The eradication of muskrats and coypu from Britain

Abstract

Introduced vertebrates can cause massive environmental damage but most attempts to remove feral populations have failed. This paper discusses the eradication campaigns against feral muskrats, Ondatra Zibethicus and coypus, Myocastor coypus in Britain. Both species were introduced in the 1920s to be farmed for pelts and feral populations became established following escapes. The risk of environmental damage by muskrats was well known from Europe and an eradication campaign started promptly in 1932 making use of overseas expertise and a control strategy designed by pest control specialists. The campaign was brought to a successful conclusion in 1939 when at least 4388 muskrats had been killed. In the 1930s, few believed that coypus would cause significant environment damage and early trapping efforts were inadequate. An early campaign achieved only limited success partly because of the lack of biological information. The eradication campaign which started in 1981, was based on a long tem study of population ecology. The effect of trapping and cold weather was quantified and detailed population simulations were used to plan the numbers of trappers, the time needed for eradication arid thus the likely cost of the campaign. An incentive bonus scheme was designed to overcome the problem that trappers would be reluctant to work themselves out of a job. Trapper deployment was planned using capture/trapping effort ratios and progress was checked by Ministry of Agriculture field staff. The muskrat campaigns succeeded because technical information to help plan the work was available and because action was taken quickly. Where an introduced population is well established, as with coypus in Britain, a closely integrated programme involving applied population ecology and a well-planned control organization may be essential for succesful removal.

Full text

Biotogical
./ournal
oJ the
Linnean Societv
(1989),38.' 39 51. \\'ith 5 figtrrcs
The eradication of rnuskrats and coypus frorn
Britain
L. M. GOSLTNG
AND S..1. BAKER
Oolpu Research [,aboratory, Ministry of Agriculture, Fisheries and
Food,
Jultiter
Road,
Jtforwich lVR6
6SP
Introduced vertcbrates can cause massive environmcntal damagr: but most att('mpts to rcrnovc lcral
populaticlns havc failcd. 'l'his papcr discusscs thc cradication campaisns against ft'ral muskrats,
Ondatra lihethicus,
and cclypus, Mytcastnr colltus
ttt Britain. Both spccics rvcrc introduced in thc 1920s
to bc larmcd for pclts and leral populations became cstablisht'd follorving cscapcs. 'l'hc risk of
environmental damagc by muskrats was rvcll known liom F)uropc and ern cradir:atit,n t'umpaiqn
startcd promptly in 1932 making usc of ovcrscas cxpcrtisc and zr control stratcg,v dcsisncd bv pcst
control specialists.'l'he campaisn was brought to a succcssfrrl conclusion in 1939 when lrt lcast'l38fl
muskrats had becn killed.
In the 1930s, lew believed that covpus would causc significant cnvironmcntal damaser and carlv
trapping ellr,rrts were inadcquatc. An carl,v campaisn achievccl onlr. limitt'd su('('css1 partly bct':rusc
of thc lack of biologir:al information.'l'he cradication campaign rvhich started in l98l, rvas bascd orr
a k;ng tcrm study ol'population ecolclgy. 'l'hc cffcct of trapping ancl cold lvcatht'r rvas quantificd and
dct:rilt:d population simulations wcrc uscd to plan tht' numbcrs o{- truppcrs, tht' timc nccdcd lirr
eradication and thus thc likcly cost of thc t'ampaign. An incentive bclnus si'hcme rvas dcsignr:d t<r
ovorcomc thc problcm that trappers woulcl bc rt'luctant to n'ork thcmsclvcs out of a job. 'l'rappcr
dcplclymernt was planned using capturc/trappirrg t:flirrt ratios and prouress was t'ht'ckcd b,v \'Iinistry
ol-
Asriculturc ficld stafI.
'l'hc mrrskrat campaigns succet'dt'd lrct'airse tet'hnir-al inftrrmalion to hclp plan the uork u'as
ar"ailablc and llct'aust'action was takcn cluicklt.. \\'hclc an intrclduced pclpulation is rvcll t'stirlllislrccl,
as u'ith ('oypus
in Britain, a closclv intcsrated pro{rrumm(' inr'olving appliccl populatirln ccologv and
a w-cll-planncd t'ontrol organizatiorl [laY bc csscntial {irr sr-rcccssfirl
rcrnoval.
KFIY WORDS: Muskrats coypus population ccologv craclication Britairr.
I]ON'I'EN'I'S
Introducti<ln
Brrckgrorrtttl lrr l lrt' slrt'r'it's
'l'hc introduction of muskrats and covpus to Britain.
Dantaq,'
Pcrccptiorr o{'tl.re
problcm :rnd thc dif}irins rcsponscs
t() muskrri{s and cor'pus
(lorrt
rrrl
stlatt'git'.
\lusknrts
(lor llrrs
'l'rappine tcchniclu
'l'rapper motivilti()
Cclnclusions
.\r'krton lt'dg('m('nls 5(l
Rtli't
cttt
t's 50
IN'fRODUC]'I'ION
'l'hc introduction of mammal specics outsidc thcir native ranse can result in
severe damase to agriculture and the natural environmcnt. Nlany native spccies
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+0 L. \{. GOSLTNG
AND S.
J. BAKER
of plants
and animals
have been
brought to extinction by introduced species
and
this process
continues.
Most attempts to remove introduced species
have f,ailed, a
British example being the attempt to eradicate mink, MusteLa uison,
which started
in 1965, employed nine trappers, and was abandoned in 1970 (Thompson,
leTl).
This paper is about the eradication campaigns against muskrat, Ondatra
<ibethicus
(Warwick, 193+, 1940; Munro, 1935) and coypus, Mltocaslor coypus
(Gosling & Baker, l9B7;
Gosling,
Baker & Clarke, 19BB)
in Britain. The muskrat
campaign was broueht to a successful conclusion
in the 1930s.
Further checkine
is needed befbre a final judgement can be made in the case of' the reccnt
campaigns
asainst coypus,
but, at the time of writing, only two isolated coypus
have been detected over the past 21 months. It seems
unlikely that a brecding
population rcmains and trappine ceased
inJanuary 1989. Herc wc outlinc the
reasons for thcsc succcsses
so ttrat, hope
fully, the lcssons can bc applied
elsewherc.
BACJKC]ROUND'I'O'I'HL, SPE,C]IL,S
Muskrats are myomorph rodents which reach up to about 1.8
kg in weight
(Walkcr, 1983). Many muskrats brought to Britain came from Canada
(Warwick, 1934; Munro, 1931).
Their breeding is seasonal and in Britain they
had two or rarely three litters between April and October (Warwick, 1940).
Gestation is 25 30 days (Walker, l9B3) and litters averase between five and
seven youns (Danell, l97B;
Clay & Clark, l9B5).
Coypus zrre hystricomorph rodents, native to South America which weigh
6 7
kg when fully srown; their biology is described
elsewhere
(Newson, 1966;
Gosling, l986; Gosling
& Baker, in press). Coypus are poorly adapted to freezing
conditions (Newson, 1966;
Goslins,
l98l) and, in Britain, would probably have
a slower rate of population increase than muskrats, particularly in years
lollowing cold winters.
Both spccies arc semi-aquatic and swim from daytime resting sites
in nests and
burrows to feeding areas in or near water.
THE, INTRODUCTION OF N{USKRA'I'S AND COYPUS TO BRITAIN
Muskrats and coypus are f,armed for their pelts and many have been exported
lrom their native ranges for this purpose (Gosling & Skinner, 1984). They were
imported to Britain during the late 1920s
(Lever, l9B5). Over 87 muskrat farms
were quickly established throughout Britain although most were small
(Warwick, 1934). Enclosures
were often inadequate and escapes were common.
All muskrat {arming ended by 1933. Over 50 coypu larms were established,
mostly in the south and east of England. Nearly all had closed by 1939
but by
this time many had escaped
(Laurie, 1946).
It is sometimes overlooked how difficult it is lor animals to colonize new areas.
Apart from any novel hazards, the animals may be unused to living in the wild
and may simply escape without potential mates. The success of furbearers in
invading new habitats may be because they sometimes escape in sufficient
numbers to overcome these problems. Whatever the reasons) separate

E,RADICATION O}'MUSKRA'I'S AND C]OYPLIS 4I
populations of muskrats became established
in the Severn valley in Shropshire,
in a central area of Scotland and in two smaller areas
in Sussex and Surrey.
Accurate estimates of the muskrat populations are not available but the
numbers
killed give some impression.
In fact these numbers are onl\. those
killed
in the official trapping campaiens: for erample,,
the Scottish
population started
from escapes in 1927
and zrt least 140
anirnals
were killed before the campai*=ns
started
in 1932
(Munro, 1934;
Lever, l9B5).
The total killed in Scotland was at
least I l0B. At least
3052
muskrats
were accounted for in Shropshire (FiS.
1) and
a further 228 were killed in Sussex and Surrev. The total causht lrom all {bur
sites
was
at least
43BB
(Warn,ick,
1934, 1940)
One group of coypus becamc
cstablished
near Slough,
remained at a low level
and disappeared
without any known control in 1956 (Norris, 1967b). A second
group probably originated lrom three farnrs near Norwich, close to the Rir,,ers
Yare and Wensum (Laurie, 1946).
The best known escape
was from a farm at
East Carlton in 1937 (Ellis, 1960). The East Anglian wetlands have many
similarities to the coypus' native swamps in South America and coypus soon
increased
and spread
into adjoinine rivers. At their peak in the 1950s most were
in Norfolk, Suffolk and Essex but significant numbers had also spread to
neiehbourins
counties (Davis, 1963;
Norris, 1967b).
'fhe size
of the coypu population is
known in detail after 1962
(Gosling, Watt
& Baker, l98l, and see Fig.5) but earlier information is anecdotal. The
population started in about 1937
and grew progessively with the first complaints
about darnaee in I9+3 (Laurie, 1946). There were major checks in the severe
winters of 1946147
and 1962i63.
In the late 1950s,
there was
a dramatic eruption
in numbcrs: Rabbit Clearance Societies were grant-aided to control coypus
and
loczrl drainage organizations increased their efforts to limit the increasins
damzrse.
About 100000 coypus were killed in 1961-62
without {reatly affecting
the population (Norris, 1963).
Centrally-organized
control started
in 1962
and
continued at various levels
until the start of the eradication campaign in 1981.
Maximum numbers were once believed
to be 200000 in the late 1950s
(Norris,
1967b) but this may have been an overestimate.
DA\{A(;F]
Coypus and muskrats
are seneralist
herbivores
which damase a wide varietl.
of native plants and crops in their introduced ranges. Coypus often destroy
monocotvledonous
plants bv selecting
basal nreristems
(Goslinq. 197+)
and large
areas
of reedswamp
were elirninated
during the late 1950s
(Ellis,
1963; Boorman
& Fuller, l98l). C)ovpr-rs
also firvour particular species and sorlle, inclr-rdine
Butomtts
umhellatus, the flowering rush, and (,'icuta
ttiro.ra, cowbane, becanre
extremelv rare (Ellis, 1963). Muskrats in North America also cause extensi','e
damase to marshland vesetation
when at hieh population densities
(Errington,
1963).
Fortunately, their numbers in Britain did not reactr
thc lcvcls
at which
large scale
damage to native plants n,ould have occurred.
Both species also damase a variety of crops: sugar beet are a fbvouritc lor
coypus but they also
eat cereals,
brassic'zrs and other root crops (Norris, 1967b;
Anon., l97B). Muskrats did not cause sienificant damage to crops in Britzrin
(Warwick, 1940)
.

42 L. N,r.
GOSLTNG
AND
S.
J.
BAKER
The most important damage in economic terms is caused by burrowins. In
the Netherlands where 289 ll6 muskrats were trapped durine l9B7 alone
(Anon., l9BB) there is extensive damage to the land drainage system.
Damase
through burrowing was also
recorded to the banks of'the River Severn
in the
1930s
(Warwick, 1940). The cost of repair in the Nethcrlands and the risk of
increased
damage
are
so
great that a current annual cxpenditurc of seven
million
pounds on control is considered cost effective (R. van Oostenbrusse, personal
communication). Coypus dig exte
nsive
burrow systems
into the banks of ditchcs
and rivers which disrupted drainase systems and posed the risk of flooding in
low-lying East Anslia (Cotton, 1963).
PF]RCI.]P'I'ION
O!..fHE PROBLENT AND THI.] DI!'!'I']RF]NT
RESPONSE,STO MUSKRA'I'S ANI)
COYPUS
The costs of future damape, both in economic terms and the natural
environment, cannot always be assessed at the early stages of an introduction.
This affects whether or not prompt action is taken. Fortunately, it was already
known from experience in Europe that muskrats
could cause serious damage in
Britain and this information was used by staff of the British Museum (Natural
History), notably the zoologist
M. A. C. Hinton, to persuade
the Ministry of
Agriculture and the 'I'reasury that money spent on eradicating muskrat would
be worthwhile (Sheail.
l9BB).
These initiatives also led to the Destructive Imported Animals Act of lgg2
which allows control over the importation and keeping of' named animals.
Initially muskrats were only allowed to be kept under licence
and in 1933 all
keeping was prohibited. By this stage strinsent keeping requirements had
reduced the holdings to six (five in England and one in Scotland) and
compensation
was paid to close
these down. The act also authorized
the Ministry
of Agriculture to carry out the eradication campaisn.
Although the coypu population was
growing steadily at thc same
time, coypus
were only added to the list of animals proscribed under the Act in 1962. There
are a number of reasons for this. The first was that relativcly little was known
about the biology of coypus and their potential for damage in the 1930s.
Coypus
also have a lower potcntial rate of incrcase
than muskrats,
particularly over cold
winters, and so early events were no so alarming. The advice from Germany,
where coypus had also been introduced, was that coypus would not cause
problems (Sheail, l9BB). Although true in Germany where the cold continental
climate keeps
coypus at a low level,
it is not the case
in Brtitain where winters are
milder. Coypus were also believed to be easy to catch if they escaped (Carill-
Worsley, 1932) and Hinton commented that they were unlikely to become a
serious pest (Sheail, 19BB).
As a result, although there were some warnings about the potential dangers
(Stearn, 198l), little was done to prevent the establishment of coypus. Some
trapping was carried out by trappers employed by the Norfolk War Agricultural
Executive Committee and 193 animals were caught between l9+3 and l9+5
(Laurie, 1946). But this trapping was localized and it was not continued. After
the cold winter of 1946147
when numbers were ereatly reduced there must have
seemed even less need to do anythine.

ERADICA'I'ION O! MUSKRA]'S AND COYPUS
CON'I'ROI- S'I'RA'I'I]G
I T,]S
Mu.rkrals
The muskrat campaign in Shropshire was planned by Hinton and a N,,Iinistry
of Agriculture pest control specialist,
E. C. Read (Public Record Office, MAF
++ll+). Fewer details are availablc frorn other areas but presumably the
experience
from Shropshire
was put to use.
Here, an oblong area was defined
usittg
the locations
of the outerrnost
known muskrats
in each area and divided
into squares each of ten square
miles.
A trapper was deployed to each of these
square.s
to survey and trap. This intensity of'trapping appears to have been
decided on the basis
of common sense
estimates
of thc amount o{'sround one
man could cover. Nlotorized transport was very limited: one van lvas
available
in Shropshire
but most trappers
either bicycled or walked to work.
The initial area trapped was
just o\rer
300 square miles (see
Fig. l). This was
enlarsed by 100 square miles in 1933 and later enlarsed again as outlyinu
muskrats wcre found. However, the maximum number of trappers \vas
39 and so
presumallly traPpers worked over larger areas at later stages
of tlie exercise.
Some men became specialized
as 'searchers'
to look for outlying muskrats.
It is
also
stated in the historical
documents (Public Record Oflrce, MAF 44114)
that
effort should be proeressively concentrated alons rivers where the population
was most
dense, as
the outlying :rnimals
were cleared. How far this
intention was
carried out is not know.
After early peaks
of captures, the numbers of muskrats declined steadily to
zero (Fie.
2). The last anirnals
in Shropshire,
Sussex
and Surrey we
re caught in
1935,
and the last
in Scotland
in 1937
(Warwick, 193+,
1940).
The last
muskrat
in England, possibly
an emisrant from the Shropshire
population, was
caueht in
Cheshire,
in 1936.
Figure l. 'l'hc distribution of rruskrars cauqht in
Sirropshirc. After Wanvick 1
1940).
1.'
..o ooo
O o
OOo\O
.OO. rOO J-5,o:-:---'
Welshpool . o^p }-+---e-f o )
Number
krlled
per
squo
re m rle
O =lo+
o -<lO
1932
and 1933
in the rrapping
c.ampaiun
in

L. M. GOSLTNG AND S.
J. BAKER
350
300
250
200
r50
roo
Scotlond
| 932 i933 | 934 1935 |
936 1937 t938
Troppers ....'.. Muskrots killed
l'igure 2. 'l'he number of trappers employed durins the campaigns against muskrats in Shropshire
and Scotland and the numbcr of rnuskrats c:ausht.
As numbers declined there was financial pressure
to reduce the trappers. Men
were gradually dismissed but with a sufficient lag that there was some effort in
the later years of the campaign when muskrat numbers were low (Fig.2). In
Scotland some trapping was maintained until March l93B even though only two
muskrats were killed in the last 28 months of the campaign.
Colpus
As damage by coypus increased alarmingly in the late 1950s there were
widespread calls
for official action. Two initiatives were taken by the Ministry of
Aericulture, Fisheries and Food (MAFF), both in 1962:
the first was to establish
the Coypu Reserarch Laboratory in Norwich and the second to launch a
trapping campaign against coypus. The aim of the campaign was to reduce
coypus and confine the remainder to the Norfolk Broads (Norris, 1967b).
6a)
!(D
=
Io
o
Ia
=
E
o
(D
-o
E
=
z
t50
U'
o
o
O.n
ov
!
o
bt5
!E
z
to roo
50
Sh
ropsh i re

ERADICATION OF MUSKRATS AND COYPUS +5
By necessity, the control strategy adopted in this initial operation was
organized in advance of any results from the laboratory. The area containing
most coypus (nearly all of Norfolk plus north Suffolk, see Da'u,is, 1968) was
divided into nine sectors
which were trapped successively by up to l4 specially
employed trappers starting at the outside and working north-east towards the
Broads. A large amount of trapping was also carried out ahead of the campaign
by Rabbit Clearance Societies and landowners. Behind the sector organnation,
permanent MAFF staff attempted to clear what were regarded as outlying
colonies
(Norris, 1967b).
It is easy to be wise after the event and in retrospect flaws are evident in this
strategy, notably that the main trapper force spent too much time in clearine
low density areas rather than attempting to maximize capture rates. While the
effect
of immigration into cleared areas was considered it was not given sufficient
weight and an increasing proportion of time was devoted to controlling
reinfestation of areas
which had been trapped previously.
Events were also complicated by the winter of 1962/63, the coldest for over
200 years,
when about 90o,6 of the coypu population perished.
At the end of the
campaign in 1965 the main objectives
had been achieved but in the absence of
basic demographic knowledge it was not clear to what extent cold weather or
trapping were responsible.
Perhaps the main achievement was to keep numbers
down to the low levels caused by the cold winter. But in the absence of an
undcrstanding of the relationship between trapping effort and population
response
the trappers were reduced to fivc men in 1965. This force could not
prevent an eruption in numbers when a run of mild winters occurred in the early
1970s
(Gosling et
al.,
lgBl).
In contrast, the l98l eradication
can-rpaien was
designed usins the results
of a
lonq term investigation
of coypu population ecolosy. Over 30
000 coypus from
the continuing trapping operation were dissected to obtain information about
reproductive biology and other data needed to understand why coypu numbers
varied. A priority was to estimate population size and, since most adults are
eventually killed by trapping, this could be done by reconstructing the numbers
alive
, each month in the past, from the numbers and ages
of the animals caught
in the control operation (Gosling et al., l98l). It was then possible to quantify
the short term effect of trapping on the adult sector
of the population and assess
the relative importance of cold weather as a limiting factor. Except in
exceptionally severe
winters, cold weather has its main effect through reduced
breeding success
and juvenile survival. Trapping intensity proved to be a more
important factor in explaining change between years (Table I ) and, together,
trapping and winter severity explained B2o/u of the variation in the change in
coypu numbers. By the late 1970s
enoush was known to provide an analytical
backsround for attempts to build simulation models of the population (Gosling
& Baker, l9B7).
These simulations were used to assess the effect of employing different
numbers of trappers on the response
of the population under various climatic
circumstances.
A range of simulations (FiS.3) were considered in the late 1970s
when an independent committee, the Coypu Strategy Group, took a fresh look
at long term policy for coypu control (Anon., l97B). The option recommended
and subsequently accepted by the MAFF was an attempt to eradicate coypus
using 2+ trappers. Taking into account reasonable expectation of improved

cloeflicient
oi- deterrninariorl
: 0.82: F: 38.6: d.t 2. 14.
trapping techniques, and an average number of cold r,r,inters, it was decided to
attempt eradication within ten years.
The existing Coypu Control Organization was reconstituted and the
eradication campaign started in l98 L Biologists
from the Coypu Research
Laboratory save tcchnical guidance throughout the campaien. An example is
the scheme used to deploy trapping effort (Fig.
a). Trapper deployment was
more flexible than in the muskrat campaign because each trapper had a car.
Recommendations
were made every three months usins past capture:trapping
L. \r. GOSLTNG AND S.
J. BAKER
'l'aurn l. Multiplc resression analysis of the effect of trapping
intensity and winter severity on the change in the mean number
ol' adult coypus alive in successive years. The number of adult
coypus rvas
obtained from a monthly reconstruction of the popu-
lation (Gosline et al., l9Bl) averaged for each year betrveen 1970
and 1987. \\'inte r ser.erin'
is expressed
using
an inder rvhich rvcights
and accumulates runs of freezine
days each winter (Gosling, l98l);
trappine intensity is the numbe r of trap-nights in vear y wherc the
changc in coypus numbers is calculated between y and y * I
lndcpcndent r,'ariable Beta t value P
Trapping ir.rtcnsit,v
\\:intcr scvt-ritv -0.73 6.06 <0.001
- 0.26 - 2.+t 0.03
O123456
Yeo
r
Fisure 3. Simulations of adult fcmalc coypu numbers in relation to various trapping intensities.
'fhesc simulations arc sonre of those considered
as options by the Coypu Stratesy Group in 1977
and
doc:umentcd
in thcir l97B rcport. Linrited in{brmation about thc cfl'cct
of c<-rld
weathcr rr,as ar':rilablc
in 1977 and the sinrulation with 24 trappers and occasional cold n,inters simply replicates the eflect
of the 1975/76 u'inter. When further data rvere collected in subsequent years it became possiblc to
explcrre the effect of cold weather in more detail (Gosling et al., l9B3).
l9 troppers
| =
Cold
wrnter
-- \*---z\ 24 tro21ers
l'.
-\.- r """""" ".. 24 troppers
L ,34troPPers
I
o
-
I
qO
E
E
o
-)
!
A
o
o)
E
J
z

ERADIC]ATION O!' MUSKRATS AND COYPUS
!'igurc '1.
'l hc distributir-,n r.rl'cot'ptts
caught rluring tht' igBl l9B9 t'l'arlication campaien ancl th<'
rcgionsust'dfbrstratcgicclc'plovnrcnto['tra1>1>irrec{Jirrt.']-he
cilt'lcsontht'm:rl)r('J)rcs(nl
<0.1.0.1
1.0 ancl > 1.0",, <rl
tht' t()titl
nunrl)<'r
t'aught (n:Ii4ti221. I.,flirrt
nirs clt'plor<'cl in lrropr)ru()n
r()
"vciehtt'd ('iIJ)turc:trilpping t'flirrt riiti()s. Althor-rch s()rn('tral)l)irrs u.as carrit'cl orrt tlrroriglrorrt tht'
ar('a th('initial aim. using stronglr ueightccl riitit.rs.
u'lrs t()
(()n(('ntrirt('cllirlt in tht lrich clr'lrsitt
I't'q-iotts'
(ll arrcl
(12, to tnaxitnize nroltalitv ancl rrrininrizt't'rrri11r'utir)n
t() l)(,r'il)ll<'r':.rl
irn.;1s.,\s tlrt'
'('ore'areas rvere reclut't'd,
thc rvciqhting lltctrlr was rcclu<'r'cl
to dcplor trappillq <'liirrt
rnrtrt'<'r.r'1ly
arrrl thus ltav rclativclv rn<trt'
attcntion t<t thc pcriphcrr'.
effbrt ratios in eight stratesic resions (Fig. a) and available eflorr was
apportioned in proportion to the macnitude of the ratio in each resion. This
ratio was weishted to different extents so that effort could bc conccntratcd on
high density areas
early in the campaign arld deployed more widely larer on.
The laboratory also monitored progress
by field checks,
by reconstructine the
population (Fig.
5) and usins simulation techniques
to detect
and analyse
trencls
(Gosling & Baker, l9B7). Numbers were reduced from around 6000 adults in
l98l to near-zero
in 1987. T'he campaign was helped by an above averase
number of cold winters. However,,
cold weather by itself
would never eradicatc
all coypus
in Britain, as
shown in the exceptional
winter of 1962/63.
The main
effect of cold weather in the campaiqn was to lower recruitment to the adult
sector of the population by reducing breedins succcss
and juvcnilc survival
(Gosling & Baker, l9B7) thus allowins trapping to make a relatively large
im"pact
on adult numbers.
TRAPPING
TECHNIqUE,S
'I'he main technique used in the muskrat campaisn was leg-hold trapping
using smooth-jawed traps, possibly followine the Canadian experience of the
field manager of the Shropshire area. The traps had long chains so rhat rrapped
animals could get to deep water and drown. Where this did not happen io-.
animals escaped with severed legs. There was also a large toll of non-target
+7
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4B L. N'I.
GOSLING AND S. I. BAKER
30
to
25
2ao
(J
()
I
1 5 a
-c
I
c
I
o
o
a)
o
An
lo
o-
>
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84 8887
86
858382
8I80
7B
77
76
75
747372
7l70
Yeo
r
...... Coypu Trop-nrghts I CotO
*,nr"r.
ligLlre 5. Recoustnrctctl rtumbt-rs
of adult c()\'l)us
ilnd trappir)g intensitv clurilq 197() l98B.'l'hc risc
in thc callv l9T0s was duc to a run o1 rrrild n'intcrs rvith inadecluatc trappins. 'l hc cradi('atioll
t:ampai{n started ofliciallv in l98l although the trappins lirrce had bccn built llp over thc prcvic'rus
r,r'itrtcr.
'I'ht' hieircr than avt'rase rtut.nbcr o{ cold u'intcrs probaLrlv accclcrat('d thc campaign lrtrt
cold rvitrtcrs
lrv thcmsclrcs u'ould r]cvcr rcmovc covpus liom Britain.'l'ht'pripulation rt'tonstruttiorr
tcclrriique is
documented br Gosli:ng
el
al. il98ll.
captures
with over 6500 mammals and birds killed in Scotland alone (Nlunro,
l
e3s).
Both campaiqns against
coypus used
cage trapping (Norris, 1967a). Each trap
was
ilrspected
daily and any coypus shot. This technique
has the advantaqe
that
non-target animals can be released unharmed. Apart fronr being ethically
desirable, this also made it possible to get the cooperation of all landowners,
including those
with conservation and game
interests.
Improvements to trappins
techniques were introduced by the Co,vpu
Research Laboratory, including the
use of traps on baited rafis. Experiments
showed these
were at least
50il; more
effective than traps set on land and fbllowing this work over 600 were deployed.
Non-tarset captures were also significantly
reduced (Baker & Clarke, l9BB).
'I'R.{PPI.]R
}I O'f I\'A1'I ON
Why should trappers try to succeed in an eradication exercise when in doine
so they lose their jobs? l'his problem is often raised but, to our knowledge
, the
attempt to o\,ercome it in the coypu eradication
campaign is the first time it has
been tackled in a pest
control operation. The scheme devised
\ /as
to restrict
the
funding to a maximum of ten years and pron-rise the trappers a bonus of up to
three times their annual salaries
if they succeeded
in eradication. The scheme
was designed so that after six years the bonus would gradually decline, thus
encourasins an early end to the campaign. Most trappers worked hard to

E,RADICA'I'ION OI MUSKRA'I'S AND COYPUS +9
achieve the maximum bonus and we believe that this incentive was an essential
element
in the campaign.
If the bonus scheme was essential
in coypu eradication then why did the
campaigns against muskrats succeed, even when the numbers of trappers were
reduced as the campaign proceeded?
The reasons
may be partly technical and
partly socioloeical. The muskrat-trapping technique was designed so that
animals were killed as they were caught and while this does not seem to have
worked universally it would have limited the opportunity for trappers to control
events.
Secondly, societal values in the 1930s
were different and perhaps people
were more ready to follow instructions than today, even when it meant their
eventual dismissal.
Bounties were paid for both muskrats (Sheail, 19BB) and coypus at various
times and pelts were also sold lor profit. It is sometimes believed that such
rewards encourage people to reduce pest numbers and this may be true under
restricted circumstances.
More often these
practices will encourase husbandry of
the animal to ensure
a continuing income. Fortunately, bounties for muskrats
were discontinued before the start of the official trapping campaigns and
although pelts were sold durine the campaigns, the profit was used to offset
costs,
not to reward the trappers. The sale of coypu pelts by trappers was lorbidden
during the coypu eradication
campaign but, in any case,
demand was
low at this
time and the potential rewards small in relation to the possible eradication
bonus.
CONCLUSIONS
What are the lessons
of these two campaigns lor other attempts to remove
introduced species? It must be borne in mind in considerins these that there is a
justifiable reluctance on the part of those who must finance any operation to take
action unless
(l ) they believe there is a problem, (2) they think it can be solved
and (3) they know, within limits, how much it will cost.
The muskrat campaien
was undertaken because
important elements of this information were available
from experience in Europe and Canada. However, this might not have been
sufficient without the skilled technical advocacy of M. A. C. Hinton. The
campaigns were organized using common sense principles and althoueh there
was little technical input into the field exercise, there was the great advantase
that action was taken quickly. Experience in Europe shows how remote the
chances of success would have been if the populations had been allowed to
spread more widely.
T'hc coypu eradication campzr"ien
would simplv not have becn undcrtakcn
without detailed technical assessments of'the efibrt and costs
required and the
likely chances of'success.
These assessments were onlv achiel'ed b,u o long term
study of population ccology, tarqeted to a particular control application. 'l'his
study included operational experience as well as applied ecolosv :rr-rd
it is
sienificant that the arsuments for such practical details as the inc'entive
bonus
scheme crame
fiom biologists.
One reason that the eradication of muskrats and coypus
could be considered
was that their populations were confined to reasonably small areas with no
immigration. However, even
in other situations
where eradication
is
not possiblc,
for example where large scale immisration from a neighbouring country

50 L. N,t.
GOSLINC; AND S.
J. BAKER
continually replenishes
a controlled population, the principles which emerge
in
considering
the removal of-muskrats and coypus can be applied. Indeed, these
principles may be applicable to the sensible
manasement of any population of
vertebrates. Most important is the need for applied population ecology,
so that
the role of various factors,
including control techniques
in limiting the subject
population can be evaluated. Next there must be a target population density
over a defined area which can be independently mor"titored. This target could be
eradication or a density low enoueh to preclude environmental damage. In the
coypu cradication exercise,
the control organization
was centralized because
the
aim was to eradicate a single population but a coordinated systen-r
of local
organizations might sometimes
be more appropriate. Next, the peoplc carrying
out the control in the field must be rewarded for achievir-rg
their objective and
not for failing to do so (as
with bounties).
However, where incentive schemes
are
used, results need to be independently monitored. In thc campaiun against
covpus, estimates
of trappins eflbrt, population trends and the results of-field
checks were passed
to the control orsanization and this
trelped
both directly and
in stimulatine the efforts o{-the
trappers.
'I'his
sr-rrt of inlormation is also
nceded
bv the cor-rtributinq orsanizations to evaluate thc results of' their financial
invcstment
at each
stagc of the exercise.
This sort of intcraction between applied biologists, policy makers, funding
ors.anizations and a well-coordinated control operation has potential for wide
application in any extensive
pest control operation. It may be essential
fbr the
removal of a well-established
introduced mammal.
AC] KNO\\IL E, T]G
I] M I-,N' I'S
'Ihe Coypu Control Orsanization supplicd inlbrmation and material for post
mortem exzrmination and the Public Record Officc, the Scottish Record Office
and colleasues in the department of Agriculture and Fisherics
provided access
tr,r
files
on the muskrat campaien.
Comments by'I'. Wanvick and D. C. Drummond
improvcd an earlicr draft.
Rlrl'tlRI'lNCI]S
ANON., 1978.
Ct1ypu: Report
q/'the
Co,ypu,9tralegt Group. Ministry of Agriculture, Fishcrics
ancl
l'oocl.
ANON., l9BB. Jaari'ersLag
1987. Pcrmanent t:r-rllcsc
van overlcs rnuskr;rttcnbcstrijdine's
CJravenhasc,
Netirerlands.
BAKER, S J. & ClLARKlr, C. N., l9BB.
Cage trapping coypus
(,M,yocastor
crypu.t) on baitcd ra['ts.
.'Journal
of
Altplied
Ecolog1t,
25: +l 48.
BOORMAN, L.A.& I''LILLER, R. \{., l98l. The changing status ol'rt'r:dswamp
in thc Norftrlk
Broads.
.lf
ournal of Applied
llcologt,.
lB: 241 269.
C]'\RILL-WORSI,F,Y.P.F]''l..,l932'.\Iirr.lirr.nlirlNtlr1illk..I'rutt'tutlitttt'stlJ|he'\orlil
,\otielt,13:
lOir I15.
(lL,'\Y, R.'l'. & CILARK, \\'. R., l9fl5. Dt'nrosraPhy ol-muskrats
orr
tlrt ul)lx'r MississiPPi ritrr..JourrnL
of
l'I:ildli/e Mana{emt'nt. 1.r/. 883 U90.
CO'l"l'ON, K.E., 1963.'l'he
coypu. Riz,er Board Association
Tearbook, II: 3l-39.
DANELL, K. D., 1978.
Population dynamics of the muskrat in a shallow Swcdish
lakc. Journat
o_f Animat
Ecology,
47; 697 709.
DAVIS, R. A., 1963.
f'eral coypus in Britain. Annals of Applied
Biologlt,5l: 315 348.
l)A\/lS. R. r\., 196U. A ll()r('()n
thc li'ral
(.(rvJ)u
irr thc L.'nitt'd Kinq(tom,
1930 1967.
I.,uropean
anrl
trIetliterranerut
Plant Proteclioil
Oreaili.ta!ion
I'uhlitatiort. ,\'eric.t
.1
. 17: 6'3 66.
ELLIS, E. A., 1960.'l'he
coypu
threat in llast
Anglia. Country LiJc,
Decembar: 1590 1591.
F]LLIS,F].A.,l963'Stlrnec1Iit'tstl|selt't'ti'"'t'fcctlinellytlrt'('oVpu(,II-},ora'sltltco1pu.tM<iil
of Broadlancl.
Tronsaclirtns
of'tlte
.\trtr/itk ard .\'uru;ich.l\"aturalists'
,Jocirlt,,2();
32 35.

E,RADICA'I'ION O}'\,IUSKRA'I'S AND COYPUS 5I
ERRING'l'ON, P. L.. 1963. ,llu.tkral Population.t..\mt's: Iorva Statc f--niit'r'sitv
l)r't'ss.
GOSLING,L.\'1.'197.l..I.hcCO\'PuirrE,astAIrgli:r..fransaction':
.19
59.
GOSLING, L. M., l98l. Climatic dctcrminants of spring
littt-ring
bl'fr:ral
covpus, ,llyocastor co_ypus../ournal
oJ
ioologt, London,
195:
2Bl '288.
GOSLINT;, L. \1., 1986.
Selectivt'
abortion
of cntirt'
litters in thc covpu: adaptirc control
ol'ofliprirrq
production
in rclation
to quality and sc'x.
American.\"alurali.yt, 127:772 7!)5.
GOSLING, L. M. & BAKITR, S.
J., 1987. Plaruring and rnonitorins an attenrpt tci er-adicatc
cor'pus {i'om
Britairr.
Slmposium
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loological
,\ociely
o.f London,5B: 99 ll3.
GOSLING, L. M. & BAKER, S.J., in press. Covpu, Ill-yocaslor
clypu.r.In
G. B. f)orbct
& S. Harris (.b:,ds,\,7he
Handbook
of British
.llanmals:
Oxlbrd: Blacku'cll Scicntific Publications.
[.;OSLING, L. M. & SKINNIIR,J. R., 1984. Cioypu. In I. L. \lasrin
(Ed.1,.Lr',r1ulion
of Domestirnled ;lninals:
2+6 '251
. London: Lonsman.
()()SLING, L. N{., tsAKL,R,
S J & CLARKL,, Cl.
N., l9BB.
An attempt
to rernove coypus
(,ll_t'otastor
co-ypu.r1
liorn a wctland habitat in Earst
Anglia. JournaL
( 'l1t1tlied
Ecologt,25:
19 62.
(;OSLING, L. \1.. B.\KFll{. S .J & SKINNI'lR,
J l{. lgt33.
'\ simulution apploat'lr 1o
invt'stigrttiris tht'
f('Sp()nscoI.actlr.pu1lclpulati<lnt<it:linlirticr.lrriatirll.
BulleLin.l3:
lB3 ltl2.
G()SLING,
L. \,I., \{ATT, A. D. & BAKER, S.J., 198
1.
flontirrut,us rctros;rccli\('rcnsus of tht'East Angliarr
coypu
pcrpulation
bctween i970 ancl
1979..'/ournal
of -lninrul
Ecolagt, Sr.'BB5
90
l.
LAURIE, E. M. ().. 1946.'fhe
coypr.r
t,.ll-yoca.rtor
coypus) in (lreat Britain.
./ournal
nf Animal Ecologt,, l5:2'2 3+.
LEVER, C., 1985.
,\'aturali<ed
Mammals
qf the l1'orld Harlon', Irsst'x: Lonsrnan.
MUNRO,'l'., 1931.
N{usquasir
in Scotland.
The
Scottish.,Naturalist,
IB9: 65 70.
N,IUNRO,
T., 1934.
NIusk
rats in Scotland. The
Scottish./ournal o.f''lgriculture,.lonuary:94 98.
M[;NRO,'f., 1935.
Notc on musk-rals and
othcr
animals killed
since thc
inccption
o{'thc
campaiqn
agzrinst
nrusk-rats
in C)ctobcr
193'2.
'fhe
,9cotli.rh
.\'aturaLi.rt.
1: ll lti.
h"E\\.SoN,R.\{.,l966.Rcprodrtt'titlItirrtlrcli'I-alCoVPu(..II.l,oca.ltortoypu.t1.,\-,t,ntosiunl
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A campaign asaitrst
the
co,vpus in F-ast
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Srrrn/rsl,
l7:625-62tt.
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control of
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\tr[,yocastor
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.'f
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I7 19.
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+21 +'25.
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of'lhe
ll'orld. Ith t'cln.
Baltirnolt':
Johns
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Plt'ss.
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'I'he
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(l'iber
zibethicrrsl
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A contribution
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I.Onrtalrn
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