BARCELONA, Spain—Some 42 migrants, including 30 women and eight children, are believed to have died when their boat capsized in rough seas shortly after setting sail from the coastal town of Dakhla, in Western Sahara, a Spanish migrants rights activist said. Helena Maleno, founder of the NGO Walking Borders, tweeted late Thursday that she had spoken to one of 10 survivors who claimed to have lost two children in the accident that occurred as the group was trying to reach Spain’s Canary Islands in the Atlantic. Moroccan officials in Dakhla could not be immediately reached for official confirmation. However, local media reported that 12 bodies had washed ashore on Thursday while 10 people were rescued by fishermen off the Dakhla coast. Morocco claims the disputed Western Sahara territory, annexed in 1975, and its navy operates there. The Polisario Front seeks the territory’s independence. Further north along the Western Sahara coast, …