Bull. Jpn. Soc. Fish. Oceanogr. 73(2), Page 105-117, 2010   Estimating gastric evacuation rate and daily ration of chub mackerel and spotted mackerel in the Kuroshio-Oyashio transition and Oyashio regions Sayaka Nakatsuka1†, Atsushi Kawabata2, Akinori Takasuka2, Hiroshi Kubota2, Hiroshi Okamura3 and Yoshioki Oozeki1,2 1 Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato, Tokyo 108-8477, Japan 2 National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan 3 National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648 Japan †e-mail: nakatsuka@n-sayaka.com Gastric evacuation rates and daily rations of chub mackerel Scomber japonicus and spotted mackerel S. australasicus were estimated using the stomach content weight and sampling time information from a data set of trawl surveys in the Kuroshio-Oyashio transition and Oyashio regions in summer, autumn and winter from 2001 to 2007. Gastric evacuation rates were calculated for three ranges of sea surface temperature (SST; SST<15°C, 15°CSST<20°C, 20°CSST) from the data collected in autumn, and daily rations (daily amounts of food consumption as % of body wet weight) were estimated for different ranges of fork length, three ranges of SST, and three seasons. Gastric evacuation rates were 0.13 to 0.16 in chub mackerel and 0.05 to 0.19 in spotted mackerel. The estimates of daily ration per body weight were 5.8 to 14.2% body weight in chub mackerel and 3.8 to 23.3% body weight in spotted mackerel. Mackerels indicated the highest daily ration in the SST range of 15-20°C in the feeding grounds in autumn. Daily rations at SST <15°C were much lower than those at SST 15°C in summer. This pattern indicated that the feeding activity could be depressed at temperature lower than 15°C in the nursery grounds in summer and feeding grounds in autumn although the daily rations were relatively high at the same SST in the wintering grounds. Daily rations of mackerels indicated high values at 15°C in summer, whereas the rations indicated a peak at 15-20°C in autumn. These trends suggested that the gastric evacuation rate and daily ration of mackerels were influenced by SST and seasonality. Overall, the estimates of gastric evacuation rate and daily ration were consistent with those from previous experimental studies and those of congeneric species in other ecosystems. Hence, the present results would provide a general pattern of these parameters for mackerels. Key words: daily ration, gastric evacuation rate, mackerel