Two federal agencies will investigate an incident in which concrete blocks dropped by the state Department of Land and Natural Resources off Keawakapu earlier this month damaged live coral.

Dan Polhemus, administrator of the department's Division of Aquatic Resources, said the department requested the federal investigation so that the inquiry would be independent. The U.S. Fish and Wildlife Service and the National Oceanic and Atmospheric Administration are expected to provide an initial "emergency assessment" of the coral damage by mid-January.

A more complete "rapid ecological assessment" is expected in March, including the total amount of damage and recommendations for penalties and corrective action. That could involve mitigation efforts that would leave the blocks in place, if the investigation concludes that more damage would be caused by attempting to move them.

The DLNR will cooperate by providing any documents or other materials requested by the federal investigators, Polhemus said. The DLNR also will conduct an internal investigation by a branch of the department that didn't participate in the original incident, with the results to be released alongside the federal assessment, Polhemus added.

"We have two parallel independent investigations," he said. "We thought that's the only credible way to do it."

On Dec. 2, the department sank 1,400 one-ton concrete blocks off Keawakapu to enhance an existing artificial reef, but reported the next day that some of the slabs had fallen on live coral.

Polhemus said last week that it appeared about 50 of the slabs had been dropped on the reef, while the remainder of the blocks fell on sand.

Divers entered the water after all the slabs had been dropped because it was unsafe for them to be in the water while the operation was under way, he said. They immediately recognized the problem and reported it to officials.

The DLNR has placed a moratorium on its artificial reef program statewide, "until we sort out what happened and why," Polhemus said.

He declined to speculate on how the mistake happened, saying he would wait for the results of the investigations.

Some divers familiar with the area said they were encouraged by the department's plan for an independent federal investigation, but they thought the state was underestimating the amount of damage to the reef.

Commercial dive instructor Gary Ahrnsbrak said that after diving in the area to look at the damage, he judged that the department's estimate that 50 blocks fell on coral was too low.

"From what I saw, I would guess it's closer to 100 slabs," he said.

The damage was extensive where they fell, he said.

"It just totally destroyed it," he said. "Each one of those slabs is a ton. It totally smashed everything beneath it."

Dive instructor Rene Umberger also estimated that between 80 and 100 blocks fell on coral, based on her own dive in the area and on photographs and videos of the damage.

"Where I dove, one slab hit the reef and slid down it like a sled," she said.

She was concerned about how the department would try to fix the damage, saying that removing the slabs might only make it worse.

"I can't even imagine how they would be able to move it," she said. "Finger coral is really fragile."

Umberger said she wanted any fines imposed to be an appropriate amount, and said the money should be applied toward conservation efforts on Maui.

"It's good they're having someone independent come in," Arnsbrak said. "We'll see what happens from there."

Commercial boat operators who have caused damage to coral in the past have faced steep fines.

Maui Snorkel Charters was fined $396,000 after its tour boat Kai Anela sank off Molokini in 2006 and salvage efforts worsened coral damage. A fine of $543,000 was proposed for Makena Boat Partners after its Kai Kanani catamaran dragged its anchor over coral off Makena in 2007, but a settlement of $130,000 is being considered.

"We're going to hold ourselves to the same standard," Polhemus said, noting that the federal investigation of the Keawakapu reef damage would include recommendations for penalties.

The department's artificial reef program involves dropping the concrete blocks to create structures on the ocean floor that attract fish. Polhemus said the program has been very popular with local fishermen, but that it had been about 10 years since there had been any project on Maui.

The artificial reef at Keawakapu was started in 1962.

Ahrnsbrak said the natural reef that was damaged in the incident was a system of "very healthy" finger coral that served as a habitat for young and small fish. It was rarely dived, leaving it largely unaffected by humans, he said.

The entire reef system is very large, so a large part of the reef escaped damage, he said.