See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/227800788 The eradication of muskrats and coypu from Britain Article in Biological Journal of the Linnean Society · August 1989 DOI: 10.1111/j.1095-8312.1989.tb01561.x CITATIONS 193 READS 3,883 2 authors , including: Leonard Morris Gosling Newcastle University 76 PUBLICATIONS 4,335 CITATIONS SEE PROFILE All content following this page was uploaded by Leonard Morris Gosling on 09 October 2017. The user has requested enhancement of the downloaded file. B i o t o g i c a l . / o u r n a l o J t h e L i n n e a n S o c i e t v ( 1 9 8 9 ) , 3 8 . ' 3 9 5 1 . \ \ ' i t h 5 f i g t r r c s The eradication of rnuskrats and coypus frorn Britain L. M. GOSLTNG AND S..1. BAKER OolpuResearch [,aboratory, Ministry of Agriculture, Fisheries and Food,Jultiter Road, JtforwichlVR66SP Introduced vertcbrates can cause massive environmcntal damagr: but most att('mpts to rcrnovc lcral p o p u l a t i c l n s h a v c f a i l c d . ' l ' h i s p a p c r d i s c u s s c s t h c c r a d i c a t i o n c a m p a i s n s a g a i n s t f t ' r a l m u s k r a t s , 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 e n v i r o n m e n t a l d a m a g c b y m u s k r a t s w a s r v c l l k n o w n l i o m F ) u r o p c a n d e r n c r a d i r : a t i t , n t ' u m p a i q n s t a r t c d p r o m p t l y i n 1 9 3 2 m a k i n g u s c o f o v c r s c a s c x p c r t i s c a n d z r c o n t r o l s t r a t c g , v d c s i s n c d b v p c s t c o n t r o l s p e c i a l i s t s . ' l ' h e c a m p a i s n w a s b r o u g h t t o a s u c c c s s f r r l c o n c l u s i o n i n 1 9 3 9 w h e n l r t l c a s t ' l 3 8 f l muskrats had becn killed. I n t h e 1 9 3 0 s , l e w b e l i e v e d t h a t c o v p u s w o u l d c a u s c s i g n i f i c a n t c n v i r o n m c n t a l d a m a s e r a n d c a r l v trapping ellr,rrts were inadcquatc. An carl,v campaisn achievccl onlr. limitt'd su('('css1 partly bct':rusc o f t h c l a c k o f b i o l o g i r : a l i n f o r m a t i o n . ' l ' h e c r a d i c a t i o n c a m p a i g n r v h i c h s t a r t e d i n l 9 8 l , r v a s b a s c d o r r 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 ' h c m r r s k r a t c a m p a i g n s s u c c e t ' d t ' d l r c t ' a i r s e t e t ' h n i r - a l i n f t r r m a l i o n t o h c l p p l a n t h e u o r k u ' a s a r " a i l a b l c a n d l l c t ' a u s t ' a c t i o n w a s t a k c n c l u i c k l t . . \ \ ' h c l c a n i n t r c l d u c e d p c l p u l a t i o n i s r v c l l t ' s t i r l l l i s l r c c l , a s u ' i t h ( ' o y p u s i n B r i t a i n , a c l o s c l v i n t c s r a t e d p r o { r r u m m ( ' i n r ' o l v i n g a p p l i c c l p o p u l a t i r l n c c o l o g v a n d a w - c l l - p l a n n c d t ' o n t r o l o r g a n i z a t i o r l [ l a Y b c c s s c n t i a l { i r r s r - r c c c s s f i r l r c r n o v a l . K F I Y W O R D S : M u s k r a t s c o y p u s p o p u l a t i o n c c o l o g v c r a c l i c a t i o n B r i t a i r r . I ] O N ' I ' E N ' I ' S I n t r o d u c t i < l n B r r c k g r o r r t t t l l r r l l r t ' s l r t ' r ' i t ' s ' l ' h c i n t r o d u c t i o n o f m u s k r a t s a n d c o v p u s t o B r i t a i n . D a n t a q , ' P c r c c p t i o r r o { ' t l . r e p r o b l c m : r n d t h c d i f } i r i n s r c s p o n s c s t ( ) m u s k r r i { s a n d c o r ' p u s ( l o r r tr r r l s t l a t t ' g i t ' . \ l u s k n r t s ( l o r l l r r s ' l ' r a p p i n e t c c h n i c l u ' l ' r a p p e r m o t i v i l t i ( ) Cclnclusions . \ r ' k r t o nl t ' d g ( ' m ( ' n l s 5 ( l R t l i ' t c t t t t ' s 5 0 I N ' f R O D U C ] ' I ' I O N ' l ' h c i n t r o d u c t i o n o f m a m m a l s p e c i c so u t sidc thcir native ranse can result in sev e r ed a m a s e t o a g r i c u l t u r e a n d t h e n a t u r al environmcnt. Nlany native spccies 3 9 -t0 .t0 1 1 T I / ( l * L + . 1 -t.) +4 1 1 a l .lfl +9 S r l c i c t v 00244066/89/090039+ r3 $03.00/0 39 O l 9 B 9 ' l ' l r t ' L i r r n r : a r r < l { ' L o n d o r r + 0 L . \ { . G O S L T N G A N D S . J . B A K E R o f p l a n t s a n d a n i m a l s h a v e b e e n b rought to extinction by introduced species and this processcontinues. Most attempts to remove introduced specieshave f,ailed,a British example being the attempt to eradicate mink, MusteLa uison, which started i n 1 9 6 5 , e m p l o y e d n i n e t r a p p e r s, and was abandoned in 1970 (Thompson, l e T l ) . This paper is about the eradication campaigns against muskrat, Ondatra <ibethicus(Warwick, 193+, 1940; Munro, 1935) and coypus, Mltocaslor coypus ( G o s l i n g & B a k e r , l 9 B 7 ; G o s l i n g , Baker & Clarke, 19BB)in Britain. The muskrat c a m p a i g n w a s b r o u e h t t o a s u c c essful conclusion in the 1930s.Further checkine is needed befbre a final judgement can be made in the case of' the reccnt c a m p a i g n s a s a i n s t c o y p u s , b u t , a t the time of writing, only two isolated coypus h a v e b e e n d e t e c t e d o v e r t h e p a s t 21 months. It seemsunlikely that a brecding p o p u l a t i o n r c m a i n s a n d t r a p p i n e ceasedinJanuary 1989. Herc wc outlinc the reasons for thcsc succcssesso ttrat, hopefully, the lcssons can bc applied e l s e w h e r c . B A C J K C ] R O U N D ' I ' O ' I ' H L , S P E , C ] I L , S M u s k r a t s a r e m y o m o r p h r o d e nts which reach up to about 1.8 kg in weight ( W a l k c r , 1 9 8 3 ) . M a n y m u s k r a ts brought to Britain came from Canada ( W a r w i c k , 1 9 3 4 ; M u n r o , 1 9 3 1 ) . Their breeding is seasonaland in Britain they h a d t w o o r r a r e l y t h r e e l i t t e r s b etween April and October (Warwick, 1940). G e s t a t i o n i s 2 5 3 0 d a y s ( W a l k e r , l9B3) and litters averase between five and s e v e ny o u n s ( D a n e l l , l 9 7 B ; C l a y & C l a r k , l 9 B 5 ) . C o y p u s z r r e h y s t r i c o m o r p h r o dents, native to South America which weigh 6 7 k g w h e n f u l l y s r o w n ; t h e i r b iology is described elsewhere (Newson, 1966; G o s l i n g , l 9 8 6 ; G o s l i n g & B a k e r , i n press).Coypus are poorly adapted to freezing c o n d i t i o n s ( N e w s o n , 1 9 6 6 ; G o s l i n s , l 9 8 l ) a n d , i n B r i t a i n , w o u l d p r o b a b l y h a v e a s l o w e r r a t e o f p o p u l a t i o n i n crease than muskrats, particularly in years l o l l o w i n g c o l d w i n t e r s . Both spcciesarc semi-aquatic and swim from daytime resting sitesin nestsand burrows to feeding areas in or near water. T H E , I N T R O D U C T I O N O F N { U S K R A ' I ' S A N D C O Y P U S T O B R I T A I N 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 i m p o r t e d t o B r i t a i n d u r i n g t h e l a t e 1920s (Lever, l9B5). Over 87 muskrat farms were quickly established throughout Britain although most were small ( W a r w i c k , 1 9 3 4 ) . E n c l o s u r e sw e r e often inadequate and escapes were common. A l l m u s k r a t { a r m i n g e n d e d b y 1933. Over 50 coypu larms were established, m o s t l y i n t h e s o u t h a n d e a s t o f E ngland. Nearly all had closed by 1939 but by t h i s t i m e m a n y h a d e s c a p e d ( L a u rie, 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 successof furbearers in invading new habitats may be because they sometimes escape in sufficient numbers to overcome these problems. Whatever the reasons) separate E , R A D I C A T I O N O } ' M U S K R A ' I ' S A N D C ] O Y P L I S 4 I populations of muskrats became established in the Severn valley in Shropshire, in a c e n t r a l a r e a o f S c o t l a n d a n d i n t w o s maller areas in Sussexand Surrey. A c c u r a t e e s t i m a t e s o f t h e m u s k r a t p o p ulations are not available but the nu m b e r s k i l l e d g i v e s o m e i m p r e s s i o n .I n f a c t these numbers are onl\. those killed in t h e o f f i c i a l t r a p p i n g c a m p a i e n s : f o r e r a mple,, the Scottish population started fro m e s c a p e s i n 1 9 2 7 a n d z r t l e a s t 1 4 0 a n i r n als were killed before the campai*=ns s t a r t e d i n 1 9 3 2 ( M u n r o , 1 9 3 4 ; L e v e r , l 9 B 5 ) . T h e t o t a l k i l l e d i n S c o t l a n d w a s a t lea s t I l 0 B . A t l e a s t 3 0 5 2 m u s k r a t s w e r e a c c ounted for in Shropshire (FiS. 1) and a f u r t h e r 2 2 8 w e r e k i l l e d i n S u s s e xa n d S u rrev. The total causht lrom all {bur s i t e sw a s a t l e a s t 4 3 B B ( W a r n , i c k , 1 9 3 4 , 1 9 4 0 ) O n e g r o u p o f c o y p u s b e c a m c c s t a b l i s h e dnear Slough, remained at a low level and d i s a p p e a r e d w i t h o u t a n y k n o w n c o n t r ol in 1956 (Norris, 1967b). A second group probably originated lrom three farnrs near Norwich, close to the Rir,,ers Ya r e a n d W e n s u m ( L a u r i e , 1 9 4 6 ) . T h e b e st known escapewas from a farm at Ea s t C a r l t o n i n 1 9 3 7 ( E l l i s , 1 9 6 0 ) . T h e East Anglian wetlands have many similarities to the coypus' native swamps in South America and coypus soon inc r e a s e da n d s p r e a d i n t o a d j o i n i n e r i v e r s . At their peak in the 1950smost were in Norfolk, Suffolk and Essex but significant numbers had also spread to n e i e h b o u r i n s c o u n t i e s ( D a v i s , 1 9 6 3 ; N o r r i s , 1 9 6 7 b ) . ' f h e s i z e o f t h e c o y p u p o p u l a t i o n i s k n o w n in detail after 1962 (Gosling, Watt & B a k e r , l 9 8 l , a n d s e e F i g . 5 ) b u t e a r lier information is anecdotal. The population started in about 1937 and grew progessivelywith the first complaints ab o u t d a r n a e e i n I 9 + 3 ( L a u r i e , 1 9 4 6 ) . T h ere were major checks in the severe win t e r s o f 1 9 4 6 1 4 7a n d 1 9 6 2 i 6 3 . I n t h e l a t e 1950s,there was a dramatic eruption in n u m b c r s : R a b b i t C l e a r a n c e S o c i e t i e sw e re grant-aided to control coypus and loczrl drainage organizations increased their efforts to limit the increasins da m z r s e .A b o u t 1 0 0 0 0 0 c o y p u s w e r e k i l l e d in 1961-62 without {reatly affecting the p o p u l a t i o n ( N o r r i s , 1 9 6 3 ) . C e n t r a l l y - o r ganized control started in 1962 and con t i n u e d a t v a r i o u s l e v e l s u n t i l t h e s t a r t o f the eradication campaign in 1981. Ma x i m u m n u m b e r s w e r e o n c e b e l i e v e d t o be 200000 in the late 1950s (Norris, 19 6 7 b ) b u t t h i s m a y h a v e b e e n a n o v e r e s t i mate. DA\{A(;F] C o y p u s a n d m u s k r a t s a r e s e n e r a l i s t h e r b ivores which damase a wide varietl. of n a t i v e p l a n t s a n d c r o p s i n t h e i r i n t r o d uced ranges. Coypus often destroy mo n o c o t v l e d o n o u sp l a n t s b v s e l e c t i n gb a s a l nreristems(Goslinq. 197+) and large a r e a so f r e e d s w a m pw e r e e l i r n i n a t e dd u r i n g t h e l a t e 1 9 5 0 s( E l l i s , 1 9 6 3 ;B o o r m a n & F u l l e r , l 9 8 l ) . C ) o v p r - r s a l s o f i r v o u r p a r t icular species and sorlle, inclr-rdine Butomtts umhellatus,the flowering rush, and (,'icuta ttiro.ra,cowbane, becanre ext r e m e l v r a r e ( E l l i s , 1 9 6 3 ) . M u s k r a t s i n North America also cause extensi','e dam a s e t o m a r s h l a n d v e s e t a t i o n w h e n a t h ieh population densities (Errington, 19 6 3 ) . F o r t u n a t e l y , t h e i r n u m b e r s i n B r i t a i n did not reactr thc lcvcls at which larg e s c a l e d a m a g e t o n a t i v e p l a n t s n , o u l d have occurred. Both species also damase a variety of crops: sugar beet are a fbvouritc lor coy p u s b u t t h e y a l s o e a t c e r e a l s ,b r a s s i c ' z r s and other root crops (Norris, 1967b; An o n . , l 9 7 B ) . M u s k r a t s d i d n o t c a u s e s i e nificant damage to crops in Britzrin (W a r w i c k , 1 9 4 0 ). 42 L. N,r. GOSLTNG AND S.J. BAKER T h e m o s t i m p o r t a n t d a m a g e i n economic terms is caused by burrowins. In t h e N e t h e r l a n d s w h e r e 2 8 9 l l 6 muskrats were trapped durine l9B7 alone ( A n o n . , l 9 B B ) t h e r e i s e x t e n s i v e damage to the land drainage system. Damase t h r o u g h b u r r o w i n g w a s a l s o r e c orded to the banks of'the River Severn in the 1 9 3 0 s ( W a r w i c k , 1 9 4 0 ) . T h e c o s t of repair in the Nethcrlands and the risk of i n c r e a s e dd a m a g e a r e s o g r e a t t h a t a current annual cxpenditurc of sevenmillion pounds on control is considered cost effective (R. van Oostenbrusse, personal c o m m u n i c a t i o n ) . C o y p u s d i g e x t ensive burrow systemsinto the banks of ditchcs and rivers which disrupted drainase systems and posed the risk of flooding in l o w - l y i n g E a s t A n s l i a ( C o t t o n , 1 9 6 3 ) . P F ] R C I . ] P ' I ' I O N O ! . . f H E P R O B L E N TA N D T H I . ] D I ! ' ! ' I ' ] R F ] N T R E S P O N S E , S T O M U S K R A ' I ' S A N I ) C O Y P U S The costs of future damape, both in economic terms and the natural e n v i r o n m e n t , c a n n o t a l w a y s b e assessed at the early stagesof an introduction. This affects whether or not prompt action is taken. Fortunately, it was already k n o w n f r o m e x p e r i e n c ei n E u r o p e that muskrats could cause seriousdamage in Britain and this information was used by staff of the British Museum (Natural H i s t o ry ) , n o t a b l y t h e z o o l o g i s t M. A. C. Hinton, to persuade the Ministry of Agriculture and the 'I'reasury that money spent on eradicating muskrat would b e w o r t h w h i l e ( S h e a i l . l 9 B B ) . T h es e i n i t i a t i v e s a l s o l e d t o t h e Destructive Imported Animals Act of lgg2 w h i c h a l l o w s c o n t r o l o v e r t h e i mportation and keeping of' named animals. I n i t i a l l y m u s k r a t s w e r e o n l y a l l o wed to be kept under licence and in 1933 all k e e p i n g w a s p r o h i b i t e d . B y t h i s stage strinsent keeping requirements had r e d u c e d t h e h o l d i n g s t o s i x ( f ive in England and one in Scotland) and c o m p e n s a t i o nw a s p a i d t o c l o s et h esedown. The act also authorized the Ministry of Agriculture to carry out the eradication campaisn. A l t h o u g h t h e c o y p u p o p u l a t i o n was growing steadily at thc same time, coypus w e r e o n l y a d d e d t o t h e l i s t o f a n i mals proscribed under the Act in 1962. There are a number of reasons for this. The first was that relativcly little was known a b o u t t h e b i o l o g y o f c o y p u s a n d t heir potential for damage in the 1930s.Coypus a l s o h a v e a l o w e r p o t c n t i a l r a t e o f incrcase than muskrats, particularly over cold winters, and so early events were no so alarming. The advice from Germany, w h e r e c o y p u s h a d a l s o b e e n i n troduced, was that coypus would not cause p r o b l e m s ( S h e a i l , l 9 B B ) . A l t h o u g h true in Germany where the cold continental climate keeps coypus at a low level, it is not the casein Brtitain where winters are milder. Coypus were also believed to be easy to catch if they escaped (Carill- W o r s l e y , 1 9 3 2 ) a n d H i n t o n c o m mented that they were unlikely to become a s e r i o u sp e s t ( S h e a i l , 1 9 B B ) . As a result, although there were some warnings about the potential dangers ( S t e a r n , 1 9 8 l ) , l i t t l e w a s d o n e t o prevent the establishment of coypus. Some trapping was carried out by trappers employed by the Norfolk War Agricultural E x e c ut i v e C o m m i t t e e a n d 1 9 3 a nimals were caught between l9+3 and l9+5 ( L a u r ie , 1 9 4 6 ) . B u t t h i s t r a p p i n g was localized and it was not continued. After the cold winter of 1946147when numbers were ereatly reduced there must have seemed even less need to do anythine. E R A D I C A ' I ' I O N O ! M U S K R A ] ' S A N D C O Y P U S 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 o f A g r i c u l t u r e p e s t c o n t r o l s p e c i a l i s t ,E . C. Read (Public Record Office, MAF + + l l + ) . F e w e r d e t a i l s a r e a v a i l a b l c f r o rn other areas but presumably the e x p e r i e n c e f r o m S h r o p s h i r e w a s p u t t o u se. Here, an oblong area was defined u s i t t g t h e l o c a t i o n s o f t h e o u t e r r n o s t k n o wn muskrats in each area and divided in t o s q u a r e s e a c h o f t e n s q u a r e m i l e s . A t rapper was deployed to each of these s q u a r e . st o s u r v e y a n d t r a p . T h i s i n t e n s i ty of'trapping appears to have been d e c i d e d o n t h e b a s i s o f c o m m o n s e n s ee s timatesof thc amount o{'sround one m a n c o u l d c o v e r . N l o t o r i z e d t r a n s p o r t w a s very limited: one van lvas available in S h r o p s h i r e b u t m o s t t r a p p e r s e i t h e r b i cycled or walked to work. T h e i n i t i a l a r e a t r a p p e d w a s j u s t o \ r e r 3 00 square miles (seeFig. l). This was e n l a r s e d b y 1 0 0 s q u a r e m i l e s i n 1 9 3 3 a nd 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. S o m e m e n b e c a m e s p e c i a l i z e da s ' s e a r c h ers'to look for outlying muskrats. It is a l s o s t a t e d i n t h e h i s t o r i c a l d o c u m e n t s ( P ublic Record Oflrce, MAF 44114) that effort should be proeressively concentrated alons rivers where the population w a s m o s t d e n s e ,a s t h e o u t l y i n g : r n i m a l s w ere cleared. How far this intention was ca r r i e d o u t i s n o t k n o w . A f t e r e a r l y p e a k s o f c a p t u r e s , t h e n u m bers of muskrats declined steadily to ze r o ( F i e . 2 ) . Th e l a s t a n i r n a l s i n S h r o p s h ire, Sussexand Surrey we re caught in 1 9 3 5 ,a n d t h e l a s t i n S c o t l a n d i n 1 9 3 7 ( W a r w i c k , 1 9 3 + , 1 9 4 0 ) .T h e l a s t m u s k r a t in E n g l a n d , p o s s i b l y a n e m i s r a n t f r o m t h e Shropshire population, was caueht in C h e s h i r e , i n 1 9 3 6 . F i g u r e l . ' l ' h c d i s t r i b u t i o n o f r r u s k r a r s c a u q h t i n S i r r o p s h i r c . A f t e r W a n v i c k 11 9 4 0 ) . 1 . ' . . o o o o O o OOo\O . O O . r O O J - 5 , o : - : - - - ' Welshpool o^p }-+---e-f o ) N u m b e rk r l l e dp e r s q u o r e m r l e O = l o + o - < l O 1 9 3 2 a n d 1 9 3 3 i n t h e r r a p p i n g c . a m p a i u n i n L . M . G O S L T N G A N D S . J . B A K E R 350 300 2 5 0 200 r 5 0 r o o S c o t l o n d | 9 3 2 i 9 3 3 | 9 3 4 1 9 3 5 | 9 3 6 1 9 3 7 t 9 3 8 T r o p p e r s . . . . ' . . M u s k r o t s k i l l e d 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 t h e l a t e r y e a r s o f t h e c a m p a i g n 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). 6 a ) ! (D = I o o I a = E o (D -o E = z t 5 0 U' o o O . n o v ! o b t 5 ! E z t o r o o 50 S hr o p s h i r e E R A D I C A T I O N O F M U S K R A T S A N D C O Y P U S + 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 successivelyby 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 c o l o n i e s ( N o r r i s , 1 9 6 7 b ) . 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,6of 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 1 9 7 0 s( G o s l i n ge t a l . , l g B l ) . I n c o n t r a s t , t h e l 9 8 l e r a d i c a t i o n c a n - r paien was designedusins the resultsof a lo n q t e r m i n v e s t i g a t i o n o f c o y p u p o p u l a t ion 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 t h e c o n t r o l o p e r a t i o n ( G o s l i n g e t a l . , l 98l). 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 successand 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/uof the variation in the change in c o y p u n u m b e rs . B y t h e l a t e 1 9 7 0 se n o u s h was known to provide an analytical backsround for attempts to build simulation models of the population (Gosling & B a k e r , l 9 B 7 ) . These simulations were used to assess the effect of employing different numbers of trappers on the response of the population under various climatic c i r c u m s t a n c e s .A r a n g e o f s i m u l a t i o n s ( F i S.3) were consideredin the late 1970s when an independent committee, the Coypu Strategy Group, took a fresh look a t l o n g t e r m p o l i c y f o r c o y p u c o n t r o l ( A n on., 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 c l o e f l i c i e n t o i -d e t e r r n i n a r i o r l : 0 . 8 2 : F : 3 8 . 6 : d . t 2 . 1 4 . trapping techniques, and an average number of cold r,r,inters, it was decided to a t t e m p t e r a d i c a t i o n w i t h i n t e n y e a rs. T h e e x i s t i n g C o y p u C o n t r o l Organization was reconstituted and the e r a d i c a t i o n c a m p a i g n s t a r t e d i n l98 L Biologists from the Coypu Research L a b o r a t o r y s a v e t c c h n i c a l g u i d a n c e throughout the campaien. An example is t h e s c h e m e u s e d t o d e p l o y t r a p p i ng effort (Fig. a). Trapper deployment was more flexible than in the muskrat campaign because each trapper had a car. R e c o m m e n d a t i o n s w e r e m a d e e v e ry three months usins past capture:trapping L . \ r . G O S L T N G A N D S . J . B A K E R 'l'aurn l. Multiplc resression analysis of the effect of trapping i n t e n s i t y a n d w i n t e r s e v e r i t y o n t h e c h a n g e i n t h e m e a n n u m b e r o l ' a d u l t c o y p u s a l i v e i n s u c c e s s i v ey e a r s . T h e n u m b e r o f a d u l t c o y p u s r v a s o b t a i n e d f r o m a m o n t h l y r e c o n s t r u c t i o n o f t h e p o p u - l a t i o n ( G o s l i n e e t a l . , l 9 B l ) a v e r a g e d f o r e a c h y e a r b e t r v e e n 1 9 7 0 a n d 1 9 8 7 . \ \ ' i n t e r s e r . e r i n 'i s e x p r e s s e du s i n g a n i n d e r r v h i c h r v c i g h t s a n d a c c u m u l a t e s r u n s o f f r e e z i n e d a y s e a c h w i n t e r ( G o s l i n g , l 9 8 l ) ; t r a p p i n e i n t e n s i t y i s t h e n u m b e r o f t r a p - n i g h t s i n v e a r y w h e r c t h e c h a n g c i n c o y p u s n u m b e r s i s c a l c u l a t e d b e t w e e n y a n d y * I l n d c p c n d e n t r , ' a r i a b l e B e t a t v a l u e P Trapping ir.rtcnsit,v \\:intcr scvt-ritv - 0 . 7 3 6 . 0 6 < 0 . 0 0 1 - 0 . 2 6 - 2 . + t 0 . 0 3 O 1 2 3 4 5 6 Yeor F i s u r e 3 . S i m u l a t i o n s o f a d u l t f c m a l c c o y p u n u m b e r s i n r e l a t i o n t o v a r i o u s t r a p p i n g i n t e n s i t i e s . ' f h e s c s i m u l a t i o n s a r c s o n r e o f t h o s e c o n s i d e r e d a s o p t i o n s b y t h e C o y p u S t r a t e s y G r o u p i n 1 9 7 7 a n d d o c : u m e n t c d i n t h c i r l 9 7 B r c p o r t . L i n r i t e d i n { b r m a t i o n a b o u t t h c c f l ' c c t o f c < - r l d w e a t h c r r r , a sa r ' : r i l a b l c i n 1 9 7 7 a n d t h e s i n r u l a t i o n w i t h 2 4 t r a p p e r s a n d o c c a s i o n a l c o l d n , i n t e r s s i m p l y r e p l i c a t e s t h e e f l e c t o f t h e 1 9 7 5 / 7 6 u ' i n t e r . W h e n f u r t h e r d a t a r v e r e c o l l e c t e d i n s u b s e q u e n t y e a r s i t b e c a m e p o s s i b l c t o explcrre the effect of cold weather in more detail (Gosling et al., l9B3). l 9 t r o p p e r s | = C o l d w r n t e r -- \*---z\ 24 tro21ers l ' . - \ . - r " " " " " " " . . 2 4 t r o p p e r s L , 3 4 t r o P P e r s I o - I q O E E o -) ! A o o) E J z E R A D I C ] A T I O N O ! ' M U S K R A T S A N D C O Y P U S ! ' i g u r c ' 1 . ' l h c d i s t r i b u t i r - , n r . r l ' c o t ' p t t sc a u g h t r l u r i n g t h t ' i g B l l 9 B 9 t ' l ' a r l i c a t i o n c a m p a i e n a n c l t h < ' r c g i o n s u s t ' d f b r s t r a t c g i c c l c ' p l o v n r c n t o [ ' t r a 1 > 1 > i r r e c { J i r r t . ' ] - h e c i l t ' l c s o n t h t ' m : r l ) r ( ' J ) r c s ( n l < 0 . 1 . 0 . 1 1 . 0 a n c l > 1 . 0 " , , < r l t h t ' t ( ) t i t l n u n r l ) < ' r t ' a u g h t ( n : I i 4 t i 2 2 1 . I . , f l i r r t n i r s c l t ' p l o r < ' c l i n l r r o p r ) r u ( ) n r ( ) " v c i e h t t ' d ( ' i I J ) t u r c : t r i l p p i n g t ' f l i r r t r i i t i ( ) s . A l t h o r - r c h s ( ) r n ( ' t r a l ) l ) i r r s u . a s c a r r i t ' c l o r r t t l r r o r i g l r o r r t t h t ' a r ( ' a t h ( ' i n i t i a l a i m . u s i n g s t r o n g l r u e i g h t c c l r i i t i t . r s . u ' l r s t ( ) ( ( ) n ( ( ' n t r i r t ( ' c l l i r l t i n t h t l r i c h c l r ' l r s i t t I ' t ' q - i o t t s ' ( l l a r r c l ( 1 2 , t o t n a x i t n i z e n r o l t a l i t v a n c l r r r i n i n r i z t ' t ' r r r i 1 1 r ' u t i r ) n t ( ) l ) ( , r ' i l ) l l < ' r ' : . r l i r n . ; 1 s . , \ s t l r t ' ' ( ' o r e ' a r e a s r v e r e r e c l u t ' t ' d , t h c r v c i q h t i n g l l t c t r l r w a s r c c l u < ' r ' c l t o d c p l o r t r a p p i l l q < ' l i i r r t r n r t r t ' < ' r . r ' 1 l y a r r r l t h u s l t a v r c l a t i v c l v r n < t r t 'a t t c n t i o n t < t t h c p c r i p h c r r ' . effbrt ratios in eight stratesic resions (Fig. a) and available eflorr was a p p o r t i o n e d i n p r o p o r t i o n t o t h e m a c n i t ude of the ratio in each resion. This ratio was weishted to different extents so that effort could bc conccntratcd on h i g h d e n s i t y a r e a s e a r l y i n t h e c a m p a i g n arld deployed more widely larer on. The laboratory also monitored progress by field checks, by reconstructine the p o p u l a t i o n ( F i g . 5 ) a n d u s i n s s i m u l a t i o n t echniquesto detect and analyse trencls ( G o s l i n g & B a k e r , l 9 B 7 ) . N u m b e r s w e r e reduced from around 6000 adults in l 9 8 l t o n e a r - z e r o i n 1 9 8 7 . T ' h e c a m p a ign was helped by an above averase n u m b e r o f c o l d w i n t e r s . H o w e v e r , , c o l d weather by itself would never eradicatc a l l c o y p u s i n B r i t a i n , a s s h o w n i n t h e e x ceptional winter of 1962/63. The main e f f e c t o f c o l d w e a t h e r i n t h e c a m p a i q n was to lower recruitment to the adult s e c t o r o f t h e p o p u l a t i o n b y r e d u c i n g b reedins succcssand juvcnilc survival ( G o s l i n g & Ba k e r , l 9 B 7 ) t h u s a l l o w i n s trapping to make a relatively large i m " p a c t o n a d u l t n u m b e r s . 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 a n i m a l s c o u l d g e t t o d e e p w a t e r a n d d r o wn. Where this did not happen io-. animals escaped with severed legs. There was also a large toll of non-target +7 a a o o o f o o o O O a a a a a a a a a o o a o o o a o o a o 1 a a a o o o o o o r f a a o o o a o o o a o o a a o o o o ? o o o ) ,. O O oo o o o o o o o o o o a a o f O o O a a a o a o o 4B L . N ' I .G O S L I N G A N D S . I . B A K E R 3 0 t o 25 2 a o (J () I 15 a -c I c I o o a) o A n l o o- > o (J = a A o o E f, z - a 8 4 8 8 8 7 8 6 8 5 8 3 8 2 8 I 8 0 7 B 7 7 76 7 5 74 7 3 7 2 7 l 70 Yeor . . . . . . C o y p u T r o p - n r g h t s I C o t O * , n r " r . l i g L l r e 5 . R e c o u s t n r c t c t l r t u m b t - r s o f a d u l t c ( ) \ ' l ) u si l n d t r a p p i r ) g i n t e n s i t v c l u r i l q 1 9 7 ( ) l 9 8 B . ' l ' h c r i s c i n t h c c a l l v l 9 T 0 s w a s d u c t o a r u n o 1 r r r i l d n ' i n t c r s r v i t h i n a d e c l u a t c t r a p p i n s . ' l h c c r a d i ( ' a t i o l l t : a m p a i { n s t a r t e d o f l i c i a l l v i n l 9 8 l a l t h o u g h t h e t r a p p i n s l i r r c e h a d b c c n b u i l t l l p o v e r t h c p r c v i c ' r u s r,r'itrtcr. 'I'ht' hieircr than avt'rase rtut.nbcr o{ cold u'intcrs probaLrlv accclcrat('d thc campaign lrtrt c o l d r v i t r t c r s l r v t h c m s c l r c s u ' o u l d r ] c v c r r c m o v c c o v p u s l i o m B r i t a i n . ' l ' h t ' p r i p u l a t i o n r t ' t o n s t r u t t i o r r t c c l r r i i q u e i s d o c u m e n t e d b r G o s l i : n ge l a l . i l 9 8 l l . c a p t u r e s w i t h o v e r 6 5 0 0 m a m m a l s and birds killed in Scotland alone (Nlunro, l e 3 s ) . B o t h ca m p a i q n s a g a i n s t c o y p u s u s ed cage trapping (Norris, 1967a). Each trap w a s i l r s pe c t e dd a i l y a n d a n y c o y p u s 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, i n c l u d i n g t h o s e w i t h c o n s e r v a t i o na n d game interests.Improvements to trappins t e c h n i q ue s w e r e i n t r o d u c e d b y t h e Co,vpu Research Laboratory, including the u s e o f t r a p s o n b a i t e d r a f i s . E x p e r i ments showed these were at least 50il; more effective than traps set on land and fbllowing this work over 600 were deployed. N o n - t a r s e t c a p t u r e s w e r e a l s o s i g n i ficantly 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 a t t e m p t t o o \ , e r c o m ei t i n t h e c o y p u eradication campaign is the first time it has b e e n t a c k l e d i n a p e s t c o n t r o l o p e r a tion. The scheme devised \ /as to restrict the funding to a maximum of ten years and pron-risethe trappers a bonus of up to t h r e e t i m e s t h e i r a n n u a l s a l a r i e s i f they succeededin 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 , R A D I C A ' I ' I O N O I M U S K R A ' I ' S A N D C O Y P U S + 9 achieve the maximum bonus and we believe that this incentive was an essential ele m e n t i n t h e c a m p a i g n . I f t h e b o n u s s c h e m e w a s e s s e n t i a l i n c oypu 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 pa r t l y s o c i o l o e i c a l . T h e m u s k r a t - t r a p p i n g 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 ev e n t u a l d i s m i s s a l . B o u n t i e s w e r e p a i d f o r b o t h m u s k r a t s ( 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 a n i m a l t o e n s u r e a c o n t i n u i n g i n c o m e . 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 du r i n g t h e c o y p u e r a d i c a t i o n c a m p a i g n b u t , in any case,demand was low at this time and the potential rewards small in relation to the possible eradication bo n u s . C O N C L U S I O N S What are the lessonsof 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 ac t i o n u n l e s s ( l ) t h e y b e l i e v e t h e r e i s a p r o blem, (2) they think it can be solved an d ( 3 ) t h e y k n o w , w i t h i n l i m i t s , h o w m u c h it will cost. The muskrat campaien wa s u n d e r t a k e n b e c a u s ei m p o r t a n t e l e m e nts of this information were available from experience in Europe and Canada. However, this might not have been su f f i c i e n t w i t h o u t t h e s k i l l e d t e c h n i c a l a dvocacy 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 th a t a c t i o n w a s t a k e n q u i c k l y . E x p e r i e n c e in Europe shows how remote the chances of successwould have been if the populations had been allowed to spread more widely. T ' h c c o y p u e r a d i c a t i o n c a m p z r " i e n w o u l d simplv not have becn undcrtakcn wi t h o u t d e t a i l e d t e c h n i c a l a s s e s s m e n t s o f ' t he efibrt and costs required and the lik e l y c h a n c e s of ' s u c c e s s . T h e s e a s s e s s m e nts were onlv achiel'ed b,u o long term stu d y o f p o p u l a t i o n c c o l o g y , t a r q e t e d t o a particular control application. 'l'his stu d y i n c l u d e d o p e r a t i o n a l e x p e r i e n c e a s well as applied ecolosv :rr-rdit is sie n i f i c a n t t h a t t h e a r s u m e n t s f o r s u c h p r actical details as the inc'entive bonus scheme cramefiom biologists. O n e r e a s o n t h a t t h e e r a d i c a t i o n o f m u s k rats and coypus could be considered was that their populations were confined to reasonably small areas with no im m i g r a t i o n . H o w e v e r , e v e n i n o t h e r s i t u a t ionswhere eradication is not possiblc, for example where large scale immisration from a neighbouring country 5 0 L . N , t .G O S L I N C ; A N D S . J . B A K E R c o n t i n u a l l y r e p l e n i s h e sa c o n t r o l l ed population, the principles which emerge in c o n s i d e r i n g t h e r e m o v a l o f - m u s k rats and coypus can be applied. Indeed, these p r i n c ip l e s m a y b e a p p l i c a b l e t o the sensiblemanasement of any population of v e r t e b r a t e s .M o s t i m p o r t a n t i s t h e need for applied population ecology, so that t h e r o l e o f v a r i o u s f a c t o r s , i n c l u ding control techniques in limiting the subject p o p u l a t i o n c a n b e e v a l u a t e d . N ext there must be a target population density over a defined area which can be independently mor"titored.This target could be e r a d i c a t i o n o r a d e n s i t y l o w e n o ueh to preclude environmental damage. In the c o y p u c r a d i c a t i o n e x e r c i s e ,t h e c ontrol organization was centralized becausethe aim was to eradicate a single population but a coordinated systen-rof local organizations might sometimes be more appropriate. Next, the peoplc carrying out the control in the field must be rewarded for achievir-rgtheir objective and n o t f or f a i l i n g t o d o s o ( a s w i t h b ounties). However, where incentive schemesare u s e d , r e s u l t s n e e d t o b e i n d e p e ndently monitored. In thc campaiun against c o v p u s , e s t i m a t e s o f t r a p p i n s e f l brt, population trends and the results of-field c h e c k sw e r e p a s s e dt o t h e c o n t r o l orsanization and this trelped both directly and i n s t i m u l a t i n e t h e e f f o r t s o { - t h e t r appers. 'I'hi