Bull. Jpn. Soc. Fish. Oceanogr. 76(2), Page 51-58, 2012   Possibility of bottom-up and top-down controls of the population fluctuations in willowy flounder, Tanakius kitaharai, in Joban Kashima-nada, Japan Masakazu Takahashi1 †, Akira Nihira2 and Yasunori Sakurai3 1 Ibaraki Prefectural Fisheries Experimental Station, Hiraiso, Hitachinaka, Ibaraki 311-1203, Japan 2 Institute of Regional Studies, Ibaraki University, Bunkyo, Mito, Ibaraki 310-8512, Japan 3 Graduate School of Fisheries Sciences, Hokkaido University, Minato-cho, Hakodate, Hokkaido 041-8611, Japan † e-mail: mas-takahashi@pref.ibaraki.lg.jp Population of willowy flounder, Tanakius kitaharai in the waters of Joban Kashima-nada fluctuate heavily throughout their recruitments although the mechanisms are poorly understood. In addition to assessing the relationship between parent and offspring flounder, we analyzed statistically the relationship between flounder recruitment size index, abiot- ic and biotic factors such as water temperature at various depths and zooplankton biomass. We found that recruitment size index was significantly linking with water temperatures at a depth of 100 m in May, suggesting bottom-up control caused by environmental factors was influencing the flounder’s population level. However, in addition to bottom-up control by environmental factors, strong influence from top-down control factor caused by competition with pelagic fish fed the same zooplankton are also suspected, due to an increase in feeding activity caused by a large number of Japanese sardine, Sardinops melanostictus, migrating in the waters of Joban Kashima-nada before mid-1990s. We con- cluded that bottom-up and top-down processes are equally important in the survival success of willowy flounder through the recruitment process. Key words: willowy flounder, catch fluctuation, recruitment, environmental effects, bottom-up and top-down control, Joban Kashima-nada