Abstract
Olfaction is the most primitive and important chemosensory system for detecting a wide variety of environmental changes and is involved in various functions required for survival. Salmon have an efficient olfactory system; their olfactory abilities are inferior to those of dogs but superior to those of humans. Salmon olfactory systems can discriminate seasonally and yearly stable compositions of dissolved free amino acids in their natal streams produced by biofilms in the riverbed, and salmon can imprint on single amino acids before and during smoltification and discriminate these amino acids in the spawning season several years later. This chapter describes the structure and functions of the olfactory systems in salmon, previous olfactory hypotheses for salmon homing, properties of natal stream odorants, and biochemical and molecular biological studies on salmon olfactory functions.