Mechanisms of Salmon Homing Migration from Molecules to Behavior

Salmon have an amazing ability to migrate thousands kilometers from the open ocean to their natal stream for reproduction after several years of oceanic feeding migration. It is now widely accepted that some specific factors of the natal stream are imprinted to particular nervous systems of juvenile salmon during downstream migration, and that adult salmon evoke these factors to recognize the natal stream during homing migration. A number of studies have investigated the amazing ability of salmon to migrate long distances from the ocean to their natal river for spawning (Ueda and Shoji 2002). For a better understanding of the mechanisms of salmon homing migration, three different analyses have recently been applied using Japanese chum salmon (Oncorhynchus keta) homing migration in Hokkaido, Japan. The first is behavioral analysis of upstream tactics of chum salmon through a rehabilitated segment of the Shibetsu River by electromyogram (EMG) telemetry and micro-data logger tracking in collaboration with Yuya Makiguchi, Masumi Akita (Hokkaido University), Hisaya Nii, and Katsuya Nakao (Hokkaido Farming Fisheries Promotion Corporation). To restore the ecological condition of a previously channelized river to its more natural, meandering state, a segment of the Shibetsu River was reconstructed. We investigated the upstream tactics of chum salmon as they migrated up the channelized segment, reached the confluence point with the restructured segment, and then entered the rehabilitated area. Fish were radio tracked with EMG telemetry and micro-data loggers. The monitored salmon preferred to hold in deep, slow moving areas of the channelized river, and EMG telemetry data revealed that these extended holding times provided the fish with efficient rest. In the reconstructed segment, the fish swam at more shallow depths and against stronger currents. Although all fish chose to enter the reconstructed segment, few fish remained in the segment for long. This implies that the reconstructed area may not provide suitable holding sites for migrating salmon. The second is endocrinological analysis of hormone profiles in the brain-pituitary-gonadal axis in collaboration with Tomoko Kitani (Hokkaido University), Masafumi Amano (Kitazato University), and Masatoshi Ban (National Salmon Resources Center). Gonadotropin-releasing hormone (GnRH) molecules produced in the various brain regions are considered to be involved in many physiological functions of teleost life cycle. In order to clarify GnRH roles on salmon homing migration, two molecular types of GnRH, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II) in different brain regions, as well as gonadotropin (GTH) and steroid hormones, were measured using …