VARIATION IN PACIFIC SALMON REPRODUCTIVE BEHAVIOUR ASSOCIATED WITH SPECIES, SEX AND LEVELS OF COMPETITION

THOMASP.QUINN Male and teniale Pacific salmon compete for different resources: females for suitable spawning sites and males for access to ripe females. Aggression should thus be primarily intra-sexual rather than inter-sexual. When different species are sympatric, males should primarily attack conspecitics whereas females should attack all females, regardless of species because they all compete for the same resource space. The level of aggression should be a function of density, being relatively low at low densities and peaking at either intermediate or high densities. These predictions were supported in most respects by data collected on the behaviour of adult sockeye (Oncorhynchus nerka), chum (O. keta) and pink (O. gorbuscha) salmon in a large, relatively homogeneous spawning channel. Males almost exclusively attacked other males, especially conspecitics. Females were more likely to attack female heterospecifics than males but still tended to attack conspecifics most often, and also directed many attacks at males. Male aggression and digging, apparently a form of intra-sexual display, were related to density of male conspecitics in the local area, and the intensity of competition from satellite males courting the female.