But the lot that attracted the most interest was a giant 342kg blue fin tuna caught off Japan's northern island of Hokkaido. The winning bid was made jointly by the owners of a restaurant in Tokyo's upmarket Ginza district, and a sushi chain in Hong Kong. Restrictions on catches have been tightened in recent years because of concerns about overfishing. Traders at Tsukiji market say growing Chinese demand for sushi is also helping to push up prices.