Action of Natural Selection on Body Size and Date of Return in a Wild Population of Coho Salmon, Oncorhynchus kisutch

In order to understand the process of natural selection, it is critical to identify life history traits that affect variation in individual fitness and to study the magnitude and direction of selection occurring on those traits. In anadromous salmonids, a number of studies have shown that body size and date of arrival to the spawning grounds affect individual fitness, such as reproductive success or survival. In this study, we estimated the magnitude, type and direction of selection acting on body length and date of arrival in a wild population of coho salmon (Oncorhynchus kisutch) at Big Beef Creek, Washington. Individual reproductive success was quantified by pedigree reconstruction using 11 microsatellite loci. Strong evidence for directional selection was observed in both traits for both sexes, favoring larger, earlier returning individuals. However, directional selection on body length in males was greater than in females. The date of arrival that maximized reproductive success was different between sexes. Specifically, the earliest returning males were less successful than males returning shortly thereafter; however, earlier returning females were consistently more successful than females returning later. This study provides baseline information on evolution in this species and has implications on management practices.