ABSTRACT: Clarification of the biological and/or physical factors determining the location requirements for spawning aggregations is necessary for understanding the population dynamics of free-spawning marine animals. We monitored the density of adult abalone Haliotis discus hannai on 2 types of artificial reefs with different heights (tall and short blocks) and the surrounding seabed area in Otsuchi Bay, northeast Japan, from February 2016 to December 2019. In addition, the number of neighboring adults located within a 50 cm radius from each individual was counted from March 2017 to December 2019. Algal succession in the 3 types of location (tall and small blocks, seabed) was monitored and compared with temporal fluctuations in adult abalone density. Our results show that the fluctuations in density were different among the 3 locations, especially during the spawning season (August-October). On tall blocks, clear increases in adult abalone density and number of neighboring adults were confirmed during the spawning seasons in 2016 and 2019. In contrast, there was no clear increase in the adult density on small blocks and surrounding bottom areas. Aggregations on the tall blocks clearly differed among the 4 years, and the difference was considered to be influenced by differences in the macroalgal species and coverage during the spawning season. This is the first study to report that a difference in phenology among algal species is a key factor in the formation of spawning aggregations of a free-spawning marine animal.