Trajectory and rate of change in female life-history traits following colonization of a freshwater, lacustrine environment by oceanic threespine stickleback

Question: What is the trajectory of change of female life-history traits following colonization of a freshwater lake by oceanic threespine stickleback? Hypotheses: Based on comparative studies of many freshwater populations, we predict that (1) reproductive effort and clutch size will be reduced from the ancestral condition, and (2) the breeding pool of females will change to include greater percentages of younger breeders. The trajectory of change in egg size cannot be predicted from current knowledge. Organism and field site: Threespine stickleback (Gasterosteus aculeatus). Loberg Lake, south-central Alaska, was poisoned in 1982 to remove all fish, and then restocked by the Alaska Department of Fish and Game with trout only; the lake was apparently naturally recolonized by anadromous stickleback between 1983 and 1989. Methods: Annual small-mesh trap collections between 1992 and 2015, and trap collections of the presumed ancestral stock in a nearby stream, the latter providing the starting point for evolutionary trajectory calculations. Our study encompassed an estimated 21 generations. Conclusions: Reproductive effort and clutch size (both standardized for female body size) declined as predicted, by 28% and 41% from the ancestral values, respectively. Although the overall downward trend in both traits was substantial and rapid, a strong cyclical pattern was also present. Female breeding age also declined as predicted, with age-1 breeders becoming more prevalent in the lake. Egg size did not change appreciably.