Variation of vertebral number in juvenile Oncorhynchus mykiss in relation to upstream distance from the ocean

Vertebral counts in fishes are set early in development by a complex interaction of genetic and environmental factors. Recently, R. M. McDowall showed that vertebral counts can differ between diadromous and non-diadromous galaxiids, and suggested similar life history-based variation might also occur in the closely related Salmonidae. We examined variation in vertebral counts of juvenile rainbow trout, Oncorhynchus mykiss, from within a single large basin where anadromous and non-anadromous forms co-occur. Juveniles were collected at 29 sites within the Klamath River basin using a nested sampling design. Mean vertebral counts varied among regions, among complexes within regions, and among sites within complexes. Regression showed that mean vertebral counts decreased with increasing distance from the ocean. This decrease could not be explained by differences in temperature within the basin during early development. However, the decrease is consistent with the expectation that non-anadromous forms have fewer vertebrae and are more common at sites distant from the ocean, and suggests that further study of the relationship between vertebral counts and life history form in O. mykiss is warranted.