Seasonal variations in and effect of incubation water temperature on vertebral number in naturally spawning chum salmon Oncorhynchus keta

Seasonal variations and reaction norms for vertebral number (VN) in response to incubation water temperature were estimated in adult and juvenile naturally spawning chum salmon Oncorhynchus keta. The mean VN of adults varied according to spawning time; the early-spawning population had higher VN values than the late-spawning population. Moreover, the mean VN values in the early-spawning population decreased with seasonal changes, whereas VN values in the late-spawning population remained stable. Chum salmon embryos in three full-sib families were incubated at five different temperatures until hatching, and the VN values of the resulting juveniles were analyzed. The VN reaction norm to incubation water temperature showed a V-shaped curve that was lowest at an intermediate temperature. The mean VN at the same incubation temperature varied among the three families. These results suggest that VN values in chum salmon are influenced by genetic components and incubation water temperatures. VN may be a useful parameter for estimating the environmental conditions during ontogenesis and the genetic background by detecting population changes.