Cost-Benefit analysis of invasive muskrat control in The Netherlands: Complete removal as financially rational strategy

Abstract

Muskrats threaten public safety in The Netherlands by burrowing into water-retention structures, and a control programme has been in effect since 1941. Recent European legislation on Invasive Alien Species requires Member States to take appropriate action in muskrat control, based on the cost-effectiveness and socio-economic aspects of control. The costs of inaction must also be considered. Possible control strategies include (i) year-round trapping to maintain numbers at a given level; (ii) no control; and (iii) complete removal. We estimate the costs of labour, the costs of repairing damage inflicted by muskrats, and investment in preventive measures of each strategy, and conclude that the Net Present Value (assuming 3% inflation and 5% interest rate) is lowest for the ‘complete removal’ option. Importantly, complete removal is achievable, but its success is dependent upon competent staff that work in a motivated and coordinated manner.

Full text

36 – WATER GOVERNANCE – 01/2020
COST-BENEFIT ANALYSIS
OF INVASIVE MUSKRAT CONTROL
IN THE NETHERLANDS
COMPLETE REMOVAL AS FINANCIALLY RATIONAL STRATEGY
Sandra Zappeij-Ploeger, Morrison T. Pot, Daan Bos*
* Sandra Zappeij-Ploeger, Economist at Waterschap Zuiderzeeland;
Morrison T. Pot en Daan Bos,
Ecologist at Altenburg & Wymenga ecological consultants.
Muskrats threaten public safety in The Netherlands by burrowing into water-retention
structures, and a control programme has been in effect since 1941. Recent European
legislation on Invasive Alien Species requires Member States to take appropriate action in
muskrat control, based on the cost-effectiveness and socio-economic aspects of control.
The costs of inaction must also be considered. Possible control strategies include (i)
year-round trapping to maintain numbers at a given level; (ii) no control; and (iii) complete
removal. We estimate the costs of labour, the costs of repairing damage inflicted by
muskrats, and investment in preventive measures of each strategy, and conclude that the
Net Present Value (assuming 3% inflation and 5% interest rate) is lowest for the ‘complete
removal’ option. Importantly, complete removal is achievable, but its success is
dependent upon competent staff that work in a motivated and coordinated manner.
In Europe, 1200-1800 invasive alien species (IAS) are
associated with an annual damage and control costs
estimated at €12 billion (Kettunen et al. 2008, Scalera
et al. 2010, Williams et al. 2010). Besides financial costs
associated with damage (e.g. to agricultural crops) and
population control, IAS are considered a significant threat
to biodiversity, and are associated with impacts on human
health, as recognized by several international agreements
(Roy et al. 2016). European legislation (EU regulation No.
1143/2014) is now in place that requires Member States to
take appropriate action against IAS listed as high-profile
through, for instance, management obligations and trade
restrictions (Genovesi et al. 2014). In such European policy,
cost-benefit analyses are recognized as an important
decision support framework in IAS management (Reyns
et al. 2017). According to the EU regulation, muskrats are
deemed to pose a high-risk, requiring EU Member States
to take appropriate action when muskrats are found on
their territory (Genovesi et al. 2015, Booij et al. 2017, Roy
et al. 2018). The control actions should be based on sound
information on the cost-effectiveness and socio-economic
aspects of control.
The muskrat (Ondatra zibethicus) is a semi-aquatic rodent
native to North America and invasive in Europe. Muskrats
are considered a threat to public safety in several low-
lying European countries due to their habit of burrowing
into water-retention structures such as dykes, dams
and levees (Ydenberg et al. 2019). The Netherlands is
a country vulnerable to flooding with a vast network
(~280.000 km) of waterways and carefully regulated water
levels. However, along with rich waterway vegetation,
few predators and a mild maritime climate, these features
offer high-quality habitat to muskrats, and their numbers
grew quickly after initial settlement. Dutch authorities
recognized the risks associated with muskrat’s burrowing
habits, and responded by setting up a control programme
immediately after invasion of the species in 1941 (van
de Peppel 1949, Barends 2002, van Loon et al. 2017a).
The Dutch muskrat control programme is carried out by
professional trappers, who spend their time looking for
signs of muskrat presence, setting and checking lethal
traps along hundreds of thousands kilometres of waterway
(Barends 2002). Regular revisits are required to check
for remnant animals. In recent years, around 400.000
person-hours were spent and in 2018 ~54.000 muskrats
were trapped (Unie van Waterschappen 2018). There has
been a declining trend in the catch since 2004 (van Loon
et al. 2017a), which population modelling (van Loon et
al. 2017b) indicates is related to a declining population.

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Associated with this decline, the control organisations
have been able to slowly diminish trapping effort over the
past decade.
The primary objective of the Dutch muskrat control
programme is to safeguard the integrity of the water
infrastructure, and to maintain public safety. Until June
2019, the strategy aimed to maintain the muskrat numbers
at or below an average trap-rate of 0.15 muskrats per
kilometre of waterway per year, a level barely attained after
several decades of effort. Because control measures are
expensive, large numbers of animals are killed annually,
and other species are (directly and indirectly) killed as well
(Zandberg et al. 2011, Bos and Gronouwe 2018), there
is ongoing debate about the rationality, desirability and
effectiveness of muskrat trapping as a control method.
Other possible control strategies exist. Other than lowering
or raising the desired capture rate, alternative strategies
for Dutch muskrat control would be no control or complete
removal. Under the latter, once all muskrats have been
removed, limited control measures are required (Gren
2008, Bos and Gronouwe 2018). Complete removal differs
from eradication in the sense that a (minor) ongoing
trapping effort will be required along the borders to
prevent recolonisation (Robertson et al. 2017). Practical
field examples at regional scale from the Netherlands,
Flanders (Belgium) and the UK (Gosling and Baker 1989)
illustrate that complete removal of muskrat is feasible
(Bos and Gronouwe 2018). Good evidence shows that the
Dutch control programme reduces muskrat population
size provided that the levels of the effort are in adequate
proportion to the population present (van Loon et al.
2017a, Bos et al. 2019). Under the strategy of no control,
preventive measures are required to discourage or prevent
muskrat burrowing (Spoorenberg 2007), for example by
applying mesh wire, concrete or steel along all dykes and
levees (BCM 2007, Zandberg et al. 2011).
In general, the political assessment of alternative
strategies should be based on careful risk assessment
and evaluation by multiple and diverse criteria, including
ethics, biodiversity, effectiveness with regard to public
safety, practicality, negative impacts, acceptability and
costs (Booy et al. 2017, Roy et al. 2018). Risk assessments
are given by Kumschick et al. (2015) and Carboneras et al.
(2018). Biodiversity criteria are debated in Bos & Gronouwe
(2018) and Bakker & Bos (2019). In this paper we aim to
analyse muskrat control from a financial point of view.
Methods
Three main types of financial cost can be identified in
relation to muskrat control: (i) ongoing costs for repair of
damage caused by muskrats; (ii) ongoing costs associated
with control activities; (iii) one-time costs for the installation
of measures to prevent damage. Some of these costs
incur to the Regional Water Authorities, and others to
third parties, but these are not distinguished here. We
distinguish two relevant time periods: short-term ( 12
years; intended to allow time for implementation); and
long-term (13-30 years). We estimated the annual cost
(2018 prices) of the components of each strategy based
on: (i) data from the Regional Water Authorities and
their member control organisations; (ii) questionnaires.
The long-term required effort in each scenario has been
calculated from the available length of waterways in the
country and in a zone along the border, in combination
with best professional judgement on required effort in
relation to muskrat presence (Bos and Gronouwe 2018).
With this information, we calculated the Net Present
Value for each strategy over 30 years, assuming 3%
inflation and 5% interest rate (see table 1). We examine
the robustness of the findings with a sensitivity analysis
changing relevant assumptions and parameter values.
Further details may be found in a technical report (in
Dutch) by Bos & Gronouwe (2018).
The strategy complete removal refers to a nationally-
coordinated effort to remove muskrats from The
Netherlands completely. We assumed that control
effort would have to be maintained for twelve years at
a level of 400.000 hours per year, after which it could
be reduced to 200.000 hours per year, concentrated
along the national borders to prevent recolonisation from
neighbouring countries. This scenario is derived from a
population modelling exercise by van Loon et al. (2017b).
No preventive measures are required. We assume that
muskrats will be eliminated in The Netherlands, and
that costs associated with damage control and damage
recovery will fall to zero.
The second strategy, maintaining numbers at a given level,
works towards low muskrat population size (as indicated
by an annual catch below 0.15 muskrat per km waterway)
by ongoing control. Based on 2018 levels, the effort can
be reduced to about 280.000 hours per year over the
long-term, and no preventive measures are required.
Given that the required low numbers are achieved,
the ongoing costs for inspection, repair, dredging and
damage to third parties (being parties other than Regional
Water Authorities) are limited.
The third strategy, no control, is implemented by investing
in preventive measures along 17.800 km of essential
water infrastructure. For practical reasons, it would not
be possible to implement these everywhere at once, and
these costs are therefore assumed to increase linearly
over a period of twelve years until all investments have
been realised. The costs for preventive measures refer
to installation of mesh wire at a unit value of €45/m (Unie
van Waterschappen 2014). Preventive measures are
assumed to have a limited lifetime and will be written
off over a period of 30 years. Capital costs have been

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activated in order to spread them over the years. Due to
the lack of any control programme, we assume that the
muskrat numbers will rise to be higher than in 2018 (Bos
et al. 2019). Thus, costs for inspection, damage repair,
dredging, zoönotic diseases and damage to third parties
along the remaining lengths of waterway, are expected
to increase. Under the strategy of no control, costs
associated with professional trapping are zero, because
no trapping would be required. There are, however, friction
costs during the transition period, for retraining the current
team of professional trappers.
Other strategies, varying in control intensity in space or
time (Bos and Ydenberg 2011), have not been included in
the analysis, because they had previously been dismissed
as unsuitable or suboptimal (Bos and Gronouwe 2018).
They are considered to lead to higher muskrat population
sizes than publicly can be accepted without large
investments in preventive measures. In addition, they are
intermediate to the three strategies studied and therefore
less informative.
Results
Table 1 summarizes the estimated annual cost (2018
prices) of each component of each strategy, over both the
short- and long-terms. Note that the estimate is given for
year 1 of the short-term period. The cost declines over
the remaining 11 years of the short-term period under two
out of three strategies. Figure 1 displays the trajectory of
annual costs (2018 prices) over the full 30-year period.
The most important contrast lies in the ongoing cost for
labour (muskrat control) and the up-front implementation
of preventive measures. To a lesser extent there are
differences in costs for maintenance of the water system,
especially the restoration of earthen banks, and inspection
related to water safety. Figure 2 compares the distribution
of costs across the main categories in years 1 and 30 of
each strategy.
Under the strategy of maintaining numbers at a given level,
the costs follow the current declining trend to stabilise
at ca. €33 million annually in the long-term (figure 1).
This is in contrast to the strategy of complete removal
in which control effort is maintained at the current level,
until complete removal is accomplished. Under complete
removal long term annual costs are estimated at ca. €21
million. Thus, after 10-15 years of investment, the annual
difference in costs between these two scenario’s amounts
to approximately €10 million. The long-term costs of the
strategy no control are highest at ca. €60 million, which is
mainly due to the high investment in preventive measures.
To a lesser extent, there are expected costs of damage to
the extensive network of earthen banks of waterways that
are not essential for water safety and thus not protected by
preventive measures.
Category Cost item Complete removal Maintaining numbers
at a given level
No control
Short-term
costs in K€
Long-term
costs in K€
Short-term
costs in K€
Long-term
costs in K€
Short-term
costs in K€
Long-term
costs in K€
Control Labour 30.660 15.190 30.660 23.280 30.660 0
Transport and trapping equipment 5.800 2.870 5.800 4.400 5.800 0
Innovation and research 300 150 150 150 150 150
Other 4.630 2.290 4.630 3.520 12.300 0
Water safety Physical preventive measures 0 0 0 0 3.890 46.690
Inspection (labour) 1.350 960 1.350 1.155 1.350 2.660
Damage recovery water retaining structures 1.630 0 1.630 160 1.630 0
Water systems
(maintenance)
Dredging for restoration water system 230 0 230 20 230 460
Restoration of banks 800 0 800 80 800 7.080
Other costs Communication 120 0 120 60 240 120
Zoönotic diseases 0 0 5 0 10 10
Damage to third parties 100 0 100 10 100 2.460
Total 45.620 21.460 45.475 32.835 57.160 59.630
Table1: Annual estimated short-term and long-term costs (in K€; price levels of 2018) of
muskrat control under the three strategies. Short-term costs refer to costs made in year 1.

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Table 2 shows the ‘Total Cost’ (i.e. the sum over the 30
year trajectory of each strategy, in 2018 €) and the ‘Net
Present Value’ (the total cost, with costs after year 1
discounted by 3% inflation, and raised by 5% borrowing
costs for capital investments). Both measures indicate
that complete removal has the lowest expected cost. The
Net Present Value of complete removal, is €740 million,
€832 million under maintaining numbers at a given level,
and €1.3 billion under the scenario of no control (table 2).
The sensitivity analysis, in which we tested for the effect
of changing parameter values (± 20%) on model outputs,
showed that these estimates are robust, and change little
in response to alterations of parameter values (table 3).
Discussion
The important finding of this study is that complete
removal of muskrats from the Netherlands is financially
Figure 1: Development of annual costs in K€ over a 30-year period.
The striped line represents the strategy of complete removal, the
dotted line the strategy of continued control at low equilibrium
and the solid line refers to no control.
Figure 2: Distribution of costs under the three scenario’s in
year 1 and year 30. Panel A represents a strategy of complete
removal, B refers to a strategy of continued control at low
equilibrium and C refers to no control.
Annual costs (Ke)
70.000
60.000
50.000
40.000
30.000
20.000
10.000
-
1 5 9 13 17 21 25 29
Year
Complete removal
Control
No control
rational. We are confident that this outcome of the analysis
is robust, though agree that the exact levels of costs
identified are open to debate. Here we devote attention to
the costs of labour, and the implementation of preventive
measures, because these are pivotal in the analysis at
large. The costs of labour in future scenarios are based
on best professional judgment of staff responsible for
coordination of control at national level. It directly relates
to historical data from The Netherlands (van Loon et al.
2017a). The historical data indicate that control costs are
higher with a higher population density. This important
finding is substantiated by population modelling (van Loon
et al. 2017b) and corroborated by practical experience
in Flanders (Stuyck 2008; pers. comm. M. vanderWeeën)
and in The Netherlands. Because each trap requires time
to set out, and must be checked regularly, the rate at
which waterways can be patrolled (km per hour) is low
when the capture rate is high, in turn presumably due
Table 2:
Total costs and Net Present
Value in K€ of the three
relevant scenarios
Table 3:
Effects of ±20% variation
in parameter values on total
costs and Net Present Value
Parameter Effect
Short-term costs for labour under no control 3%
Speed at which physical preventive measures are installed under no control 3%
Interest rate in calculation of capital charges under no control 6%
Costs for inspections under complete removal 1%
Length of water retaining structures requiring preventive measures under no control
13%
Costs per meter of preventive measures under no control 12%
Costs of third parties under no control 1%
Costs for bank recoveries under no control 2%
Scenario Total costs (K€) Net Present Value (K€)
Complete removal 915.735 739.763
Maintaining numbers at a given level 1.067.210 832.393
No control 1.707.832 1.298.674
Annual costs (Ke)
60.000
50.000
40.000
30.000
20.000
10.000
-
Year
1
Year
30
Year
1
Year
30
Year
1
Year
30
Other cost Water systems Water safety Control
panel Apanel Bpanel C

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to a high population level. As a consequence, control
organisations need to invest less in labour as the catch
rate declines
The costs for preventive measures strongly depend on
the timeframe and the extent (length of dikes and levees,
km) over which they are implemented. We have opted
for gradual implementation (over a period of 12 years),
using mesh wire, along the entire length of essential
dikes and levees (17.800 km). The use of mesh wire as
a preventive measure is among the cheapest options
and we assume that it is technically feasible everywhere.
The use of other materials, such as concrete or steel,
would result in higher costs. Lowering the annual
costs for preventive measures by protecting less of the
essential water-retaining infrastructure will lower public
safety (Bayoumi and Meguid 2011, Ydenberg et al.
2019). Given the fact that the investments in safety from
flooding in The Netherlands exceed billions of euro’s, we
believe that such a compromise cannot be acceptable
for the Dutch Water Authorities, the Dutch government
or the general public.
The results of the analysis presented here are generally
consistent with Bomford & O’Brien (1995) and Clark
(2010), who show that control can be an economically
rational activity, depending on the rate of inflation, the
damage caused by muskrats and the cost of trapping
them. Reducing the population is an investment that can
be regained in the longer term. Reinhardt et al. (2003)
conclude that eradication of muskrats on a national scale
in Germany is likely to be economically sound, taking
into account the maintenance costs for waterways and
water infrastructure, as well as costs for public health,
agriculture and fisheries. Panzacchi et al. (2007) show
that eradication of the coypu (Myocastor coypu) in Italy
presumably has a very favourable cost-benefit ratio.
The successful eradication campaigns for muskrats and
coypu in England (Gosling and Baker 1989, Baker 2010)
were carried out because it was clear at the time that
this investment would prove effective in the long run. In
retrospect, that is also the case.
In addition to the financial arguments presented above,
the difference between each of the strategies has also
been weighed for other criteria, as has been mentioned
in the introduction (Bos and Gronouwe 2018). Each
Regional Water Authority has been informed and has
debated the pro’s and con’s of the different strategies.
Finally, the information has supported a policy decision
by the Dutch Water Authorities in June 2019 to change
the previous management objective from maintaining
numbers at a given level to complete removal.
Now there may be a strong economic incentive for
complete removal, as indicated by the large difference
in Net Present Value between the strategies (table 2),
but this has little relevance for the individual trapper,
unless their job perspectives are taken into account
properly. Thus, given that the success of any muskrat
control programme is to a large extent dependent
upon competent staff that works in a coordinated and
motivated way, the personal interests of the trappers
need to be taken serious by the control organisations.
If not, the analysis presented has limited relevance.
Conclusion
We have shown, based on a cost-benefit analysis,
that complete removal would be a financially rational
strategy for Dutch muskrat management. Under compete
removal the investments required are lower than future
costs to maintain a strategy of control at low equilibrium
population size, or to apply a strategy of no control
coupled with preventive measures to protect against
flooding and maintain public safety.
Acknowledgements
We highly appreciate the input from many of the
professional trappers for their contribution to our
understanding of professional muskrat control. This
analysis has been supported by a project team of the
Dutch and Regional Water Authorities with members
J. Boontjes, S. Dol, R. Kleinman, D. Moerkens, H. Post,
B. Rietman en E. de Wit. We also thank several experts
for their willingness to share their insights: S. Baker,
P. Robertson, M. Vanderweeën, M. Maas en P. van
Tulden. R.C. Ydenberg gave very useful comments on
an earlier version of the manuscript.
ABSTRACT
Door graverij in waterkeringen bedreigen muskusratten de
Nederlandse waterveiligheid en daarom wordt de soort al
sinds de vestiging in 1941 bestreden. Recente Europese
wetgeving met betrekking tot invasieve exoten verplicht de
lidstaten van de EU om passende maatregelen te treffen
ten aanzien van het beheer van muskusratten, afhankelijk
van kostenefficiëntie en socio-economische factoren. Ook
dient het alternatief van een beheerstrategie waarin geen
muskusratten worden bestreden te worden overwogen.
Mogelijke strategieën in het muskusrattenbeheer zijn (i)
bestrijding waarbij de dichtheden op een bepaald laag
niveau worden gehouden; (ii) geen bestrijding; (iii) volledige
verwijdering. In dit artikel schatten we de kosten voor arbeid,
de kosten voor het repareren van schade aangebracht door
muskusratten en investeringen in preventieve maatregelen
voor elk van deze strategieën op korte en lange termijn in.
We concluderen dat de Netto Contante Waarde (uitgaande
van 3% inflatie en 5% rente) het laagst is voor ‘volledige
verwijdering’. Volledige verwijdering van de muskusrat is
realistisch en haalbaar, maar het succes van deze strategie
is afhankelijk van competente bestrijders die gemotiveerd
en gecoördineerd kunnen werken.

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