THE Agriculture and Fisheries Department yesterday claimed that a 1990 consultancy report had given the wrong figure for the amount of nitrogen contributed by fish culture zones in Hong Kong. The department's director, Lawrence Lee Hay-yue, told legislators that the total nitrogen loading from mariculture in Hong Kong could not be 698.5 tonnes a year - including 624.4 tonnes from fish waste and 35.8 tonnes from unconsumed feed - as estimated by the consultants. He argued that the amount of nitrogen produced was only 126.3 tonnes a year, of which 29 tonnes went into Tolo Harbour. The amount of nitrogen produced by mariculture zones in Tolo Harbour was the focus of a row between the Environmental Protection Department (EPD) and the Agriculture and Fisheries Department over whether the zones should be removed to clean the harbour, according to the Auditor's report. The EPD said the zones in Tolo Harbour had created 670 kilograms of nitrogen per day, rendering it impossible to reduce nitrogen to 500 kilograms per day to meet the target to reduce pollution. The Director of the EPD, Stuart Reed, proposed phasing out the zones in Tolo Harbour within three years. The Secretary for Economic Services, Gordon Siu Kwing-chee, said removal of the zones by non-renewal of licences was not an option. Tolo Harbour contributed about a quarter of mariculture production and about 1,200 people depended upon this activity for their livelihood, Mr Siu said at the public hearing. He said new types of feed which reduced wastage and thus pollution had been developed and were being introduced to fish farmers. Mr Lee said the consultancy report was wrong because it had conflicting figures. He quoted the report's figures that the major contributors to nitrogen loading from mariculture in Hong Kong were 35.8 tonnes from unconsumed feed and 624.4 tonnes from fish waste. He said nitrogen in fish waste could come from feed consumed by the fish. Based on the consultants' figure that 30 per cent of feed would be unconsumed, amounting to 35.8 tonnes of nitrogen, Mr Lee calculated that the total amount of nitrogen in the feed would be 119.3 tonnes a year. Since the fish would at most consume 119.3 tonnes of nitrogen, Mr Lee said it was impossible for them to produce 624.4 tonnes of nitrogen a year through waste. His department put the correct figure at 74.7 tonnes a year through fish waste resulting in total nitrogen production of 126.3 tonnes a year. Another way of illustrating that the consultants' estimates were high was that their estimate of fish waste at 624.4 tonnes a year would convert to a feed cost of $166 to produce one kilogram of live fish which sold at only $80. Mr Reed said the different figures had been attributed to different points of view. Legislator Emily Lau Wai-hing asked what would happen if the two departments did not agree on this issue. Mr Siu said the Government would seek advice from university experts.