Langara Island, at the north-western tip of British Columbia's Queen Charlotte archipelago, was once nesting grounds for an estimated 500,000 seabirds. However, infestations of Norway rats (Rattus norvegicus) and their predation of eggs and breeding adults have caused extirpation or serious declines of all seabird species. By 1993, the breeding population of ancient murrelets (Synthliboramphus antiquus) had declined to 10% of its historical size. Successful eradication of rats on smaller New Zealand islands using the anticoagulant brodifacoum prompted its application on Langara Island. The island is also home to breeding bald eagles (Haliaeetus leucocephalus), peregrine falcons (Falco peregrinus), and other wildlife. No comprehensive studies of non-target impacts and potential for secondary poisoning were done during similar operations elsewhere; thus, in 1994 and 1995 we initiated a two-year study into the risk of secondary poisoning to non-target species. During 1994, rat carcasses were laid out with motion sensor cameras to identify potential scavengers. Ravens, northwestern crows and bald eagles were photographed at carcasses, and therefore at risk of feeding on rats that die above ground. During the baiting program, 19 rats were radio-tagged to determine the proportion dying above ground, and thus available to predators/scavengers. Ravens were found poisoned both from feeding directly on the bait, and predating/scavenging poisoned rats. Bald eagles were trapped and blood sampled for brodifacoum residue analysis and prothrombin time evaluation; 15% of the sampled population showed detectable residues but no adversely-affected birds were found. We conclude that the use of brodifacoum for rat removal on seabird islands poses a clear risk of secondary poisoning to avian scavengers. This risk must be weighed against the benefit of rat removal programs.
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