Write a question about the following article: Most Abu Sayyaf victims have been Filipinos; however, in recent years , Australian, British, Canadian, Chinese, Dutch, French, German, Indonesian, Japanese, Korean, Malaysian, Norwegian, Swiss and Vietnamese nationals have been kidnapped or attacked. Previously, Americans were particularly targeted. An unnamed ASG spokesman allegedly stated, "We have been trying hard to get an American because they may think we are afraid of them". He added, "We want to fight the American people." In 1993, Abu Sayyaf kidnapped an American Bible translator. In 2000, Abu Sayyaf captured an American Muslim and demanded that the United States release Sheikh Omar Abdel Rahman and Ramzi Yousef, who were jailed for their involvement in the 1993 World Trade Center bombing in New York City. Between March 2016 - July 2017, the majority of Abu Sayyaf kidnap for ransom operations shifted to the high seas. Seventeen ships were boarded and some sixty-five hostages from six countries were taken. In total, thirty hostages have been released , seven escaped, three were rescued by Philippine security forces, and four were executed.  Two others were killed during the attacks while eight seamen escaped during the shipjackings. An additional forty seamen were not taken hostage.
For how many months did Abu Sayyaf focus on kidnapping people from ships?