Discussion
We show for the first time that the size of the
adipose fin varies among wild spawning salmon
populations after controlling for other known
sources of variation (e.g., migration distance,
reproductive state). Thus, the adipose fin joins
the list of other morphological traits (body depth,
jaw length) that differ among populations of
sockeye salmon. Furthermore, population specific
differences in adipose fin size of mature male
salmon provide support to the hypothesis that the
trait has functional significance in reproduction,
thereby helping to explain its evolutionary
persistence in this group of eutelost fishes. An
obvious and intriguing extension of these findings
would be to investigate the potential reproductive