Among-population variation in adipose fin size parallels the expression of other secondary sexual characteristics in sockeye salmon (Oncorhynchus nerka)

The small, non-rayed adipose fin is present in eight extant orders of fishes, including the Salmoniformes (salmon and trout) but the functional significance of the trait is unknown. Recent evidence suggests a hydrodynamic function in juvenile salmonids, and observations of sexually dimorphic adipose fin expression and female preference for males with large fins indicate a role in reproduction by mature individuals. To the extent that the adipose fin functions in reproduction, it might be expected to evolve in parallel with other sexually dimorphic traits, such as body depth and jaw length. To test this hypothesis, we quantified adipose fin size of mature male sockeye salmon, Oncorhynchus nerka, among five populations. Populations differed significantly in adipose fin size after correcting for variation in body length and body depth. Adipose fin size tended to parallel the development of other secondary sexual characteristics, but was more closely related to body length and body depth than jaw length. Interestingly, shallow bodied populations from small creeks with high brown bear, Ursus arctos, predation during spawning tended to have smaller size-adjusted adipose fins than populations spawning in deeper water. However, it remains unclear whether adipose fin size is being selected independently of other traits or if it is pleoitropically linked to a trait under selection (e.g., body size or shape).