Unsaturated fish assemblages in rivers of the North-Western ......We used two extensive data sets representing two different spatial scales. The first was a set of 10 tributaries and

Bull. Fr. Pêche Piscic. (1997) 344/345 :193-204 — 193 —

UNSATURATED FISH ASSEMBLAGES IN RIVERS OF NORTH-WESTERN FRANCE : POTENTIAL

CONSEQUENCES FOR SPECIES INTRODUCTIONS.

D. B E L K E S S A M (1), T. O B E R D O R F F ( 1 , 2)* , B. H U G U E N Y (3)

(1) Muséum National d 'Histo i re Naturel le, Laboratoire d ' Ichtyo log ie Générale et Appl iquée, 43 rue Cuvier, 75231 Paris Cedex 5, France.

(2) Consei l Supérieur de la Pêche, Délégat ion Régionale de Compiègne, 3 rue Sainte-Marie,

60200 Compiègne , France.

(3) Antenne O R S T O M , Laboratoi re d 'Ecolog ie des Eaux Douces, Universi té Claude Bernard Lyon I, 43 boulevard du 11 novembre 1918, 69622 Vi l leurbanne Cedex, France.

A B S T R A C T

Int roduct ions of f reshwater f ish into French rivers have been carr ied out wi th increasing

f requency in the last decades . Thus, the potent ial impact of these in t roduct ions is o f major

concern for biologists and f ishery managers . Knowledge of the degree of saturat ion of a

commun i t y provides an impor tant basis for understanding how the commun i t y reacts or will

react to the int roduct ion of new species. In this paper, we compare local and regional freshwater

f ish species r ichness in similar sizes tr ibutar ies and coastal r ivers located in the same

b iogeographic region (North-Western France). We show 1) that species r ichness is greater for

tr ibutaries than for coastal rivers on bo th local (station) and regional (catchment) scales, and 2)

that, whatever the rivers s tud ied (tributaries or coastal rivers), there is a signif icant, positive

linear relationship between local species r ichness and regional spec ies r ichness. These f indings

suggest that local, and probably regional, f reshwater f ish commun i t i es in Nor th-Western French

rivers are unsaturated and thus , that major impac ts on the communi ty , such as short- term

ext inct ions, are not to be expec ted . However, compet i t ion for f o o d and space , prédat ion,

int roduct ion of exot ic parasites and diseases, cou ld also affect some native species.

K e y - w o r d s : species r ichness, f ish assemblages, insaturat ion, local r ichness, regional

r ichness, species in t roduct ions.

I N S A T U R A T I O N D E S P E U P L E M E N T S P I S C I C O L E S D A N S L E S

C O U R S D ' E A U F R A N Ç A I S D U N O R D - O U E S T : C O N S É Q U E N C E S

P O T E N T I E L L E S P O U R L E S I N T R O D U C T I O N S D ' E S P È C E S .

R É S U M É

Evaluer les conséquences des int roduct ions d 'espèces dans les cours d 'eau reste

diff ici le, car il est généralement reconnu que l 'espèce introduite et la c o m m u n a u t é réceptr ice ne

peuvent être étudiées séparément et que les conséquences liées à l ' introduct ion de l 'espèce

sont suscept ib les de se mani fester à dif férents niveaux d 'organisat ion éco log ique. A l'échelle

des communau tés pisc icoles, la première quest ion à se poser afin d 'aborder le problème des

int roduct ions concerne leur degré de saturat ion ; en d 'autres te rmes : jusqu 'à combien

d 'espèces peut suppor ter un écosys tème ? Bien que non suff isante, la réponse à cet te quest ion

permet t ra de savoir si oui ou non le mil ieu est potent ie l lement colonisable.

* Corresponding author, auteur pour correspondance.

Article available at http://www.kmae-journal.org or http://dx.doi.org/10.1051/kmae:1997022

http://www.kmae-journal.org

http://dx.doi.org/10.1051/kmae:1997022

Bull. Fr. Pêche Piscic. (1997) 344/345:193-204 — 194 —

Dans ce t article, en se replaçant dans les perspect ives théor iques générées par la

b iogéograph ie insulaire, nous comparons , à deux échelles spatiales que sont le bassin versant

et la s ta t ion, la r ichesse en espèces de fleuves côt iers et d'aff luents de tailles similaires et situés

dans une m ê m e région b iogéographique. Nous mont rons 1) qu 'à l'échelle du bassin versant, la

r ichesse en espèces des f leuves côt iers est inférieure à la r ichesse en espèces des affluents

(r ichesse régionale) ; 2) qu ' à l 'échelle de la station, la r ichesse en espèces des fleuves côt iers

est inférieure à la r ichesse en espèces des affluents (richesse locale) ; 3) qu' i l existe une relation

l inéaire pos i t ive entre la r ichesse locale et la richesse régionale et cela quel que soit le type de

cou rs d ' eau considéré (affluents ou f leuves côtiers). Ainsi , il est permis de supposer qu 'à

l 'échel le locale le milieu reste insaturé en espèces. Des effets impor tants sur la structurat ion des

c o m m u n a u t é s , tels que des ext inct ions à court terme, sont donc peu probables. Néanmoins,

ce la ne veut pas dire que toute int roduct ion irréfléchie sera sans conséquence sur les espèces

rés idantes . En effet, la prédat ion, la compét i t ion pour l 'espace et l 'al imentat ion, l ' introduction

de paras i tes o u de maladies exot iques sont également suscept ib les d 'engendrer des effets

dé lé tères sur certaines espèces autochtones.

M o t s - c l é s : r ichesse en espèces, peuplements piscicoles, insaturat ion, r ichesse locale,

r ichesse régionale, in t roduct ions d 'espèces.

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

The int roduct ion of f ishes into European rivers began wi th the carp (Cyprinus carpio L.)

dur ing the R o m a n per iod and cont inued until the second part of the nineteenth century (BALON,

1974). The number of in t roduced species has increased over the years, especial ly since the

beg inn ing of the twent ie th century. The species most frequent ly in t roduced in Europe are

cypr in ids , fo l lowed by sa lmonids and coregonids (HOLCIK, 1991).

The impact of species introduction into different ecosystems has been extensively

d o c u m e n t e d throughout the wor ld and many authors have commented on the difficulty of

documen t ing the specif ic role of introduced species on native communit ies, particularly in

f reshwater f ish communi t ies (ARTHINGTON, 1991). However, it is widely agreed that the

int roduct ion of fish usually has a damaging effect on the receiving aquatic ecosystem and its

ind igenous f ish fauna (HERBOLD and MOYLE, 1986 ; BROWN, 1989 ; BIANCO, 1990 ; HOLCIK,

1991 ; ELVIRA, 1995). Nevertheless, there is a lack of any valid evaluation of introduction

exper iments based on thorough ecological analysis (HOLCIK, 1991). In France, early publications

wh ich cal led attention to fish introduct ions, are those of CESTAS (1992) and KEITH era / . (1992).

These publ icat ions essentially compi le lists of fish transfers and provide very global analyses of the

risks of introduct ions. One way for highlighting possible effects of introduced species on natural

f ish communi t ies is to evaluate the degree of saturation in a given community. In a saturated

communi ty , a new (introduced) species must either go extinct or cause the extinction of another

species (which the introduced species then replaces). Conversely, in a nonsaturated community,

in t roduct ion of new species may be successful and cause no systematic extermination of existing

species (PIANKA, 1983). CORNELL (1985) has devised a simple test to check for communi ty

saturat ion. If local species richness shows no tendency to reach a clear upper limit in regions with

the highest species richness, but rather increases proport ionately with regional richness, then local

communi t ies are presumably not saturated. Another important point is to assess the effects of

in t roduced species at the regional scale (the term regional refers to the spatial scale at which

b iogeographic processes predominate). A positive relationship between river size and total

number of resident fish species has been demonstrated in many cases, suggesting that, in some

way, river size sets a limit for the number of species supported by a river (HUGUENY, 1989 ;

W E L C O M M E , 1990 ; OBERDORFF et al., 1995). There are two main, non-exclusive, hypotheses

to explain the species-area relationship : 1) area-dependent extinction rates, which form part of the

theory of island biogeography (Mc ARTHUR and WILSON, 1963,1967) and 2) the habitat diversity

hypothesis (Mc ARTHUR and WILSON, 1967). According to the first hypothesis, the probabil ity of

ext inct ion of a species increases as island size decreases, due to a decrease in total population

size. The second hypothesis suggests that larger areas contain a larger array of habitat


configurat ions and food resources, thereby providing more niches and support ing more species.

Unfortunately, it is likely that in most insular systems these processes are together and their effects

are mingled. To disentangle these two hypotheses, HUGUENY (1989) suggests that, in the light of

the insular theory, t w o kinds of rivers must be considered : tr ibutaries and fully isolated rivers (e.g.

coastal rivers). Tributaries are similar to mainland areas since they have connect ions with other

sect ions of the catchment area and can be colonized freely f rom surrounding parts of the river, in

the same way that a mainland area can be colonized f rom its ne ighbourhood. Consequently there

should be a larger number of species in a tr ibutary than in a fully isolated river (e.g. coastal river)

of similar size (HUGUENY, 1989 ; OBERDORFF et al., 1997) because extinction rates are not

balanced by immigrat ion. Conversely, under the habitat diversity hypothesis the same number of

species is expected in the two kinds of similar size rivers (tributaries and coastal rivers).

In th is paper, w e address three pr imary quest ions : (i) at the ca tchment scale (regional

scale), are there more spec ies in t r ibutar ies than in comparab le coasta l r ivers ? (ii) at the local

scale (station scale), are there more species in tr ibutar ies than in comparab le coastal rivers ? (iii)

for the comp le te data set (tr ibutaries and coastal rivers), is there a posit ive linear relationship

between local and regional spec ies r ichness ? Consequences of the results for the management

of in t roduced species wil l be emphas ized .

M A T E R I A L A N D M E T H O D S

Data s o u r c e

Two main cond i t ions have been respected to make the test val id : 1) tr ibutaries and

coastal rivers s tud ied were similar in size ; 2) assessment w a s carr ied out in geographic areas

that were homogeneous w i th respect to a combina t ion of envi ronmenta l character ist ics,

Figure 1

Area studied and its location in France.

Figure 1

Région étudiée et sa localisation en France.


inc lud ing c l imate, physiography, soi ls and vegetat ion in order to avoid potential inf luence of

these fac to rs on species r ichness (OMERNIK, 1987 ; HUGHES et al., 1987 ; HUGHES and

LARSEN, 1988 ; WHITHER et al., 1988). With th is a im, w e l imited our invest igat ion to rivers

be long ing to a same b iogeograph ica l unit (Seine-Normandie Basin ; region IID, a ring of upper

c re taceous , auréole de crétacé supérieur) as defined by DUPIAS and REY, 1985 (Figure 1).

We used t w o extensive data sets representing two different spatial scales. The first was

a set of 10 tr ibutar ies and 11 coasta l rivers (catchment scale) (Table I). The second was a set of

17 (sampl ing) s tat ions in t r ibutar ies and 20 (sampling) stat ions in coastal rivers (station scale)

(Table II).

Table I

TSADB : total surface area of the drainage basin ; RSR : regional species richness (basin

richness). Only native species are considered. Estuarine species are omit ted.

Tableau I

TSADB : surface totale du bassin versant ; RSR : richesse totale en espèces du bassin (échelle régionale). Seules les espèces autochtones sont considérées. Les espèces estuariennes sont exclues.

COASTAL RIVERS TSADB (square km) RSR

RISLES 1660 13 DURDENT 44 6 SAANE 117 6 YERES 64 4 SCIE 50 6 BRESLE 460 14 LEZARDE 46 6 ARQUES 660 12 SOMME 6000 16 VALMONT 45 4 AUTHIE 750 7

TRIBUTARIES TSADB (square km) RSR

EPTE 1336 16 THERAIN 990 14 AUSTREBERTHE 32 4 CAILLY 90 6 ANDELLE 286 14 VAUCOULEUR 135 13 EURE 5238 19 BRECHE 245 14 VIOSNE 110 10 RANÇON 12 5


Table II

SADB : surface area of the drainage basin ; LSR : local species richness (local scale) ; RSR : regional species richness (basin richness). Successfully introduced species are included. Estuarine species are omitted.

Tableau II

SADB : superficie du bassin versant drainé ; LSR : richesse en espèces de la station (échelle locale) ; RSR : richesse régionale (richesse totale du bassin). Les espèces introduites acclimatées sont incluses. Les espèces estuariennes sont exclues.

C O A S T A L RIVERS SADB (square km) LSR RSR

LEZARDE 10 4 7 LEZARDE 29 6 7

RISLE 295 6 17

RISLE 565 9 17

CHARENTONNE 360 7 17

CHARENTONNE 372 8 17

GUIEL 57 5 17

DURDENT 20 5 7

DURDENT 25 4 7

SAÂNE 10 5 7

VIENNE 18 4 7 SCIE 61 4 7

VARENNE 25 4 15

VARENNE 131 6 15 VARENNE 152 8 15 YERES 40 4 5

YERES 70 5 5

BRESLE 89 5 17 BRESLE 390 6 17

BRESLE 502 6 17

TRIBUTARIES SADB (square km) LSR RSR

BRECHE 92 9 30 THERAIN 13 4 30 PETIT THERAIN 41 6 30 RU ONS BRAY 3 6 30 AVELON 62 5 30 VIOSNE 71 13 30 VAUCOULEUR 24 6 30 EPTE 140 9 30 EPTE 228 10 30

MESANGUEVILLE 30 8 30 ANDELLE 100 11 30

CREVON 8 5 30

CREVON 17 5 30 AVRE 400 9 30 ITON 276 8 30

CAILLY 11 5 30 AUSTREBHERTE 8 4 30


Values for the r ichness of native freshwater fish species (estuarine species were omitted)

f rom ent i re drainage basins (number of species in drainage basin) were ext racted f rom the

da tabase managed by the Consei l Supér ieur de la Pêche (Banque Hydrobio logique et Piscicole)

and cover ing 10 years of survey. Consequent ly, these values can be considered as quite

rel iable. The status of each f reshwater fish species (whether native or introduced) was obtained

f rom BELLIARD, 1994. Data for the r ichness of local freshwater fish species (number of species

at a s i te, inc lud ing successful ly in t roduced species) were col lected between 1989 and 1991.

Si tes were samp led by electrof ishing, dur ing low f low periods, and using standardized methods.

This p rocedure al lows a compar ison between sites.

The ca tchment area for each drainage basin and each site was measured wi th a digital

p lan imeter on a 1 : 500,000 scale map of the Seine Basin provided by the Agence de l'Eau

Se ine -Normand ie .

S t a t i s t i c a l m e t h o d s

We per formed regressions using the method of least squares. We used analysis of

covar iance (SOKAL and ROHLF, 1981) to examine patterns of variation of number of freshwater

f ish present among tr ibutar ies and coasta l rivers (coded as d u m m y variables), and drainage

bas in area at the catchment and the stat ion scales. We also analysed local species r ichness as

a func t ion of surface area of the drainage basin, regional species r ichness and square regional

spec ies r ichness. The last variable was used to model a saturation curve between local and

regional spec ies r ichness (CORNELL, 1985). The contr ibut ion of independent variables to the

between- r iver and between-s i te variat ion in species r ichness was derived by s tepwise mult iple

regression procedures (DRAPER and SMITH, 1981). At each iteration, only the variable showing

the h ighest part ial correlat ion wi th the dependent variable was included in the model only if its

corre lat ion was signif icant at the 5 % level. All statist ical analyses were per formed using Systat

5 vers ion 5 .2 .1 . , Evanston, IL for the Macintosh (WILKINSON, 1990).

4

C a t c h m e n t area (In)

Figure 2 A

Relationship between species richness and total surface area of the drainage basin for tr ibutar ies and coastal rivers (regional scale).

Figure 2 A

Relation entre la richesse totale en espèces du cours d'eau et la taille de son bassin versant (échelle régionale).

Tributaries (Seine Basin)

Coastal rivers

(u|) sseuip

u s

epads


RESULTS

At the regional scale, species r ichness was posit ively correlated wi th total surface area

of the drainage basin for both coastal rivers and tr ibutaries (Figure 2A). The relat ionships

obta ined are :

Coastal rivers (1) InSR = 0.249 InS + 0.682 ; r 2 = 0.750 ; p = 0.001

Tributaries (2) InSR = 0.260 InS + 0.929 ; r 2 = 0.758 ; p = 0.001

where SR is species r ichness of freshwater f ish and S is the total surface area of the drainage basin in km 2 .

• Tributaries (Seine Basin)

o Coastal rivers

B

3 5

C a t c h m e n t a r e a ( I n )

Figure 2 B

Relationship between species richness and surface area of the drainage basin for tributaries and coastal rivers (local scale).

Figure 2 B

Relation entre la richesse en espèces de la station et la superficie du bassin versant drainé (échelle locale).

The same study carried at stat ion scale showed similar t rends (Figure 2B), with the

fol lowing relat ionships :

Coastal rivers (1) InSR = 0.149 InS + 1.036 ; r 2 = 0.594 ; p = 0.001

Tributaries (2) InSR = 0.185 InS + 1.235 ; r 2 = 0.519 ; p = 0.001

Our da ta clearly show that the species-area s lopes are homogeneous among coastal

rivers and tr ibutar ies at both scales (catchment scale : c o d e (coastal river or tr ibutary) -by-area

interact ion : p = 0.880 ; stat ion scale : code (coastal river or tr ibutary) -by-area interact ion : p =

0.500). A mode l wi th area and code (tr ibutaries or coasta l rivers) f i t ted to these data (at both

scales) indicated that both basin area and code expla ined signif icant amounts of variation in

total f ish numbers (Tables III, IV).

(u|)

sseuu.a

u sajo

eds

Bull. Fr. Pêche Piscic. (1997) 344/345 :193-204 — 200

Table i l l

(regional scale). Summary of multiple regression analysis of fish species richness (In scale) versus total surface area of the drainage basin and code (coastal rivers = 0, tributaries = 1). Also given are the slope coefficients, the standardized regression coefficients (magnitudes of standardized regression coefficients indicate the relative importance of each variable), and the probabi l i ty associated with the partial F of individual terms of the model. The final model is highly signif icant (dF = 2, R 2 = 0.774, p<0.001, n = 21).

Tableau III

(échelle régionale). Résumé des résultats de la régression multiple de la richesse en espèces (In) en fonct ion de la superficie totale du bassin versant et du type de cours d'eau (fleuves côtiers = 0 ; affluents = 1). Le modèle final est hautement significatif (dF = 2, R 2 = 0.774, p<0.001, n = 21).

V a r i a b l e s S l o p e S t a n d a r d P

V a r i a b l e s c o e f f i c i e n t c o e f f i c i e n t (2 T a i l s )

C o n s t a n t 0.653 0.000 0.005

In ( total sur face a rea of 0.254 0.831 0.0001

t h e d ra inage bas in)

C o d e 0.305 0.297 0 .016

Table IV

(local scale). Summary of multiple regression analysis of fish species richness (In scale) versus surface area of the drainage basin and code (coastal rivers = 0, tributaries = 1). Also given are the slope coefficients, the standardized regression coefficients (magnitudes of standardized regression coefficients indicate the relative importance of each variable), and the probability associated w i th the partial F of individual terms of the model. The final model is highly significant (dF = 2, R 2 = 0.602, p<0.001, n = 37).

Tableau IV

(échelle locale). Résumé des résultats de la régression multiple de la richesse en espèces (In) en fonct ion de la superficie du bassin versant drainé et du type de cours d'eau (fleuves côtiers = 0 ; aff luents = 1). Le modèle final est hautement significatif (dF = 2, R 2 = 0.602, p<0.001, n = 37).

V a r i a b l e s S l o p e

c o e f f i c i e n t S t a n d a r d

c o e f f i c i e n t

P

(2 T a i l s )

C o n s t a n t 0 .960 0.000 0.0001

In (sur face a r e a of the

d r a i n a g e basin)

0.166 0.704 0.0001

C o d e 0.343 0.530 0.0001

We can conc lude that species r ichness in tr ibutar ies is always greater than in comparab le

coasta l r ivers whatever the scale of investigation (catchment or stat ion scales). If, for the

comp le te da ta set, we analyse the relationship between local species r ichness (station richness)

and regional species r ichness (basin richness) after control l ing for the ups t ream-downst ream

grad ient (e.g. surface area of the drainage basin) the best fit is obtained wi th a linear model

(Figure 3). This result demonst ra tes that local species r ichness does not strictly depend upon

local cond i t ions but also relies upon the regional pool of potential colonizers.


^ 4

CO

3

T 3

CO o

O -4 H • 1 ' 1 ' r-0 1 0 2 0 3 0

Regional species richness

Figure 3

Relationship between local species r ichness (residuals) and regional species richness (tributaries and coastal rivers). The best f i t is obtained with the fol lowing relationship : local species richness (residuals) = -2.184 + 0.108 x regional species richness ( R 2 = 0.313 ; p<0.01). A second order does not contribute significantly to this equation when integrated (p>0.05).

Figure 3

Relation entre les résidus de la richesse locale et la richesse régionale (affluents et fleuves côtiers). La relation est de type linéaire avec richesse locale (résidus) = -2.184 + 0.108 x richesse régionale ( R 2 = 0.313 ; p<0.01). Le carré de la richesse régionale n'améliore pas significativement le modèle (p>0.05).

D I S C U S S I O N

Our results do not provide evidence suppor t ing f ish communi ty saturat ion at the local

scale, since local species r ichness increased wi th regional species r ichness. A prev ious study

deal ing wi th f reshwater f ish species r ichness at the local scale (station scale) cor roborates these

results (HUGUENY and PAUGY, 1995). This suggests that local fish communi t ies in rivers are

shaped by processes operat ing on a broader spatial scale, the most probable p rocess being

recolonizat ion f rom neighbour ing sites, wh ich prevents long- term interspecif ic exc lus ion .

Knowledge of the degree of saturat ion of a commun i t y provides an impor tant basis for

unders tand ing h o w the commun i t y reacts or will react to the int roduct ion of new species. The

da ta analysed here suggest that species in t roduct ion wil l have reduced shor t - term impact on

local communi t ies through species ext inct ion. Thus, examinat ion of this prob lem should be

shi f ted f rom the local to the regional scale and f rom shor t - term to long- term effects. The

compar i son per formed in the present s tudy be tween tr ibutar ies and coastal rivers (fully isolated

rivers) leads to the conclusion that species r ichness at the regional scale is regulated, in part,

by an interplay between ext inct ion and immigrat ion and that there is no upper limit in species


r ichness determined only by river size (and its correlates such as habitat diversity). As it is

a c c e p t e d that the r ichness of the West European freshwater fish fauna is low, partly due to

histor ical processes (MAHON, 1984 ; MOYLE and HERBOLD, 1987 ; BANARESCU, 1989 ;

W O O T T O N , 1991 ; OBERDORFF etal., 1997), we can hypothesize that the insaturation pattern

is l ikely t o be t rue at the regional scale.

Managemen t of species in t roduct ion is a diff icult task because it is generally assumed

that i n t roduced species and target communi t ies cannot be studied independent ly (LODGE,

1993) and that in t roduced species can have detr imental effects at different organizat ion levels.

However, intensive studies deal ing wi th all these points are not cost effective and are diff icult to

imp lemen t , wh ich makes t hem unlikely as attainable goals. Consequently, research priorit ies

mus t be de f ined . Our data suggest that, for the rivers s tud ied, local and probably regional

c o m m u n i t i e s are unsaturated and thus, that there is no need to devote great effort in assessing

sho r t - te rm commun i t y -w ide consequences of species introduct ion because large detr imental

ef fects are not expec ted at these scales. If, for example, we consider the total species r ichness

w i th in the Seine Basin, nineteen exot ic species (about 40 % of the total r ichness of the basin)

are success fu l l y establ ished (BELLIARD, 1994) and have had to date, no clear negative effect

on the survival of native species (e.g. no species ext inct ion). Only d iadromous species have

been ex t i rpa ted in recent years f rom the native fauna of the Seine Basin, presumably because

of d a m s and water pol lut ion. Thus, efforts should rather focus on other consequences of

spec ies in t roduct ion, such as the detr imental ef fects on some target species (e.g. endemic

spec ies) , t he in t roduct ion of parasites and diseases, the effects on populat ion genetic structure,

etc. (KOHLER and COURTENAY, 1986). For example, in the Euro-Mediterranean region, wh ich

is cons ide red as a refuge zone wi th a high number of endemic species (BANARESCU, 1989),

in t roduc t ions are likely to reduce biodiversity on a global scale (world scale) should these

endemic spec ies become ext inct .

A C K N O W L E D G E M E N T S

This w o r k was funded joint ly by the Conseil Supérieur de la Pêche and by the Agence de

l 'Eau Se ine-Normand ie (subvent ion n° 1184). We hasten to add , however, that this paper does

not necessar i ly reflect the v iews of those organizations and no official endorsement should be

inferred. We thank J.L. MAROUSÉ for help w i th draft ing the f igures and three anonymous

referees for helpful c o m m e n t s on an earlier draft of the manuscr ipt .

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Documents

Unsaturated fish assemblages in rivers of the North-Western ......We used two extensive data sets representing two different spatial scales. The first was a set of 10 tributaries and