Bull. Jpn. Soc. Fish. Oceanogr.  68(1), Page 9-19, 2004   Recent Increase of Jellyfish Populations and their Nuisance to Fisheries in the Inland Sea of Japan Shin-ichi Uye †1 and Uka Ueta1 1Graduate School of Biosphrere Sciences, Hiroshima University, 4-4 Kagamiyama 1 Chome, Higashi-Hiroshima 739-8528, Japan. †e-mail: suye@hiroshima-u.ac.jp We conducted a poll of fishermen in order to survey the recent increase of jellyfish (e.g. Aurelia aurita and Chrysaora melanaster) populations and their nuisance to fisheries in the Inland Sea of Japan. We sent questionnaires to 161 fisheries cooperatives and interviewed face-to-face in respondent’s ports and homes. A total of 1152 respondents, with >20-year-experience as net-fishermen (85%), angling-fishermen (7%) and others (8%), was obtained. During the survey, 65% of total respondents indicate that A. aurita population have increased in the last 20 years. The increase is most remarkable, particularly in the last 10 years, in the western Inland Sea of Japan. An elongation of the period of occurrence as medusa is obvious (in extreme cases, there are over-wintering medusae) in both the eastern and western Inland Sea of Japan, due to recent increase of annual minimum water temperature (>11°C). On the other hand the seasonal occurrence pattern is more or less the same as before in the central Inland Sea of Japan, where the winter water temperature is <10°C. Annual landings of zooplanktivorous fish have decreased to about 1/3 of the peak landings in the mid-1980s, suggesting that A. aurita can utilize more zooplankton as food than before to enhance its population. From these facts, we conclude that the recent increase of A. aurita population might be attributed to water temperature elevation in the winter and over-fishing of zooplanktivorous fish. Although the seasonal occurrence of C. melanatus is largely the same as before, 47% of respondents indicate that its population has increased in the last 20 years. The nuisance of jellyfish to fisheries, particularly to net-fisheries, has been increasingly severe, according to reports by fishermen. The productive ecosystem of the Inland Sea of Japan has been degraded by the proliferation of jellyfish, i.e. the process of so-called "jellyfish spiral". Key words: Inland Sea of Japan, jellyfish, biomass-increase, nuisance to fisheries, poll survey