Shifts in life-history traits of two introduced populations of threespine stickleback

Background: Anadromous threespine stickleback (Gasterosteus aculeatus) from Rabbit Slough were introduced into Cheney and Scout Lakes, Alaska in 2009 and 2011, respectively. The introductions were intended to model colonization of freshwater habitats by oceanic stickleback following the deglaciation of the Cook Inlet region, which resulted in a widely studied system of highly variable populations. Hypothesis: Life-history traits of females in the colonizing lake populations will change from the ancestral phenotype within the first few generations after the experimental introductions, consistent with expectations for an opportunistic life-history strategy based on models for life-history change in invasive fishes. Females in the lakes are expected to reproduce at an earlier age and smaller size than anadromous females, and to exhibit greater reproductive effort (size-adjusted clutch mass) and clutch size (size-adjusted numbers of eggs). Changes in egg mass are difficult to predict because complex factors influence this life-history trait. Methods: We quantified length, body mass, clutch mass, clutch size, and egg mass of wildcaught females from the anadromous source population and the two introduced lake populations during the first few years after introduction. In analyses of clutch mass, clutch size, and egg mass, we used body mass to correct for female size. Results: As expected, age at reproduction and adult body size decreased while size-adjusted clutch size and reproductive effort increased. These changes occurred abruptly in the first year after introduction. Egg mass did not change in the first year post-introduction. Changes in life-history traits in subsequent years included larger mean egg mass and a greater proportion of females reproducing at age 2 instead of age 1.