Tokyo stocks opened lower Monday as investors locked in profits after five straight days of gains, while some exporters were sold on a firmer yen against the U.S. dollar. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 158.42 points, or 0.42 percent, from Friday to 37,904.25. The broader Topix index was down 11.08 points, or 0.41 percent, at 2,667.52. On the top-tier Prime Market, decliners were led by mining, service and retail issues. At 9 a.m., the dollar fetched 147.96-98 yen compared with 147.54-64 yen in New York and 149.03-05 yen in Tokyo at 5 p.m. Friday. Th…