The 85-meter vessel, named Just Reema, was sailing towards the eastern Mediterranean under a Congolese flag on Sunday when it was intercepted by police in an operation dubbed "Urca", 45 nautical miles south of Malaga.
Spanish authorities had been given a tip from Europe's police agency Europol, which had in turn received an alert from French authorities.
Investigators searched the boat at the scene, initially finding only a cargo of about 1,500 tons of salt. However, once the ship was brought into port at Malaga for a more thorough search, officers found that the ship had double bottom beneath which 15.7 tons of cannabis resin were stashed.
Europol estimates the value of the cannabis to be about 24 million euros (about US$ 26.5 million), if sold on the Spanish market.
The ship's crew, composed of six Syrian and three Indian nationals, were arrested.
This case is only the latest in a series of cannabis seizures from boats intercepted by Italian, French and Spanish authorities in the past two and a half years – about 22 ships carrying up to 30 tons of cannabis each.
Investigators believe that the route is mostly used by organized crime groups based in North African countries who aim to distribute cannabis to European countries.
