on Hatchery-Reared Salmonids

The occurrence of Ichthyobodo neeator, Trichodina truttae and Chilodonella piscitola wasinvestigated on juvenile Pacific salmon (chum, pink, masu and sockeye saImon) reared at 204 hatcheries in northern Japan. These ectoparasitic proto-zoans were widespread in the area at water temperatures between 2 " and 15'C. Ichthyobodo necator was most common among them, being recorded from all four salmonid species. Chilodonella piscicola was found on the former three fish species, and T. truttae was encountered on only chum salmon. The percentage of positive hatcheries was 37.3 % for I. necator, 15.2 % for T. truttae, and 8.8 % for C. piscicola. The occurrence of Ichthyobodo infections was not related to type of water supply (spring, well, infiltrate, or river waters), which impBes that there are other mecha-nisms ta spread the parasite among host populations besides direct transmission. In contrast, the majority of Trichodina and Chilodmella infections occurred at the hatcheries supplied with river water, suggesting that wiId fish may serve as the reservoirs of infectian. StatisticaI comparisons indicated the existence of a possible interference competition between I. necator and T. truttae on the host body surface