Otolith Techniques in the History of Eel Discoveries

Major parts of the age-old mysteries about eel migration and reproduction that have lasted since the time of the ancient Greek philosopher Aristotle, 2400 years ago, have now been solved with the help of otolith research. Otolith microchemistry studies (Sr/Ca) have revealed the plasticity of diadromy of freshwater eels, and the various types of habitat use between seawater and freshwater has been documented using this technique for both temperate and tropical eels in recent years. The spawning season of the Japanese eel, which had long been believed to be in winter, was estimated to be as late as during summer by aging of glass eels with otolith daily rings. The ages in daily precision of eel leptocephali collected in the North Equatorial Current could accurately estimate approximately how far upstream in the current the spawning area of the species was from the collection site. Hatching date analysis of leptocephali collected near the estimated spawning area led scientists to the New Moon Hypothesis that the Japanese eel spawns in the darkness of the new moon of each month during their spawning season in late spring and summer. Environmental temperature and spawning depth were estimated by the analysis of oxygen isotopes in the otolith core of glass eels to be < 200 m, a much shallower depth than ever speculated. These estimates resulted in the first success of collecting anguillid eel eggs at the southern part of the West Mariana Ridge in the spawning area of the Japanese eel. Migratory history analysis of more than 600 silver eels leaving the Japanese coast on the way to their spawning area revealed that the majority of migrants (> 60% or more) are derived from sea eels and estuarine eels, while river eels are rare in some areas, which is consistent with the results of analysis of the otoliths of adults collected in the spawning area. Further isotopic analysis (Sr and O) of the original geographic localities of growth habitat for individual adult eels from the spawning area and the type of habitats they used will help to reveal the contribution of eels from various regions and habitat types to the next generation, which can assist with effective conservation of this important fisheries and aquaculture species.