THE hotels, restaurants, wholesale and retail trades survey by the Census and Statistics Department has started. About 9,000 establishments will be covered by the survey. Commissioner for Census and Statistics Frederick Ho appealed to the management of all sampled establishments to fulfil their legal and social responsibilities by promptly returning the completed questionnaires. He also emphasised that information relating to individual establishment would be treated in confidence. ''Only aggregate information that does not reveal details of individual establishments will be released,'' he said, adding that audited accounts were not essential for the supply of income and expenditure data. The survey aims to upgrade statistical data for evaluating the contribution of various economic activities to the territory's gross domestic product and for ascertaining their cost structure, operating character and sales levels.