Bull. Jpn. Soc. Fish. Oceanogr. 69(4), Page 263-270, 2005   Sudden increase and the factor in Japanese sardine catch in Hiuchi-Nada, the Central Seto Inland Sea, observed during a period of declining stock size Akio Watanabe†1 and Hiroaki Hashimoto2 1 Toyo Branch Ehime Prefectural Chuyo Fisheries Experimental Station, 1188 Kawarazu, Saijo, Ehime 799-1303, Japan 2 Graduate School of Biosphere Science, Hiroshima University, 1-4-4 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8528, Japan † e-mail: watanabe-akio@pref.ehime.jp The biological aspects of the Japanese sardine found in Hiuchi-Nada, the Central Seto Inland Sea, were studied in terms of catch fluctuation, egg production, maturation, and growth. Since the end of the 1980s, sardine population around Japan began to decline drastically. However, the sardine catch in Hiuchi-Nada has increased in the 1990s. The peak of catch in the center of the Seto Inland Sea was apparently later than the catch in the entrance of the Seto Inland Sea by tracing the pattern of sardine fishing in the Seto Inland Sea. The fishing of 0-year-old and adult sardines was started in Hiuchi-Nada, in 1995, and 1996. On the basis of egg production, it was noted that the number of adult sardines that migrated to Hiuchi-Nada had increased in 1993. The characteristic of length frequency distributions of adult sardine in Hiuchi-Nada was similar to that of the fishes in Tosa Bay, which is one of the main spawning grounds on the Pacific coast of Japan. Therefore, adult sardine in Hiuchi-Nada probably migrated from the Pacific coast. Though anchovy catch had been more dominant than the other pelagic catch in Hiuchi-Nada, former was poor when sardine migrated to Hiuchi-Nada. The sardine migrations to Hiuchi-Nada were not continued by the decline of the population along the Pacific coast. Key words: Hiuchi-Nada, sardine, catch fluctuation, egg production