Sea turtles are generally found in the waters over continental shelves. After taking to the water for the first time, males will not return to shore ever again. During the first three to five years of life, sea turtles spend most time in the pelagic zone floating in seaweed beds. Green sea turtles in particular are often found in Sargassum beds, a brown seaweed in which they find shelter and food. Once the sea turtle has reached adulthood it moves closer to the shore. Females will come ashore to lay their eggs on sandy beaches during the nesting season. Of the seven species of marine turtles four are listed as endangered or critically endangered. Although sea turtles usually lay around one hundred eggs at a time, on average only one of the eggs from the nest will survive to adulthood. While many of the things that endanger these hatchlings are natural, such as sharks, raccoons, foxes, and seagulls, many new threats to the sea turtle species have recently arrived and increased with the ever-growing presence of humans.