THE EFFECTS OF BODY-SIZE AND SEXUAL DIMORPHISM ON THE REPRODUCTIVE-BEHAVIOR OF SOCKEYE-SALMON, ONCORHYNCHUS-NERKA

Factors associated with the evolution of sexual dimorphism in sockeye salmon were examined by relating individual behavioural correlates of reproductive success with morphological measurements in both sexes. Ad ult salmon were captured, measured and tagged, then released and obser ved in Iliamna Lake, Alaska. Males were generally larger than females (447 versus 428 mm) and had larger dorsal humps and more elongated jaw s for their length. Indices of male reproductive success varied consid erably among individuals (e.g. 0-12 observations in dominant status). Male social status was positively associated with both length and dors al hump size, independent of length. Ah resident females completed spa wning and only nine of 82 had their nests dug up by other females. How ever, size-related variation in spawning-site location was observed; l arger females tended to spawn in shallower water than did smaller fema les. Thus reproductive success apparently varied much more in males th an in females. Consistent with the theory regarding the evolution of s exual dimorphism, the greater variability in male reproduction was dir ectly associated with the greater average size and exaggerated shape o f males.