NATURAL SELECTION RESULTING FROM FEMALE BREEDING COMPETITION IN A PACIFIC SALMON (COHO:ONCORHYNCHUS KISUTCH)

Abstract

We studied breeding competition among wild female coho salmon (Oncorhynchus kisutch) and quantified natural selection acting on two important female characters: body size and kype size (a secondary sexual character used for fighting). We found that body size contributed to adult female fitness in three ways, through 1) an increased initial biomass of egg production, 2) the ability to acquire a high-quality territory for egg development, and 3) success in nest defense. These factors together resulted in as much as a 23-fold fitness advantage to the largest females in the population. The initial investment into egg production accounted for 50-60% of the measured intensity of natural selection on female body size. The effective investment into egg production (after female competition for territories) accounted for 40-50% of natural selection on female body size. Therefore, success in breeding competition is about as important as egg production in the current evolution of female body size. This is contrary to the expectation based on most fisheries literature. The size of a female's kype was also important to female reproductive success, although its contribution could not be separated from that of body size in our study. The strong natural selection that we have found for female competitive ability is presumably the basis for the evolution of female parental care in salmonids.