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Aggregation of the abalone Haliotis discus hannai during the spawning season, and its associations with seasonal and interannual changes in the macroalgal community

Jun Hayakawa*, Kenta Nakamoto, Masafumi Kodama, Tomohiko Kawamura

*Corresponding author:

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. In the present study, we monitored the density of adult Haliotis discus hannai on two types of artificial reefs with different heights (tall and small blocks) and the surrounding seabed area in Otsuchi Bay, northeast Japan from Feb 2016 to Dec 2019. In addition, the number of neighboring adults located within a 50 cm radius from each individual was counted from Mar 2017 to Dec 2019. Algal succession on the three types of location were monitored and compared with the temporal fluctuations of the adult abalone density. Our results show that the fluctuations of the adult abalone density were different among the three locations, especially during the spawning season (Aug–Oct). On the tall blocks, clear increases in the adult abalone density and number of neighboring adult abalone were confirmed during the spawning seasons in 2016 and 2019. In contrast, there was no clear increase in the adult density on the small blocks and surrounding bottom areas. Aggregations on the tall blocks clearly differed among the four 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 research finding that the difference of phenology among algal species is a key factor for the formation of the spawning aggregation of a free-spawning marine animal.