Spreading of River Water Guides Migratory Behavior of Homing Chum Salmon in Otsuchi Bay, a Narrow Inlet with Multiple River Flows.

The Sanriku-ria coast of Japan, a homing area for chum salmon, , is characterized by a large number of small closed bays into which one or multiple short rivers flow. The present behavioral investigation of chum salmon in this region was designed to gain deeper insight into the migration of chum salmon to their natal rivers. Eighty-three fish caught at the middle part of Otsuchi Bay were tracked using an acoustic transmitter in the narrow inlet into which flow three rivers: the Otsuchi, Koduchi, and Unosumai. The majority of 18 fish that entered the Unosumai River, which flows into the southwest side of the bay, directly approached the river along the southern coast. More than half of fish that entered the Otsuchi and Koduchi Rivers, which flow into the northwest side, also migrated into the inner bay via the southerly route, and then entered these rivers frequently after passing the mouth of the Unosumai River. In the inner bay, the salinity of sea surface water suggested that water from the three rivers circulates in a counterclockwise direction at a depth of less than 1.0 m, flowing eastwardly along the southern coast. The observed migratory paths of homing salmon in Otsuchi Bay thus correspond well with the counterflow of surface river water in the bay. The present results suggest that homing migration of salmon in the Sanriku narrow inlet is guided by natal river flows.