Status and trend of the Japanese eel resources ecological research for eel conservation Mari Kuroki
The Japanese eel is an important food resource and have been intensively studied on their ecological aspects such as life history, migration and spawning area since 1930s. Big progress in knowledge has been obtained in these decades, e.g. discovery of “Sea eel”, spawning area of the species, molecular phylogeny and recruitment mechanism of glass eels etc. However, the catches of glass eel have been decreasing for these 40 years to be only some 5% of their peak levels in the 1960s, and catches of yellow and silver eels in rivers are also a mere fraction of what they were in the 1960s. Overfishing of glass eels and silver eels and the degradation of river environments undoubtedly cause long-term decreases in eel stocks by reducing the number of silver eels migrating back to sea. Construction of channelling structures and riverbank repairs has reduced habitats for eels and their prey by covering them with concrete structures. Climate change and global warming may also have a long-term effect on the resource, whereas short-term fluctuations in eel resources can be caused by monthly and annual changes in oceanographic conditions such as El Niño events, changes in seawater temperature, and changes in the latitude of the salinity front at the spawning area. Catches of glass eels tend to be low in El Niño years, when there is a southward shift in latitude of the salinity front, affecting the latitude of spawning sites of adult eels in their spawning area. A southward shift of just 1 degree in the spawning site may cause the entrainment of eel larvae into the south-flowing Mindanao Current, reducing the percentage of larvae able to transfer from the North Equatorial Current to the Kuroshio, which normally carries the larvae to their ongrowing habitats in East Asia. All these factors influence the recruitment of glass eels to East Asia in a particular year, so the fluctuations in eel resources we see today are almost certainly a mixture of these shortand long-term changes.
|B1801742||Koleksi Digital||Archivelago Indonesia Marine Library - Perpustakaan Kementerian Kelautan dan Perikanan||Available|
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