Bull. Jpn. Soc. Fish. Oceanogr. 82(3), Page 121-126, 2018   Evaluation of incidental ingestion by stomach contents analysis of chub mackerel Scomber japonicus caught by purse seine Makoto Taga1,2† and Yoh Yamashita2 1 Ibaraki Prefectural Fisheries Research Institute, 3551–8, Mitsuzuka, Hiraiso, Hitachinaka, Ibaraki 311–1203, Japan 2 Field Science Education and Research Center Kyoto University, Kitashirakawa-Oiwakecho, Kyoto 606–8502, Japan † E-mail: m.taga@pref.ibaraki.lg.jp The body of chub mackerel Scomber japonicus is covered with small scales and mucus, and it has been pointed out that the stomach contents analysis for chub mackerel caught by purse seine may be biased due to incidental ingestion of them in the net during fishing. In this study, the stomach contents of mackerels caught by purse seine were compared to those caught by pole and line fishing, which are not affected by the incidental ingestion, at the same locations and time. Scales and mucous substances were found in 90 out of 99 (90.9%) stomachs of mackerels caught by purse seine, whereas they were found in a limited fraction (8.3%) of stomachs from mackerels caught by pole and line fishing. The scales found in the stomachs had the characteristics of mackerels. Furthermore, stomach contents index of the mackerels caught by purse seine was significantly higher than that from pole and line fishing when the weights of scales and mucous substances were included, while no difference was found when they were not included in the calculation. It has been concluded that the scales and mucous substances were ingested incidentally during fishing, and they should be excluded from the dietary data. Key words: chub mackerel, purse seine, stomach contents, scale, incidental ingestion