Bull. Jpn. Soc. Fish. Oceanogr. 81(4), Page 271-283, 2017   Reconsidering Trans-Pacific “synchrony” in population fluctuations of sardines Akihiko Yatsu1†, and Atsushi Kawabata2 1 Hokkaido National Fisheries Research Institute, Fisheries Research Agency, 2–2 Nakanoshima, Toyohira-ku, Sapporo, Hokkaido 062–0922, Japan Present affiliation: Japan Fisheries Information Service Center, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan 2 National Research Institute of Fisheries Science, Fisheries Research Agency, 2–12–4 Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236–8648, Japan Present affiliation: Resources and Environmental Research Division, Fisheries Agency of Japan, 1–2–1 Kasumigaseki, Chiyoda-ku, Tokyo 100–8950, Japan † E-mail: yatsu@jafic.or.jp The causes of Trans-Pacific synchrony and asynchrony in sardine populations were evaluated by examining commercial catch, fishing mortality, spawning stock biomass, and productivity, defined as ln-transformed recruitment residuals (LNRR). Anomalies of basin-scale and regional sea-surface temperatures (SSTs) are key environmental drivers of population dynamics of some stocks of sardines. Key results were: 1) productivity was synchronized between the Humboldt sardine (Sardinops sagax sagax) and the Japanese sardine (S. melanostictus) (Pacific stock) during the period for which data were available (1970–99), 2) sudden recoveries in recruitment and productivity of the Humboldt and Japanese sardines during the early 1970s were associated with favorable regional SST anomalies that were enhanced under the favorable PDO regime that followed after 1977, 3) recruitment failures coincided with a climate regime shift in 1988/89 (Humboldt and Japanese sardines and probably California sardine (S. sagax caeruleus) in the Gulf of California) and negative anomalies of PDO and CalCOFI SST during 2008–12 (the northern subpopulation of California sardine) caused natural stock declines, and 4) high fishing mortalities coupled with generally unfavorable regional SST anomalies during the 1990s and 2000s prevented recoveries of the Humboldt and Japanese sardines, under a generally favorable PDO regime for sardines. Finally, implications for fisheries management, such as long-term management coping with mismatch between economic and ecological scales of variability, were discussed. Key words: California sardine, Humboldt sardine, Japanese sardine, PDO, regime shifts, sea-surface temperatures, fisheries management