Diverse downstream migration patterns of the anadromous Japanese grenadier anchovy Coilia nasus in the Chikugo River estuary and Ariake Sea, Japan

Abstract

Variations in the downstream migration patterns of the anadromous Japanese grenadier anchovy Coilia nasus in the Chikugo River estuary and the inner part of the Ariake Sea, Japan were investigated using otolith Sr:Ca ratios over life history transects. Although most individuals migrated from freshwater or low salinity areas (i.e., upper estuary; salinities <10) to brackish water or sea water areas (i.e., lower estuary or sea), the timing of the downstream migration was considerably different among individuals. Based on the otolith Sr:Ca profiles, four migratory groups were distinguished and their mean standard length (SL) at the initial downstream migration were inferred: (1) the Late Migration Group (LMG), in which individuals remain within freshwater or low salinity areas, where the estuarine turbidity maximum (ETM) is well developed at salinities <10, during the larva and juvenile stages until initial downstream migration at a size of 154 mm in SL; (2) the Early Migration Group (EMG), in which individuals immediately migrate from freshwater or low salinity areas to relatively high salinity brackish water areas at the early juvenile stage (45 mm in SL); (3) the Middle Migration Group (MMG), in which the initial downstream migration of the fish occurs at 90 mm in SL; and (4) the Brackish-Sea water Resident Group (BSRG), in which individuals are hatched in relatively high salinity brackish water areas and spend time there during the early life period. The relative contribution of the LMG and BSRG to adult females were low, whereas adult females were dominated by the EMG and MMG (72.4% of n = 58). Based on previous studies and our current results, C. nasus may greatly depend on the ETM zones during the early life stage, but there are significant differences in the timing and downstream migration patterns of C. nasus.