The BSE benchmark Sensex fell by over 158 points in opening trade today. The 30-share index, which had lost 121 points in yesterday's trade, moved down further by 158 points, or 1 per cent, to 15,723 in the first few minutes of trade.In a similar fashion, the wide-based National Stock Exchange Nifty index declined by 48 points, or 1.01 per cent, to 4,715.