First assessment of the Caulerpa racemosa (Caulerpales, Chlorophyta) invasion along the French Mediterranean coast

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Marine Pollution Bulletin 50 (2005) 1061–1068

First assessment of the Caulerpa racemosa (Caulerpales,Chlorophyta) invasion along the French Mediterranean coast

Sandrine Ruitton a,*, Fabrice Javel b, Jean-Michel Culioli c, Alexandre Meinesz b,Gerard Pergent d, Marc Verlaque a

a UMR 6540 CNRS, Centre d’Oceanologie de Marseille, Campus de Luminy, Case 901, 13288 Marseille Cedex 09, Franceb EA 3156, Gestion de la biodiversite, Laboratoire Environnement Marin Littoral, Universite de Nice-Sophia Antipolis, Parc Valrose,

06108 Nice Cedex 02, Francec Reserve Naturelle des Bouches-de-Bonifacio, Office de l’Environnement de la Corse, Avenue Jean Nicoli, 20250 Corte, France

d Equipe Ecosystemes Littoraux, Universite de Corte, BP 52, 20250 Corte, France

Abstract

The introduced green alga Caulerpa racemosa var. cylindracea has been rapidly spreading in the Mediterranean Sea since 1990. It

was first observed in France in 1997 (Marseilles). In early 2004, the stretch of the French Mediterranean coastline and the surface

area affected by the invasion were estimated at about 83 km and 4014 ha, respectively. The depth range of colonized areas was usu-

ally 10–35 m depth. Shallow (0–10 m) and deep (down to 40 m) dense meadows were rarely observed. In contrast to the dead matte

of Posidonia oceanica, which constituted the most widely colonized substratum, dense P. oceanica meadows and fine sand with large

ripple-marks were not invaded. Few rocky areas were colonized and coarse sand bottoms were usually colonized below 20 m depth.

All the colonized areas were exposed to human activities and more than 40% were fishing areas. Mild climate, suitable substrata,

presence of vectors of dispersal and absence of efficient biological control make the French Mediterranean coast particularly

vulnerable to the further spread of the alga.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Introduced species; Biological invasion; Caulerpa racemosa var. cylindracea; France; Mediterranean Sea; Distribution

1. Introduction

Caulerpa racemosa var. cylindracea (Sonder) Verl-

aque, Huisman et Boudouresque, a south-western Aus-

tralian variety newly introduced into Europe, iscurrently achieving dramatic and continuous expansion

throughout most of the Mediterranean Sea and the

Atlantic (Verlaque et al., 2000, 2003, 2004). This inva-

sive variety differs from the two other varieties already

present in the Mediterranean Sea [C. racemosa var. tur-

0025-326X/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2005.04.009

* Corresponding author. Tel.: +33 4 91 829136; fax: +33 4 91 411265.

E-mail address: [email protected] (S. Ruitton).

binate-uvifera, an intermediate variety between var.

turbinata (J. Agardh) Eubank and var. uvifera (C.

Agardh) J. Agardh, and C. racemosa var. lamourouxii

f. requienii (Montagne) Weber-van Bosse] mainly by its

upright axes with clavate, more or less inflated branch-lets, which are uncrowded and radially to distichously

disposed and its thin rhizoids (Verlaque et al., 2000,

2003). Observed for the first time off the coast of Libya

(Nizamuddin, 1991), the introduced C. racemosa var.

cylindracea has so far been reported from at least eleven

Mediterranean countries (Albania, Croatia, Cyprus,

France, Greece, Italy, Libya, Malta, Spain, Tunisia

and Turkey), all the major Mediterranean islands (Bale-aric Islands, Corsica, Crete, Cyprus, Sardinia and Sicily)

and from the neighbouring areas of the Atlantic (Canary

1062 S. Ruitton et al. / Marine Pollution Bulletin 50 (2005) 1061–1068

Islands) (Verlaque et al., 2003, 2004; Piazzi et al., in

press).

In France, C. racemosa var. cylindracea was first ob-

served in 1997 in the Gulf of Marseilles (Provence,

France) where a large population was already thriving

in the north of the gulf. The probable year of introduc-tion was estimated to be 1993 or 1994 (Verlaque et al.,

2000) and, by the end of 2004, C. racemosa var. cylindr-

acea had become common in the gulf. Since 1999, other

colonized areas of the French Mediterranean coast have

been discovered in eastern Provence, the French Riviera

and, recently, in Corsica. A similar rapid spread has al-

ready been reported in Italy (Piazzi et al., 1997, 1999,

2001b,c; Buia et al., 1998; Piazzi and Cinelli, 1999; Cec-cherelli et al., 2000; Ceccherelli and Piazzi, 2001).

The spread of C. racemosa var. cylindracea along the

French Mediterranean coast has been monitored since

1997 and the colonized areas inventoried. At a smaller

scale, the colonization of the Gulf of Marseilles has been

studied. The aims of the present study are (1) to provide

a first survey of the colonization of the French Mediter-

ranean coast; (2) to describe the ecological characteris-tics of areas colonized by C. racemosa var. cylindracea;

and (3) to discuss the possible outcome of the invasion.

2. Materials and methods

2.1. The French Mediterranean coast

For the monitoring campaign for the spread of Caul-

erpa racemosa var. cylindracea along the French Medi-

terranean coast, two networks of observers carried out

several hundred SCUBA dives: the ‘‘Observatoire sur

l’expansion des Caulerpes envahisantes en Mediterranee’’

(Provence and French Riviera) and the ‘‘Caulerpa Net-

work’’ (Corsica). The observations were taken all year

round. Date of observation, geographical position,depth (0–10, 11–20, 21–30, 31–40 and 41–50 m), length

of coastline and surface area affected, substratum [rocky

substratum, Posidonia oceanica (L.) Delile meadows,

dead matte of P. oceanica, fine sand or coarse sand

and co-occurrence with the other invasive Caulerpa,

C. taxifolia (Vahl) C. Agardh were noted. The coloniza-

tion levels were estimated according to the standardized

procedure proposed for C. taxifolia by Vaugelas et al.(1999) and Meinesz et al. (2001):

• Level I: area where one or several small colonies

cover a total surface area of less than 1000 m2.

• Level II: area where several colonies cover a total sur-

face greater than 1000 m2 with small colonies dis-

persed over a surface area of less than 10 ha.

• Level III: area with several large colonies greater than1000 m2 dispersed over a surface area greater than

10 ha.

Field data were input into a Geographic Information

System (ArcView microcomputer program, Version 8.3,

Esri�) and the Caulerpa online web database (http://

www.caulerpa.org, Meinesz et al., 2004).C. racemosa var. cylindracea spreads by vegetative

multiplication (cuttings, vegetative propagules) (Cec-

cherelli et al., 2000; Ceccherelli and Piazzi, 2001; Renon-

court and Meinesz, 2002) and sexual reproduction

(Panayotidis and Zuljevic, 2001). Human activities such

as fishing, anchoring and ballast water unloading are

probably the main vectors of remote dispersal of propa-

gules. Thus, in each colonized area, fishing, diving,anchorages and harbour vicinity were noted.

The data were analyzed per area and for the whole

French Mediterranean coast.

2.2. Gulf of Marseilles

The spread of C. racemosa var. cylindracea has been

monitored in the gulf of Marseilles since 1998. Two fieldsurvey techniques are used in SCUBA diving, either pin-

point observation (punctual observation) or transects.

The nature of the substratum, depth and density of C.

racemosa var. cylindracea meadows (0: uncolonized; 1:

sparse individuals with only few creeping horizontal

axes; 2: small patches less than 1 m in diameter; 3:

patches more than 1m in diameter; 4: continuous mead-

ows with only one layer of stolons and 5: continuousmeadows with a thick layer of stolons) are noted. The

estimation of the colonized areas in 1998 and 2004

was performed using kriging as gridding method. The

kriging method uses spherical functions as a variogram

model (Surfer microcomputer program, Version 8,

Golden Software�).

3. Results

3.1. Distribution of C. racemosa var. cylindracea

meadows along the French Mediterranean coast

In early 2004, C. racemosa var. cylindracea colonized

26 distinct areas in Provence and the French Riviera and

3 in Corsica (Fig. 1 and Table 1). The number of colo-nized areas has increased, in particular since 2002 (Table

1). Field data and detailed maps are available on the

Caulerpa online web site. The increase in the length of

coastline and surface area affected by the invasion of

C. racemosa var. cylindracea is given in Table 2. In early

2004, the length of coastline and surface area affected by

C. racemosa var. cylindracea reached 82.9 km and

4014 ha respectively (Table 2). C. racemosa var. cylindr-acea co-occurred with C. taxifolia in three large bays:

Villefranche-sur-Mer, Hyeres and Toulon.

In western Provence (Marseilles), C. racemosa var.

cylindracea presented a strongly marked annual cycle

Fig. 1. Map of Caulerpa racemosa colonized areas along the French Mediterranean coast. Numbers correspond to areas colonized by Caulerpa

racemosa according to Table 1. Level of colonization according to Vaugelas et al.�s classification (1999).

S. Ruitton et al. / Marine Pollution Bulletin 50 (2005) 1061–1068 1063

of development with a growth period from late spring to

early autumn, a decay period in late autumn and early

winter, and a resting period in late winter and early

spring during which only some very inconspicuous small

fragments scattered on the substrate were present. Con-

versely, in eastern Provence and the French Riviera, theseasonal decline of C. racemosa var. cylindracea mead-

ows was limited to a more or less complete fall of erect

axes without disappearance of the stolons. Field data in

Corsica were insufficient to establish a seasonal cycle.

3.2. Characteristics of the colonized areas

Although observed down to 50 m depth, the usualdepth range of C. racemosa var. cylindracea was 10–

35 m depth. Shallow (0–10 m) and deep (down to

40 m) dense meadows were rare (Table 1, Fig. 2). How-

ever in the vicinity of large and dense colonized areas, C.

racemosa var. cylindracea was usually present, although

very discrete in shallow waters.

The dead matte of P. oceanica was the most fre-

quently colonized substratum (Table 1, Fig. 2). OnlyP. oceanica meadows with low density of shoots were

colonized. Dense P. oceanica meadows and the soft sub-

strata under strong currents (fine sand with large ripple-

marks) were not colonized. A few rocky areas, above all

those constituted of boulders surrounded by dead matte

of P. oceanica or sand, were colonized. Development on

vertical rocky substratum was rarely observed. Coarse

sand areas were usually colonized below 20 m depth

(Table 1).

Most of the colonized areas were exposed to human

activities and more than 40% were fishing areas (Table

1, Fig. 2). Moreover, most of the first settlements were

found in the vicinity of fishing areas, anchoring areasor harbours (Table 1).

3.3. The gulf of Marseilles

As in other French Mediterranean areas, at Mar-

seilles Caulerpa racemosa var. cylindracea mainly colo-

nized the P. oceanica dead matte between 5 and 30 m

depth, and coarse sand below 20 m depth. The cumu-lated field data gave a gross estimation of colonized sur-

face area in the Gulf of Marseilles and vicinity of

1120 ha in early 2004. By the kriging method, the esti-

mation of colonized surface area in 1998 and 2004 was

900 ha and 3180 ha, respectively, corresponding to an

increase of more than 350% in six years (Fig. 3).

4. Discussion

4.1. Distribution and depth range along the French

Mediterranean coast

The spread of C. racemosa var. cylindracea along

the French Mediterranean coast was faster and more

Table 1

Characteristics of areas colonized by Caulerpa racemosa on French Mediterranean coast (from east to west)

No. Areas Date of first

observation

Coastline (m) Depth Substratum Level Human

activities

1 St-Jean-Cap-Ferrat, south of Pointe

de Crau de Nao

Sep-2003 2 6 DM/FS I F/A/D

2 St-Jean-Cap-Ferrat, bay of

Villefranche-sur-Mer

Sep-1999 6500 1–45 RS/PO/DM/ FS/CS III F/A/D/H

3 Nice, Coco Beach May-2003 2 3 RS I –

4 St-Tropez, west of the harbour Nov-2002 170 12–14 DM/FS I F/A/H

5 La Londe, Hyeres, north of

the bay of Hyeres

Sep-2003 4300 18–25 PO/DM III F/A/H

6 Hyeres-les-Palmiers, south-east

of the Gapeau mouth

Jun-2003 200 13–16 PO/DM I F

7 Hyeres-les-Palmiers, centre of

the bay of Hyeres

Sep-2002 2600 20–32 PO/DM/CS III F/A/H

8 Port-Cros island, Pointe du Vaisseau Jul-2001 15 20–28 PO/DM I F/D

9 Port-Cros island, west of the bay of Port-Cros Oct-2002 6 18 DM I F

10 Port-Cros island, Gabiniere islet md md 26–36 RS md D

11 Porquerolles island, south-west

of Cap des Medes

Feb-1999 15 12 DM I D

12 Porquerolles island, Pointes du Gros

Baou and Galeasson

Jul-2001 40 20–25 PO/FS I F

13 Porquerolles island, south-west of the

Pointe du Galeasson

Jan-2001 20 12 DM I F/A

14 Porquerolles island, Langoustier Shoal Aug-2003 100 20–35 CS II F/D

15 West of Giens peninsula, Fourmigue islet Jul-2002 20 35 CS I F/D

16 Hyeres-les-Palmiers, gulf of Giens Nov-2003 1700 28–35 DM/FS III F/A

17 From Carqueiranne (Salettes harbour)

to the bay of Toulon

May-2000 22,200 1–40 RS/PO/DM/FS/CS III F/D/A/H

18 Marseilles, south of the Calanque de l�Escu Nov-2003 30 48 CS I F

19 Marseilles, north of Plane island Oct-2003 30 27 DM/CS I F/D

20 Marseilles, Plateau des Chevres Oct-2003 30 30 CS I F

21 Marseilles, Planier island Aug-2003 1 28 RS I F/D

22 Marseilles, from Callelongue to

Pointe Rouge harbour

Jan-1998 8500 3–50 RS/PO/DM/CS III F/A/D/H

23 Marseilles, bay of Prado Jan-1999 6200 4–42 DM III F/A/H

24 Marseilles, Estaque Nov-1997 500 12–22 RS/DM/FS II F/A/H

25 Marseilles, Pointe of Corbiere Dec-1998 150 5–15 DM II F/H

26 Le Rove, from Cap Ragnon

to Niolon harbour

Jan-1998 1600 7–20 RS/DM/FS II F/D/A

27 Bastia, from Golo mouth to Pietra Nera 2002 25,000 32–45 DM/CS III F/A

28 Bonifacio, gulf of Sant�Amanza Nov-2002 3000 11–41 PO/DM/FS III A

29 Propriano, gulf of Valinco Jun-2004 500 15–25 DM/FS II F/H

Numbers (No.) correspond to Fig. 1. Level of colonization according to Vaugelas et al.�s classification (1999); coastline affected; md = missing data.

Substratum: RS: rocky substratum; PO: Posidonia oceanica meadow with low shoot density; DM: dead matte of P. oceanica; FS: fine sand; CS:

coarse sand. Human activities in the area: F: Fishing; D: Diving; A: Anchorage; H: Harbour vicinity. All the detailed information about the

colonized zone is available on the web site www.caulerpa.org (Meinesz et al., 2004).

1064 S. Ruitton et al. / Marine Pollution Bulletin 50 (2005) 1061–1068

widespread than that of C. taxifolia, which has not yet

been sighted at Marseilles or in Corsica (Meinesz

et al., 2004). Since the first sighting in 1984, the coastline

affected by the C. taxifolia invasion had reached 22.0 km

in 1994, 77.3 km in 2001 and 122.7 km in 2004 (Meinesz

et al., 2004). In comparison, the coastline affected by the

C. racemosa var. cylindracea invasion reached 82.9 km

after only 10 years of colonization (if we consider itsprobable introduction at Marseilles in 1994; Verlaque

et al., 2000). At Marseilles, the colonized area increased

by 350% in six years, which is less than in Italy where a

similar increase was observed in only one year (Piazzi

et al., 2001c). However, this difference could be related

to the mapping scale.

Depth range distribution of C. racemosa var. cylindr-

acea appears to be more restricted along the continental

French Mediterranean coast than in the southern and

eastern Mediterranean where the species was observed

down to 70 m (Southern Italy and Cyprus) (Piazzi

et al., in press). In the present study, although C. race-

mosa var. cylindracea was observed down to 50 m depth

(Marseilles), the areas colonized were usually at between

10 and 35 m depth. The poor development of C. race-

mosa var. cylindracea close to the sea surface is probably

Table 2

Length of coastline and surface area affected by the invasion of

Caulerpa racemosa between 1997 and 2004 on the French Mediterra-

nean coast

Year Coastline (km) Area (ha)

1997 0.4 10

1998 2.0 43

1999 2.3 46

2000 12.3 656

2002 28.0 1014

2004 82.9 4014

Fig. 2. Frequency of encounter of C. racemosa var. cylindracea in

relation to depth, substratum and human activity in the 29 colonized

areas. RS: rocky substratum; PO: Posidonia oceanicameadow with low

density of shoots; DM: dead matte of P. oceanica; FS: fine sand; CS:

coarse sand; F: Fishing; D: Diving; A: Anchorage; H: Harbour

vicinity.

Fig. 3. Colonization by Caulerpa racemosa in the Gulf of Marseilles

and vicinity (France, Mediterranean Sea) in autumn 1998 and 2004.

The grey scale on the left indicates the density of C. racemosa

colonization from level 0 (uncolonized) to level 5 (continuous meadow

of C. racemosa with a thick layer of stolons). The geographic

coordinates are in the ED50 reference system (European Datum

1950, unit: degree, decimal). Open circles and crosses indicate the

absence/presence of Caulerpa racemosa at the field survey points.

S. Ruitton et al. / Marine Pollution Bulletin 50 (2005) 1061–1068 1065

related to high hydrodynamism and sediment abrasion,

which strike off and pull up the plants.

4.2. Substrata colonized

The diversity of substrata colonized along the French

Mediterranean coast appeared to be lower than in some

other Mediterranean regions such as Italy or Croatia

where a wide array of colonized substrata have been re-

ported (Zuljevic et al., 2004; Piazzi et al., in press). As in

other Mediterranean regions (Piazzi and Cinelli, 1999),

the dead matte of P. oceanica appears to be the mostfavourable substratum for C. racemosa var. cylindracea

colonization. Conversely, as in Italy (Ceccherelli et al.,

2000; Piazzi et al., in press), no colonized dense P. oce-

anica meadows were observed. This preferential coloni-

zation provides an example of an invasive species that

takes advantage of ecosystem degradation (Occhipinti-

Ambrogi and Savini, 2003).

4.3. Seasonal changes

The seasonal changes in C. racemosa var. cylindracea

meadows are increasingly marked in the Mediterranean

Sea along a southeast–northwest gradient. Along the

continental French coast, this gradient runs from east

to west. In the Marseilles region, C. racemosa var.

cylindracea presented the most marked seasonal cycle

in the Mediterranean, with a quasi-complete withdrawal

in late winter and early spring. On the other hand, in

eastern Provence and the French Riviera as in the south-

ern Mediterranean (Giaccone and Di Martino, 1995;

Piazzi et al., 2001a), the C. racemosa var. cylindraceameadows did not disappear during the resting period.

The growth rate of Caulerpa spp. is correlated with

temperature (Meinesz et al., 1995; Komatsu and

Meinesz, 1997; Meinesz, unpublished data) so these

geographical differences in seasonal cycle are readily

explained by the winter seawater temperatures at Mar-

seilles, which are among the coldest in the north-western

Mediterranean.

1066 S. Ruitton et al. / Marine Pollution Bulletin 50 (2005) 1061–1068

4.4. Competition with Caulerpa taxifolia

At Toulon and Hyeres, the dense meadows have a

different depth range distribution: C. taxifolia meadows

thrive between 2 and 15 m depth and C. racemosa var.

cylindracea meadows between 20 and 30 m depth (Bel-sher et al., 2003). At Villefranche-sur-Mer (France), at

16–20 m in depth, C. taxifolia almost completely elimi-

nated C. racemosa var. cylindracea over a wide area

(ca. 1 ha) in only one year (Meinesz, unpublished data).

Conversely, near Livorno (Italy), in mixed beds (Piazzi

et al., 2003) and through manipulative experiments

(Piazzi and Ceccherelli, 2002), C. racemosa var. cylindr-

acea showed higher competitive ability than C. taxifolia.In the Mediterranean Sea, C. taxifolia is taller than C.

racemosa var. cylindracea (Meinesz and Hesse, 1991)

and has a less marked seasonal cycle, a higher stolon

growth rate and a higher annual cumulated growth

(Komatsu and Meinesz, 1997; Ruitton et al., in press-

b; Capiomont et al., 2005). However, C. racemosa var.

cylindracea could have a higher capacity to occupy the

substratum than C. taxifolia, because the length of sto-lons and number of apices per square meter are higher

in C. racemosa var. cylindracea meadows (Meinesz and

Hesse, 1991; Meinesz et al., 1995; Ruitton et al., in

press-b; Capiomont et al., 2005). More investigations

are needed for a better understanding of the competition

between the two Caulerpa.

4.5. Human activities and spread vectors

Several potential natural and anthropic vectors of

dispersal of C. racemosa var. cylindracea have been

clearly identified in the Mediterranean Sea. They give

rise to two distance scales of spread:

(1) Long to medium distance (>1 km): zygotes trans-

ported by currents or ballast waters; cuttings andpropagules carried with ballast waters, ship

anchors and fishing equipment (nets, fish pots

and trawls) (Magri et al., 2001; Zuljevic et al.,

2004).

(2) Short distance (<1 km): zygotes, cuttings and

propagules transported by water movements, mac-

rophytes and benthic invertebrates or disseminated

by ship anchors and fishing equipment (Ceccherelliand Piazzi, 2001; Ceccherelli et al., 2001; Renon-

court and Meinesz, 2002; Zuljevic et al., 2004).

Like C. taxifolia, cuttings of C. racemosa var. cylindr-

acea could probably survive for several days in cold, wet

anchor chain lockers and among fishing equipment, so

transport by ships might generate long or medium spread

depending on vessel movements. These vectors are high-lighted by the fact that more than 40% of colonized areas

along the French Mediterranean coast were fishing areas

and most of first settlements were found in the vicinity of

fishing areas, anchoring areas or harbours.

4.6. Accuracy of the mapping

The present study showed that the standardizedmethod used to describe theC. taxifolia invasion can also

be used to monitor the spread of C. racemosa var.

cylindracea. However, the estimation of colonized sur-

face area from the cumulated field data gave a much

lower value than those obtained by kriging method (Gulf

of Marseilles). Consequently, the total gross estimation

for the whole French coastline (4014 ha) probably under-

estimates the true colonized area. In fact, the mapping ofC. racemosa var. cylindracea meadows is more difficult

than C. taxifolia. Conversely to C. taxifolia which has

a mainly contagious pattern of colonization from vegeta-

tive fragments fallen to the bottom (Hill and Coquillard,

1999), C. racemosa var. cylindracea has a diffuse pattern

of colonization that is very difficult to map with the cur-

rent methods (meadows with complex limits). The kri-

ging method, which is probably a good method formapping the diffuse populations could be improved in

the future for example by mapping the dead matte of

P. oceanica, and by increasing the sampling effort.

5. Conclusion

The rapid spread of C. racemosa var. cylindracea inthe Mediterranean Sea has few equivalents in other

introduced marine macrophytes (Verlaque et al., 2004).

Since its discovery at Marseilles, C. racemosa var.

cylindracea quickly spread along the French Mediterra-

nean coast where it found suitable conditions for devel-

opment. Despite a greater winter withdrawal than in

southern areas, probably related to the colder winter

seawater temperatures of the north western basin, largeC. racemosa var. cylindracea meadows already thrive in

several major bays of the littoral. Moreover, to date, all

maps probably underestimate the true colonized area.

In conclusion, the French Mediterranean coast fea-

tures a wide range of favourable characteristics for the

development of C. racemosa var. cylindracea a climate

similar to its native region (south-western Australia;

Verlaque et al., 2003), large areas of dead matte of P.oceanica, numerous vectors of spread in particular

human activities and no efficient biological enemy

(Ruitton et al., in press-a). Consequently, the further

spread of C. racemosa var. cylindracea is to be expected.

Acknowledgements

This work was supported by Rhone—Mediterra-

nee—Corse Water Agency, the Provence-Alpes-Cotes

S. Ruitton et al. / Marine Pollution Bulletin 50 (2005) 1061–1068 1067

d�Azur Region, the DIREN PACA (Direction Regio-

nale de l�Environnement Provence-Alpes-Cote d�Azur),

the Environment Office of Corsica and ALIENS (‘‘Algal

Introductions to European Shores’’), a 5th Framework

Programme of the European Union.The authors are in-

debted to Michael Paul for correcting the English. Theauthors also thank Roland Graille, Frederique Zuberer,

Bernard De Ligondes and Eric Pironneau for field

assistance.

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