Comparison of return rates of chum salmon Oncorhynchus keta released at various times into the Gakko River, Yamagata Prefecture, Japan

Abstract

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 (0.62%-1.02%) 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.