Juvenile bimodal length distribution and sea-run migration of the lower modal group in the Pacific Ocean form of three-spined stickleback

Juvenile three‐spined stickleback Gasterosteus aculeatus in their nursery pond had a bimodal length distribution after October in their first year of life. A sea‐run migration of all individuals belonging to the lower modal group was observed, with a peak in early November. These data suggest that partial migration based on the early status of individual juveniles occurs in the Pacific Ocean form of three‐spined stickleback.