Abstract
ABSTRACTThe muskrat (Ondatra zibethicus) is an invasive species in Europe. The extensive waterways of the Netherlands provide ideal habitat for muskrats, and a large population established itself after arrival in 1941. A control program was put into effect immediately because muskrat burrowing can compromise the integrity of dikes and, hence, poses a significant public safety risk. The current (2015) annual catch of approximately 89,000 individuals is equivalent to approximately 0.30 muskrats/km of waterway, well above the national objective in spite of decades of effort. The control program is expensive (€35 M annually) and contested by animal rights groups. These factors created the need for a careful evaluation of the full range of control possibilities, from ‘no control’ to ‘extermination.’ As part of this, we experimentally evaluated the validity of a previously published correlation (based on historical data) between catch and effort. We raised or lowered removal effort (2013–2016) in a stratified random sample of 117 5‐km × 5‐km ‘atlas squares’ from the national grid. We found that catch‐per‐unit effort (CPUE) decreased after effort was increased, and rose after effort was decreased, by amounts slightly greater than expected based on the correlational data, though confidence intervals enclose zero. As anticipated, CPUE varied consistently and strongly between seasons. The biggest (and unanticipated) effects were those of the catch in the preceding 3 years (‘history’), and surrounding area (‘neighborhood’). Our experiment confirms estimates of intensity of control required to lower muskrat populations. These results will help with more effective allocation of control effort, and better‐informed evaluation of the economic costs of various control options. © 2020 The Authors. Wildlife Society Bulletin published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.
Highlights
The muskrat (Ondatra zibethicus), native to North America, was introduced to Europe as a furbearer early in the 20th Century (van den Bosch et al 1992)
Virgl and Messier 2000) supports predictions from source–sink theory (Pulliam 1988) that dispersal into trapped areas is a mechanism by which muskrat populations recover from high mortality rates induced by trapping
The extensive shallow, linear waterways bordered by abundant vegetation, the lack of many predators, carefully controlled water levels, and a mild climate make much of The Netherlands ideal muskrat habitat
Summary
The muskrat (Ondatra zibethicus), native to North America, was introduced to Europe as a furbearer early in the 20th Century (van den Bosch et al 1992). Reproduction and mortality (especially in winter) are both high (Errington 1963, Moens 1978) In their native range, muskrats show strong and irregular population fluctuations, attributed to high annual variation in predation, food abundance, disease, and the amount of habitat available as a result of strong variation in water level, which influences access to food resources and safety (Errington 1956, 1963; Messier et al 1990; Clark and Kroeker 1993; Clark 1994; Virgl and Messier 2000). The extensive shallow, linear waterways bordered by abundant vegetation, the lack of many predators, carefully controlled water levels, and a mild climate make much of The Netherlands ideal muskrat habitat Their number and distribution grew rapidly after their arrival in 1941. This is (barely) within the range considered ‘sufficiently under control’ (see table 1 in van Loon et al 2017), and well above the official national management objective of the Water Authorities of
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