Drug smuggling, money laundering, prostitution – and sea cucumber poaching. Japan’s yakuza gangs are adding the unlikely – but lucrative – act of illegally exporting sea cucumbers to their portfolio of criminal activities.

A growing number of yakuza gangs are reportedly diversifying into Japan’s flourishing sea cucumber market, by fishing for the sea creatures before selling them abroad, mainly in Hong Kong and China.

Sea cucumbers, widely regarded as a culinary delicacy in Asia, have long been popular overseas, with Japan's total exports valued at £143.8 million (21 billion yen) last year.

The nation's yakuza have increasingly focused on this market in recent years, with sea cucumber poaching creating as big a revenue for some gangs as amphetamine trafficking, according to a source cited in the weekly magazine Shukan Bunshun.

The government is keen to clamp down on illegal exports, with the Fisheries Agency recently unveiling plans to create the first catch certificate system of its kind in order to regulate domestically-caught sea cucumbers.