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Chum salmon (Oncorhynchus keta) is an important marine resource in Japan. Hatchery release are actively carried out in Hokkaido and the Tohoku region in order to maintain and increase these resources. However, the returns have reduced in recent years. The sustainable use of chum salmon resources requires understanding their depletion mechanisms.
Chum salmon life history
Chum salmon (Oncorhynchus keta) are hatched in fresh water from autumn to winter. After living in the river during the winter, they descend into the ocean in the spring. The juveniles that descend into the ocean move to the Sea of Okhotsk, where they feed and grow in the summer. They then further migrate to the Gulf of Alaska and the Bering Sea. Once they have sufficiently grown, sexual maturation starts, and they return to Japan. In the case of chum salmon, all juveniles descend to the ocean in the spring of the first year, but the returning age varies since the marine life varies at two, three, four, or more years. It is important to clarify factors influencing their life history patterns and the life history stage where depletion is significant.
Growth hypothesis explaining mortality of chum salmon
Field surveys on salmon and trout to date have suggested that over 90% of the juvenile fish that descend into the sea are depleted by the first winter of marine life. Achieving a certain size by the time they leave the coast is an important factor in whether the juvenile fish will die or survive. Small-sized fish demonstrate inferior swimming ability and are thus either preyed upon or exhaust their physical strength when they move to the feeding area. Furthermore, the critical period/growth hypothesis states that individual fish who cannot store enough energy will die due to the lack of food in winter.
How to measure growth in chum salmon?
As shown above, it is important to measure the growth of juvenile fish during their movement from the river to estuary to coast to offshore in order to estimate chum salmon stock recruitment. The most secure way of doing so is to label an individual fish, recapture it after some time, and observe changes in size. However, it is extremely difficult to re-capture a fish released into the wild. Therefore, methods for indirectly measuring the growth of fish are generally used. The ring patterns in the otoliths and scales record the growth and habitat of each fish, which is useful for reconstructing past growth history. Furthermore, the amount of ribosome RNA in the muscles is said to reflect the degree of protein synthesis, that is, the growth of the muscles and the ratio per cell (DNA) (RNA/DNA ratio) is used as an index of current or recent growth. Our research group focuses on the insulin-like growth factor (IGF)-I, a hormone that controls growth, as a growth index of either current or recent growth.