The composition of the stomach contents found in our study is consistent with that reported by previous studies conducted on anguillid eels in which crustaceans and other invertebrates were shown to be the major prey item, with small fishes sometimes being consumed by larger eels [28, 37–40]. A. japonica individuals captured in the estuary and the bay had similar diets, but these diets differed from that of A. japonica in freshwater habitats [28], which is also thecase for the European eel [41]. For C. myriaster, the findings of this study are also consistent with studies on conger eels in much deeper marine habitats where they also eat many crustaceans, although a higher proportion of fishes are sometimes consumed [5, 8, 34, 42–44].The stable isotope signatures showed that the diet of four other fish species analyzed in this study (tonguefish, urohaze-goby, pike eel, and sea bass) may be considerably different from that of A. japonica, while there appeared to be considerable diet overlap between A. japonica andC. myriaster. However, the d13C and d15N values ofA. japonica and C. myriaster were slightly different, although not significantly different. The higher proportion of fishes in the diet of C. myriaster could account for their slightly higher d15N values compared to A. japonica. Even though C. myriaster appeared to consume a higher proportion of fish than A. japonica, the high value of the Pianka’s index for food (0.96) indicates that the food types of both species highly overlapped.Although the prey types were similar between A. japonica and C. myriaster captured in the bay, the prey size of the mud shrimp appeared to be considerably different. The larger A. japonica appeared to be able to eat a wider size range of mud shrimp than the smaller A. japonica andC. myriaster. The mean dactylus length of mud shrimp collected from A. japonica stomachs (8.3 mm) was 1.7- fold larger than that from C. myriaster stomachs (4.8 mm), and these means were significantly different. On the basis of the clear difference in the size of U. major prey, the food resources of the two types of eels appears to be consider- ably different.