Large-scale frequency dynamics of alternative male phenotypes in natural populations of coho salmon (Oncorhynchus kisutch): patterns, processes, and implications

Abstract

Using over 20 years of annual spawner survey data collected from 46 natural populations of coastal Oregon coho salmon (Oncorhynchus kisutch), we examined large-scale spatial and temporal dynamics of alternative male pheno types (age-3 hooknoses and age-2 jacks). Fluctuations in jack and hooknose abundance were synchronous among populations (i.e., region-wide synchrony), similar to patterns seen previously in Oregon hatchery populations and indicative of oceanic processes operating at large spatial and temporal scales. By contrast, regional-scale synchrony in the ratio of jack to hooknose males was likely attributable to the influences of the freshwater environment. Moreover, within-population variation in phenotype ratio was lower by brood year than by return year (i.e., weak cohort relationship), contrary to previous patterns in hatchery populations. The spatial and temporal patterns of phenotype ratio observed in natural populations but not in hatchery populations, where freshwater effects were statistically controlled, indicate that the frequency dynamics of coho male alternative phenotypes are influenced more strongly by freshwater than by marine processes. The environmentally induced variability in phenotype ratio suggests that there may be constant perturbations of fitness functions for alternative phenotypes, such that the system should be less stable than predicted from the status-dependent selection model with fixed fitness functions.