An investigation of ecological isolation and female life-history traits in a species pair of threespine stickleback (Gasterosteus aculeatus ) in Mud Lake, Alaska

A sympatric pair of anadromous and resident freshwater threespine stickleback (Gasterosteus aculeatus) occurs in Mud Lake in the Matanuska-Susitna Valley, Alaska. The two forms differ in an array of morphological traits. Time and place of breeding was investigated as possible barriers to reproductive isolation between the two forms. Female life-history traits, such as size and age at reproduction, clutch size, clutch mass and egg mass were examined in both populations.;Partial temporal and spatial isolation in breeding was detected between the two forms. Although ethological isolation was not examined in this study, discrete distribution of morphological traits in the two populations suggests that assortative mating contributes to reproductive isolation to a large extent.;Resident fish in Mud Lake are making very small eggs and moderate sized clutches, while anadromous fish are making small clutches and moderate sized eggs. Both populations have low reproductive output compared to that of other threespine stickleback populations in Alaska. Females might optimize their reproductive effort by laying several clutches over a long breeding season.