HOMING IN PACIFIC SALMON - MECHANISMS AND ECOLOGICAL BASIS

Pacific salmon (Oncorhynchus sop.) are famous for their homing migrati ons from oceanic feeding grounds to their natal river to spawn. During these migrations, salmon travel through diverse habitats (e.g. oceans , lakes, rivers), each offering distinct orientation clues and, perhap s, requiring distinct sensory capabilities for navigation. Despite the se challenges, homing is generally precise and this philopatry has res ulted in reproductively isolated spawning populations with specialized adaptations for their natal habitat. This paper reviews the mechanism s underlying all aspects of salmon homing but emphasizes the final, fr eshwater phase governed by olfactory recognition of homestream water. Prior to their seaward migration, juvenile salmon learn (imprint on) o dors associated with their natal site and later, as adults, use these odor memories for homing, Our understanding of this imprinting process is derived primarily from studies using artificial odorants and hatch ery-reared salmon. Recent findings suggest, however, that such studies may underestimate the complexity of the imprinting process in nature.