Relative contribution of migratory type on the reproduction of migrating silver eels, Anguilla japonica, collected off Shikoku Island, Japan

In order to understand the reproductive contribution among migratory types in the Japanese eel, Anguilla japonica, otolith strontium (Sr) and calcium (Ca) concentrations by X-ray electron microprobe analysis were examined for 37 silver eels collected in Kii Channel off Shikoku Island during the spawning migration season. The wide range of otolith Sr:Ca ratios indicated that the habitat use of A. japonica was not obligatory but facultative among fresh, brackish and marine waters during their growth phases after recruitment to the coastal areas as glass eels. Three migratory types, which were categorized as river eels, estuarine eels and sea eels were found. The estuarine eels were dominant (59%), followed by sea eels (22%) and river eels (19%). The low proportion of river eels from the spawning migration season suggested that the estuarine and sea eels inhabiting the nearby coastal areas might make a larger reproductive contribution to the next generation in this area.