As juveniles and adults, they settle into coastal waters where they forage on algae and sea grasses. Hawaiian green turtle feeding pastures are found primarily around the major Hawaiian Islands and Johnston Atoll. By tagging and measuring turtles over the years in these areas, Balazs has found that they grow extremely slowly, taking an average of 25 years to reach sexual maturity. About every two to three years, the sexually mature female turtle returns to her birthplace to nest on land at night. Nesting typically occurs from May through August.
Males make the journey every year or two, mating with the females offshore.
About 90 percent of Hawaii's green turtles nest at East Island and its neighboring islets. Known collectively as French Frigate Shoals, these islands are the remains of an ancient volcano that sank over time, leaving a series of sand spits and a basalt pinnacle around which a crescent-shaped barrier reef has formed. French Frigate Shoals is also an important habitat for endangered monk seals and seabirds. Both males and females engage in this activity, possibly as a way to warm themselves and relax out of reach of sharks. Mass killings of turtles on the beaches at French Frigate Shoals occurred as late as 1959, when a fishing company took more than 25 percent of the nesting turtles in one season.