Seasonal variation in the migratory history of the Japanese eel Anguilla japonica in Mikawa Bay, Japan

The seasonal variation in the migratory history and biological characteristics of Japanese eels Anguilla japonica was examined using 554 eels collected with set nets that were fished all year in Mikawa Bay from May 2000 to April 2002. The largest catches occurred in November and December of both years and consisted mostly of female silver eels (71.2%) with relatively high gonadosomatic index (GSI) values (0.4 to 4.3) that were beginning their spawning migration. Otolith strontium and calcium concentrations of 176 females and 23 males, measured by X-ray electron microprobe analysis, indicated that 40% of the eels were categorized as 'sea eels', which may never have migrated into freshwater throughout their life history, 43% were 'estuarine eels', which stayed in the estuary or frequently moved between freshwater and marine habitats, and 17% were 'river eels', which showed a typical catadromous migration. River eels occurred only in autumn and early winter during the spawning migration season, but at least a few sea eels or estuarine, eels occurred throughout the year, with their numbers increasing dramatically in November and December. There were no major differences in the size or age among the different migratory types of eels, but the growth rates of both sexes appeared to be slightly lower in freshwater. These findings show that river, estuarine, and sea eels begin their migration out of Mikawa Bay toward the open ocean at about the same time, and that the estuarine and brackish water habitats of Mikawa Bay appear to be producing the majority of spawning migrants in this area.