L'étude du régime alimentaire du Grand Cormoran Phalacrocorax carbo dans le sud-ouest de la France a été réalisée grâce à l'analyse de 385 contenus stomacaux issus d'oiseaux prélevés dans le cadre du plan de régulation national de cette espèce. Suite à l'augmentation des effectifs nicheurs dans le nord-ouest de l'Europe au début des années 80, le nombre d'oiseaux hivernants dans le sud-ouest de la France a fortement augmenté. Cette hausse a provoqué de vives réactions notamment chez les pêcheurs qui ont obtenu des tirs de contrôle du Grand Cormoran. Dans cette étude nous analysons les contenus stomacaux d'oiseaux provenant de différentes rivières de Midi-Pyrénées et d'Aquitaine. 22 espèces de poissons dont la taille varie de 2 à 50 cm ont été répertoriées. Les proies principales sont constituées par la famille des cyprinidés. Le gardon Rutilus rutilus est l'espèce ayant la plus forte occurrence, mais en terme de biomasse, le barbeau Barbus barbus est la proie la plus importante. Le poisson-chat Ictalurus melas joue aussi un rôle important dans le régime alimentaire du Grand Cormoran. Ces premiers résultats dans des rivières françaises et l'impact du Grand Cormoran sur ces milieux sont discutés

CSP (Conseil Supérieur de la Pêche)

SANTOUL, Frédéric

I-Revues

—  83  — 
THE DIET OF GREAT CORMORANTS PHALACROCORAX CARBO  WINTERING 
IN SOUTHWESTERN FRANCE
Frédéric SANTOUL1
In collaboration with the CSP (Conseil Supérieur de la Pêche)2
RÉSUMÉ. — Le régime alimentaire de Grands Cormorans Phalacrocorax carbo hivernant dans le
sud-ouest de la France. — L’étude du régime alimentaire du Grand Cormoran Phalacrocorax carbo
dans le sud-ouest de la France a été réalisée grâce à l’analyse de 385 contenus stomacaux issus
d’oiseaux prélevés dans le cadre du plan de régulation national de cette espèce. Suite à l’augmentation
des effectifs nicheurs dans le nord-ouest de l’Europe au début des années 80, le nombre d’oiseaux hiver-
nants dans le sud-ouest de la France a fortement augmenté. Cette hausse a provoqué de vives réactions
notamment chez les pêcheurs qui ont obtenu des tirs de contrôle du Grand Cormoran. Dans cette étude
nous analysons les contenus stomacaux d’oiseaux provenant de différentes rivières de Midi-Pyrénées et
d’Aquitaine. 22 espèces de poissons dont la taille varie de 2 à 50 cm ont été répertoriées. Les proies
principales sont constituées par la famille des cyprinidés. Le gardon Rutilus rutilus est l’espèce ayant la
plus forte occurrence, mais en terme de biomasse, le barbeau Barbus barbus est la proie la plus impor-
tante. Le poisson-chat Ictalurus melas joue aussi un rôle important dans le régime alimentaire du Grand
Cormoran. Ces premiers résultats dans des rivières françaises et l’impact du Grand Cormoran sur ces
milieux sont discutés.
In aquatic ecosystems, predator-prey interactions between birds and fish can play an
important role in the structure and dynamics of the food-chain. Cormorants Phalacrocorax
carbo represent top-predators and are therefore good indicators of the general health of the
water environment. They are important predators of fish and they have received considera-
ble attention in the literature. Studies on the diet and distribution of Cormorants have been
done in particular during the breeding period (Warke et al., 1994; Veldkamp, 1995; Kirby
et al., 1996; Musil & Janda, 1997; Grieco, 1999). Data from the non-breeding period have
been published from fish farms in France (Im & Hafner, 1984), water bodies in Switzerland
(Suter, 1995) and lake Ketelmeer in the Netherlands (Platteuw et al., 1992). Seasonal varia-
tion in the prey species are also studied all over the year (e.g. Keller 1998). In southwestern
France the Cormorant was a rare species seldom seen in the early 70’s (Affre & Affre,
1978). It is presently a common and regular wintering waterbird. The increasing number of
regional wintering birds is highly correlated with the demographic explosion of the Danish
and Dutch colonies (Bousquet, 1992; Marion, 1997). The debate about Cormorant predation
of fish stock has been sufficiently strong to have resulted in reductions of Cormorant popu-
1  Laboratoire d’Ecologie des Hydrosystèmes, CNRS/UPS, Université Paul Sabatier, 118, Route de Narbonne,
F-31062 Toulouse Cedex 04 France. Tél. : (33) 05 61 55 65 75, Fax : (33) 05 61 55 60 96. E-mail : santoul@cict.fr
2  CSP (Conseil Supérieur de la Pêche) Délégation Régionale Midi-Pyrénées-Aquitaine, 34, boulevard Riquet,
31000 Toulouse, France.
Rev. Écol. (Terre Vie), vol. 60, 2005. 
NOTE BRÈVE
—  84  — 
lations by shooting. However, particularly in southwestern France, little is known about
species and size composition of the diet of wintering Cormorants. In this area, commercial
fisheries are few; even if some conflicts between man and fish eating birds were noticed,
most of them came from interference with recreational angling.
This paper provides a brief overview of the diet of Cormorants in southwestern France,
based on stomach contents analyses. The impact of wintering Cormorants on fish stock is
also discussed.
MATERIAL AND METHODS
A total of 385 stomachs were analysed from 49 different sites located in Midi-Pyrénées area (Ariège, Aveyron,
Haute-Garonne, Lot, Tarn and Tarn-et-Garonne) and in Aquitaine area (Dordogne, Landes, Lot-et-Garonne and
Pyrénées-Atlantiques). Only the cyprinid dominant part (middle basin) of rivers were taken into account. Salmonid
dominant parts will be analysed elsewhere (Santoul et al., in press). The main rivers of southwestern France area are
represented in this study (Adour, Ariège, Aveyron, Dordogne, Garonne, Lot and Tarn). 
Cormorant’s diet was assessed by examining the stomach contents of birds shot under license by agent of the
French Environment Minister (CSP: Conseil Supérieur de la Pêche and ONCFS: Ofﬁce National de la Chasse et de la
Faune Sauvage) in winter during the period 1997-2002. Only recent meals where included in the analyses. Some
stomachs also contained the remains of previous well-digested meals but these were excluded, differences between
relatively fresh ﬁsh and older retained items in the posterior part of the stomach are often clear. Whole ﬁsh were
identiﬁed and their lengths measured. Fish lengths were converted to fresh weight by the series of regression equations
we had validated (for method see Carss & Marquiss, 1997). This study aims at characterizing the global diet of
wintering Cormorants in southwestern France. We do not intend to assess daily food intake. There are several reasons
for this, ﬁrst Cormorants where shot in the morning and as well as in the evening at roosting places, second quantifying
food intakes from stomach contents analyses has several biases, quick digestion of some prey being the most obvious
one (Carss & Marquiss, 1997). A percent occurrence (i.e. the percentage of individual Cormorants in which each ﬁsh
species was recorded) was calculated for every ﬁsh species. We also calculated the biomass of ﬁsh items in the
stomachs expressed as the mean of biomass percentages (aggregate percent).
RESULTS
Identifiable fish remains were found in 48% of all stomachs (n = 186). In all 846 iden-
tifiable individual prey items were found, belonging to 22 different fish species (Table I).
Cyprinids represent 50% of the total number of species found in this study. The size range
of the fish taken is considerable: 2-50 cm in length and 2-900 g in weight.
Table II shows that the dominant prey species in terms of occurrence were cyprinids.
Within this family the Roach Rutilus rutilus (28.6%), Barbel Barbus barbus (24.8%) and
Bleak Alburnus alburnus (9.7%) were the most common. Catfish Ictularus melas (22.7%)
was also an important prey. In weight (aggregate percent biomass) cyprinids still dominate
with 62.2%: Barbel (22.2%), Roach (15.5%) and Chub (5.1%) are the most important of this
family. Catfish (18.8%) is also important.
Predators (Pike Esox lucius, Perch Perca fluviatilis and Pikeperch Sander lucioperca)
account respectively for 2.1, 8.6 and 2.3% of occurrence and 1.5, 5.1 and 1.6% of mean biomass.
For the three principal species of fish caught (Roach, Barbel and Catfish), the mean
size varied from 11.6 cm (4 to 25 cm) for the Roach to 22.2 cm (5 to 50 cm) for the Barbel
(Table I).     
DISCUSSION     
The stomach contents of collected birds have been commonly used to estimate diet
(e.g. Rae,1960; Carss & Marquiss, 1997; Carss et al., 1997). In our study, a total of 22 prey
species eaten (in the cyprinid zone) reflects the great plasticity of the Cormorant foraging
behaviour (Van eerden & Zijlstra, 1997). Cormorants are almost entirely piscivorous, but
no high commercial or fishing value fish species were found as main prey in the present
study. The fish fauna of southwestern France rivers is dominated by cyprinids which repre-
sent 50% of the fish species caught by Cormorants which most commonly take Roach, Bar-
bel and Catfish in Midi-Pyrénées and Aquitaine area These species have been frequently
—  85  — 
TABLE I
Prey species of wintering Cormorants in southwestern France (N = number of ﬁsh measured)
Fish length, cm Fish mass, g N %
Scientific name Name Mean Min Max  Mean Min Max   
Abramis brama (L., 1758) Bream 14.6 5 23 45.9 3 125 29 3.4
Alburnus alburnus (L., 1758) Bleak 8.6 5 13 8.2 4 22 156 18.4
Anguilla anguilla (L., 1758) Eel 30.0 144.0 1 0.1
Barbus barbus (L., 1758) Barbel 22.2 5 50 165.0 4 900 74 8.7
Micropterus salmoides (Lacépède, 1802) Largemouth bass 17.7 11 28 76.5 12 240 6 0.7
Blicca bjoerkna (L., 1758) Silver bream 11.0 11 11 15.0 15 15 3 0.4
Carassius carassius (L., 1758) Crucian carp 16.0 12 26 58.0 20 220 25 2.9
Cyprinus carpio (L., 1758) Carp 15.7 6 40 81.0 5 420 19 2.2
Esox lucius (L., 1758) Pike 24.0 19 32 106.0 60 205 4 0.5
Gobio gobio (L., 1758) Gudgeon 8.8 5 18 12.9 5 65 16 1.9
Gymnocephalus cernua (L., 1758) Ruffe 10.0 12.0 1 0.1
Ictalurus melas  (Raﬁnesque, 1820) Catﬁsh 13.1 4 27 45.4 5 210 147 17.4
Lepomis gibbosus (L., 1758) Pumpkinseed 5.1 4 12 3.9 3 11 79 9.4
Leuciscus cephalus (L., 1758) Chub 23.4 9 29 179.9 10 310 14 1.7
Leuciscus leuciscus (L., 1758) Dace 24.4 23 30 186.0 150 280 5 0.6
Mugil cephalus (L., 1758) Mullet 24.0 23 25 166.0 142 190 2 0.2
Perca ﬂuviatilis (L., 1758) Perch 11.6 6 25 23.3 4 120 29 3.4
Rutilus rutilus (L., 1758) Roach 11.6 4 25 24.7 2 160 197 23.3
Salmo trutta fario (L., 1758) Brown trout 24.0 17 31 145.1 60 280 15 1.8
Scardinius erythrophtalmus (L., 1758) Rudd 14.7 10 20 41.7 11 89 16 1.9
Sander lucioperca (L., 1758) Pikeperch 26.5 21 30 195.0 115 215 4 0.5
Tinca tinca (L., 1758) Tench 20.5 12 31 133.3 23 290 4 0.5
TABLE II
Percentage occurrence and biomass of ﬁsh items in the stomachs of Cormorants wintering in southwestern France
Scientific name Name
Occurrence of 
stomachs%
Mean biomass of total 
ﬁsh weight%*
Abramis brama (L., 1758) Bream 8.1 4.4
Alburnus alburnus (L., 1758) Bleak 9.7 3.1
Anguilla anguilla (L., 1758) Eel 0.5 0.5
Barbus barbus (L., 1758) Barbel 24.8 22.2
Micropterus salmoides (Lacépède, 1802) Largemouth Bass 2.7 0.7
Blicca bjoerkna (L., 1758) Silver Bream 0.5 0.5
Carassius carassius (L., 1758) Crucian Carp 5.4 3.5
Cyprinus carpio (L., 1758) Carp 5.4 3.2
Esox lucius (L., 1758) Pike 2.1 1.5
Gobio gobio (L., 1758) Gudgeon 2.2 1.3
Gymnocephalus cernua (L., 1758) Ruffe 0.5 0.1
Ictalurus melas  (Raﬁnesque, 1820) Catﬁsh 22.7 18.8
Lepomis gibbosus (L., 1758) Pumpkinseed 4.3 2.1
Leuciscus cephalus (L., 1758) Chub 5.9 5.1
Leuciscus leuciscus (L., 1758) Dace 1.6 1.2
Mugil cephalus (L., 1758) Mullet 1.1 0.7
Perca ﬂuviatilis (L., 1758) Perch 8.6 5.1
Rutilus rutilus (L., 1758) Roach 28.6 15.5
Salmo trutta fario (L., 1758) Brown Trout 5.4 5.4
Scardinius erythrophtalmus (L., 1758) Rudd 3.8 2.4
Sander lucioperca (L., 1758) Pikeperch 2.2 1.6
Tinca tinca (L., 1758) Tench 1.1 1.1
*  Aggregate percent (see text).
—  86  — 
recorded in the diet of Cormorants in similar habitats (e.g. Van dobben, 1952; Cramp &
Simmons, 1977; Keller, 1995; Boudewijn & Dirksen, 1997). Numerous studies have found
that the main prey species of Cormorants are cyprinids, representing between 50% and more
than 90% of the species found in Cormorant’s diet (Keller, 1995; Van eerden & Zijlstra,
1997; Privileggi, 2003; Trauttmansdorff, 2003). Cormorants exploit a wide range of fish
from 4 to 50 cm, but mainly 10-22 cm in length (mean size of 11.6 cm for the principal prey,
the Roach). These sizes are also much alike those of the most frequently eaten individuals
elsewhere (e.g. Keller, 1995; Martyniak et al., 1997).
It is difficult to make a general classification of species of value for anglers in the
cyprinid zone. However within the cyprinid family anglers take mostly Carp (Cyprinus car-
pio), whereas the birds take only few (5.4% of occurrence). Predators especially Pike, Perch
and Pikeperch are also highly valuable for anglers. In the stomachs analysed, the proportion
of predators was quite low, especially for Pike and Pikeperch. For Perch, 8.6% of occur-
rence in stomachs do not seem to be a significant impact on the Perch stock which is well
represented in the Adour-Garonne basin (Mastrorillo et al. 1998).
However, accurate estimation of fish stock is not possible in most large rivers of our
study site. In order to estimate Cormorant impact on fish stock we have used an indirect
method. Using recent studies from southwestern France (e.g. CSP unpublished data,
Hutagalung et al., 1997; Santoul et al., 2004), we have compared principal fish (e.g. roach)
present in stomachs in a given area and class size of this fish caught by gillnets. In all
rivers studied no impact of Cormorants can be found on fish class structure (e.g. lack of
one class). The good balance of these classes shows that fish stock seems to be strong
enough to support wintering Cormorant predation in most of southwestern rivers (open
wetlands).
However some Cormorants fed in adjacent wetlands of the Garonne River (e.g. gravel
pits) and the proportion of birds foraging on theses places is difficult to estimate. In such
closed wetlands, the impact of Cormorants seems to be higher (Santoul, 2000). Catfish often
caught by Cormorants are well represented in gravel pits (Santoul, 2000). For Warke et al.
(1994), the distribution and abundance of Cormorants was controlled by fish availability.
During the wintering period they are highly mobile, they move rapidly from place to place
(Kirby et al., 1996); these numerous wetlands, by providing numerous fish with numerous
individuals, are important places for wintering Cormorants in southwestern France (San-
toul, 2000). The provision of alternative prey like Catfish and Pumpkinseed with no com-
mercial or angling value could protect river stock, even if also this has the disadvantage of
increasing the total density of potential prey available to Cormorants and may result in lar-
ger numbers of birds being attracted (Draulans, 1987). In some circumstances, there may be
merits in sacrificing the fish to Cormorants in one particular area (e.g. unmanaged gravel
pits) in order to conserve fish stock in other areas, though this would need to be part of a
wider management strategy (Kirby et al., 1996).
Because of our important sample size we are able to characterize globally the diet of
Cormorants in southwestern France. However, more work is required to better estimate the
impact of Cormorants on fish stocks in each wetland.
ACKNOWLEDGEMENTS
I am very grateful to M. Philippe Derenne (Conseil Supérieur de la Pêche) for his collaboration. I also thank two
referees for their useful comments.
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The diet of great cormorants Phalacrocorax carbo wintering in southwestern France

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The diet of great cormorants Phalacrocorax carbo wintering in southwestern France

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