山形県月光川において異なる時期に 放流したサケ Oncorhynchus keta の 回帰率比較

To determine the optimal time for releasing chum salmon into the Gakko River, otolith-marked juveniles reared to more than 1 g body mass were released in late February, mid-March, and late March of 2009 to 2011. Although factors that determine return rates are yet unknown, the return rates were highest for the mid-March (0.62ô-1.02ô) groups of all three brood years. Return rates for the late February and late March groups were approximately half that of the mid-March groups. These results suggest that releasing chum salmon in mid-March is optimal for increasing return rates.