The profile of homing behavior in chum salmon (Oncorhynchus keta) that migrate from coastal sea to their natal river was not known well. We thus investigated temporal behavioral profiles of pre-spawning chum salmon in terms of water depth and temperature in Ishikari Bay using a micro data logger in 1997 and 1998. Fish were caught by a set net, tagged and attached with a data logger under MS222 anesthesia, and were released at the points 5 and 3 km off from the mouth of the Ishikari River in 1997 and 1998, respectively. A temporal profile of water depth and ambient temperature along the migratory pathway of recaptured salmon indicated that they usually stayed near the surface where water temperature was relatively low. Conductivity-temperature-depth recorder (CTD) data and the sea surface temperature estimated with National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) indicated that the influent of river water formed a low-temperature brackish subsurface layer near the mouth of the Ishikari River. These results indicate that chum salmon homing to the Ishikari River prefer low water temperature, and wander in the subsurface layer of nearshore area close to the mouth of the Ishikari River until they are motivated to migrate upstream. The main factor that regulates behavioral pattern of returned chum salmon in coastal sea should be distribution of water temperature.
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