FLUCTUATIONS IN FISH CATCH OF MIDDLE-SIZED PELAGIC FISH IN THE SETO-INLAND-SEA, JAPAN

Catch fluctuations of middle-sized pelagic fish such as sardine, ancho vy, young clupeoid fish (juvenile sardine and anchovy) horse mackerel and mackerel in the Seto Inland Sea were analysed from 1964-'91. Since 1975, the sardine catch rapidly increased and became the most dominan t catch from 1978-'82, whereas catches of the other fish decreased. Fr om 1983-'86 the sardine stock decreased gradually, while catches of th e other fish recovered. Recently, however, the anchovy catch has decre ased again whereas catches of young clupeoid fish have fluctuated shar ply at high levels. The connection between fish catches and environmen tal factors (water temperature, salinity, COD and chlorophyll-a) was e xamined. However, no correlation between trends in catch and environme ntal factors was found. It is said that the pelagic fish form their ow n community and competition occurs between fish in that community. Alt hough the sardine were not distributed typically in the sea, the recen t increase in abundance due to migration from the Pacific Ocean has af fected the structure of the pelagic fish community. Consequently, the relative abundances of the pelagic fish lay have influenced the compos ition of the catches. The increase in the catch of young clupeoid fish in recent years may have resulted from sardine spawning in the sea.