Bull. Jpn. Soc. Fish. Oceanogr. 84(2), Page 89-99, 2020   Detection of pair-trawling ships using ALOS-2/PALSAR-2 data Kenji Takasaki1,Tsutomu Saito1,Yoshioki Oozeki1,Denzo Inagake1,Hiroshi Kubota2, Tadafumi Ichikawa3,Hiroya Sugisaki4 and Shuji Shimizu5 1 Headquarters, Japan Fisheries Research and Education Agency, 2–12–4 Fukuura, Kanazawa, Yokohama, Kanagawa 236–8648, Japan 2 Japan Sea National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 1–5939–22 Suido-cho, Chuou-ku, Niigata, Niigata 951–8121, Japan 3 National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, 2–12–4 Fukuura, Kanazawa, Yokohama, Kanagawa 236–8648, Japan 4 Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3–27–5 Shinhama-cho, Shiogama-city, Miyagi 985–0001, Japan 5 Satellite Applications and Operations Center, Japan Aerospace Exploration Agency, 2–1–1 Sengen, Tsukuba, Ibaraki 305–8505, Japan In this study, we detected pair-trawling ships using a synthetic aperture radar that can observe fisheries without a fishing lamp. Automatic detection of pair-trawling ships is possible by extracting all ships from the image, examining the distance to the surrounding ships from each ship, and using the combination that minimizes the distance between ships as a pair. For the cases where the ship pair is incorrect, correction methods are presented. In addition, in order to understand the activities of the pair-trawling ships in the target area, the density of pair-trawling ships and distance between ship pairs is calculated. In this study, if sufficient data can be obtained during the fishing season, there is a possibility to estimate fishing effort. Key words: Pair-trawling ship, ALOS-2/PALSAR-2, Synthetic Aperture Radar, Japan Sea, Japanese common squid