RETAIL sales rose 13 per cent in volume year-on-year in May, buoyed by mainland shopping sprees, government figures released yesterday show. Value of sales was about $15 billion, up 19 per cent before discounting for inflation. Sales of clothing and footwear saw the largest increase, up 46 per cent in value and 34 per cent in volume compared with May 1992. Sales of jewellery and watches were up 32 per cent in value and 30 per cent in volume. Consumer durable goods other than motor vehicles rose 23 per cent in value and 22 per cent in volume. Barclays de Zoete Wedd economist Catherine Chou said: ''The increases of sales in these areas, combined with the fact that arrivals of short-stay mainland tourists have increased, suggest that retail sales are rising because of rising demand from mainland shoppers, who come here to buy electronic goods, jewellery and clothing and footwear products that are not available in China.'' Sales of staples dropped off, with food sales down six per cent and fuel sales off five per cent. Motor vehicle sales were worst hit, down 11 per cent in value and 17 per cent in volume. This follows the substantial growth of last year due to promotion deals and lower prices offered by leading brands. Both department store and supermarket sales increased, by 11 per cent and three per cent, respectively. From January to May, total retail sales increased in value by 13 per cent. representing a real increase in volume of nine per cent compared with the same period last year. Hong Kong's negative real interest rates are continuing to support high levels of spending although analysts suggest the Government's clampdown on property speculation could slow retail sales.