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Wildlife Society Bulletin 44(2):314–322; 2020; DOI: 10.1002/wsb.1099
Original Article
A Large‐Scale Experiment to Evaluate Control
of Invasive Muskrats
DAAN BOS ,
1,2
Altenburg and Wymenga Ecological Consultants, Suderwei 2, 9269 TZ Feanwâlden, The Netherlands
E. EMIEL VAN LOON , Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, P.O. Box 94240, NL‐1090 GE,
Amsterdam, The Netherlands
ERIK KLOP, Altenburg and Wymenga Ecological Consultants, Suderwei 2, 9269 TZ Feanwâlden, The Netherlands
RON YDENBERG ,Centre for Wildlife Ecology, Simon Fraser University, Burnaby, B.C. Canada V5A 1S6
ABSTRACT 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. © 2020 The
Authors. Wildlife Society Bulletin published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.
KEY WORDS catch‐per‐unit effort, management experiment, muskrat, Ondatra zibethicus, pest species, spatial
context, The Netherlands, trapping.
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). Burrowing by musk-
rats can undermine the integrity of dikes, essential to public
safety in parts of the continent (Barends 2002, Bayoumi and
Meguid 2011). Significant resources are spent to control
muskrat populations in The Netherlands (van Loon
et al. 2017), Flanders (VMM 2010), and Germany (Pelz
1996). Although there was and still is broad support for
muskrat control (Ritzema‐Bos 1917, van Wijngaarden
1955, Doude van Troostwijk 1976), explicit doubts have
been raised by animal welfare organizations (Zandberg
et al. 2011) as well as by scientists (Pelz 1996) about the
need for and effectiveness of these control programs.
Muskrats are aquatic and herbivorous, requiring bodies of
water with good access to nutritious vegetation, and shore-
lines in which to excavate burrows (Boutin and Birkenholz
1987, Heidecke and Seide 1990). Reproduction and mor-
tality (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 re-
sources and safety (Errington 1956, 1963; Messier et al.
1990; Clark and Kroeker 1993; Clark 1994; Virgl and
Messier 2000). Muskrats are generally site‐faithful and ter-
ritorial (Errington 1963, Marinelli and Messier 1993, Virgl
and Messier 1997), and dispersal is density‐dependent
(Simpson and Boutin 1989, Virgl and Messier 1996).
Received: 25 February 2019; Accepted: 18 December 2019
Published: 9 June 2020
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction
in any medium, provided the original work is properly cited.
1
E‐mail: d.bos@altwym.nl
2
Joint affiliation: Conservation Ecology Group—Groningen Institute
for Evolutionary Life Sciences, University of Groningen,
P.O. Box 11103, 9700 CC Groningen, The Netherlands
314 Wildlife Society Bulletin •44(2)
Studies in Europe and North America have measured
muskrat dispersal distances of hundreds of meters to several
kilometers (Aldous 1947, Caley 1987, Adelberg 2008),
taking place mostly between successive breeding seasons in
autumn and spring (Mallach 1971, Verkaik 1987). Work by
Simpson and Boutin (1989, c.f. 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. Dispersing animals are highly
vulnerable to predators, and inspired the ‘doomed surplus’
hypothesis (Sinclair and Pech 1996, Boyce et al. 1999). This
concept holds that compensatory mechanisms in the pop-
ulation dynamic system cause the mortality from one source
to offset that from another. Hence, it is not at all inevitable
that heightened harvest intensity lowers either the
population or catch.
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. Al-
though a control program was immediately initiated
(Barends 2002), the annual numbers trapped climbed
steadily for a half century, reaching a peak of 434,000 in
1991, representing a catch of approximately 1.5/kilometer
waterway/year (#/km/yr). Thereafter, ongoing annual
effort of 1.5–2.0 trapping‐hours/kilometer waterway/year
(hr/km/yr) have reduced the catch rate to approximately
0.30 km/year (~89,000 animals in 2015). 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 <0.15 km/year.
The actual population size of muskrats in The Netherlands
is unknown. Population size and trend are inferred from
measures of the number trapped and invested effort. The
situation resembles many fisheries, with important differences
in that trappers are generally not competitors, and especially
that sustainable harvest is not the objective. Rather, the
program aims to drive numbers down and maintain them at a
low level, or even to exterminate this invasive species.
Muskrat populations have been successfully eradicated or
almost completely removed in other jurisdictions (Gosling
and Baker 1989, VMM 2010).
Bos and Ydenberg (2011) used a stage‐structured sto-
chastic dynamic meta‐population model to evaluate dif-
ferent strategies of control, and identify gaps in knowledge
that hamper muskrat management. Their model compared
year‐round, time‐and space‐differentiated harvest strategies,
at various levels of harvest intensity under a wide range of
parameter values. They concluded that harvest intensity has
a strong effect on population level, and year‐round har-
vesting compares favorably to a time‐differentiated strategy
in which harvesting is restricted to part of the year, at least
under the seasonal harvest proportions studied. This is be-
cause, with these proportions, the metapopulation is able to
recover during the no‐harvest seasons, and because the total
annual effort is lower. However, harvesting animals in
winter and spring was predicted to affect population via-
bility more per animal harvested, than harvesting in summer
and autumn. Thus, an allocation of effort toward winter and
spring, while maintaining total annual effort, is predicted to
result in relatively stronger effects on the population. More
intense harvesting (the model compared scenarios with 10%
and 25% of the population harvested per season) reduces the
population size more quickly, and is also more effective at
reducing the number of animals killed over the longer term.
Although larger numbers are killed initially, numbers killed
in later years are much reduced. Minimizing the total
number of deaths (and associated suffering) of trapped an-
imals, as well as considerations such as bycatch, are im-
portant elements of the broader social discussion in the
Netherlands.
van Loon et al. (2017) analyzed catch and effort based
on the extensive historical records. They found that relative
to the preceding year, catch declined once trapping effort
exceeded 1.4 hours/km/year. The relative catch fell by
0.295/hour of trapping effort. The y‐intercept (i.e., the
relative change in catch at zero trapping effort) had a value
of 0.42 (95% CI =0.33–0.51), suggesting that without
trapping, the population would increase rapidly. However,
these results are correlational and based on data aggregated
on a large (provincial level) scale. More careful investigation,
preferably including an experiment with manipulated effort
levels, is required to evaluate causality in these relationships,
assess how to lower costs, increase effectiveness, reduce
the number of animals killed, and limit the risk to
infrastructure.
Bioeconomic evaluation (Clark 2010) of the full range of
control possibilities, from ‘no control’to ‘extermination’
requires information on 1) population dynamics of muskrats
(Bos and Ydenberg 2011), 2) the relation between catch and
the amount of damage (Ydenberg et al. 2019), and 3) on the
relation between effort and catch. We experimentally eval-
uated the relation between catch and effort, testing the
validity of the previous correlational analysis (van Loon
et al. 2017). This correlation predicts that catch‐per‐unit
effort (CPUE) will fall after effort is increased, and rise after
effort is decreased, changing by a factor of 0.295. We also
anticipate, as described in the literature on muskrat natural
history that there will be strong effects of season on the
catch (Verkaik 1987). Finally, based on Bos and Ydenberg
(2011), we predict that a seasonal concentration of trapping
effort in winter and spring is more effective than year‐round
harvesting at the same total annual effort, resulting in
stronger effects on the population over time.
STUDY AREA
The Netherlands (50–54°N, 3–8°E; 33,700 km
2
) was char-
acterized by flat topography, with nearly 26% of the land
area below sea level. The low‐lying western regions were
characterized by peat and clay soils, while sandy soils pre-
dominated in the east and south. Peat extraction (mostly in
the medieval period) lowered many areas to below sea level,
and land reclamation projects have since the late 16th
Bos et al. •Large‐Scale Muskrat Control Experiment 315
Century created ‘polders’by building elaborate drainage
systems including dikes, canals, and pumping stations. The
Netherlands had a dense human population of >17 M. It
had a mild maritime climate, with average annual precip-
itation of approximately 780 mm. Intensive agriculture do-
minated land use, occupying >55% of the area. Muskrats
were found throughout the country but numbers were es-
pecially high in low‐lying regions with peat and clay
soils (Fig. 1).
METHODS
We manipulated the trapping effort assigned to individual
‘atlas squares’located across the Netherlands. Atlas squares
are 5‐km ×5‐km areas fixed on a national reference grid
(Vogelbescherming Nederland 2007; there are 2,202 atlas
squares in the country). Use of atlas squares enabled direct
reference to the national muskrat trapping database in
which catch and effort are registered, and eased the con-
siderable task of organizing the experiment. Atlas squares
compare favorably in area with field studies of muskrats
(e.g., Clark and Kroeker 1993), and exceed the size of
muskrat home ranges reported in the literature (Caley 1987,
Marinelli and Messier 1993).
In contrast to the largely isolated potholes and marshes
in native muskrat habitat on North American prairies,
aquatic habitats in the Netherlands are largely composed
of highly connected linear landscape elements. The extent
of muskrat habitat was estimated for each atlas square by
the total length of waterways (km), calculated as the sum of
1) the length of linear waterways that carry water during
Figure 1. Location of experimental atlas squares (5 km×5 km) in The Netherlands in which muskrat‐trapping effort was manipulated from winter 2012/
2013 to winter 2015/2016. The map also shows the 3 (high, medium, low muskrat density) strata within which atlas squares were stratified and randomly
assigned to treatments.
316 Wildlife Society Bulletin •44(2)
>3 months of the year; 2) double the length of linear wa-
terways wider than 6 m and deeper than 1 m; and 3) the
circumference of lakes and ponds (unpublished data of the
Dutch Muskrat Control Programme 2008). These 3 varia-
bles operationalize estimating the quantity of muskrat
habitat based on the length of shoreline. We also classified
each atlas square by the prevailing soil type (Alterra soil type
map 2006).
We selected 117 atlas squares (from the total of 2,202 on
the national grid) for the experiment, as follows. We first
excluded 965 atlas squares dominated by water or urban
areas, as well as those in which no muskrat had ever been
trapped. We used the 2‐step cluster algorithm in SPSS 20
(International Business Machines Corporation, Armonk,
NY, USA) to classify the remaining 1,237 atlas squares into
high (n=76), medium (n=460), or low (n=701) strata,
based on the mean annual muskrat catch and trapping effort
over 3 years prior to the experiment (2009–2011) and the
length of waterways, all of which co‐vary strongly.
From each stratum, we randomly selected 39 atlas squares
and assigned them to a combination of ‘effort’and ‘temporal’
treatments (Fig. 1, Table 1). The even allocation to the 3
strata meant that the few high‐stratum atlas squares, which
we assume are most influential to the overall population, both
in numbers and as source populations, are well‐represented in
the experiment. In the ‘effort’treatments, we increased or
decreasedtheallocatedannualeffort by 30%, relative to the
level during the reference period, defined as the 12 months
preceding the start of the experiment. In the ‘temporal’
treatments, trappers either adhered to their normal annual
routine (‘year‐round’), or, in the ‘seasonally‐concentrated
treatment’, the hours expended during summer were limited
to 20% of the total annual effort assigned to that atlas square.
We conducted the experiment from December 2012 through
January 2016. Trapping and the registration of catch and
effort went on as usual in each of the atlas squares that were
not included in the experiment.
Our aim was to estimate the effect of raising (or lowering)
the quantity of the trapping effort invested, but not to alter
the behavior of trappers. Hence, we made no prescriptions
regarding the type of traps or trapping strategy. We in-
structed trappers to follow their normal routines, registering
catch and effort following established standard procedures.
We surveyed regional trapping team leaders after the con-
clusion of the experiment and asked them to score the
quality of the trapping effort during the experiment, ranging
from 1 (poor) to 10 (excellent).
Data Analysis
In the current data registration procedure (implemented
1987), each trapper records on a standard form the date,
atlas square, number of muskrats trapped, as well as a record
of time devoted to various task categories. The forms are
processed centrally and entered into a database. For the
purposes of our analysis, trapping effort includes the time
spent in the field setting and checking traps, and the travel
time between locations, but excludes holidays, admin-
istrative time, overhead, and time used in preparation and
maintenance of equipment.
For each atlas square, we aggregated records by ‘season,’
defining seasons by the solar calendar. The experiment
ran for 13 successive seasons, from winter 2012/2013
(‘time’=1) through winter 2015/2016 (‘time’=13). There
were thus 1,521 observations (117 atlas squares in each of
13 seasons). We tabulated for each observation ‘catch’and
‘effort,’and calculated ‘catch rate’(muskrats trapped per
kilometer of waterway); CPUE (muskrats trapped per hour
of trapping time); the ‘historical CPUE’(average CPUE for
the same season in that atlas square in the 3 years prior to
the experiment 2010–2012); and ‘neighboring CPUE’(the
average CPUE for the same season in the 8 atlas squares
surrounding each atlas square). Considering the 8 adjacent
atlas squares enlarges the scale of the analysis from
5‐km ×5‐km (25 km
2
)to15‐km ×15‐km (225 km
2
) and
thus provides a step between the atlas‐square scale analysis
here and larger scale analysis of van Loon et al. (2017).
To assess whether there may have been some bias in the
selection of atlas squares for the experiment, we asked
Table 1. We experimentally evaluated the validity of a previously published correlation (based on historical data) between catch and effort in regard to
effectiveness of muskrat control by trapping in The Netherlands. We assigned each of the 1,237 eligible atlas squares in The Netherlands to the Low,
Medium, or High stratum of muskrat density
a
. We randomly selected 117 of the 1,237 and assigned them to the experiment, with the number of atlas
squares assigned to each combination of the effort and temporal treatments given in the right portion of the table. ‘yr’=‘year‐round’treatment;
‘sc’=‘seasonally concentrated’treatment.
Effort manipulation
b
Decrease Control Increase
Temporal manipulation
c
Stratum nWaterway (km)Catch (n)Effort (hr)yr sc yr yr sc
Low 701 93 27 145 7 6 13 7 6
Medium 460 285 124 480 7 6 13 7 6
High 76 506 796 1,730 7 6 13 7 6
a
Strata were based on the mean annual muskrat catch and trapping effort over 3 yr prior to the experiment (2009–2011) and the length of waterways, all of
which co‐vary strongly.
b
In the ‘effort’treatment, the allocated annual effort was increased or decreased by 30% or maintained (Control), relative to the level during the reference
period, defined as the 12 months preceding the start of the experiment.
c
In the ‘temporal’treatment, the allocated trapping effort was adjusted so that more was expended in winter and spring and less in summer (‘seasonally
concentrated’), or else expended ‘year‐round’, following the regular pattern.
Bos et al. •Large‐Scale Muskrat Control Experiment 317
whether the change in CPUE between the experiment
(2013–2016) and 3 years preceding the experiment
(2010–2012) differed between control atlas squares and
nonexperimental atlas squares using a linear mixed‐effects
model (accounting for repeated measures over the 13 periods/
atlas square). To evaluate effects of the effort and temporal
treatments, we compared linear mixed models of CPUE. The
null model contained ‘atlas square’as a random factor and
‘season’as a fixed factor. Other models included, in various
combinations, the ‘effort’and ‘temporal’treatments, time,
interaction of treatments and time, stratum (high, medium,
low), predominant soil type, the Regional Water Authority
responsible, neighboring CPUE, historical CPUE, and the
quality of invested effort (from the postexperiment ques-
tionnaire; Table 2).
We assessed model performance using Akaike’sIn-
formation Criterion, adjusted for small sample sizes (AIC
c
).
After fitting the models, residuals of the model(s) best
supported by these data were checked for normality, and we
inspected residuals of all variables. We tested for spatial
autocorrelation in the residuals and slopes using semi-
variograms. We performed analyses in Program R (R
Foundation for Statistical Computing; https://www.r‐
project.org/), using the package lme4 (Bates et al. 2015).
We report parameter estimates from models with
ΔAIC
c
<4.0, using the value for ‘autumn’as intercept. We
did not model average because the 2 top models were
similar.
We further investigated effects of neighborhood and his-
tory to assess the scale of these influences. We calculated the
correlation coefficient of CPUE estimates between each
possible pair of experimental atlas squares in relation to the
distance between them. We also calculated the correlation
coefficient between the CPUE measured during the ex-
periment in each atlas square in relation to the temporal
separation (no. of seasons) between the measures. We
use correlograms to display these spatial and temporal
autocorrelations.
RESULTS
The number of muskrats trapped annually in The Netherlands
declined during the experiment, dropping by approximately
8%, from approximately 97,000 (2013) to 89,000 (2015).
These numbers are the lowest in recent decades—total
annual catch has been >100,000 since 1978, and (though
fluctuating) has declined since the 1991 peak of 434,000.
Matching this decline, mean CPUE in nonexperimental
atlas squares fell by approximately 7.7% (equivalent to
0.021/hr; SE =0.005). There were no indications that
suggested a biased assignment of atlas squares to the
experiment, Linear mixed‐effects model indicated no dif-
ference between control atlas squares and nonexperimental
atlas squares (t
1,438
=1.18, P=0.24).
The overall average CPUE in the 117 atlas squares was
0.274 muskrats/hour (SE =0.039). This was significantly
greater than in nonexperimental atlas squares (0.147
muskrats/hr: t
1,438
=3.25, P=0.001), which is almost en-
tirely explicable by the overrepresentation of ‘high stratum’
atlas squares in the experiment relative to The Netherlands
as a whole (39 of 117, or 33.3% in the experiment vs. 8%
nationally; Table 1). The seasonal pattern was strongly
evident, with CPUE peaking in spring and autumn and
troughs in summer and winter (Fig. 2).
Implementation of Treatments
Effort in the reduced effort treatment was on average
16.7% ±54.6 (SD) lower, and in the increased effort
treatment 23.8% ±24.5 (SD) larger, than the reference
period (objectives were to raise or lower effort by 30%).
Accordingly, we expected that the experimental decrease in
effort would increase the annual CPUE by 0.295 ×
16.7% =4.9%, while the experimental increase in effort
would decrease CPUE by 0.295 ×23.8% =7.0%. The
change in effort in control atlas squares relative to the ref-
erence period was a mere 1.7% ±17.5 (SD).
The implementation of the ‘seasonally concentrated’tem-
poral treatment was as intended, with only 17.5% ±5.0 (SD)
Table 2. Competition evaluating models of variation in muskrat‐trapping catch‐per‐unit effort (CPUE; no./hr), in relation to season, time, neighborhood
(Neighb; CPUE in the 8 neighboring atlas squares), history (Hist; CPUE in the 3 yr before the experiment for the same season), effort (EFF; increased or
decreased), treatment (TEMP; year‐round or seasonally concentrated), Regional Water Authority (RWA), and soil (predominant soil type), from winter
2012/2013 to winter 2015/2016 in The Netherlands. All models with any support (AIC
c
weight >0) include a random slope per atlas square (Time|AS).
Model
a
K
b
AIC
c
c
ΔAIC
c
d
Weight Cum.Weight
e
Season +Hist +Neighb +EFF +(Time|AS) 12 220 0.0 0.62 0.62
Season +Hist +Neighb +EFF ×Time +(Time|AS) 12 223 2.2 0.20 0.82
Season +Hist +Neighb +(Time|AS) 10 224 4.1 0.08 0.90
Season +Hist +Neighb +TEMP ×Time +(Time|AS) 13 225 4.2 0.08 0.98
Season +Hist +Neighb +Stratum +(Time|AS) 12 228 7.8 0.01 0.99
Season +Hist +Neighb +Soil +(Time|AS) 13 228 7.8 0.01 1.00
Season +Hist +Neighb +RWA +(Time|AS) 30 247 26.4 0.00 1.00
Season +Hist +Neighb +EFF +Time +(1|AS) 11 356 135.6 0.00 1.00
Season +Hist +Neighb +EFF ×Time +(1|AS) 13 359 138.6 0.00 1.00
Season +Soil +RWA +(Time|AS) 31 460 239.3 0.00 1.00
Season +(1|AS) 6 617 397.1 0.00 1.00
a
For the model best supported by the data, the marginal R
2
equals 0.40, while the conditional R
2
equals 0.59.
b
K=no. of free parameters in the model.
c
AIC
c
=Akaike Information Criterion, adjusted for small sample sizes.
d
ΔAIC
c
=difference between model AICc and AICc value of the best model
e
Cum.Weight =Cumulative weight.
318 Wildlife Society Bulletin •44(2)
of the annual effort expended during summer (20% intended
at maximum). However, the effort expended during summer
in the ‘year‐round’treatment was only 21.6% ±5.0 (SD).
Considering this small difference in relation to the large
variation among atlas squares, any effect of this treatment is
in hindsight not expected to be detectable.
Experimental results
The best performing model includes the effort treatment,
the variables season, historical CPUE, neighborhood
CPUE, and contains a random slope of time per atlas square
(Table 2). The model provides a good fit to the data
(marginal R
2
=0.40, conditional R
2
=0.59). The residuals
were normally distributed and showed no structural devia-
tions in relation to the response variable, predictor variables,
or length of waterway or stratum. The second‐best model
was nearly identical, differing only in that the effort
treatment showed an interaction with time. Not included in
either of the top models are the temporal treatment, the
variables ‘time’, Regional Water Authority, soil type, the
quality of invested effort, or the stratum.
Parameter estimates for the seasons range from −0.004
to −0.036 muskrats/hour (Table 3). Both top models in-
dicate an effect of the effort treatment. As predicted, CPUE
increased when effort was lowered (by 0.032/hr, or 11%;
prediction 4.9%) and lowered when effort was raised (by
0.059/hr, or 21–26% in the top and second models; pre-
diction 7.0%). These estimates are larger than predicted, but
note that the confidence intervals for both include zero.
Both top models assigned the largest parameter estimates to
‘history’(~0.32 muskrats/hr) and ‘neighborhood’(~0.63
muskrats/hr).
Neither of these large effects was anticipated, and we
carried out a post hoc analysis to assess the scale of these
influences in more detail (Fig. 3). These results show that
the similarity of atlas squares extends, though in a dimin-
ishing fashion, to a distance of almost 40 km. Successive
measures tend to be very similar (high values at temporal
separation of 1–2 seasons), while the strong seasonality is
evident by the peaks at 4 and 8 seasons (i.e., 1 and 2 yr).
DISCUSSION
We used an experiment manipulating trapping effort, car-
ried out over 3 years in 117 5‐km ×5‐km ‘atlas squares,’to
test the correlation between trapping effort and muskrat
catch identified by van Loon et al. (2017). As in their native
range, muskrats in The Netherlands have low survival
(Bos et al. 2019 estimate 0.26–0.34/half‐year) but high re-
productive potential (Clay and Clark 1985, Clark and
Kroeker 1993, Virgl and Messier 2000). We thus had ex-
pected that population size, and hence the catch, would
quickly change in response to a 30% change in trapping
effort. The results were consistent with the correlational
analysis of van Loon et al. (2017), in that CPUE changed by
about the magnitude predicted, though we note that the
confidence intervals around the parameter estimates en-
closed zero. Nevertheless, the effect of the effort treatment
Figure 2. Basic experimental results, showing the catch‐per‐unit effort
(CPUE; no./hr) over the successive 13 seasons (from winter 2012/2013 to
winter 2015/2016) of the experiment in which muskrat‐trapping effort was
manipulated in The Netherlands. Portrayed is CPUE in the each of
3 experimental treatments, and in the nonexperimental atlas squares
(yellow ‘no‐experiment’line). Note the strong seasonal variation: the peaks
correspond to spring and autumn.
Table 3. Parameter estimates
a
with 95% confidence intervals (CI) in the 2 top‐ranked models
b
of variation in muskrat‐trapping effort, as measured from
winter 2012/2013 to winter 2015/2016 in The Netherlands.
Top model (weight =0.62)Second model (weight =0.20)
Coeff. Estimate 2.5% CI 97.5% CI Estimate 2.5% CI 97.5% CI
(Intercept) 0.033 −0.021 0.090 0.054 −0.008 0.118
Winter −0.004 −0.037 0.029 −0.002 −0.035 0.031
Spring −0.036 −0.071 −0.001 −0.032 −0.067 0.002
Summer −0.024 −0.060 0.012 −0.023 −0.059 0.013
Historical 0.322 0.243 0.401 0.320 0.241 0.399
Neighborhood 0.633 0.534 0.732 0.637 0.538 0.736
Decreased effort 0.032 −0.031 0.094 0.030 −0.047 0.106
Increased effort −0.059 −0.122 0.003 −0.073 −0.150 0.004
Time 0.026 −0.010 0.062
Interactions
Decreased effort ×time −0.003 −0.053 0.048
Increased effort ×time −0.015 −0.066 0.035
a
Parameter estimates are given with respect to the control treatment in autumn.
b
Model specification given in Table 2. The second model contains the interaction terms.
Bos et al. •Large‐Scale Muskrat Control Experiment 319
on CPUE was small compared with that of the factors
‘neighborhood’and ‘history’(0.633 and 0.322 muskrats/hr,
respectively). These parameters had the largest effects on
the experimental outcome, and show that the scale of an
atlas square, though convenient for the experimental pro-
cedure and data registration, is relatively small for trapping
effort to have a large effect as long as neighboring squares
are not managed in the same way. Our findings illustrate at
what scale a control program needs to be organized to be
more effective. The dispersal into trapped areas is a powerful
mechanism by which muskrat populations recover from
high mortality rates induced by trapping over multiple
kilometers (Mallach 1971, Simpson and Boutin 1989), in
spite of the fact that muskrats are generally site‐faithful,
territorial (Errington 1963) and dispersal distances may on
average be only hundreds of meters (Caley 1987). Indeed,
van den Bosch et al. (1992) estimated the velocity of pop-
ulation expansion for muskrats to be 10.9 and 5.1 km/year,
respectively, for 2 phases of population expansion in
Europe, before and after the start of large‐scale trapping
programs around 1925–1930. The correlational analysis of
van Loon et al. (2017) was based on data aggregated by
province (12 in all), on average approximately 100 times
larger than the individual atlas squares on which the ex-
periment was based. Thus, it was perhaps not surprising
that experimental effects were noisy and difficult to detect.
The expected seasonal pattern is strongly evident, however,
with relatively high CPUE in spring and autumn, when
there is more movement of animals (due to mating and
dispersal; Errington 1963, Verkaik 1987).
We were unable to evaluate the prediction of Bos and
Ydenberg (2011) that a time‐differentiated strategy with
‘seasonally concentrated’trapping is more effective than
year‐round harvesting. The temporal treatment was not
included in either of the top‐ranked models; thus, without
evidence suggesting an influence on CPUE. We propose
this is likely because the ‘temporal’treatments intended to
test this idea were not sufficiently contrasting.
The historical variation in control effort has been con-
siderable over the 8 decades that muskrats have been present
in The Netherlands, ranging from almost zero in some
places to >4 hours/km in several locales over shorter pe-
riods. Part of that variation is logically related to differences
in history (time since local muskrat invasion; van Loon
et al. 2017) and landscape. But much has been unguided by
quantitative evaluation of the effect of trapping on muskrat
populations. The general management strategy has been to
increase the effort allocated where the catch rate rose, on the
reasonable premise that this indicates the muskrat pop-
ulation is increasing. Our study makes clear that the catch
rate changes in relation to several factors, which implies that
the recent history in any atlas square should be seen only as
an approximate guide for what is required to maintain
control. The true required effort could deviate strongly from
this estimate as a result of recent history, or the neighbor-
hood. For example, if the effort allocated during the refer-
ence period drove numbers down, it likely overestimates
that required to maintain control (or vice versa). The strong
effect of history may be due partially to such effects.
The experimental design fixed the total annual effort as-
signed to each atlas square, but each trapper retained other
means to exert control over their effort (e.g., by shifting the
timing of their field work). Trappers often related that their
effectiveness varies greatly with environmental circumstances.
For example, it is greater after mowing of shoreline vegeta-
tion, when the water table is lower, or during weather such as
frost; and trappers were clear that they routinely adjust their
schedules to take advantage of such conditions. Trappers are
professional and highly dedicated, and many were reluctant
participants in the experiment. By randomly assigning atlas
squares to treatments, the imposed experimental design drove
the allocation of effort away from that what would have
Figure 3. Correlograms showing spatial and temporal autocorrelation in muskrat CPUE (catch‐per‐unit effort; no./hr, with standard errors) in 117
experimental atlas squares over 13 successive seasons (from winter 2012/2013 to winter 2015/2016) of the experiment in which muskrat‐trapping effort was
manipulated in The Netherlands. The left panel A) displays the correlation between CPUE in pairs of atlas squares in relation to the distance between their
centers. The right panel B) displays the correlation between CPUE measures in each atlas square, in relation to the number of seasons between the measures.
320 Wildlife Society Bulletin •44(2)
chosen based on the usual procedure. Many trappers felt this
created an unwarranted hazard and to some extent, trappers
may have felt a necessity to make adjustments to hedge
against muskrat outbreaks (e.g., by timing visits to the atlas
square concerned under ideal field circumstances). This
would result in reduced contrast among treatment effects.
Any effect on our results is difficult to quantify, but it is
noteworthy that CPUE in the nonexperimental atlas squares
declined during the experiment (by 0.021 muskrats/hr, con-
tinuing the national trend of the previous decade), whereas
CPUE in the experimental atlas squares rose (by 0.026
muskrats/hr).
Concerns about liability related to muskrat outbreaks have
long kept the responsible authorities very averse to field
experiments. But in this case, the Dutch Water Authorities
were persuaded to permit experimental variation in trapping
effort, the parameter considered most influential in affecting
muskrat population development. Fortunately, only a few
situations (<5—about the usual rate) with ‘large’damage
(defined as repair costs exceeding €10,000, or compromising
public safety) occurred during the experiment. Now, having
finished the experiment and the associated studies (van
Loon et al. 2017, Bos et al. 2019, Ydenberg et al. 2019), the
authorities have greatly enhanced the evidence‐base to
support future management decisions.
MANAGEMENT IMPLICATIONS
Control of muskrat numbers is a means to reduce the risk
that their burrowing activities pose to dikes and other water
control infrastructure. However, it is also expensive (cur-
rently requiring €35 M annually in The Netherlands).
Ultimately, the total national investment in muskrat control
is a political decision, influenced by the acceptable level of
safety risk (Doude van Troostwijk 1976), economic costs
and benefits (Clark 2010), availability of other methods to
prevent damage, ethical considerations (Warren 2007,
Zandberg et al. 2011), effects on biodiversity (Danell 1979,
Vermaat et al. 2016), and potential for transfer of disease
(Ulrich et al. 2009). These aspects almost certainly vary
among landscapes in northwest Europe. For the low‐lying
landscapes of The Netherlands, vulnerable to flooding, we
have strong indications that large muskrat populations
compromise public safety (Ydenberg et al. 2019). Thus, we
strongly recommend continuing the control program until
the risks, costs, and other considerations have been sub-
stantiated and debated. This is in line with recom-
mendations of the global Convention on Biological Diver-
sity (Aichi Target 9; https://www.cbd.int/sp/targets/),
scientific sources (Genovesi 2005, Lambertini et al. 2011,
Luque et al. 2014), and recent European legislation (Reg-
ulation (Eu) no 1143/2014 of the European Parliament and
The Council of 22 October 2014 on the prevention and
management of the introduction and spread of invasive alien
species). Our results confirm estimates of the intensity of
control required to lower muskrat populations. These ex-
perimental results will support more effective allocation of
control effort, given the emphasis on spatial scale of im-
plementation. In combination with other information (e.g.,
on the relation between damage to infrastructure and
muskrat numbers; Ydenberg et al. 2019), a careful bio‐
economic analysis of control strategies is now possible.
ACKNOWLEDGMENTS
We thank the trappers and staffof the Water Authorities
who endured the experiment and discussed the outcome
with us. We also appreciate the input we received from our
colleagues at the Dutch Mammal Society. Thanks to the
Unie van Waterschappen for funding the study and Asso-
ciate Editor and 2 anonymous reviewers for their helpful
comments. A special thumbs up to D. Moerkens for his
help in coordinating many aspects of the research. The
authors declare that they have no conflict of interest. These
data have been deposited at https://zenodo.org/record/
3827952.
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