POLICE believe they have smashed a long-term triad-controlled extortion racket in Tuen Mun after arresting 18 people in an operation early yesterday morning. About 80 officers from Tuen Mun police district and the Organised Crime and Triad Bureau raided the fish market at Sam Shing Estate, which officers believe has been controlled by the powerful Sun Yee On triad group for about 10 years. Fifteen men and three women, aged between 18 and 51, were arrested. Police believe the group, claiming to be drivers and odd job workers at the fish market, had been involved in blackmailing and extorting money from fishermen and fish hawkers in the area. Tuen Mun assistant district commander (crime), Superintendent Ray Pierce, who led the operation, said the triad members used a common ruse telling fishermen and fishmongers that one of their relatives had died and asking for ''funeral donations''.