EVOLUTION OF ADULT FEMALE LIFE HISTORY AND MORPHOLOGY IN A PACIFIC SALMON (COHO: ONCORHYNCHUS KISUTCH)

Female competition on the spawning grounds can generate strong natural selection on female coho salmon (Oncorhynchus kisutch). We examined the morphology and investment into egg production of 13 wild and five hatchery populations. For each population, the degrees of breeding competition and migration arduousness were quantified. The results support the importance of breeding competition in the evolution of female morphology and life history. Female secondary sexual characters, including body coloration and length of kype (an extension of the upper jaw used for fighting), increase significantly with degree of breeding competition among populations. In contrast, egg size and female investment into egg production tend to decrease as competition increases, probably as a result of life-history trade-offs. The difficulty of migration to the spawning grounds also molds female morphology and life history. Salmon become more streamlined with increasing migration arduousness, and the biomass of eggs produced appears to decline. In hatcheries, where breeding competition is relaxed, characters associated with breeding competition and spawning performance are reduced. In contrast, egg size is increased. These results indicate that the morphology and life history of adult female salmon respond evolutionarily to local selection pressures, including both the biotic demands of breeding competition and the abiotic demands of migration.