The world should safeguard coral reefs with networks of small no-fishing zones to confront threats such as climate change, and shift from favoring single, big protected areas, a U.N. study showed.

“People have been creating marine protected areas for decades. Most of them are totally ineffective,” Peter Sale, a leader of the study at the U.N. University’s Institute for Water, Environment and Health, told Reuters.

“You need a network of protected areas that functions well,” he said. “It’s important to get away from single protected areas which has been the common approach.”

Fish and larvae of marine creatures can swim or be carried large distances, even from large protected areas.