Prevalence and transmission capability of Didymocystis wedli (Digenea; Didymozoidae) in cage-reared young Pacific bluefin tuna Thunnus orientalis in the Amami area of Japan

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We investigated the prevalence and transmission capability of Didymocystis wedli Ariola, 1092 (Digenea; Didymozoidae) in the gills of young Pacific bluefin tuna (PBT) Thunnus orientalis in the Amami area of Japan between November 2004 and May 2005. All young PBT caught in the Japan Sea in November 2004 were infected with D. wedli, while no infection was observed in young PBT caught in the Pacific between August and September 2004. However, the prevalence of the former group of PBT decreased during cage-rearing in Amami and finally reached 0% in April 2005. The parasitism of D. wedli was not confirmed in young PBT from the Pacific or adult PBT reared in the same cages as PBT from the Japan Sea or adjacent cages throughout this experiment. These results suggest that the introduction of young PBT from other areas does not cause transmission of D. wedli in the Amami area.