ICES 1990 POSTER C.M. 1990/L:36Sass. Q

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A TROPHIC MODEL OF A MEDITERRANEAN LAGOON,ETANG DE THAU, FRANCE

by

M.L Palomaresa

P. Reyes-Marchant, N. Lairb

M. Zainure, G. Barnabe, and G. Lasserrec

aLaboratoire d'Ichtyologie AppliqueeEcole Nationale Superieure Agronomique

145Avenue de Muret, 31076 Toulouse Cedex, France

bLaboratoire de Zoologie-ProtistologieUA CNRS 138, Hydrobiologie des Eaux DouceUniversite Blaise Pascal, Clennont-Fe"and II

B.P. 45, 63177Aubiere, France

cLaboratoire d'Hydrobiologie Marine, UA CNRS 694Universite des Sciences et Techniques du Languedoc

Place E. Bataillon, 34060 Montpellier, France

ABSlRACf

An attempt was made to model a littorallagoon in the French Meditterranean, the Etang deThau. The model was structured around cornmercially imp0t;lant fish groups with a toppredator (i.e., Anguilla anguilla) biomass assumed at 11 t/km. Reasonable estimates ofbiomass for the other species groups were obtained.

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INTRoDucnON

.Tbc Etan~ de Thaü (Fig•. 1) is arelatively deep (mean == 5 rii, maximum =10 m) littoral lagoon situated in tbeMediterranean and bordered by the townsof Sete, Meze~d Marseillan., It covers anarea of 88 km and measures 19.5 x 4.5 km(Audouin 1962). Seawatsr exchan%e~cursin a diUrnal cyc1e at 7xlO to 35xl0 m Iday.Mean annual water .temperatrire is.15.3°Cand mean salinity at 37.6 ppt (Bach 1985).

Thau is areiatively, well mvestlgatedarea. Recent. ecologicaI studies initiated intbe lri~oonwere centered around the idea cfa statistical modelling scheine. using pathanaIysis that wouId .lead to a knowledge ofhow the lrigoon functioris(Arilanieu et 31.1989). These studies, however, have justbegun and their results are still umivailable.

.It was possible, though, to extracteriough information frorii the publishedliterature . and use it in the context ofECOPATII II to abtain a preliminarytrophic model of the lagoon. In thefollowing, abrief description is given of howthis information was standardized' andassembled for use in thc model.

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MATERIALS ANO METHOOS. . .

ECOPATII II uses a baIancedequation of the form:

~ \, '. ' ,.

Bi • Pi/Bi • EEi =(Bj • QjlBj • DCij)

which. requires . estimätes . of biomass,prodriCtion, ecotrollhic efficiencies, foodconsumption and dlet compositions of thespecies groups being considered.. Thismodel is dlScussed. in more detail byChristensen and Pauly (this vol.) to wbichwe refer for details.

. 'fable i lists the input paranieters usedfor the model of tbe lagoon for the 1980s.Note that tbe parameter values (e.g., B, P/Band Q/B) were averaged to represent annualmeans (ar each .species group.. Missingvalues indicate eitner that no informationwas available in. the literature or thatliterature data could not be used because it

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could not be staiidardized~ .This niissInginformation was . later.. estimated... byECOPATII n. All ecotrophic effiClencyvalues were assumed.

The biomass estimate. of 11.0 t/km2was obtained froni an earlier run.of themodel based on known biomass .valries fOf .molluscs, ,zcioplankt~n, phytoplankton ~dbenthie producers. Smce to~ ee! catch wasestimated to be ~O t/km, the biomassestirilate of 11 t/km was accepted riild usedin the final model presented below.

RESULTS AND olscussioN

"fable 2 lists the biomass esiiIiiatesobtained from ECOPATII n, while Table 3listS tbe model's summary statistics.. Thebiomasses for adult, and young predatorsap'peär reaSonable. However, these valuesmlght well be underestiniates given tbatFarrugio and Le Corre (1986) estim~ted acumulative biomass of 56 t/km. forEuropean bar,. sea ,bream and. sole.Fui'thermare, Qwgnard (984) menti0I!S .atotal fish catch ,of 26 t/km for .197L This 2sthree times the total fish catch (9 t/kIn )used in this model of the 1980s. It should benoted that there has been an increase infishing effort in tbe lagoon (Bach 1985), andthus one should expeet lower biomassestimrites in recent years. Moreover, thegroups given eIriphas~s .bY. FaiTugi~ ~d !--eCorre (1986) were mlgratory specleswh,lchstar in the lagoon f~r ~ll1Y a part ~f th~ year•This furtber comphcates theestlmation ofmean annual fish biomass in the lagoon.

. The eccitrophIc" efficiencfes (i~E)estimated by. ECOPATII II seem to bereasonable. In eels, an EE value .of 0.84migbt be compatible with cyelie migr.ationsoccurring in die eel pop~latlOn from inlandwaters to the sea. The high zooplankton BEof 0.98 is acceptable given that the lagoonserves as a. nurseIY. ground for Jry,. andjuveniles of both nugratory and sedentaryspecies. Thus, ,tber~ is a considerableamount,. of pI'edatlOn pressur.e onzooplankton.When a dep1etion of. thezooplankton population occurs. (perhapsonly during a certain season of the year), asubsequent surplus of phytoplankton might

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occur. Given its high tumover rate, theJ?hytoplankton would die off and join theoetritus. Thus the low EE vaIue of 0.50. :

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Overall, the ECOPATII n modelpresented in Fig. 2 seems to be reasonablybalanced. However, the seriscinal shirts inthe ecosrstems are not taken intoconsideratlOn. It . is. recommended,therefore, tbat the next modelling attemptsshould foeus on separating .tbe fisb boxesin!O seasonal grC?ups and taking theil'rmgratory patterns mto aeeount.

REFERENCES

Amanieu et al. 1989. .

Audoin. 1962.

Bach, P. 1985. La peche dans l'~tang de Thau.Application de quelques notions d'tcologielhtrique aux communaut~ de poissons et ä.leur exploitation. Strattgie de quelquespopulatio.ns ichthyologiques capturtes. USTL;Montpellier/n 316 p. + annexes. Th~e,Doctorat 3' e cycle. .

CambronYr M. 1983. Recrutement et biolo~e desstaaes juveniles de multilidae .(POlSSOns­Ttltosttens) dans trois mifleux lagunaires duRoussillon el du Narbonnais (SalSes-Leucate;Lapalme, Dourdi~ou). Univ. Pierre et MarieCurie. Pariä UDlV. Perpignan. 285 p. These.Doctorat3' e cyde.

Charbonnier, D. and S. Garcia, editors. 1984. Reportto lhe third GFCM technical consultation onstock assessment in the Balearic and Gulf ofLion's statistical. divisions. AnnexeClAppendix C. Rapport ISTPM conccrnantl'exploitation ella recherche dans la sous-zone37-2; pp. 41-54, 110 p.

.' Farrugio, H. and G. Le Corre. 1986. Jl'iteractionsentre pccheries de lagunes, pecheries cötiereset peChe au chalut dans le Golfe du Lion:Rapport F'mal Convenlion CEEXlV-Bl-8S12/MI0 P. 208 p. .

Gerbal, M. 1990. Repartition du macrophytobenthosde l'~tang de Thau, p. 163. InD. Jouffre (cd.)ECOTHAU, 3,m, Compte-Rendu d'Activitts,1988-1989.174 p.

GrentZ, C. 1990. Quantiflcation et destinte desbiodtpols dans des zones de. productionconchylicoles intensive en MMiterran~e..PJi1'70-121. In D. JouIfre. (ed.) ECOTHAU, 3 .eCompte-Rendu d'AcUV1tts, 1988-1989. 174 p.

Henard, D. 1978. Production primaire d'une lagunem~diterran~cnne; ttang de Thau (H~r<luJt).USTK. Montpcllict, 85 p. Th~e 3cmc cycle.

Hoenig, iM. 1984. Empiricai use oClongevity data toestimate mortality rates. US rlSh. Bult Sl(4).

Jouffre,O. 1989. Etude de l'organisation spatiale duzooplankton dans l'ttang de Thau (France) etde l'inf!uence des tcbanges entre la lagune clla met. USTL, MontpeUier, 243 p. TheseDoetorat.

Monteiro, C.C. 1989. La faune ichtyologique dc lalagune Rio FormoSa (sud. Portugal). USTL.Montpellier, 219 p. Th~e Doctorat.

Outm. .V. A. Fiala-Medioni, J.C. Colomines and 1.Mabit. 1988. Ecophysiologie de l'huitreCrassostrea gigas en conditins naturelles dansune lagune meditcrrantenne (Etang de Thau):role dans les lransCerts tnergttiques ct impactdes populations sur le milieu, pp. 124-1S3~M. Amanieu (ed.) ECOTHAU,CompLe-Rendu d'Activilts, 1987-1988. 168 p.+ annexes.

Pauly, D •.. 1989. A simple mcthod Cor cSliniating tbcfood consumpLion.of flSh populations fromgrowth data and food conversion experiments.U.S. Fish. Bull. 84(4):827-842.

Polovina, JJ. 1984. Model of a coral reef cc6systcmI. Thc ECOPATH model and its applicationto French Frigate Shoals. Cora! Rccfs3(1):1-11.

Quignard, I.P. 1984. Lcs caractcrisLiques biologiquesct environmentales des laguncs en. tanl quebase biologique de l'amenagement des~~~eries, p. 3-38•. In JM. Kapclsky and G.

erre (eds.) Management of coasÜu JagoonflSheries•. FAO Stud. and Rev. 61. Vel. I,Rome, 4381'.

Rosecchi, E. 1985.. Ethologie alimentairc dessparid~s (Diplodus annularis, Diplodus sargus,Diplodus vulgaris, Pagellus erythrinus, Sparusaurata) du Golfe du Lion et des ttangsPalavasiens. USTL; MontpcUier, 282 p. TheseDoctorat 3,me cycle.

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lable.l. Inputs used In ECOPATH 11 for the Thau lagoon system tor the 1980s.

Specles Catc~ Sjom~s( PISa) Q/Sb)Eec)considered Groups (t/km t/km) (annual) (annual) GE Remarks

Anguilla anguilla eels 7.80d) 11.00c) 0.80 4.00 0.20 eel from 4-10 yearB

Dlcentrarchus labrax adult five In lagoon; QlB

Sparus auram predators 1.20e) 1.00 4.80 0.90 0.21 assaumed at GE

Solea volgaris value

Mugilidae adult O.lSt) 1.00 5.00 0.90 0.20

mugiJidsDlcentrarchus labraxSparus aurata young 0.32t) 3.00 8.00 0.90 0.38Solea volgarls predators

• MugilidaeAtherIn!dae sUversides 0.201) 1.60 11.60 0.90 0.14

molluscs 267.009) 0.70g) 5.00 0.90 0.14 OIB ass. at GE value

benthos 1.00e) 4.00 36.00 0.90 0.11 P/B. QJB ass. generalvalues (Moreau. pers.comm.)

zooplankton 267.00h) 20.00 60.00 0.33 P/8 from Polovlna(1984);

phytoplankton 600.00') 40.00 OIB assumed at GE

valuebenthlc 1500.001) 20.00 OIB assumed at GEproducers value

a)

b)

c)• d)

e)

f)

g)

h)

I)

j)

P/B - ZIn fiah groupa; In absence ofliteratur. data, Zwai estimated using InZ .. 1.44· 0.984 In tma.e (Hoenig 1984)

OIB In fish groups estimated from log 0/8 ... -0.102+O.444IogT-o.11510gW+0.4271ogA+0.577 logD-O.46410gP (Pauly 1989)

Assumed values

Total catch =0 26 t/km2 ln 1971 (Quignard 1984) and 18.4 t/km2 ln 1982 (Charbonnier and Garcia 1984).1f eel ~'l(, oftotalcatch (OuJgnard 1984), then !el catch =15.6 t/km2 and 11.1 t/km2 respeetively. Annual decrease of 0.41 t/km Iyear.Eel catch predieted at 7 t/km In laIe 1980S.

Oata from Farrugio and La Corre (1986)

Oata from Bach (1985)

Mean production 30,000 tlyear (Charbonnier and Garcia 1984) and mean biomass 42,000 tlyear (Grentz 1990) of molJuscsIn cullure.

Mean annual biomaSa cf 3,000 tonnes dry weight (Jouffre 1989); assumed dry to wet weight conversion for zooplankton Is 15.

Henard (1978) glves a mean phytoplankton produetion of 204 gClm2/year (about 600 t/km2 wet welght).

Biomass estimate of macrophytobenthos at 150 t/km2 dry weight (GerbaI 1990); assumed dry 10 wet coversion faetor of 10 formacrophytes.

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Table2. Estimated parameters of specles groups of the Thau Lagoon system obta[ned from ECOPATH 11for the 19805.

Blom~s Flowto FoodGroup t/km EE Detritus Intake Respiration Assimilation

Eels 0.840 10.2 44.0 26.4 35.2Adult predators 6.87 6.5 29.4 17.5 23.6Adult mugillds 9.17 10.1 45.8 27.5 36.7Young predators 11.50 21.8 91.9 39.1 73.5Atherlnlds 9.07 22.5 105.0 69.6 84.1Molluscs 426.00 456.0 2,128.0 1,405.0 1,703.0Benthos 13.10 99.8 472.0 326.0 378.0Zooplankton 3,332.0 16,020.0 7,476.0 12,816.0Phytoplankton 0.980 13,026.0Benthlc producers 0.460 29,892.0 •Detritus 0.004

Table 3. Summary statlstlcs for the ECOPATH 11 model obtalned for the Etang de Thau In the 1980s.

Statlstlc Unlt Estlmate

Sum of all production t/km2/year 19,214

Sum of all respiratory flows t/km2/year 53,723

Sum of all f10ws Into detritus t/km2/year 46,875

Total system throughput t/km2/year 119,813

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---------------=--------:-1

0L.'--..L_-.J~'-_~.a--_4..., _-.l~ Km

Mediterranean

..': ••. I. ...:.,\:;.:..\.~·I:I·~~~~;.;~~.:·:.··~

: ..:",:.;'::. ",:: :;.:........;.~.:;. :"/.": ":,:·:i';· i ~:;"f"':':I;:\:'1-~'::!!/:.:.,.,,:~ I.•.. :..:.,.... t~.:.:.:~-::-

"'f",

.'~~':I.',. ':\.

Fig. 1. Map showing the Thau lagoon region. Area in boxes indicate zones of oysterculture. Inset shows the lagoon area in France (adapted tram Outin et al. 1987).

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Adultpredators

8c6.1EE=O.9

Benthos8=13.1EE=O.9

AdullmugilidsB=9.2

EE=O.9

467

Young predators8=11.5EE=O.9

45.

~~--I04 ------+---1

:~--43.G--------r---;==±=::;:---1

Atherinids8==9.1

EE=O.9

Eels8=11

EE=O.B45

Zooplankton8=267

EE=0.98

PhytoplanktonB..600

EE=0.46

1214Benthic

producers8=1500EE=.004

j.------r----,r-- 2128

Molluscs8=426EE=O.9

Fig. 2. A quantitative representation o~ the trophic interaction in Lagoon Eta2g de Thau.France. All flows are expressed in g/m Iyear. while biomasses (8) are in g/m .

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