MADRID, Spain - Police across Europe have seized 25 tonnes of eels and baby eels destined for Asia in a huge operation targeting smugglers of the critically endangered creatures, Spanish police said Thursday.

Officers arrested 256 suspected smugglers as part of the "historic" operation coordinated by the European Union's police agency Europol that involved 32 European countries including France and Portugal, Spain's Guardia Civil police force said in a statement.

The bulk of the eels, 18 tonnes, were seized in Spain where police smashed three separate organised crime groups involved in the trade of the creatures with the arrest of 30 people, the statement added.

"Police identified shell companies created expressly to export eels to Asia, which disguised the exports among other refrigerated products," the statement said.

Spanish police released nearly two tonnes of baby eels, and over a tonne of eels, which were confiscated alive as part of the operation back into the wild.

The rest of the eels -- prized as a delicacy in Spain and parts of Asia -- were found had not been subject to any food safety checks.

Baby eels can fetch up to 9,000 euros ($9,800) per kilo (2.2 pounds) on the black market, the statement said.

Smugglers often put the tiny fish in plastic bags and hide them in luggage where they can survive up to 42 hours, it added.

Stocks of the European eel (Anguilla anguilla) have plummeted 90 percent in three decades as development has encroached on the wetlands and dammed the rivers it needs to grow and feed in.

Experts fear criminal gangs who are smuggling the lucrative fish are pushing it towards extinction.

European eels have been protected since 2009 by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).