Bleaching, Crown-Of-Thorns Starfish Moving Through Japan's Coral Reefs

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Hiroshi Misaki, director of Kushimoto Marine Park in Kushimotocho, Wakayama Prefecture, laments the huge losses to the coral reef off the Kii Peninsula. "More than half of the coral has been eaten by crown-of-thorns starfish," Misaki said. About 120 species of coral live in the waters off the Kushimoto area, which was registered as a Ramsar wetland in November.

Coral reefs, often called "underwater tropical forests," support marine ecosystems with nutritious seaweeds that live inside. Though located at a relatively high latitude--33 degrees north--the temperature of the water of the reef habitat off the Kushimoto area is nearly 20 C, even in winter, because of the Kuroshio Current that flows nearby. A large number of crown-of-thorns starfish started to appear in the waters off Kushimoto in 2004. In cooperation with local divers, Misaki has removed more than 30,000 of the creatures, which average 30 centimeters in diameter. On Jan. 17, in just one hour, seven divers caught nearly 500 starfish in waters six to seven meters deep.

Many seas around the globe have been assaulted by armies of crown-of-thorns starfish. There are said to be several causes: rises in water temperature; an overexploitation and a consequent decrease in the number of shellfish that are predators of the starfish; and the depletion of oxygen in the water due to excess plant growth, known as eutrophication. In Japan, almost all the water surrounding Okinawa Prefecture suffered from an explosion in the number the of starfish in the 1970s. The phenomenon subsided for a while, but has reemerged in recent years. Before the 1990s, the water temperature off Kushimoto in winter commonly fell below 15 C, making it difficult for tropical creatures to survive.

"At that time, crown-of-thorns starfish soon died when winter came, with only a few appearing," Misaki said. In recent years, however, the temperature has often stayed over 16 C, even in winter. As a result, 20 species of coral that generally live in warmer parts of the globe have been discovered in the area. Though the water temperature fell under 15 C last winter for the first time in 15 years, the starfish population did not significantly decline. The starfish had apparently acquired greater resistance to cold temperatures.

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http://www.yomiuri.co.jp/dy/features/science/20060126TDY04003.htm