The spawning territorality of male and female non-anadromous sockeye salmon, Oncorhynchus nerka, was examined experimentally by comparing the resources sought after, the behaviours used in intrasexual competition, and the correlates of success in intrasexual competition for each sex. Females expressed territorial behaviour both in the presence and absence of males, defending specific, repeatedly selected, spawning sites. In contrast, males exhibited clear territorial behaviour only in the presence of females, defending access to specific females and positions over time. Size was a factor in determining which females established territories with preferred areas, but this advantage was largely negated by prior residency. Size was a major factor in determining which males gained closest access to individual females, with smaller males adopting subordinate, sneak, positions. Prior residence was also a significant factor in determining the outcome of contests for access to females, with the weight of this factor dependent on the relative sizes of competing males. The behavioural differences between sexes appear to arise from differences both in the resources defended and the relative costs of potential losses for each sex.
