Department of Ag Stalls, But Finally Yields On Permit For Lehua Rodenticide Drop

posted in: September 2017 | 0
A helicopter dropping rodenticide–laced pellets on Lehua Island last month.

The state Department of Land and Natural Resources’ (DLNR) effort to rid its Lehua Island bird sanctuary of invasive rats got off to a rocky start last month. The agency did not receive notice from the National Oceanic and Atmospheric Administration (NOAA) until August 21 that the DLNR’s plan to kill the rats by aerially applying an anticoagulant rodenticide would not jeopardize any threatened or endangered species on or around the island, located about a mile off Ni`ihau.

The day after NOAA’s “no jeopardy” determination, the state Department of Agriculture (DOA) granted the DLNR a permit allowing it to broadcast diphacinone-laced bait pellets via helicopter. By then, the DLNR had already staged its operation on Ni`ihau and was ready to drop the bait as soon as the permit was signed and winds permitted.

Keen to complete its project during the dry summer months, the DLNR originally planned to start dropping bait on August 8, with subsequent drops on August 18 and 29. By dropping the bait when there is the least amount of vegetation on the island, the agency increases the odds that rats will 1) eat the bait rather than the vegetation, and 2) be less able to counteract the anticoagulant effects by eating vegetation (The antidote to diphacinone is vitamin K, which is found in plants).

The permit the DOA granted on August 22 expired on August 27 and allowed for just a single bait application, which occurred on August 23. A new permit will be required for each subsequent drop. A second drop occurred a week later.

In the weeks leading up to the permit being signed, many expressed concerns over potential adverse impacts to non-target species, including humans. Local news outlets detailed worries voiced by some Ni`ihau fishermen about impacts to fish; Kaua`i state Rep. Dee Morikawa called for the project to be delayed, and the DOA’s former Pesticide Branch chief, Robert Boesch, beseeched multiple government agencies to consider the possibility that the rodenticide could bioaccumulate in pelagic fish, not just reef fish, and the potential dangers to humans that posed. Much of their concern stemmed from the DLNR’s 2009 aerial rat bait drop on Lehua.

A large fish kill around Ni`ihau and a couple of humpback whale calf deaths occurred shortly after the bait was dropped, leading the DOA, where Boesch worked at the time, to put an end to any further aerial bait drops before all rats could be eradicated. Out of concern for marine species, the DOA’s permit for that effort had also prevented the DLNR from aerially applying any bait along the island’s coastal areas.

Although testing found no traces of diphacinone in the dead fish, Boesch has said he believes the fish tissues contained a chemical signature that could have been a metabolite of the rodenticide. (The U.S. Fish and Wildlife Service, a partner in the eradication effort, disputes this. It states in its environmental assessment for the project that there is “a total lack of supporting evidence” that diphacinone killed the fish.)

While the results of inert bait trials on Lehua in 2015 suggest that any pellets that fall into the water will disintegrate within 30 minutes and disappear altogether within 24 hours, the DOA included a number of permit conditions to reduce the likelihood that bait will enter the ocean, and to ensure that any birds, fish, or federally listed marine mammals found dead following the bait application are properly tested to determine cause of death.

For example, the DLNR and FWS proposed to allow helicopters to drop bait only when wind speeds were 35 miles per hour or less. The DOA permit, however, reduces that maximum wind speed to 25 mph. Also, any samples of dead fish, birds, or marine mammals must be collected and stored in accordance with a “proper chain of custody maintained by USDA, and shall be provided to the [DOA] Pesticides Branch,” according to the permit. Also, any death of ten or more birds, 25 or more fish, any endangered or threatened birds or marine mammals, or any human illness or injury  suspected of being related to pesticide exposure must be immediately reported to the DOA Pesticides Branch.

For now, the DLNR is only planning to drop diphacinone, but should those applications fail to fully eradicate the rats, the agency may switch to a more potent rodenticide, brodifacoum. Because brodifacoum is also known to kill a wide range of non-target species, it’s possible any DOA permit, if one is issued, would include additional conditions.

As the U.S. Environmental Protection Agency noted in its comments on the FWS’s draft environmental assessment for the project, “If brodifacoum is used, rodenticide residues in seafood could persist much longer [than a week]. Masuda et al. (2014) tested for brodifacoum residues in fish and shellfish after an aerial broadcast project and detected residual concentrations of brodifacoum in three of ten species of coastal fish or shellfish sampled 43-176 days after bait application commenced. The proposed Farallon Island mouse eradication project that would use brodifacoum proposed a two-month closure of all non-essential access in the National Wildlife Refuge.”

The state Department of Health’s Hazard Evaluation and Emergency Response (HEER) branch expressed similar concerns. HEER program manager Fenix Grange stated in his comments, “Reviewing the literature, detections of brodifacoum in game fish associated with local applications is a potential human health concern.” Given the uncertainty, he recommended expanding the notification and fishing restrictions to “at least 30 days after brodifacoum use or until fish tissue and sediment data demonstrate no hazard.”

In their final EAs, DLNR and FWS did not propose any fishing moratorium around the island following the bait applications of either diphacinone or brodifacoum. The FWS noted in its final EA that when fish and invertebrates have been found to be exposed to bait, “brodifacoum residue levels are at or below the NOAEL [no observed adverse effect level] for the most sensitive mammals tested, which would be protective of humans (i.e., a dose or exposure level of a toxicant that produces no measurable toxic effects on the test group of animals).”

However, given the public’s concern over potential impacts to fish, “we would place greater emphasis on fish sampling than in the 2009 effort,” the FWS stated. The agency plans to try to collect samples of reef fish around the island that had in the past been observed interacting with placebo pellets; black triggerfish, which is the species that showed up dead on the coast of Ni`ihau after the 2009 eradication attempt, and “prized near-shore game fish, particularly higher trophic level predators more likely to bioaccumulate toxins.”

(For more information on this project and other efforts to control rodents via the aerial application of pesticide, see our April 2017 cover story and sidebars, available at

— Teresa Dawson

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