By Jan TenBruggencate
Advertiser Columnist

Scientists are using a new aircraft-borne laser to identify rafts of marine debris at sea, where they can be removed more cheaply than if they are allowed to wash ashore, snagging reefs and damaging marine life.

"I think a pickup at sea can be done fairly inexpensively — a lot less expensively than cutting it off the reef," said James Churnside, a physicist with the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory.

Churnside has been working with a technology called lidar, or light detection and ranging.

A laser is pulsed from an aircraft to the sea at 30 bursts per second. The equipment times how long it takes for the light signal to reflect back to the plane.

The equipment gives a different reading if it is able to enter the water and reflect off plankton, sediment or even bubbles than if it hits debris floating on the surface. "Debris on the surface shows up as a shadow," Churnside said.

He and his associates tested the system in Alaskan waters, first identifying from satellite images the areas where they believed oceanic debris rafts would be found, then confirming it with lidar testing.

"We were able to find concentrations of debris where the satellite data predicted they were," he said.

While lidar work hasn't been done around Hawai'i, Churnside said satellite predictions suggest there are large fields of debris floating in the subtropical convergence zone that normally lies north of the Islands.

The field is likely to be denser in winter, when the currents "are conspiring to sweep stuff together," he said. During El Niño weather conditions, the field can move far enough south that the debris runs ashore in vast quantities on the Northwestern Hawaiian Islands, he said.

Debris including all kinds of floating matter is perhaps most dangerous to marine life when it is made up of great clumps of netting, rope, plastic strapping and fishing gear. Birds, seals, turtles and fish can get tangled in it, and if big piles of it wash ashore, it can rip up coral heads as it rolls in the surf.

The Alaskan work has been supported in part by native communities affected by the marine trash.

"The problem of ocean debris is very real for remote Alaskan native communities," said Tim Veenstra, president of Airborne Technologies, the firm that conducted some of the aerial survey work with lidar over Alaskan waters.

"For instance, the beach at St. Paul in the Pribilof Islands is regularly affected by this debris. Not only is it an eyesore, but it has an impact on returning salmon runs."

Jan TenBruggencate is The Advertiser's Kaua'i Bureau Chief and its science and environment writer. Reach him at jant@honoluluadvertiser.com or (808) 245-3074.