Japanese eel Anguilla japonica and aquatic animals collected with Ishi-kura net in the Iroha and Katsura Rivers, Oita Prefecture, Japan

Local people in Oita Prefecture perform a traditional fishery called the Ishi-kura fishing method in estuaries. Ishi-kura is an underwater pile of approximately 0.5 tons of stones 15-30 cm in diameter, and wild eels dwell inside the pile and are caught using nets.We examined the relationship between Japanese eel Anguilla japonica and aquatic animals caught with an Ishi-kura net which was modified for aquatic animal research, in the Iroha and Katsura Rivers, Oita Prefecture, Japan. Based on community structure analysis, the aquatic animal community in the Ishi-kura net consisting of 54 taxonomic animals was divided into two groups by distance from the river month and substrate; the upstream group mainly contained the genus Hemigrapsus and genus Batillaria, and the downstream one predominantly contained Macrobrachium nipponense and Clithon retropicta. Based on stomach content analysis, 39 out of 79 eels consumed the same aquatic animal groups found in Ishi-kura. However, eels inhabiting downstream mainly preyed on mud shrimp, and those inhabiting upstream predominantly fed on crab, suggesting that eels may not use Ishi-kura for feeding but for living space. These findings suggest that Ishi-kura nets may be useful for monitoring eels as well as aquatic animals as their potential prey items.