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.
That means it is often best to set up a network of small no-fishing zones covering the most vulnerable reefs, with catches allowed in between. Closing big zones can be excessive for conservation and alienate fishermen who then ignore bans.
Reefs from the Pacific Ocean to the Caribbean are nurseries for fish and vital for food supplies since about 40 percent of the world's population lives within 50 km (30 miles) of the coast.
Climate change, pollution and over-fishing are among threats to reefs. Warmer oceans can damage corals, sometimes irreversibly. The U.N. University study is in a new handbook to help planners cooperate with marine scientists.
On land, planners can usually be confident that plants and animals will stay in areas set aside as national parks, Sale said. At sea, park limits are far less relevant.
In the past, he said, countries had sometimes set up large protected areas for reefs but then cleared mangroves along nearby coastlines to make way for hotels and beaches for scuba-diving tourists. That can damage some fish stocks.
"In the Caribbean, snappers and groupers spend their lives as juveniles in mangroves and sea grass beds," Sale said. As adults the fish go back to live on the reefs, creating a need for protected zones on both reefs and in mangroves.
Scientists recently discovered that the spiny lobster, the most valuable fishery in the Caribbean, has a larval stage lasting seven months, shorter than widely believed.
Understanding ocean currents can help to show how far they get dispersed within seven months before settling on the seabed. That can also help in deciding where to site protected zones.
Sale said Australia's Great Barrier Reef was a good example of management, with a network of no-fishing zones and others open to tourism or fishing. That system meant a balance between the needs of people and the reef.
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