CNN  —  Lake Naivasha, northwest of Nairobi, Kenya is becoming increasingly unnavigable. Water hyacinth, the world’s most widespread invasive species , is blanketing the lake, choking its fish and leaving people stranded. “Sometimes it becomes very serious,” says Simon Macharia, a local fisherman, about the weed problem. “There was this incident where fishermen were trapped by hyacinth inside the lake for three days. We had to seek help from the government (who) used a helicopter to rescue them.