Inflexibility of vertebral number in chum salmon Oncorhynchus keta

Chum salmon Oncorhynchus keta eggs from the Chitose and Shiriuchi rivers in Hokkaido, northern Japan, were incubated under identical conditions with the aim of comparing vertebral number (V_N) to that reported archived data from the 1950s. The comparison revealed that mean V_N of salmon from the Shiriuchi River has remained unchanged for 50 years and is higher than that from the Chitose River. The V_N in the Chitose River salmon was found to be significantly higher than that recorded in the archival data. The variation in the mean V_N of chum salmon fry collected during different seasons and years in the Chitose River falls within a range that has remained constant, suggesting that the difference between the values obtained in our study and the archival data may be due to differences in the timing of egg collection. These results indicate that the mean V_N of chum salmon has been stable for a long period. Incubation experiments under identical conditions are a useful tool for examining changes in genetic structure and detecting the essential properties of populations.