Yamanashi Prefectural Fisheries Technology Center, Kai, Yamanashi 400-0121, Japan
Multiple characteristics of two breeds of hatchery-reared ayu Plecoglossus altivelis (YD, Yamanashi Dam breed; YS, Yamanashi Sea breed) as a fishing resource were assessed in a natural river. The breeds were reared under identical conditions from egg fertilization to stocking. From June to October 2009, recapture experiments were conducted for fin-clipped and stocked ayu using casting nets and “tomo-zuri” fishing, i.e., using a live decoy, in the studied river. The results indicated that YD was more vulnerable to fishing than YS, especially early in the fishing season. However, as YD matured earlier, it had a shorter fishing season than YS. Conversely, YS exhibited symptoms of bacterial cold water disease (BCWD) less frequently than did YD, showing that YS better tolerated BCWD. This finding was consistent with the results of experimental infection. We concluded that the characteristic traits of the two breeds of hatchery-reared ayu differed after stocking in a natural river, although both breeds were raised under identical hatchery conditions.