State officials and conservation groups are declaring their yearslong effort to rid Lehua Island of invasive rats a success — and a milestone that they hope will help restore the once-thriving seabird sanctuary.

The announcement Wednesday comes after multiple attempts to eradicate the rodents, which eat seabird chicks and eggs, by dropping poison-laden pellets from the air. During those operations, staged from Kauai and Niihau, helicopters carrying large buckets would scatter the bait across Lehua.

No rats have been detected on the island since December 2018, according to Patty Baiao, U.S. operations head for Santa Cruz, Calif.-based Island Conservation. The nonprofit group partnered with state and federal agencies, as well as Niihau’s owners, to carry out the most recent operations in 2017.

Drone aerials of Rat Free Lehua from Hawaii DLNR on Vimeo.

Now, the partners in the effort are watching closely to see whether the endangered Newell’s Shearwater and Laysan Albatross will return to the traditional nesting grounds on Lehua, joining the Red-Footed Boobie birds that have persisted there despite the rats, according to Baiao.

It’s not clear when humans first introduced rats to Lehua, which is a state-designated wildlife sanctuary, but officials say the rodents have been there since at least the 1930s. Crews rid the island of invasive rabbits in 2006, but the rats proved to be much harder to remove.

A previous attempt to eradicate the rodents in 2009 failed. Several years ago, Niihau co-owner Keith Robinson called the 2009 operation a “complete disaster” because it had taken place in the winter, offering the rats plenty of vegetation to feed on other than the bait pellets. The bait also had been dropped in heavy winds, he added.

The 2017 aerial baiting, by contrast, took place in August and September, and officials checked that there would be light winds before sending the helicopters across Lehua, according to the state’s Department of Land and Natural Resources.

Acoustic monitoring has detected the shearwaters and albatross getting closer to the island, a step that would precede them using it as a nesting area, Baiao said. The hope is that eventually more of Hawaii’s seabirds could use the island, which boasts tall rocky cliffs, as a nesting refuge amid the sea level rise caused by climate change.

Nonetheless, the strategy to coat the entire island of Lehua with tons of bait laced with trace amounts of the rodenticide diphacinone concerned many in the community due to the potential harm to surrounding marine life.

One of the reasons the 2009 drops failed, Baiao said, was that the helicopters had to observe a 50-foot buffer zone with the shoreline where they couldn’t drop bait in order to prevent scores of it drifting into the water. The rats in that zone survived, she added.

The 2017 drops didn’t have that buffer. Instead, the bait buckets were equipped with deflectors designed to keep bait from drifting into the ocean, Baiao said.

Monitoring of fish, soil, water and carcasses by the U.S. Department of Agriculture did find traces of diphacinone in fish after the 2017 aerial drops, but the agency concluded that the compound didn’t linger there and eventually disappeared, according to Baiao.

“It didn’t last very long, as expected,” she said.

Further, the USDA monitoring didn’t detect any marine mortalities linked to the compound, she added. Muscle samples taken from dead mullet fish in tide pools showed traces of diphacinone, but it wasn’t clear if the fish had actually ingested it, she said.

“It’s only worth (it) if the long-term benefits outweigh the short-term risks” of exposure, Baiao said. In addition to the seabirds’ return, the team hopes the rats’ disappearance will help restore the island’s native plants and the broader ecosystem there.

Officials have continued to check closely for any signs of rats on Lehua, using cameras and other equipment. Just a single pregnant female could swiftly repopulate the island, Baiao said.