Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 573 —
DIEL A N D S E A S O N A L PATTERNS O F HABITAT U S E BY F ISH IN A N A T U R A L S A L M O N ID B R O O K : A N A P P R O A C H T O
T H E F U N C T I O N A L R O L E O F T H E R I F F L E - P O O L S E Q U E N C E .
J . M . R O U S S E L , A . B A R D O N N E T
INRA, Laboratoire d 'Ecologie Aquat ique, 65 rue de Saint-Br ieuc, 35042 Rennes Cedex, France.
A B S T R A C T
The spat io - tempora l use of the riff le-pool sequence in a natural salmonid brook w a s
studied by day and night for one year on b rown trout (Salmo trutta L.), bul lhead (Cottus
gobio L.), European minnow (Phoxinus phoxinus L.) and s tone loach {Barbatula barbatula L.).
Young-of- the-year b rown trouts were more numerous in riffles in spr ing, whereas o lder trouts
chose pools. A diel pat tern of habitat use between riffle and pool was observed for one-year-old
individuals, w h o moved towards riffle dur ing dayl ight in summer and early fall. Seasonal
var iat ions in t rout densi t ies were related to movements at a large spatial scale before and after
the spawn ing per iod . Bul lhead preferred riff les to poo ls , especial ly young-o f - the-year
individuals, even if th is preference decreased dur ing ontogenesis. On the contrary, minnow
chose pools th roughout the year and seemed to leave the ri f f le-pool sequence for spawning,
whereas no preference was observed for loach except for riffle dur ing the spawning period.
Results and an approach to the funct ional role of the r i f f le-pool sequence in lowland salmonid
brooks are d iscussed.
K e y - w o r d s : b rown trout, bul lhead, European minnow, s tone loach, sa lmonid brook,
r i f f le-pool sequence, spatial heterogeneity, diel preferences, seasonal preferences, funct ional
habitat.
V A R I A T I O N S J O U R N A L I È R E S E T S A I S O N N I È R E S D ' U T I L I S A T I O N D E L ' H A B I T A T
P A R L E S P O I S S O N S E N R U I S S E A U À S A L M O N I D É S : U N E A P P R O C H E D U RÔLE
F O N C T I O N N E L D E L A S É Q U E N C E R A D I E R - P R O F O N D .
La success ion rad ie r -p ro fond cons t i t ue le pr inc ipa l é lément d 'hé térogéné i té
géomorpho log ique en pet i ts cours d 'eau à truites peu pentus. Ces deux faciès présentent des
caractér ist iques d 'écou lement radicalement dif férentes qui supposent des condi t ions d'habitat
elles aussi t rès dif férentes. Dans ce travail, l 'uti l isation spat io- temporel le de la séquence
radier-profond a été étudiée de jour et de nuit tout au long d e l 'année pour les espèces les plus
souvent rencontrées dans ce type de ruisseau, à savoir : la t rui te (Salmo trutta L.), le chabot
(Cottus gobio L.), la loche (Barbatula barbatula L.) et le vairon (Phoxinus phoxinus L) . Les
juvéniles de trui te de l 'année occupaient préférent iel lement les radiers (au moins au printemps)
alors que les truitel les d 'au moins 1 an ont mont ré un net choix pour le profond. Les
compar t iments radier et pro fond ont été uti l isés d i f féremment au cours du nycthémère par les
truitelles d 'un an, don t une partie se déplaçai t vers les radiers le jour en pér iode estivale. Les
f luctuat ions saisonnières de densi tés ont été expl iquées par des di f férences de recrutement
Reçu /e 23 mai 1997
Accepté le 17 juillet 1997
Received 23 May, 1997
Accepted 17 July, 1997
R É S U M É
Article available at http://www.kmae-journal.org or http://dx.doi.org/10.1051/kmae:1997005
Bull. Fr. Pêche Piscic. (1997) m : 573-588 — 5 7 4 —
annuel et des mouvements de popula t ion avant et après la reproduct ion. Le chabot préférait le
radier au profond, cet te préférence était très marquée pour les plus jeunes individus puis
s 'a t ténua i t au cours de l 'ontogenèse. Au contraire, le vairon a nettement préféré le profond et
aucun changement n'a pu être mis en évidence ; cependant, les faibles densités en juin étaient
p robab lemen t liées à des dép lacements hors secteurs pendant la reproduct ion. La loche quant
à el le était répartie à part égale entre les deux types de faciès, sauf en juin où sa préférence
pour le fac iès radier était p robab lement liée à la recherche d 'un habitat de reproduct ion.
L'analyse éco-é tho log ique des résultats obtenus dans ce type de mil ieu permet d 'approcher le
rôle fonc t ionne l de la séquence radier-profond à trois des échelles bio logiques de la structure
des ich tyocénoses : peuplement , popula t ion et individu.
Mots -c lés : trui te fario, chabot , vairon, loche, ruisseau, séquence radier-profond,
hétérogéné i té spatiale, préférences journalières, préférences saisonnières, habitat fonct ionnel.
I N T R O D U C T I O N
Natural salmonid streams provide a large set of physical habitat condi t ions for the
deve lopmen t of aquatic fauna, and f ish usually inhabit most available habitats. The stream fish
c o m m u n i t y is generally c o m p o s e d of benthic and nectonic species exhibi t ing different habitat
pre ferences and segregated accord ing to water dep th , velocity and subst ra tum size gradients
(GRIFFITH, 1972 ; JONES, 1975 ; BAGLINIÈRE and ARRIBE-MOUTOUNET, 1985 ; GLOVA and
SAGAR, 1 9 9 3 ; GIBSON ef a / , 1993). Spatial heterogeneity and physical f ramework are
therefore important fac tors in descr ib ing stream fish habitat. In their review, NIEMI era / . (1990)
c o n c l u d e d that the loss of habitat diversi ty due to physical alterations of the stream channel was
the mos t c o m m o n impact assoc ia ted wi th long recovery t imes for fish populat ions. From
FRISSELL ef al. (1986), physical habitat in streams can be def ined as hierarchically nested
subd iv is ions , f rom the largest scale of the entire river basin and drainage network, to the
smal lest microhabi tat scale wh ich refers to point condi t ions in the channel . At a median and
loca l sca le , an impor tan t source of physical habi tat heterogenei ty d e p e n d s on
geomorpho log ica l processes in the st ream channel . The channel type, w id th and slope,
c o m b i n e d w i th f low obst ruc t ions in the stream bed (blocks or coarse woody debris), generate
part icular hydraul ic cond i t ions that favour scour and deposit ional processes. These general
mechan i sms contr ibute to the fo rmat ion of channel geomorphic units, also cal led habitat types,
de f ined as areas of relatively homogeneous depth, velocity and subst ra tum character ist ics.
App l i ca t i ons in fishery management have led to the proposal of several c lassi f icat ions of habitat
t ypes (BISSON ef a/., 1982 ; FRISSELL ef a / , 1986 ; MALAVOI, 1989 ; BRYANT ef a / , 1992 ;
HAWKINS ef al., 1993 ; JOWETT, 1993 ; RABENI and JACOBSON, 1993). In spite of variability
be tween observers w h e n naming habitat units (ROPER and SCARNECCHIA, 1995), two basic
uni ts co r respond to the c o m m o n l y used terms « riffle » and « pool », a general level of resolution
tha t segregates shallow fast - f lowing and deep s low-f lowing habitats. In most alluvial valleys, the
r i f f le-pool sequence is a natural habi tat feature o f the river channel that occurs regularly (see
B ISSON and MONTGOMERY, 1996 for synthesis). Therefore, the riff le-pool sequence as an
ent ire habi ta t unit could be of great interest for f ishery biologists.
In a one-year study, we evaluated diel and seasonal variations in riffle and pool util ization
by f ish in a typical ly alluvial sa lmonid brook in Brittany, France. Two quest ions were examined
in detai l : 1 - H o w is the f ish commun i t y (species and life stages) d iv ided between riffle and pool
habi ta ts ? 2 - Are there tempora l changes (diel or seasonal) in their habitat type preference (riffle
or pool) ? The role of the riff le-pool sequence as a funct ional habitat feature for fish in salmonid
s t reams and management impl icat ions are then d iscussed.
S T U D Y S I T E
The s tudy was conduc ted in 1995-96 in the Kerlégan brook, a tr ibutary of the Scorff river,
Bri t tany, France (Figure 1 ). Its total length is 7.2 k m and it drains an area of 20.4 k m 2 . The mean
gradient is 13 m.knv 1 and the bo t tom substratum is mainly gravel-sand, which corresponds to
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 575 —
the « pool-r i f f le reach » accord ing to BISSON and MONTGOMERY 'S s t ream classi f icat ion
(1996). Bri t tany has a temperate oceanic c l imate, and mean daily water temperatures recorded
in the brook varied f rom 17°C (Aug-95) t o 5.5°C (Dec-95). M in imum (Jun-95) and m a x i m u m
(Sep-95) month ly precipi tat ions were 10 m m a n d . 180 m m , respectively. As in most sa lmonid
rivers in Brit tany, the deve lopment of macrophytes (especially Ranunculus spp) a lso contr ibutes
to seasonal modi f icat ions on channehhydraul ics (HAURY and BAGLINIÈRE, 1996). Carex (Carex
paniculate) f o rms the most f requent banks ide emergent vegetat ion and is essential ly d is t r ibuted
along pool margins of the brook.
F i g u r e 1
M a p o f t h e s t u d y s i t e .
F i g u r e 1
C a r t e d u s i t e d 'é tude.
M A T E R I A L S A N D M E T H O D S
Four surveys were conduc ted in June 1995, Oc tober 1995, January 1996 and Apr i l 1996.
In the ki lometre before the conf luence wi th the Scorff river, three similar r i f f le-pool sequences
spaced 50 to 150 metres apart (see Table I for habi tat descr ipt ion) were electrof ished with an
AC generator (200 W, cont inuous output) us ing the removal method (DE LURY, 1951). In order
to compare day and night densit ies in the same sequence, in each season, t w o sequences
were electrof ished by day (at least 3 h after dawn) and one by night (at .least 3 h af ter dusk).
Two weeks later, an inversed sampl ing (one unit by day, t w o units by night) was per formed. At
night, the sampl ing area was lit wi th a 500. W halogen spot light. There w a s no electrof ishing
dur ing full m o o n nights. A special d ipnet design (mesh 3 mm) pre-pos i t ioned across the channel
10 k m
O c e a n
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 576 —
and above the water surface made it possible t o parti t ion each sequence in c losed riffle and
poo l hab i ta ts synchronously wi thout wad ing just before electrof ishing. When riffles and pools
were c l o s e d , an operator carry ing a 30 c m ring anode waded f rom the downst ream to the
ups t ream end of each sequence. Two other peop le netted fish with 20 c m ring net handles
(mesh 3 mm) and special 60 c m semi-c i rcular sect ion net handles (mesh 3 mm) held flat against
the b o t t o m in the riffle. All f ish caught were anesthetized (phenoxyethanol), measured (nearest
mm) and then released inside the habi tat t ype where they were caught. Each t rout w a s
indiv idual ly marked wi th a miniature Passive Integrated Transponder (PIT), and scales were
s a m p l e d to determine age in the laboratory.
A g e st ructure in non-sa lmon id populat ions was evaluated by graphic analyses of modes
in b o d y length f requencies (BHATTACHARYA, 1967). Mean populat ion densit ies in pool and riffle
were es t ima ted for day and night samples by the CARLE and STRUB method (1978) using
Micro f ish 3.0 sof tware (VAN DEVENTER and PLATTS, 1989). Hypotheses about dif ferences in
day and night use of riffle and poo l were tested wi th non parametr ic Wi lcoxon-Mann-Whi tney
tes ts ( W M W test).
T a b l e I
M e a n r i f f l e a n d p o o l c h a r a c t e r i s t i c s a n d s e a s o n a l h a b i t a t v a r i a t i o n s i n t h e t h r e e r i f f l e - p o o l
s e q u e n c e s s t u d i e d .
T a b l e a u I Carac tér is t iques méso log iques m o y e n n e s e t v a r i a t i o n s saisonnières s u r l e s t r o i s s é q u e n c e s
r a d i e r - p r o f o n d d e l 'étude.
R I F F L E P O O L
Total length (m) 24 26
Jun. 95 2.9 2.8
M e a n width (m) Oct. 95 2.4 2.1
Jan. 96 3.1 2.9
Apr. 96 3J 2.9
M e a n surface area (m 2 ) 69 73
Jun. 95 8.6 27.2
M e a n depth (cm) Oct. 95 7.1 24.2
Jan. 96 22.5 41.7
Apr. 96 15 33
Jun. 95 23 8
M e a n velocity Oct. 95 14 5
(cm.s" 1) Jan. 96 51 27
Apr. 96 20 8
Jun. 95 gravel silt - sand
Bottom substrate Oct. 95 and silt - sand
(Wentworth scale) Jan. 96 small sand-gravel
Apr. 96 cobbles sand-gravel
Jun. 95 83 25
Macrophyte Oct. 95 42 8
cover (%) Jan. 96 33 25
Apr. 96 75 25
Jun. 95 58
Stream flow ( I . s 1 ) Oct. 95 28
Jan. 96 335
Apr. 96 85
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 5 7 7 —
R E S U L T S
A total of 11 species were caught on each sampl ing date. Roach (Rutilus rutilus L.), dace
(Leuciscus leuciscus L.), gudgeon (Gobio gobio L ) , At lant ic sa lmon (Salmo salar L.) and eel
(Anguilla anguilla L.) were minori ty species., Numerous juveniles of sea (Petromizon marinus L.)
and brook (Lampetra planer! Bloch) ' lamprey had bur ied themselves into the silty habitats of
pools , and so inadapted electrof ishing prec luded correct est imat ions of their populat ion
densit ies. Bul lhead (Cottus gobio L.), European m innow (Phoxinus phoxinus L.), s t one loach
(Barbatula barbatula L.) and b rown trout (Salmo trutta L.) represented over 90 % of all individuals
caught dur ing the four surveys. These were then more precisely invest igated. For each survey,
no signif icant dif ference was found between the total number of fish caught dur ing the first and
the second electrof ishings wh ich took place t w o weeks later (p>0.10, W M W test). This indicates
that there was no inf luence of successive electrof ishings on species popula t ion densit ies.
B u l l h e a d
Bullhead was the most numerous species in the brook, and represented 57 % of all
individuals caught during the entire exper iment . No diel dif ference was observed in its
d istr ibut ion wi th in the riff le-pool sequences (Figure 2). Densit ies in the poo l were constant and
signif icantly lower (1 ind/m 2) than in riffle (p<0.05, W M W test). On the contrary, densi t ies in riffle
var ied f rom 6 i nd /m 2 (Oct-95) to 1 . 5 i n d / m 2 (Jan-96) over the year. These dif ferences were related
to demograph ic changes within the popula t ion. Three life stages were clearly identi f ied by body
length f requency analysis (Figure 4) : young-of - the-year (0+), one-year-o ld (1+) and^older fish (*1).
0 + appeared in electrof ishing samples in Jun-95 , but densit ies were probably underest imated
because of their small body size (20 m m on average). After a first summer, their mean body size
was 38 m m (Oct-95) making them easier t o ca tch. This explains why the highest densit ies of
bul lhead were est imated in riffles in Oc t -95 (6 ind/m 2 ) , 58 % of wh ich were 0 + . More than 90 %
of all 0* electrof ished came f rom riffles. During winter, densit ies strongly decreased for all life
s tages (1.5 i n d / m 2 in Jan-96). The 0 + still represented 55 % of the total populat ion, but their
cho ice for riffle habitat was less marked (78 % ) . In spring (Apr-96), 32 % of the new one-year-old
cohor t was caught in pools, wh ich cor responded to the proport ion of the tota l populat ion that
was electrof ished in pools. Growth a lmost s topped dur ing winter, as shown by the mean body
size in Apr-96 (39 mm). For older bul lheads, mean body sizes showed little increase f rom
summer to winter (from 47.5 to 49.8 m m and f rom 64.5 to 65.6 m m for 1 + and ' 1 , respectively),
suggest ing a higher growth rate in spr ing for this species.
M i n n o w
No diel dif ference was found in the distr ibut ion of minnow wi th in the riff le-pool
sequences (Figure 2). Unlike bul lhead, m innow densit ies were a lways lower in riffles (from
0 i nd /m 2 in Jun-95 to 0.4 in Apr-96) than in pools , and this was signif icant in Oct -95 , Jan-96 and
Apr-96 (p<0.05, W M W test). However, densit ies in pools varied f rom less than 0.1 i nd /m 2 (Jun-95)
to 2 i nd /m 2 (Oct-95) over the seasons. Like for bul lhead, these di f ferences were related to
demograph ic changes with in the populat ion. Only the young-of - the-year cohor t (0+) w a s clearly
identi f ied f rom analysis on body length f requencies. 0 + appeared in the Oct -95 sampl ing and
represented only 32 % of the minnow populat ion (Figure 4). Their mean body size w a s 30 m m
and it was not clear whether the electrof ishing had been efficient or not. No 0 + was caught in
Jan-96, and their growth almost s topped in winter as shown by their mean body size in Apr-96
(33 mm) when 29 % of m innow caught were 0 + . No 0* and only a few older individuals were
electrof ished in Jun-95 (Figure 2).
S t o n e l o a c h
Stone loach densit ies were global ly lower than those of bul lhead and minnow (Figure 2).
No signif icant habitat cho ice was recorded, except in Jun-95 when densi t ies were highest in
riffle (0.5 ind /m 2 ) and lowest in pool (0.1 ind /m 2 ) . No diel dif ference was found in the distr ibut ion
Bull. Fr. Pêche Piscic. (1997) 346 : 573- — 5 7 8
M i n n o w
S t o n e l o a c h
B u l l h e a d
i
1 I 1 1 1
J u n .
. O c t .
. J a n .
* Apr .
J u n .
= J
i i
i I i
i = J
i i
i I i
i = J
i i
i I i
i
1** O c t .
* J a n .
' * * Apr .
+ +
Jun .
Oct .
J a n .
Apr.
0 ,8 0,6 0 ,4 0,2
D e n s i t i e s i n p o o l ( i n d / m 2 )
0,2 0,4 0,6 0,8
D e n s i t i e s i n r i f f l e ( i n d / m 2 )
f
[ 1
^ : :
— ' 1
F i g u r e 2
D i e l a n d s e a s o n a l d e n s i t i e s e s t i m a t e d i n r i f f le a n d p o o l f o r b u l l h e a d , m i n n o w a n d s t o n e l o a c h .
B l a n k a n d h a t c h e d b a r s r e p r e s e n t d a y l i g h t a n d n i g h t s a m p l i n g s , r e s p e c t i v e l y . H o r i z o n t a l l i n e s
r e p r e s e n t a 9 5 % c o n f i d e n c e l e v e l f o r e s t i m a t e d p o p u l a t i o n s ( p < 0 . 0 5 , S t u d e n t t d i s t r i b u t i o n ) .
S t a t i s t i c a l d i f f e r e n c e s i n d e n s i t i e s b e t w e e n r i f f le a n d p o o l , b y d a y a n d n i g h t , a r e e s t i m a t e d b y n o n
p a r a m e t r i c W i l c o x o n - M a n n - W h i t n e y t e s t s (* : p < 0 . 0 5 ; ** : p < 0 . 0 1 ) .
F i g u r e 2
E s t i m a t i o n s journal ières e t saisonnières d e s densités e n r a d i e r e t e n p r o f o n d p o u r l e s c h a b o t s ,
v a i r o n s e t l o c h e s . L e s b a r r e s b l a n c h e s e t hachurées représentent l e s échant i l lonnages d e j o u r
e t d e n u i t , r e s p e c t i v e m e n t . L e s t r a i t s h o r i z o n t a u x représentent l e s i n t e r v a l l e s d e c o n f i a n c e à
9 5 % ( p < 0 , 0 5 , d i s t r i b u t i o n t d e S t u d e n t ) d e s densités es t imées . L e s différences s t a t i s t i q u e s d e
dens i tés calculées e n t r e r a d i e r e t p r o f o n d , d e j o u r e t d e n u i t , s o n t évaluées p a r d e s t e s t s n o n
p a r a m é t r i q u e s d e W i l c o x o n - M a n n - W h i t n e y (* : p < 0 , 0 5 ; ** : p < 0 , 0 1 ) .
of s tone loach within the ri f f le-pool sequences. Unfortunately, the small amount of individuals
caught per season meant that a correct analysis of cohorts in body length frequencies was not
poss ib le .
B r o w n t r o u t
Three age classes were identi f ied for brown trout by scal imetr ic analysis : young-o f - the-
year (0 +), one-year-o ld (1+) and older f ish (*1). Three-year-old t routs represented 21 % of '1
caught for all sampl ing dates. During the year, mean size-class for 0 + ranged from 28 m m after
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 579
0,4
J u n .
Oct .
J a n .
Apr.
I I I I I I
;
i 0,3 0,2
D e n s i t i e s i n p o o l ( i n d / m 2 )
0,1 0,2
D e n s i t i e s i n r i f f l e ( i n d / m 2 )
0,3 0,4
F i g u r e 3
D i e l a n d s e a s o n a l a g e - c l a s s e s d e n s i t i e s f o r b r o w n t r o u t s e s t i m a t e d i n r i f f l e a n d p o o l ( s e e F i g u r e 2
f o r e x p l a n a t i o n s ) .
F i g u r e 3
E s t i m a t i o n s journal ières e t saisonnières d e s densités d e t r u i t e s e n r a d i e r e t e n p r o f o n d p a r
c l a s s e s d 'âge ( v o i r F i g u r e 2 p o u r p l u s d e détails).
emergence to 85 m m in their f irst winter, f rom 103 m m to 138 m m for 1 + , and f rom 164 mm to
185 m m for M (Figure 4). Analysis of the seasonal evolut ion in fork length for individual ly marked
and recaptured 1* and »1 t routs showed that individual mean g rowth was high in summer and
s topped in fall and early winter except for one trout (Figure 5). Thereafter, mean body size
increased f rom winter to spr ing.
T h e t rbuf 'populat ion mainly compr ised 1 + fish f rom Jun-95 to Jan-96 (Figure 3). In 1995,
recrui tment was very low because of an except ional ly high f lood (one per hundred years) during
the winter 1994-95. However, the 1996 recrui tment was much better and consequently,
densi t ies of 0* were high in Apr-96 samples. For this per iod, soon after emergence, 0* were
signif icantly more numerous in riffle (p<0.05, W M W test). The 1* cohor t signif icantly preferred
pools f rom Jun r95 - to : Jan -96 (p<0.05, W M W test), whereas no preference between the t w o
habitat types was observed in Apr -96. Older t routs (M) showed the same tendencies as 1 + , even
if results were not statist ical ly signif icant. Signif icant diel di f ferences in 1* t rout distr ibut ions in
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 580 —
B u l l h e a d
Oct . 95 Jan. 96 Apr. 96
M i n n o w
Oct. 95 Jan. 96 Apr. i
S 0,5
80
™ r r r | g - i g i .60
' 4 0 + -
20 J_ o: _6.
— < > -
spawning
A p r . 96 Jun. 95 Oct. 95 Jan. 96 A p r . 96
80,5
200i
E150
sioo o u_
50
0
0*
B r o w n t r o u t
Oc t . 95 Jan.
emergence
Jun. 95 Oc t . 95 Jan. 96 A p r . 96
F i g u r e 4
S e a s o n a l v a r i a t i o n s i n a g e s t r u c t u r e f o r b u l l h e a d , m i n n o w a n d b r o w n t r o u t p o p u l a t i o n s ( d a r k
b a r s ) , a n d i n m e a n b o d y s i z e s f o r e a c h a g e c l a s s . V e r t i c a l l i n e s r e p r e s e n t s t a n d a r d d e v i a t i o n ( S D ) .
F i g u r e 4
V a r i a t i o n s saisonnières d e l a c o m p o s i t i o n d e s p o p u l a t i o n s d e c h a b o t s , d e v a i r o n s e t d e t r u i t e s
s e l o n l e s c l a s s e s d 'âge ( b a r r e s n o i r e s ) , e t d e s l o n g u e u r s m o y e n n e s p o u r c h a q u e c l a s s e d 'âge.
L e s t r a i t s v e r t i c a u x représentent l e s écar ts t y p e s a u t o u r d e l a m o y e n n e .
riffle were recorded in Jun-95 and Oc t -95 . For this per iod, almost all t routs were in pools at night
wh i le they used both riffles and pools during dayl ight. This was signif icant for 1 + , but '1 t routs
s h o w e d the same tempora l pat tern.
Analys is of individually marked trouts indicated that 49 % (Jun-95) and 61 % (Oct-95) of
f ish caugh t dur ing the first electrof ishing were recaptured two weeks later in the same riff le-pool
sequence (Figure 6). This r i f f le-pool fidelity d ropped in winter (22 % in Jan-96) and then
increased again in spr ing (30 % in Apr-96). This general pattern was also conf i rmed in riff le-pool
f idel i ty be tween seasons.
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 581 —
J u n . 95 Oct . 95 J a n . 9 6 Apr. 9 6
F i g u r e 5
S e a s o n a l v a r i a t i o n s i n f o r k l e n g t h f o r i n d i v i d u a l l y m a r k e d a n d r e c a p t u r e d t r o u t s . L i g h t l i n e s
r e p r e s e n t i n d i v i d u a l g r o w t h , h e a v y l i n e s m e a n v a l u e .
F i g u r e 5
V a r i a t i o n s saisonnières d e l a l o n g u e u r f o u r c h e d e s t r u i t e s m a r q u é e s e t recapturées l o r s d e s
différentes pêches . L e s t r a i t s f i n s représentent l a c r o i s s a n c e d e c h a q u e i n d i v i d u , l e s t r a i t s épais
l a c r o i s s a n c e m o y e n n e .
100
Jun. 95 Oct. 95 Jan. 96 Apr. 96
F i g u r e 6
P e r c e n t a g e o f i n d i v i d u a l l y m a r k e d a n d r e c a p t u r e d t r o u t s i n t h e s a m e r i f f l e - p o o l s e q u e n c e t w o
w e e k s a p a r t f o r e a c h s e a s o n ( d a r k b a r s ) , a n d b e t w e e n s e a s o n s ( h o r i z o n t a l l i n e s ) .
F i g u r e 6
P o u r c e n t a g e d e t r u i t e s m a r q u é e s e t recapturées d a n s l a m ê m e s é q u e n c e r a d i e r - p r o f o n d à d e u x
s e m a i n e s d ' i n t e r v a l l e ( b a r r e s n o i r e s ) , e t d ' u n e s a i s o n à l ' a u t r e ( t r a i t s h o r i z o n t a u x ) .
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 582 —
D I S C U S S I O N
Bul lhead, minnow, b rown trout and stone loach are widely d ist r ibuted along the
longi tud ina l axis in s t reams in Bri t tany (OBERDORFF and PORCHER, 1992). In the Scorff river
bas in , sympat r i c populat ions of these species can occur in the main river (BAGLINIÈRE and
ARRIBE-MOUTOUNET, 1985) and in tributaries, where they are the predominant species with
eel (BAGLINIÈRE, 1979). Some field and experimental studies have been concentrated on the
potent ia l over lap of d iets and/or habi tat requirements (MAITLAND, 1 9 6 5 ; M A N N and ORR,
1 9 6 9 ; LIEN, 1981 ; NEVEU, 1981 ; BALTZ et al., 1 9 8 2 ; WELTON et al., 1 9 8 3 ; GLOVA, 1 9 8 7 ;
B R O W N , 1991), but little is known about the tempora l aspect of habitat use. On a seasonal
scale, habi tat shifts are closely related to the life history of species, like ontogenet ic changes in
habi ta t requi rements (ELLIOTT, 1986 ; BAGLINIÈRE era/ . , 1989 ; BALTZ et al., 1991 ; SEMPESKI
and G A U D I N , 1995 ; KOCIK and TAYLOR, 1996), spawning (BAGLINIÈRE era/ . , 1987 ; MEYERS
et al., 1992) and overwinter ing migrat ion (CUNJAK and POWER, 1986 ; SWALES ef a/., 1986 ;
B R O W N and MACKAY, 1995), whereas diel habi tat shifts usually cor respond to circadian
rhy thms of rest ing and feeding act iv i t ies (CAMPBELL and NEUNER, 1985 ; KWAK et al., 1992 ;
HEGGENES e ra / . , 1993 ; ROUSSEL and BARDONNET, 1996).
S e a s o n a l c h a n g e s i n d e n s i t i e s
For bul lhead, t he decrease in win ter did not affect the structure of the populat ion and the
respect ive percentages of each age c lass. The densi ty of 1* and older individuals in April 1996
is simi lar to those observed in June 1995. This w o u l d indicate that a high winter mortal i ty (more
than 5 0 % ) affects the bul lhead populat ion. In April 1996, the 0 + cohor t is still missing in
e lect rof ish ing samples, but their catchabi l i ty is probably null because of a total body length of
less than 10 m m after hatching (SMYLY, 1957). In trout, the decrease recorded in January for
the 1994 cohor t (1*) can be expla ined by the general pattern of upstream spawning migrat ion
(which inc ludes 1* mature individuals), as descr ibed by BAGLINIÈRE ef al. (1987). Thereafter,
there is a smal l increase in Apri l 1996 for the 1995 cohort densi ty (the new 1 + cohort). This
re-equi l ibr ium in 1 + densi t ies in the low part of t he brook cou ld be the result of a downst ream
movemen t f rom the upper part of the brook initiated in the previous au tumn by 0 + , as studied
by BAGLINIÈRE ef al. (1989) in another tributary of the Scorff river. For minnow, a winter
decrease w a s also observed , wi th 0 + individuals absent f rom electrof ishing samples. Accord ing
to d iv ing observat ions in Kerlégan pools (ROUSSEL and BARDONNET, unpubl ished data),
0 + m innow may exhibit a s t rong winter concealment in carex f ine roots a long pool margins. This
makes ca tch ing them by electrof ishing very difficult, because of their quiescent behaviour and
the low water temperature. Thereafter, the increase in densit ies in Apri l 1996 is related to the
re -appearance of 0* in samples . Dur ing the breeding per iod, very few individuals were caught in
June 1995. M innow is a l i thophil spec ies that breeds on gravel bars (BALON, 1975). Such
spawn ing areas are character ized by shal low, swift waters (PONCIN, 1996). Because of the high
dens i ty of spawn ing shoals, their space partit ion on spawning areas is very patchy and possibly
none of t h e m w a s present in the samp led areas. For loach, the decrease in densi ty in January
and Apri l canno t be expla ined by popula t ion dynamics since densit ies were too low to identify
age c lasses.
H a b i t a t t y p e c h o i c e
Minnow
Accord ing to the l i terature, habitat preferences in European minnow depend on stream
size. On a 15 m w id th s tat ion in the Scorff river, BAGLINIÈRE and ARRIBE-MOUTOUNET (1985)
observed that densi t ies of m innow in October were the highest in shal low and lotie habitats
(about 20 c m dep th and f rom 20 to 40 cm/s). Under similar condi t ions in the river Nivelle
(France), NEVEU (1981) hypothes ized that they avo ided the deepest habi tats (more than 50 cm
depth) because of the presence of predators. The increased use of shal low habitat by minnow
under prédat ion risk has been recently pointed ou t under exper imental cond i t ions (EKLÔV ef al.,
1994). However, results on smal l s t reams (less than 6 m width) showed that minnow usually
inhabi t the deepest (more than 30 cm) lentic areas (MAITLAND, 1 9 6 5 ; JONES, 1 9 7 5 ;
Bull. Fr. Pêche Piscic. (1997) 346 : 573-588 — 583 —
BAGLINIÈRE, 1979). This corresponds to a preference for pools, as observed in Kerlégan brook
for all life s tages. The minnow's affinity for cover and shelter st ructures, especial ly for coarse
w o o d y debr is (NEVEU, 1981), macrophytes (LIEN, 1 9 8 1 ' ; MASTRORILLO ef al., 1996) o r stony
bo t tom substrate (JACOBSEN, 1 9 7 9 ; BAGLINIÈRE and ARRIBE-MOUTOUNET, 1985), has
often been reported by authors. A long the pool margin in the Kerlégan brook, over hanging
carex provides a large amount of overhead cover and shelter (underbank, f ine roots).
Sfone loach
As many loaches were found in pools as in riffles, except in June. This w ide distr ibut ion
in different habi tat types was also noted by MAITLAND (1965) and ZWEIMÛLLER (1995) in
lowland brooks, even if MAITLAND not iced a sl ight preference for pools. However, JONES
(1975) observed a marked preference for riffle or run (depending on the stream studied), with in
all cases the lowest densit ies in pools. Cont rad ic t ions in the literature about loach habitat use
are c o m m o n and diff icult to clarify. The s tone loach is a very discreet species and little is known
about its habitat requirements. Despite its s t rong associat ion wi th the bo t tom as a benthic
species, SMYLY (1955) d id not f ind any preference for sandy, gravelly and m u d d y bo t toms. The
use of shelter dur ing daylight has been descr ibed by BURDEYRON (1981), and s tone loach d id
not seem to need any speci f ic structures to exhibit th is behaviour s ince they are able to bury
themselves in the sandy bo t tom. BURDEYRON (1981) observed a seasonal d isplacement
towards areas wi th aquat ic macrophytes in spr ing and depar ture at the beginning of fal l , and
suggested that f ish moved in spr ing to reproduce on macrophytes . This cou ld explain why in
June they were more numerous in riffle than in poo l , s ince Ranunculus spp mainly developed
on riffles (Table I).
Bullhead
Bul lheads, like freshwater Cottidae in general , are morphological ly adapted to hold a
posi t ion on the bo t tom in high velocit ies wi thout energy constra ints (FACEY and GROSSMAN,
1992). Depend ing on the study, they inhabi t e i ther fas t - f low ing (BAGLINIÈRE, 1 9 7 9 ;
BAGLINIÈRE and ARRIBE-MOUTOUNET, 1 9 8 5 ; COPP, 1992) or s low- f low ing habi tats
(GAUDIN, 1981 ; GAUDIN and CAILLÈRE, 1990). In October, a lmost all ( 9 0 % ) young-o f - the-
year individuals were electrof ished in riffles. Riffles in Kerlégan brook may be attractive to
bul lhead for t w o different reasons : the use of unembedded cobb les as shelter is well known in
f reshwater Cottidae (BROWN,.1991 ; GREENBERG and STILES, 1993 ; HARO and BRUSVEN,
1994) and stone shelters are in l imited number in poo ls (Table I). The other reason may be
t rophic : indeed, PETTY and GROSSMAN (1996) repor ted that patchy distr ibut ion in mot t led
sculp ins (Cottus bairdi Girard) in large st reams was constant ly related to prey abundance. A
similar patchy dist r ibut ion cou ld cor respond to riffle habi tat in the Kerlégan brook, s ince riffles
are known to be higher benthos-produc ing areas (LOGAN and BROOKER, 1983 ; BROWN and
BRUSSOCK, 1991). The use of riffle was more p ronounced in 0 + f ish, even if it decreased
through the seasons to reach 70 % in Apri l , wh i ch cor responded to the total populat ion
propor t ion in riffle. GREENBERG (1991) reported that small benthic f ish avoided deep habitats
because of a higher prédat ion risk. As they grow, bul lheads become less and less vulnerable t o
f ish prédat ion, and at the same t ime are able to eat larger prey (ANDREASSON, 1971). A shift
in feeding habits cou ld lead them to explore deeper habi tats. It is then possible that the slight
seasonal decrease in preference f rom riffle nursery areas towards pool habitats may result from
a progressive d rop in prédat ion risk associated wi th a change in feeding mot ivat ions as the f ish
grow. Trophic capaci t ies or the number of s tone shelters cou ld have f ixed the upper density in
the pool to about one individual per square metre.
Brown trout
The new 0 + cohor t of b rown trout also preferred riffle to poo l . As hypothesized for
bul lhead, young 0* might avoid deep areas because of the p isc ivorous f ish prédat ion risk
(BARDONNET and HELAND, 1994). On the contrary, 1 + and older individuals were more
numerous in pool habi tats for most of the t ime. This spat ial inter-cohort segregat ion along the
water dep th gradient has already been descr ibed for b rown trout in other s tudies (BOHLIN,
Bull. Fr. PêchePiscic. (1997)m:573-588 — 5 8 4 —
1977 ; EGGLISHAW and SHACKLEY, 1982 ; BAGLINIÈRE and ARRIBE-MOUTOUNET, 1985). A shift f r om 0 + riffle habitats towards deeper areas is establ ished through the seasons, and can be ini t iated f rom the beginning of summer (KOCIK and TAYLOR, 1996) or in winter (ELLIOTT, 1986). It cou ld be related t o ontogenet ic changes in habitat requirements. Accord ing to HARVEY and STEWART (1991), as they grow, juveni le brown trouts may become more exposed to diving or wad ing predators in shal low riffle, and so need t o seek shelter in deeper pool habitats. However, habi ta t change can a lso co r respond t o reaching a cri t ical size wh ich may limit compet i t ion or even prédat ion f rom older t routs.
The only observat ion of a diel use of the riff le-pool sequence came f rom 1 + and older t routs . Dur ing the growing season (June-October) when the highest r i f f le-pool f idelit ies were recorded for individual ly marked t routs, they exhibi ted an increased use of riffle dur ing dayl ight. A similar movemen t f rom pool to riffle during dayl ight has already been shown under exper imenta l condi t ions on 1 + t routs and related t o feeding (ROUSSEL and BARDONNET, 1995). BRIDCUT and GILLER (1993) observed that trouts marked in riffles showed more movements f rom these habitats than t routs marked in pools, indicat ing that riffles are occasional rather than regular habi tats. Habitat cho ice wi th in the riffle-pool sequence could then be regarded as a t rade-of f be tween potent ia l energy intake and prédat ion risk (FAUSCH, 1984 ; METCALFE ef al., 1987), s ince faster water in riffles is supposed to provide better drift feeding condi t ions than pools , but is a lso cons idered as a more risky area.
T h e f u n c t i o n a l r o l e o f t h e r i f f l e - p o o l s e q u e n c e
At the riff le-pool scale, two types of temporal changes were identif ied. Stone loach, 0* trout and bul lhead represented examples of seasonal changes, probably l inked wi th a movement to join the spawn ing si te for loach and wi th ontogenesis for t routs and sculp ins. The other tempora l change was diel and concerned the increased use of riffle by 1 + and older t routs dur ing dayl ight . These results conf i rm that f ish habitat management in sa lmonid st reams should not be assessed w i thou t consider ing the tempora l dimension of fish habitat requirements.
The r i f f le-pool sequence is an important habitat feature in sa lmonid st reams providing spat ia l heterogenei ty to suppor t f reshwater fish deve lopment at different biological levels. At the f ish c o m m u n i t y level, it may enhance biodiversity, al lowing species wi th different habitat requi rements to cohabi t . A t the popu la t ion level, it authorizes age classes exhib i t ing different habi tat preferences t o deve lop in adjo in ing habitat types. Lastly, at the individual level, it a l lows the express ion of daily behaviour w i th pools providing nocturnal resting areas for t rout feeding in riffle. For these reasons, more at tent ion should be paid to protect ing the ri f f le-pool sequences and spat ia l heterogenei ty on channel hydraulics, since habitat diversity loss appears to be the major cause of long- term al terat ions in f ish populat ions (NIEMI ef al., 1990). Accord ing to the l i terature, the natural dynamics of instream coarse woody debris plays an impor tant role in hydraul ic p rocesses assoc ia ted w i th riff le-pool format ion and stabil ization of the channel in dif ferent t ypes of streams, including lowland rivers (GREGORY and DAVIS, 1992 ; GREGORY ef al., 1992 ; LANGFORD, 1996). Thus, the removal of instream w o o d y debr is, wh ich is a usual pract ice in French st ream management , would need to be reconsidered owing to the harmful consequences this cus tom has on the natural hydraul ic processes that are closely related to the r i f f le-pool sequence format ion.
A C K N O W L E D G E M E N T S
We wou ld like to thank N. JEANNOT, F. LEFEVRE, F. MARCHAND and many students for their he lp dur ing the f ield exper iments . We are also grateful to J.Y. MOËLO and to the Scorff Angl ing Assoc ia t ion for electrof ishing and exper iments facil it ies. This work was partially f inanced by the French Consei l Supérieur de la Pêche.
Bull. Fr. Pêche Piscic. (1997) 346:573-588 — 585 —
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