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). Consequences of hydraulic works - kmae- · PDF filebull. fr. pêche piscic. (1998) 34 1299 :129-139 spatial distribution of an eel population {anguilla anguilla l.) in a small coastal

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  • Bull. Fr. Pche Piscic. (1998) 349 :129-139 129

    SPATIAL D ISTRIBUTION O F A N E E L P O P U L A T I O N {ANGUILLA ANGUILLA L.) IN A S M A L L C O A S T A L C A T C H M E N T OF

    N O R T H E R N BRITTANY ( F R A N C E ) . C O N S E Q U E N C E S O F H Y D R A U L I C W O R K S .

    E. F E U N T E U N (1) , A . A C O U (1) (2 ) , J . G U I L L O U T (2) , P. L A F F A I L L E (1)

    e t A . L E G A U L T (2)

    (1) Labora to i re d 'vo lu t ion d e s S y s t m e s Na tu re l s et Mod i f is , U M R 6 5 5 3 E C O B I O ,

    Univers i t de R e n n e s 1, 3 5 0 4 2 R e n n e s C e d e x , France.

    (2) F ish -Pass , 91 avenue de Sa in t -Br ieuc , 3 5 0 0 0 R e n n e s , France.

    A B S T R A C T

    T h e Frmur is a 6 0 k m 2 ca t chmen t of Br i t tany w h e r e m a n y hydrau l ic w o r k s (dams,

    we i r s , g a u g i n g s t ruc tu res , etc.) have r e d u c e d t h e recru i tment by e lvers a n d g lass eels

    s ince at least 5 0 years . In 1992, an eel lift w a s bui l t on the ma in d a m (14 m h igh) and in

    1996 , t w o o ther d a m s (5 m h igh) loca ted d o w n s t r e a m w e r e e q u i p p e d w i th ee l passes to

    improve u p s t r e a m migra t ion . In 1995, be fore the cons t ruc t i on of the passes , ee l popu la t ion

    pa rame te rs (densi ty, b i omass a n d s ize d is t r ibu t ion) we re a s s e s s e d at the sca le of the

    w h o l e w a t e r s h e d by e lec t ro f ish ing ( remova l m e t h o d ) in 3 3 s ta t ions. T h e average b iomass

    w a s h igh (19 g /m 2 ) desp i te the n u m e r o u s phys ica l obs t ruc t ions . However , t he re w a s a n

    a c c u m u l a t i o n of ee l s d o w n s t r e a m m a n y hyd rau l i c w o r k s a n d v e r y l ow d e n s i t i e s

    immed ia te l y u p s t r e a m . Moreover , d o w n s t r e a m the first obs t ruc t ion , at 2 km f rom t h e sea,

    the popu la t i on is d o m i n a t e d by boot lace ee ls a n d e lvers (< 100 m m ) , w h e r e a s th is size

    c lass is absen t in u p s t r e a m reaches . T h e s e resu l ts sugges t tha t t he s tand ing capac i ty of

    t he w a t e r s h e d is not r eached a n d that the popu la t i on cou ld be e n h a n c e d in the nex t years

    by the recent ly insta l led eel passes .

    K e y - w o r d s : Anguilla anguilla, b i o m a s s , s ize d is t r ibut ion, hydrau l ic work , stock

    m a n a g e m e n t .

    D I S T R I B U T I O N S P A T I A L E D ' U N E P O P U L A T I O N D ' A N G U I L L E

    (ANGUILLA ANGUILLA L.) D A N S U N P E T I T B A S S I N V E R S A N T D U N O R D D E LA

    B R E T A G N E ( F R A N C E ) . C O N S Q U E N C E S D ' A M N A G E M E N T S H Y D R A U L I Q U E S .

    R S U M

    Le Frmur es t un bass in versant c t ie r de B re tagne de 60 k m 2 c o m p o r t a n t de

    n o m b r e u x a m n a g e m e n t s hydrau l iques (ba r rages , c h a u s s e s de mou l i n , s ta t ions de

    j a u g e a g e , etc. ) ayant fo r temen t rdui t l 'accessib i l i t par les c ive l les depu i s au moins

    c i nquan te a n s . En 1992 , un a s c e n s e u r angu i l l es a t instal l sur le pr inc ipa l bar rage

    Article available at http://www.kmae-journal.org or http://dx.doi.org/10.1051/kmae:1998038

    http://www.kmae-journal.orghttp://dx.doi.org/10.1051/kmae:1998038

  • Bull. Fr. Pche Plscic. (1998) 349 :129-139 130

    ( 14 m d e hau t ) , puis en 1996 deux au t res obstac les (5 m de haut) ont t a m n a g s p lus

    e n ava l pou r res taurer la l ibre c i rcu la t ion des angu i l les dans le bass in versant . En 1995,

    a v a n t l ' a m n a g e m e n t d e s passes , l ' abondance et la s t ructure de la popu la t ion sont

    e s t i m e s l 'chel le du bass in ve rsan t par p c h e lec t r ique dans 33 s tat ions (m thode

    d ' p u i s e m e n t des s tocks ) . La b i o m a s s e moyenne appara t leve (19 g /m 2 ) ma lg r la

    p r s e n c e d e s obs tac les la m ig ra t ion . Toutefois, on note une accumu la t i on d 'angu i l les en

    ava l d e n o m b r e u x a m n a g e m e n t s hydrau l iques et de t rs faibles a b o n d a n c e s en a m o n t

    d e c e s d e r n i e r s . De plus, l 'aval du premier obs tac le , 2 k m de l 'estuaire, la popu la t ion

    es t d o m i n e par les ind iv idus de m o i n s de 100 m m , a lors que cet te c lasse de tai l le est

    q u a s i m e n t a b s e n t e en amon t . C e s rsul tats sugg ren t que le potent ie l d 'accue i l du bass in

    v e r s a n t n 'est sans d o u t e pas at te int et que les b i omasses m o y e n n e s pour ra ien t a u g m e n t e r

    a v e c l 'aml io ra t ion d e s cond i t i ons de migrat ion g rce aux passes po issons .

    M o t s - c l s : Anguilla anguilla, b i o m a s s e , s t r uc tu re d e ta i l le , a m n a g e m e n t

    h y d r a u l i q u e , ges t ion d e s s tocks .

    I N T R O D U C T I O N

    T h e Frmur (F igure 1) is a sma l l river of nor thern Br i t tany (France) wh i ch o p e n s into

    t h e C h a n n e l next to Sa in t -Ma lo . Its ca tchment covers abou t 60 k m 2 and the overal l length

    of t h e r iver a n d its t r ibu tar ies is 4 5 k m , compr is ing 17 km for the main s t r eam. T h e s lope

    v a r i e s b e t w e e n 0.1 % a n d 2 % for an average of 0.6 %. Desp i te its smal l s ize, the Frmur

    p r o v i d e s for a w ide range of hab i ta ts f rom h igh veloc i ty s t reams of the t rout z o n e to lent ic

    w a t e r s of t h e b ream z o n e in d o w n s t r e a m areas , m a n m a d e ponds a n d reservo i rs ,

    w e t l a n d s , e tc . The to ta l ex tent of wa te r is rough ly 50 ha a m o n g wh ich 5 ha of runn ing

    w a t e r s ( s t reams) a n d 45 ha of stil l waters (ponds and reservo i rs) There fo re , th is r iver

    a p p e a r s to be representa t ive of sma l l coasta l c a t c h m e n t s of Wes te rn France.

    Th i s natura l continuum h a s been d is turbed by several hydraul ic w o r k s wh i ch reduce

    t h e a v e r a g e velocity, i nc rease the depths a n d have conduc ted the c o m m u n i t y to be

    d o m i n a t e d by lentic wa te r spec ies of the b r e a m zone . T h e s e works a lso pena l i ze eel

    m i g r a t i o n s a n d reduce rec ru i tmen t by elvers a n d by ye l low eels . Abou t 12 f lavour mil l we i rs

    w e r e bui l t du r ing the X V t h cen tu ry but nowadays on ly 3 remain . More recent ly, at the

    b e g i n n i n g of t he century , a 4 m h igh d a m was built at 2 km f rom the river m o u t h to p rov ide

    t h e counc i l w i th d r i nkab le water. Dur ing Wor ld W a r II, several 5 m high conc re te d a m s

    w e r e a l so bui l t for s t ra teg ic reasons , a single o n e rema ins nowadays . In 1 9 9 1 , a 3 mi l l ion m 3

    c a p a c i t y reservo i r w a s c rea ted to p rov ide Sa in t -Malo for d r inkab le water : the 14 m h igh d a m

    of Bo is Jo l i w a s e q u i p p e d wi th an eel lift to pe rm i t e lvers to migra te ups t ream.

    In 1 9 9 3 , a sho r t su rvey by the local f i shery author i ty s h o w e d that on ly 2 to 3 000

    e l ve rs w e n t past t he Bo is Jol i d a m using this lift. T h e resul t ing recru i tment indice

    (50 e l ve rs / km 2 / yea r ) is abou t 10 t imes lower than those of s imi lar c a t c h m e n t s in Bri t tany,

    for i n s t a n c e the rec ru i tmen t is 563 e lvers /km 2 / year in the A r g u e n o n , a ne ighbour ing river

    ( L E G A U L T , 1994) . Th i s abno rma l l y low recru i tment indice w a s possib ly d u e to 2 we i rs

    l oca ted d o w n s t r e a m the Bo is Jol i ee l lift. There fo re , both these obs tac les w e r e e q u i p p e d

    w i t h e lver passes in 1996 .

    A n in teres t ing ques t ion rose f r om this s i tuat ion : w h a t are the ef fects of th is k ind of

    e q u i p m e n t on the d y n a m i c s a n d the eco logy of eel popu la t ions at the sca le of w h o l e

    c a t c h m e n t s ? If severa l su rveys of e lver or ee l recru i tment are now avai lab le ( J E L L Y M A N ,

    1 9 7 7 ; M O R I A R T Y , 1986 ; L E G A U L T , 1994 ; G A S C U E L et ai, 1995 ; M A R T I N , 1995 ;

    e tc . ) o n l y a few s t u d i e s have re la ted r ec ru i tmen t t r ends to p o p u l a t i o n d y n a m i c s

  • Bull. Fr Pche Pisclc. (1998) 349 :129-139 1 3 1 -

    Sectors

    Reservo i r

    Electrof ishings'^ Fyke nets Obstruct ions (see Tab le I)

    Figure 1 Geograpliic location of Frmur's watershed, sampling stations and obstructions to upstream migration.

    Figure 1 Situation gographique du bassin versant du Frmur, emplacement des stations de pche et des obstacles la migration anadrome.

    ( V O L L E S T A D a n d J O N S O N , 1 9 8 8 ) a n d n o n e a i m e d t o c o n d u c t a l o n g - t e r m s u r v e y o f the e f f e c t s o f m i g r a t i o n w a y s r e s t o r a t i o n o n t h e d y n a m i c s o f a n e e l p o p u l a t i o n at t h e s c a l e of a w h o l e c a t c h m e n t . S u c h p r o g r a m m e s t a r t e d in 1 9 9 5 o n t h e Frmur a n d is st i l l c o n d u c t e d by F i s h - P a s s c o m p a n y a n d t h e U n i v e r s i t y o f R e n n e s 1 .

    T h e p u r p o s e o f t h i s p a p e r is t o d e s c r i b e t h e c h a r a c t e r i s t i c s o f t h e e e l p o p u l a t i o n ( s i z e s t r u c t u r e , d e n s i t y a n d b i o m a s s ) in t h e Frmur's c a t c h m e n t , b e f o r e t h e i n s t a l l a t i o n of e l v e r p a s s e s o n t h e d a m s . S p e c i a l e m p h a s i s is g i v e n t o a n a l y s e t h e e f f e c t s o f h a b i t a t , of t h e d i s t a n c e f r o m t h e