Physiological Mechanisms of Imprinting and Homing Migration of Pacific Salmon

© 2016 TERRAPUB, Tokyo. All rights reserved. doi:10.5047/absm.2016.00901.0001 pink salmon are phylogenetically the most advanced salmon species, while masu salmon are considered to be the more primitive species (Murata et al. 1996). Pink salmon are also the most widely distributed species and have the largest population size, while masu salmon appear to have the most restricted distribution and the smallest population (Kaeriyama and Ueda 1998). Although the homing accuracy of these salmon has not been compared in detail, it is believed that masu salmon return to their natal stream with the highest precision, and that pink salmon are more likely to stray into a non-natal stream. If most salmon might show a highly accurate homing to the natal stream, there would be little chance to enhance their distribution area as well as to increase their population size. And, they might encounter the dangerous possibility to reduce their genetic diversity. The relationship between salmon evolution and homing accuracy is one of the most interesting questions from a viewpoint of biological evolution. Abstract Salmon are recognized for their amazing abilities to precisely migrate thousands of kilometers from their feeding habitat in the ocean to their natal stream for reproduction, but many mysteries are still unsolved in the mechanisms of imprinting and homing migration. Physiological mechanisms of imprinting and homing migration of Pacific salmon were investigated using three different research approaches. Homing behavior of adult chum salmon from the Bering Sea to Hokkaido as well as lacustrine sockeye salmon and masu salmon in Lake Toya (serves as a model ocean) were examined using physiological biotelemetry techniques, demonstrating that salmon can navigate in open water using different sensory systems. Hormone profiles in the brain-pituitary-thyroid and brain-pituitary-gonad axes were analyzed in chum salmon and sockeye salmon during their imprinting and homing migration, suggesting that thyrotropin-releasing hormone and salmon gonadotropin-releasing hormone in the brain are involved in imprinting and homing migration, respectively. The olfactory memory formation and retrieval of Pacific salmon were investigated using several neurophysiological techniques, suggesting that long-term stability of dissolved free amino acid compositions in natal streams are crucial for olfactory imprinting and homing, and N-methyl-D-aspartate receptor can be a useful molecular marker for olfactory memory formation and retrieval. These topics are discussed with physiological mechanisms of imprinting and homing migration of Pacific salmon. Physiological Mechanisms of Imprinting and Homing Migration of Pacific Salmon