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Effectiveness and efficiency of chlorophacinone poisoning for the control of muskrat (Ondatra zibethicus) populations

Abstract A muskrat‐poisoning campaign, using carrot baits containing the first generation anticoagulant chlorophacinone, was carried out by professional governmental trappers in a study area in Flanders (North Belgium). A single poisoning campaign in one half of the study area (2.07 km2) was carried out between 17 and 19 April 2000, while a similar campaign was carried out in the second half of the study area (2.66 km2) a fortnight later. The survival rate of radio‐tagged muskrats was compared between the treatment group consisting of animals that were known to have been potentially exposed to the poison baits (because they were resident inside either study area half during, and at least 5 days after, the poisoning campaign), and the non‐treatment group consisting of animals that had clearly not been exposed to the poison baits. The survival rates did not differ between the treatment and non‐treatment animals prior to the poisoning campaigns. After the poisoning campaigns, however, the survival rate of the treatment animals dropped dramatically while that of the non‐treatment animals remained constant. All treatment animals (first campaign: n = 3; second campaign: n = 7) died between the fifth and tenth day after the distribution of the baits in their environment, and all showed haemorrhage on postmortem. The majority of these animals (73%) had died above ground illustrating the danger of secondary poisoning. The study confirms that chlorophacinone, despite having been used for more than 30 years, still appears to be effective against muskrats. The labour required for both baiting campaigns, however, was substantially more than anticipated.