Stock size fluctuations in chub mackerel (Scomber japonicus) in the East China Sea and the Japan ⁄ East Sea YOSHIAKI HIYAMA*, MARI YODA AND SEIJI OHSHIMO Seikai National Fisheries Research Institute, Fisheries Research Agency, 3-30 Kokubu-machi, Nagasaki 850-0951, Japan ABSTRACT The stock size of chub mackerel (Scomber japonicus) in the East China Sea and the Japan ⁄ East Sea has shown a continuous decrease, with fluctuations, from 1975 to 1990. After a remarkable increase in 1992–96, the stock size fell to its lowest level in 1997–99. The spawning success (number of recruits ⁄ spawning bio- mass) fluctuates periodically with a period of several years, although there are no clear relationships between the spawning biomass and the number of recruits. Spawning success was inversely correlated with sea surface temperature (SST). On the contrary, the catch amount and the catch per unit effort (CPUE) of spotted mackerel (Scomber australasicus) has increased since 1997. Key words: chub mackerel, Scomber australasicus, Scomber japonicus, spawning success, spotted mackerel, virtual population analysis INTRODUCTION The chub mackerel (Scomber japonicus) is distributed in temperate and subtropical zones throughout the world, although Scoles et al. (1998) stated from their genetic analysis that the Atlantic and Indo-Pacific populations may need to be recognized as separate species. For the chub mackerel around Japan, it is presumed that there are two populations, in the Pacific and the Tsushima current. The latter inhabits the East China Sea and the Japan ⁄ East Sea and its spawning ground spreads from the southern part of the East China Sea to the western part of Japan Sea (Fig. 1). Ouchi and Hamasaki (1979) suggested that there are three subpopulations in the Tsushima current popu- lation. However, some tagged fish migrated from the northern part of the Japan Sea to the south of Japan (Hasegawa et al., 1991). In this study, we assumed that the chub mackerel in the East China Sea and the Japan Sea forms one population (Tsushima current population). The chub mackerel is an important fishery resource, especially for the purse seine fisheries. In the East China Sea and adjacent waters, Japan, Korea, and China exploit the stock intensively. TAC (Total Allowable Catch) management began in Japan in 1997 and TAC is set as a combined catch for both chub and spotted (Scomber australasicus) mackerel and grouped as mackerel. Limbong et al. (1988) calculated the stock size of chub mackerel in the East China Sea and the Japan Sea using length cohort analysis of catch statistics of the Japanese large- and medium-type purse seine fishery. They showed that the stock size of chub mackerel decreased between 1974 and 1984. For the spawner-recruit relationship of the chub mackerel stock in the Japan Sea from 1969 to 1978, Nishida (1997) analysed the residuals from a Ricker spawner-recruit model and suggested that higher temperatures in the East China Sea favour high recruitment. Hwang (1999) pointed out that 0-year- old fish are abundant in Korean waters in the years when the water temperature is relatively high during June to August. To clarify the cause of stock size fluctuations, it is important to analyse recent catch data and evaluate the effects of fishing pressure and environmental factors on the stock conditions. We trace the trends shown in stock size by analysing catch statistics of the Japanese large- and medium-type purse seine fishery, which takes about 80% of Japan’s catch of mackerel in the East China Sea and the Japan Sea. Furthermore, we discuss the relationship between SST and fluctuations in spotted mackerel and chub mack- erel. MATERIALS AND METHODS Catch at age In Japanese catch statistics, the category name Ômackerel’ usually includes both the chub and spotted *Correspondence. e-mail: hiyama@affrc.go.jp Received 30 June 2001 Revised version accepted 15 July 2002 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 11:6, 347–353, 2002 Ó 2002 Blackwell Science Ltd. 347