Young European eels are routinely smuggled to China, raised there for about half a year, then shipped to the Japanese market as demand for the fish spikes in the summertime, a senior Spanish police officer has said.

The details came to light after Spanish police arrested 10 people in April, including Spanish and Chinese nationals, on suspicion of smuggling a huge amount of juvenile European eels into China during the species' fishing season between last autumn and this spring. The European Union banned exports of the fish in 2010.

At the time of the arrest, police said Japan is a major destination for illegal exports of European eel, which is protected under the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention.