Bull. Jpn. Soc. Fish. Oceanogr. 77(2), Page 68-82, 2013   Trophic positions of jellyfish, comb jellies and epipelagic fish in Hiuchi-nada, the central Seto Inland Sea, Japan: Evaluation based on carbon and nitrogen stable isotope analyses Hiromu Zenitani1 †, Naoaki Kono1, Shingo Watari1 and Youichi Tsukamoto2 1 National Research Institute of Fisheries and Environment of Inland Sea, 2-17-5 Maruishi, Hatsukaichi, Hiroshima 739-0452, Japan 2 Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Tairacho, Nagasaki, Nagasaki 851-2213, Japan † E-mail: zenitani@fra.affrc.go.jp To evaluate the impact of jellyfish and comb jellies on the fishery stock, the trophic positions of these animals as well as epipelagic fish species in marine ecosystems are to be clarified. Trophic positions of copepods, jellyfish and comb jellies, and epipelagic fish spices collected from the Hiuchi-nada, the central Seto Inland Sea, Japan, were examined based on carbon and nitrogen stable isotope distributions. The carbon and nitrogen stable isotope ratio （δ13C, δ15N） for （1） copepods （Acartia omorii, Corycaeus affinis, Oithona similis, Paracalanus parvus）, （2） jellyfish and comb jellies （Aequorea coerulescens, Aurelia aurita, Chrysaora pacifica, Bolinopsis mikado）, and （3） Japanese anchovy （Engraulis japonicus） was （−21.9 to −18.0‰, 7.6 to 11.9‰）, （−21.1 to −15.7‰, 10.1 to 16.4‰）, and （−21.0 to −15.2‰, 10.9 to 18.7‰）, respectively. We assumed that an average δ15N value of 13.5‰ for 10-20 mm SL larval anchovy corresponds to trophic level 3 for calculation of trophic level for copepods, jellyfish and comb jellies, and other fishes. The 10-20 mm SL larval anchovy-based trophic position （TP） for copepods, and jellyfish and comb jellies was 1.7-2.0, and 2.6-3.0, respectively. The TP for Japanese anchovy, Japanese jack mackerel Trachurus japonicus, white croaker Pennahia argentata, largehead hairtail Trichiurus lepturus, Japanese Spanish mackerel Scomberomorus niphonius, and chub mackerel Scomber japonicus was 2.7-3.7, 4.2, 4.3, 4.1, 3.9, and 3.6, respectively. Overlaps in feeding habits between jellyfish+comb jellies, and larval anchovy were detected. Regarding the impact of jellyfish and comb jellies on anchovy stock in Hiuchi-nada, it is suggested that the effect of competing for the prey was stronger than that of the direct predation by jellyfish and comb jellies on larval Japanese anchovy. Key words: jellyfish, comb jellies, epipelagic fish, stable isotope, trophic position, Seto Inland Sea