Japanese eels (Anguilla japonica) cultured in commercial aquaculture ponds are often released back into Japanese rivers for the purpose of enhancing natural stocks. However, environmental conditions in commercial aquaculture ponds are different from those of natural habitats, and survival of stocked eels or their contribution to restocking efforts are not clear. In this study, we marked and released 352 eels cultured in ponds having brackish water and ambient temperature, and 312 eels cultured in conventional commercial ponds which had freshwater and were artificially heated. After we managed to only recapture 3 eels which were all previously cultured in the brackish water, non-heated ponds while no eels cultured from conventional commercial ponds were recaptured. Our result suggests that eels cultured in brackish and non-heated ponds had a higher survival rate, as these ponds initially had similar environmental conditions to those of the natural habitats of juvenile eels soon after recruitment and eels may have acclimated to natural conditions of the river much faster and with greater ease.
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