Wednesday, January 2, 2008

Black coral: old threats, some good news

Hawai'i's deepwater black corals have been declining in population, due to fishing for the precious coral industry and an invasive white coral, the snowflake coral.

(Photo: Black coral "tree" with oysters attached. This is Antipathes grandis, one of the prime precious corals harvested for jewelry in Hawai'i. Hawai'i Undersea Research Laboratory photo.)

But there is hope that tougher fishing regulations and the reduced spread of the snowflake coral—at least in the prime black coral habitat of the 'Au'au Channel, between Maui and Lāna'i—will help preserve the coral beds.

The snowflake coral problem is yet another example of the threats of invasive alien species in the Islands, even in the dark depths around 200 feet below the ocean's surface, although recent surveys seem to indicate that at least off Maui, snowflake coral expansion seems to have stopped.

Black corals have declined 25 percent over several decades in the 'Au'au Channel, wrote Willow Hetrick, a University of Hawai'i Marine Option Program graduate and editor of the program's newsletter, Seawords, in its December 2007 issue.

Of 200 species of black corals worldwide, 15 are found on Hawai'i's deep reefs and two of these are the primary targets of the black coral jewelry industry: Antipathes dichotoma and Antipathes grandis.

She said the snowflake coral Carijoa riisei, was first spotted in the Islands in Pearl Harbor in 1972, and may have arrived growing on the bottom of a ship. It is now found throughout the main Hawaiian Islands, but not yet in the Northwestern Hawaiian Islands.

The invasive white soft coral moved rapidly through the waters of the main islands.

“Evidence shows that snowflake coral is accelerating its spread and exploding in abundance,” Hetrick writes.

She said that deepwater surveys from 2001 to 2004 using the Hawai'i Undersea Research Laboratory's submersible Pisces V showed that more than half the black coral colonies deeper than 230 feet were covered with snowflake coral.

But in a report last year to the Western Pacific Regional Fisheries Management Council, University of Hawai'i researcher Samuel Kahng said that a follow-up survey in 2006 showed that the invasive expansion had stopped and perhaps snowflake corals had dropped somewhat.

“It is possible that the situation at the Keyhole Pinnacle (in the 'Au'au Channel) has stabilized or even improved,” he wrote in “Ecological impacts of Carijoa riisei on black coral habitat.”

But researchers have not published their thoughts on what's causing the possible decline, and Kahng cautioned that the survey results are not as thorough as they could be, since it is difficult to be certain that precisely the same areas were being surveyed in 2006 as in earlier assessments.

Kahng said the threat of the snowflake coral partly a result of its weed-like growth habit. Its growth is 10 times faster than the black coral Antipathes dichotoma, he said.

But fishing is a significant threat as well, in part because the precious black corals are slow-growing and are not aggressive re-seeders. NOAA Fisheries in October 2007 issued a new regulation to protect the corals from overfishing. It limits all fishing to coral “trees” at least four feet tall and at least an inch thick at the stem. Previous regulations had allowed some harvesters to take smaller corals.

In the Federal Register, the Western Pacific Regional Fishery Management Council said the regulations are not expected to bear immediate fruit in terms of restoring coral populations.

“A long period of reduced fishing effort is required to restore the ability of the stock to reproduce at the maximum sustainable yield if a stock has been over-exploited for several years,” the report said.

Copies of the rule and associated information on the fishery are available at

© 2007 Jan W. TenBruggencate

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