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