SHAU Kei Wan and Aberdeen typhoon shelters are black spots for illegal fish trading, officials have said. The Agriculture and Fisheries Department said about 20,000 tonnes of marine fish were believed to be sold illegally each year rather than through the official body, the Fish Marketing Organisation. Fisheries officer Edward Wong Cheuk-kee believes half of the catch is sold to other fishermen in international waters or to neighbouring Chinese ports such as Shekou. ''We estimate a portion of the other half of the catch - the more valuable fish - are marketed illegally near Hong Kong's typhoon shelters. ''The remaining low to medium-price fish are then sold through our wholesale markets. This has led to a decline in the commission we charge, which is based on the wholesale price of the fish,'' he said. Mr Wong said illegal traders had a close relationship with the Hong Kong fishermen involved. ''When the fisherman is returning to Hong Kong after fishing in the South China Sea he will contact the trader beforehand. ''When he arrives, an arrangement will be made to allow the trader to look at the goods. ''After striking a deal, the bulk of the fish will then be conveyed onto the trader's boat either inside or outside the typhoon shelter,'' he said. The illegal trader will then drive the fish to the retail markets himself or pass them to lorry drivers. Mr Wong said known black spots for the illegal marketing of marine fish included Shau Kei Wan and Aberdeen typhoon shelters. ''You can see people moving boxes of fish onto the lorries after they have been off-loaded from a vessel inside the typhoon shelter between 4 am and 6 am,'' he said. The Marine Fish (Marketing) Ordinance gives the department control of landing, transport, wholesale marketing, and import and export of marine fish. The department has joined forces with police to combat the problem with marine patrols in the black spots at sea and road-blocks to intercept lorries. Anyone found transporting more than 60 kilograms of marine fish without a permit is liable to a maximum fine of $10,000 and six months' jail plus the usual forfeiture of the goods. From 1985 up to last month, the department had prosecuted 801 cases arising from 2,680 operations involving 260,000 kilograms of fish worth $2.47 million. Last year, there were 421 operations which resulted in 110 prosecutions involving 39,000 kilograms of fish valued at $444,000. In 1992, there were 58 prosecutions arising from 301 operations involving 32,000 kilograms of fish worth $178,000.