U.S. Department of Agriculture: Animal and Plant Health Inspection Service
Brodifacoum residues in fish three years after an island-wide rat eradication attempt in the tropical Pacific
Date of this Version
Siers et al.
Invasive rats are known to threaten natural resources and human health and safety. Island-wide rat eradication attempts have been increasing in number and scale during the past several decades, as has the frequency of eradication success. The most common method to remove all rats from an island is to broadcast anticoagulant rodenticide bait into every rat’s home range on the island. Broadcast of toxicants can put humans and other nontarget species in marine and terrestrial environments at risk of exposure. The persistence of anticoagulant residues is somewhat unknown, particularly in marine environments. Three years after ~ 18,000 kg of 25 mg/kg brodifacoum bait was broadcast across Wake Atoll to eliminate rats, we collected whole-body fish samples from six near-shore sites and one intermittently land- locked pond to test for brodifacoum residues. Of the 69 samples tested using high- performance liquid chromatography with fluorescence detection, 20 were suspected of brodifacoum contamination and therefore subject to more selective liquid chromatography-mass spectrometry analysis. Of those 20 fish, brodifacoum was detected in two individuals of blacktail snapper (Lutjanus fulvus), although at levels too low to be accurately quantified. Both fish containing detectable brodifacoum residues were caught within an intermittently land-locked pond in an area of the island that received heavy brodifacoum baiting, and not truly in the “marine environment”. Brodifacoum was not detected in any of the samples collected within the lagoon of the atoll or within near-shore waters outside the lagoon. These results demonstrate that under some circumstances very low levels of brodifacoum can occur in a low proportion of fish tissues for as long as three years after the application of the rodenticide to an environment. Such information is valuable in assessing the relative environmental risks associated with rodenticide use in rodent eradications for protection of threatened species and restoration of island ecosystems. The overall result is one of declining incidence and magnitude of residue concentrations over time and within limited environmental circumstances.
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Siers et al. (2020), Management of Biological Invasions 11(1): 105–121, https://doi.org/10.3391/mbi.2020.11.1.08