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The risk of non-target species poisoning from brodifacoum used to eradicate rats from Langara Island, British Columbia, Canada Howald, Gregory Robert

Abstract

In 1995, the Canadian Wildlife Service attempted to eradicate introduced Norway rats (Rattus norvegicus) from Langara Island and adjacent Cox and Lucy Islands with the application of the second generation anticoagulant rodenticide brodifacoum. However, anticoagulant rodenticides are non-species specific pesticides and pose a poisoning risk to non-target species. This thesis addresses the short term poisoning impacts to non-target species from brodifacoum used to eradicate the rats from Langara and Lucy Island. In 1994, during testing of the baiting protocol on Lucy Island, the native dusky shrew (Sorex monticolus elassodon) population size fell from an estimated 25 unique shrews/ha before the baiting to four unique shrews/ha after the baiting. This prompted a monitoring program in three regions on Langara Island in 1995. While shrews were attracted to bait in stations, the decline in their population was non-significant. Shrews in breeding condition were at greater risk of poisoning likely due to their ability to range widely. Shrews entered and chewed on bait blocks in up to 80% of bait stations. The risk of secondary poisoning to avian scavengers from poisoned toxic rat carcasses was investigated. In 1994, common ravens (Corvus corax) were identified as the most significant scavenger of rat carcasses. In 1995, two of 15 radio-collared Norway rats poisoned with brodifacoum died above ground and one was scavenged. Whole body brodifacoum residues from other rats found dead above ground ranged from 2.40-16.51 mg/kg. Between 1995 and 1996, 20 raven remains were found or reported. In 1995, 13 raven livers tested positive for brodifacoum. Ravens were secondarily poisoned from scavenging rat carcasses and primarily from raiding bait stations. Brodifacoum was detected in Northwestern crows (Corvus caurinus) 9 months after the cessation of baiting on Lucy Island in 1994, but before the baiting on Langara Island. Brodifacoum residues were detected in the plasma of 15% of bald eagles (Haliaeetus leucocephalus) sampled (0.037-1.74 ppm). The invertebrates as a source of brodifacoum to non-target species was investigated. Snails (Vespericola sp. and Haplotrema sp.) and banana slugs (Ariolimax sp.) were common and abundant invertebrates found feeding on bait in stations. The blue coloured bait could be seen through the translucent bodies and the molluscs tested positive for brodifacoum. Carrion insects readily consumed rat carcasses containing brodifacoum. Blowfly larva (Calliphora sp.) tested positive for brodifacoum residues. The invertebrates found feeding on the bait and carrion insects were a secondary and tertiary poisoning risk to non-target species such as the song sparrow (Melospiza melodia).

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