvenile rainbow trout Oncorhynchu s mykiss (17-44 g) stocked in a continuous-production water recirculat ion system became infested with gill amoebae. The amoebae were introdu ced into the recirculation system, as evidenced by their presence on g ills of fish held in quarantine tanks. Based on their morphology, as s een in histological sections and by electron microscopy, the amoebae a ppeared to be more closely related to the family Cochliopodiidae than to other taxa of free-living amoebae. Attempts to culture the amoebae in different media, at different temperatures of incubation, and in fi sh cell culture were not successful. Initial treatment of the recircul ation system with formalin at 167 parts per million (ppm) for 1 h elim inated amoebae from the gills. Subsequent treatments of the entire sys tem with formalin at 50-167 ppm reduced the intensity of further infes tations.