Japan imports vast quantities of juvenile Japanese eels from Hong Kong despite international efforts to control trade in the endangered species, with conservation groups calling for tighter regulations.

Japan imported 4,364 kilograms of young Japanese eels from Hong Kong in January and February, and 1,657 kg in November and December, Finance Ministry foreign trade data shows.

That means about 40 percent of the young eels put into Japanese aquaculture ponds over that four-month period came from Hong Kong, the Fisheries Agency said.

Japan, China, South Korea and Taiwan agreed in 2014 to reduce the volume of young eels put into their aquaculture ponds, but Hong Kong was not involved in those talks.

Conservation groups point to an increase in shady eel trades involving Hong Kong as evidence of a regulatory loophole and weakness in conservation efforts.

Japanese imports of eels from Hong Kong have increased sharply since 2007 when Taiwan banned such exports. But Hong Kong is not known for large-scale eel production, and experts suggest that the eels Japan imports from Hong Kong are originally from Taiwan or China.

According to Traffic, a civil group that monitors trade in wild life, more than 300 kg of young European eels, another endangered species that the European Union has banned from export, were confiscated in the first three months of the year in Hong Kong from traders that attempted to export them to China.

Many European eels put into Chinese aquaculture ponds are believed to be exported later to Japan.

Japanese eels, mainly found in East Asia, have significantly declined in number due to deteriorating habitat conditions and overfishing. They have been designated as an endangered species by the Japanese Environment Ministry as well as the International Union for Conservation of Nature.