An unprecedented eradication project on the tiny islet Mokapu off north Moloka'i is making rats more scarce than the rare native plants and seabirds on which they feast.

Last month's aerial dropping of rodenticide pellets on the 10-acre state seabird sanctuary was the first of its kind for Hawai'i's offshore islands, said Chris Swenson, Pacific Islands Coastal Program coordinator for the U.S. Fish and Wildlife Service, which partnered in the project with the state Department of Land and Natural Resources.

He said officials plan to use the method in similar cases where access is difficult because of steep terrain.

Mokapu rises to a narrow ridge 360 feet above sea level and supports some of the best native coastal plant habitat in Hawai'i, including 29 native plant species.

Polynesian rats are the biggest threat to some of these rare plants and to three seabird species known to nest on Mokapu, Swenson said.

The rodents eat the eggs and chicks of wedge-tailed shearwaters ('ua'u kani), red-tailed tropicbirds (koa'e 'ula), and white-tailed tropicbirds (koa'e kea), and also munch on the seeds and shoots of two rare plant species, loulu lelo (Pritchardia hillebrandii) palms and ho'awa (Pittosporum halophilum).

Only 14 ho'awa plants exist in the wild and 11 of them are on Mokapu.

Volcano Helicopters from Hilo was contracted to drop fish-flavored, compressed cereal grain pellets containing 50 parts per million of diphacinone on Feb. 6 and 12. Swenson said two applications were needed to ensure the bait was evenly distributed throughout Mokapu and because it must be eaten multiple times over an extended period to be effective.

Diphacinone, an anticoagulant, has undergone extensive testing and is considered less toxic than other compounds, reducing the risk that "nontargeted species" will be affected, he said.

Before it was used on Mokapu, officials held public meetings on Moloka'i and conducted an environmental assessment. Some residents expressed concern about the rodenticide spreading into the ocean, where it might contaminate fish and other marine resources in nearshore waters, according to the environmental assessment.

The aerial operation was conducted during the winter, before the rat breeding season, when rat populations are at their lowest and when seabirds are not present or are present in low numbers on Mokapu, the assessment said.

There also are fewer humans using Moloka'i's north shore waters at that time because of rough ocean conditions, the report said.

The assessment noted studies concluding that fish were unlikely to ingest bait pellets and that there would be no impact to marine animals from direct or indirect ingestion of diphacinone. In addition, the report said the pellets were not expected to persist for more than two to three days in winter seas.

"Exposure levels of marine invertebrates to toxins in the bait would be at such low levels and for such a short time that no tissue accumulation is anticipated and no effects to humans," the assessment said.

Swenson said water, fish and 'opihi samples were taken and are awaiting laboratory analysis for rodenticide residue. Results of the tests will be made public, he said.

Mokapu will be monitored for rats over the next two years, and if none is found, the eradication can be officially declared a success, according to Swenson. Scientists also will do long-term monitoring of the islet's rare plants and seabirds to measure the benefits of rat eradication.

Swenson said an environmental assessment already has been prepared for aerial application of diphacinone on Lehua, a 290-acre island northeast of Ni'ihau.

Also involved in the project were the federal Animal and Plant Health Inspection Service's Wildlife Services, under the Department of Agriculture, the Moloka'i/Maui Invasive Species Committee, The Nature Conservancy, Hacco Inc. and United Agriproducts.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.