SIYABONGA SISHI and ROGAN WARD SOUTH African authorities in the port city of Durban said on Saturday they were investigating a coastal chemical spill that may have been caused by a warehouse fire during unrest this week. Other possible sources are also being investigated as the cause of the spill, which is affecting marine and bird life, the eThekwini municipality said late on Friday, urging local residents not to use beaches in the area. "Extensive environmental impacts are being reported at uMhlanga and uMdhloti lagoons and beaches in the vicinity, that have killed numerous species of marine and bird life,"…