Bull. Jpn. Soc. Fish. Oceanogr. 85(1), Page 14-23, 2021   Massive occurrences of Japanese littleneck clam (Ruditapes philippinarum) juveniles at the river mouth tidal flat in Mikawa Bay, Japan Kenji Nakamura1,† and Teruaki Suzuki2 1 Chateau Marine Survey Co., 1�13�14 Tenma, Kita, Osaka 530�0043, Japan 2 Graduate School of Environmental and Human Science, Meijo University, 1�501 Shiogamaguchi, Tempaku, Nagoya, Aichi 468�8502, Japan † E-mail: matsu@chateau-kaiyou.co.jp Massive occurrences of juvenile clams Ruditapes philippinarum have been appearing at Rokujo tidal flat in the Toyo River mouth at the inner part of Mikawa Bay every year. These have been collected by permitted fishermen and have been released to replenish about 3,000 tons of resources into the coastal area of Ise and Mikawa Bay since 2003. However, the mechanism of the massive occurrences has not been fully elucidated. Therefore, we investigated temporal and spatial changes in the shell length composition and number of juvenile clam individuals from May 2015 to March 2017 in Rokujo tidal flat (Case1) and from March 2017 to June 2018 in Rokujo tidal flat and river mouth part (Case2). As a result, a rapid increase in the number of settled individuals in the spring was observed during the spring in both Case 1 and Case 2. The number of individuals that increased rapidly in spring was larger than those that settled in Rokujo tidal flat in the fall and survived. The main resource for the release was considered to be individuals that increased rapidly in spring. Although the source of this massive occurrence was not clear, they might be born in the tidal area of the Toyo River. The stock born in the tidal area of the Toyo River might have an important role for the massive occurrence of juvenile clams in the Rokujo tidal flat in spring. Key words: Rokujo tidal flat, river mouth, Ruditapes philippinarum, juvenile, massive occurrence