The European sea squirt, Ascidiella aspersa was first found as an alien species in 2008 from Funka Bay, Hokkaido, northern Japan, causing serious damage to the scallop aquaculture industry. We investigated A. aspersa on cultured scallops and larval occurrence from July 2010 to June 2014 to clarify life history traits and population dynamics, and consider the relation between the life history of A. aspersa and the process of scallop aquaculture. Larvae of A. aspersa were found from June to December, and recruitment on cultured scallops occurred mainly between July and October. The ascidians grew well and their weights increased until February. We found that 60–80% of A. aspersa that had settled in summer had eggs or sperm in autumn, and 90–100% of A. aspersa matured early the following summer. Maturity size in September was 17–20 mm as male, 22–24 mm as female. Scallops in Funka Bay are hung in the spring and harvested from winter to the next spring. Ascidiella aspersa settle as larvae in early summer, and grow well until winter, resulting in overgrowth on scallops in the harvest season. The linking of the process of scallop aquaculture and the life history of A. aspersa explains why this invasive ascidian has caused serious damage to the aquaculture industry in the bay. In comparison to the earlier descriptions of the native population, A. aspersa in Funka Bay has longer reproductive and growth periods, earlier initiation of reproduction, and possibly smaller maturity size.