Fifteen sheep from a flock of around 70 were found dead last week at a private ranch in Nicasio, prompting an investigation by the Marin County Sheriff’s Office. A sheepherder and the owner, whose property borders Skywalker Ranch, initially reported that the animals appeared to have gunshot wounds, but they may have fallen prey to a puma. Dr. Quinton Martins, director of True Wild Conservation, a nonprofit that studies mountain lions in the North Bay, said the attack could have been carried out by a lone lion or a mother training an adolescent to hunt. True Wild estimates that as many as 25 pumas live in or pass through Marin; six to 10 are year-round residents, including a collared lion, P60, whom Mr. Martins confirmed was not the perpetrator, based on its tracked movements. “If it was a mountain lion, it could have been one that isn’t collared,” he said. “These mass depredation events are not uncommon at all, so we encourage people to look after their animals and to avoid any kind of interaction, because one interaction could lead to multiple deaths.” Sheep owners in Nicasio are on high alert. Mark Pasternak, owner of nearby Devil’s Gulch Ranch, where a small flock of sheep grazes his vineyard in winter, noted that although it is deer season, “you don’t usually hear multiple shots.”