The event has been billed as "sex in the tropics," occurring in summer months during a new moon.

It’s spawning time for rice coral off Oahu, with peak activity expected tomorrow night.

Scientists can predict coral spawning periods, but they do not know where the floating larvae go—a mystery they hope to clear up with studies this weekend at Maunalua Bay.

"It’s very important to understand where the larvae is produced, where it is transported to and whether they become part of the reef of the area," said Robert Richmond, principal investigator at the University of Hawaii’s Kewalo Marine Laboratory and a world leader in coral reef conservation.

Corals are born and die like other living organisms, and if new corals are not produced to replace those dying, the reefs and related resources will disappear, he said.

Richmond has been working with the community-based group Malama Maunalua to conserve and restore the bay, degraded by polluted runoff and sediment, invasive algae and unsustainable harvesting. This affects the corals and ability of larvae from elsewhere to replenish the depleted populations.

Curt Storlazzi, with the U.S. Geological Survey in Santa Cruz, Calif., brought his expertise and equipment here for the spawning study.

He said they will scuba-dive at night, and when they observe spawning they will release drifters—orange balls about 10 inches in diameter—with a satellite transmitter and high-resolution global positioning system transmitters.

The drifters will float at the speed of the coral larvae, following their dispersal along the south shore, he said.

Swimmers and boaters are asked to look out for the drifters and try to avoid them, as well as "USGS" yellow floats marking instrument packages.

When corals spawn, Storlazzi said, they release egg sperm bundles with a fat layer on them that makes them float to the surface.

"They’re a little bigger than BBs; you can see them with the naked eye," Storlazzi said. "They float and break apart and fertilization takes place. Developing larvae float away with the currents."

Eggs and sperm from different colonies have a chance to intermingle at the surface, "and if the combination is right, fertilization will take place," he said.

Coral eggs are selective, he said. "They usually reject sperm which originate from their own parent colony but readily accept sperm from a different individual of the same species."

In an interview, Richmond said, "If quality is good, eggs begin to develop, and larvae begin to look for a place to settle in 24 to 72 hours. Some may drift for a week or two."

Some reefs are self-feeding, with larvae actually settling there, he said. "But there are a number of highways, connections between bays and islands. That’s how corals got here in the first place. … They came from other places."

Storlazzi said USGS did a similar study in 2003 off West Maui, where coral reefs were not doing well. They were trying to determine there, as at Maunalua Bay, whether land-based pollution is causing more coral deaths than natural stressors and if not enough new coral is replacing them, he said.

What they found was the larvae spawned off Maui did not stick around, but were being transported to Lanai.

Studies done as part of the community effort to restore Maunalua Bay raised concern about where larvae feed because of watershed discharges that dump into the ocean, Richmond said. He said there seems to be an eddy out of Maunalua Bay that turns a corner into Hanauma Bay, but a discharged sediment plume interferes with the flow.

The scientists are concerned about the quality of the water and bottom of the receiving site, he said. "If it’s covered with mud and invasive algae, coral larvae can’t find a place to settle."

The data collected on current patterns also have broader application for search-and-rescue operations and oil spills, Storlazzi said.