A large-scale experiment to evaluate control of invasive muskrats.
The 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.