1

1

1

ASSESS~ENT OF HUMAN IMPACT ON GIANT CLAMS

(TKIDACNA MAXI~lA) NEAR JEDDAh, SAUDI ARABIA

ALAIN BODOY*

Faculty of ~Iarine Sciences, King Abdulaziz University,

• Jeddah, Saudi Arabia

AbSTRACT

A reprint from

PROCEEDINGS OF THE SYMPOSIUM ON COR1\.L REEF ENVIRONMENT OF

THE RED SE 1\.

Edited by

MASSOUD A. H. SAAD

1984

2/ -

1

1

1

ASSESS~ENT OF HUMAN IMPACT ON GIANT CLAMS

(TKIDACNA MAXIMA) NEAR JEDDAH, SAUDI ARABIA

ALAIN BODOY*

Faculty of ~Iarine Sciences, King Abdulaziz University,

• Jeddah, Saudi Arabia

AbSTRACT

A reprint from

PROCEEDINGS OF THE SYMPOSIUM ON CORllL REEF ENVIRONMENT OF

THE RED SEll

Edited by

MASSOUD A. H. SAAD

1984

z 1 -

Proc. Symp. Coral Reef Environ. Red Sea, Jeddah, Jan. 19H4:472-490.

ASSESS~ENT OF HUMAN IMPACT ON GIANT CLAMS

(TKIDACNA MAXIMA) NEAR JEDDAH, SAUDI ARABIA

ALAIN BODOY*

Faculty of ~Iarine Sciences, Kin9 Abdu1aziz University,

Jeddah, Saudi Arabia

AbSTRACT •

A quantitative, eco10gica1 study was carried out on severd1

populations of the bivalve Tridacna maxima, in order to assess

the chdnyes due to the collection of this species. Four

transects were made across fri ngi n9 reefs whi ch differed from

their accessibi1ity, but exhibiting the same orientation and wave

exposure.

In an undisturDea area, the number of inoividua1s found

a10ng a transect was 63, the average 1 ength, 134 lTIil and the

biomass, expressed in ary weight of tissues and re1ated to one

metre of shore, B8 g.m-1. In se1àorn uti 1ized area, those

parameters were 19 individua1s, 113 mm and 18.6 ~.1,,-1 respective1y, while, in an area subjecteà to intermediate

exploitation, 4U individua1s were found, with an average size of

b9 mm and a biomass of 5.7 g.m- 1. At a high1y utilized

beach, on1y 6 individua1s were found, with an average 1ength of

82 mm and a biomass for one metre of shore of 1.5 g.m-1

.

The effect of such human impact is aiscussed, accorainy to

. the eco10gy ana the bio10gy of this species.

(*) Permanent address: Laboratoi re Cultures l'iari nes IFKL~IEk

b.P. 19u,17 39U La Tremblade, FRANCE.

473

INTRODUCTION

Among the different species of invertebrates co10nizing

coral reefs in the Indo-Pacific area, the giant clams of the

genus Tridacna are a dominant feature of the sha110w waters were

they are living (Rosewater, 1965). They are probab1y the 1argest

bivalves which have ever existed, and the Pacific species,

1. giyas, can reach one metre in 1ength. From a bio10gica1 point

of view, their association with symbiotic a1gae, which was

studied by severa1 authors (Yonge, 1936; Jaffrey and Haxo, 1968;

Goreau et al. 1973 and others), enab1es us to consiaer species -- '. of the genus Tridacna as phototrophic, primary producers.

Indeed, the metabo1ism of the zooxanthe11ae contributes to their

nutrition (Ricara and Sa1vat, 1977).

They are a1so of an economic interest, since they are eaten

in many countries and the adductor musc1 e has a very high

commercial value in South-East Asia (~lunro and Gwyther, 1981).

Some atternpts were made to eva1uate their aquaculture feasibility

(Beckvar, 1981; Munro and Hes1inga, 1983), and the possibility of

rearing of tridacnid's 1arvae (La Barbera, 1975; Jameson, 1976;

Gwyther and Munro, 1981).

In spite of being essentially reef-top inhabitants,

re1ative1y few works have been carried out on their growth,

population size and biomass (Sa1vat, 1971; t1cMichae1, 1974;

Richard, 1977 and 1981; A1ca1a, 1981; Beckvar, 1981) .

Along the coasts of the Red Sea, two species are usually

found, T. maxil,la and T. squamosa (Hughes, 1977; ~leryner and

Mastaller, 19BO), the first one being the most abundant. These

species are submitted to heavy exploitation in the vicinity of

Jeddah, Saudi Arabia, and they are often collected on the reef

li zi ng

f the

were

rgest

Cl es,

point

was

1968;

ecies

cers.

thei r

=aten

high

J81 ) •

il ity

:y of

976 ;

.nts,

wth,

974;

ally

and

he se

1 of

reef

-- .

flat, both for food and for decorative purposes .

The present study aims to assess on quantitative basis the

human impact on T. maxima. Therefore, abundance and biomass were

estimated for several populations of this species living in areas

differing for their accessibility. Similar studies, following the

same goal , have been carried out on corals (Antonius, 1984) and

on fishes (Oakley, 1984).

•

~IATERIALS AND I1ETHOD

The analysis of different populations of T. maxima has been

carried out using a transect technique . Four different transects

were studied in sites differing in the extent to which they are

affected by human impact (Fig. 1). The site located North of the

port of Tuwwal, because of its military importance, is strictly

protected by the coast-guard, and may be consi dered to be an

unai sturbed ar ea . The second site studied i s about 180 km South

of Jeddah. 80th the distance and the state of the road leading

to it make its access difficult. A third site was on the reef,

North of Sharm-el -Abhur, whi ch i s one of the favourite beach

spots for Jeadah people auring week-ends, and the last transect

was studi ea on Jeadah northern Corni che, 3 km North of the

desalination plant. The latter area, turned into an artificial

beach, is very popular, due ta its easy access. Since this work

was carried out, a development scheme on the reef flat has

destroyed the studied area.

I:.ach transect cons i stea of a band, 10 m wi de, across the

reef flat, from the shore-line to a depth of 2 m at the

beginniny of the outer slope. Along each transect, all the

indiviauals of T. maxima were counted and the maximum antero-

475

Tuwwal

•

•

20 Km

N

t Shoiba

RED SEA

Fi~re 1. Location of tIle four transects a10ng tI1e œasts of tIle kea Sed.

476

posterior length measured with a maryin of error of + 2 Iffi]. The

transect was divided into five metres wide sections, each of

50 m2 area.

Abuut 20 individuals, chosen as so to coyer the whole span

of length found in the field, · were also sampled in order to

establish the relationships between length and weight leading to

IIleasures of biomass. For that purpose, the specimens were

arained, to obtain a total weight accurate to 0. 01 g, and then • opened. The flesh was weighed to obtain the fresh weight and

then dried for 24 hours at 85°C and rewei ghed, to get the dry

weight. The logarithmic relationships between length and total

weight, fresh weight and dry weight were calculated using

regression techniques. The values of biomass were then

calcul ated for each unit area of 50 m2

. To improve on the

accuracy of these quantitative data, additional perpendicular

transects were carriea out in areas of high density, such as the

edye and the inner flat, near the algal crest.

Length-frequency distributions of the clams were calculated

by includir.g all the animals for each main transect.

RE5UL T5

The constants in l ength-wei ght rel ati onshi ps are shown in

Table 1. Using these coefficients, the dry weight of each animal

was calculated from its length. Fig. 2 shows the results for the

undisturbed area of Tuwwal, including the number of individuals

obtained for each unit area of 50 m2 along the transect, the

biomass (dry weight) of T. maxima corresponding to each unit area

(in 9.10-2), and the depth profile of the reef flat.

From a morpholoyical point of view, the reef flat

neighbouring the shoreline is composed of a sandstone tile,

477

Number 01 indiyidu.ls / 50 m 2

':t~~mL..l..LL..LJ.LLL~o m Biomass

3

2

Depth (m)

2

1 gure

50 100 150 300 m

----------~a 1 gal r id ge

Inn", reel liaI undance an lomass 0 rl acna

transect across the reef flat in the undisturbed area

(Tuwwa 1) .

Number of indiyiduals /50 m 2

': t,-------,-----L--L..LO --'..LU...LL-O 1 m ~O db --LL-.WJ..LjO JJ~ Biomass dry wei ght (g / m 2 )

3 -

2

lf-•

1 1

50 100 Deplh (m)

-------1 -

21--

1

150

1

algal

.+ n n-J1 1

200 250

, out er : reef

300 m

:id9~; 1

inn e r r eet fi at 'fiat Figure 3. Abundance and biomass of Tridacna maxima along a

transect across the reef flat in a slightly disturbed

area (Shoiba).

478

slight1y covered with si1t, and is popu1ated with Cau1erpa

racemosa, which first is scattered and then becomes more numerous

as one gets nearer the end of the sandstone ti1e (150 ml. Rocks,

uneven1y scattered and surrounded by sandy patches, were then

found, as were the first T. maxima. Further towards the open sea,

rock formations become denser whi1e C. racemosa disappears. The

highest densities of T. maxima were found in this zone, between • 200 to 240 m from the shore. Further on, a very dense belt of

seaweeds (Sargassum sp.) is located at the top of the a19a1 rioge

(Fig. 2), as defineo by Battistini et al. (1975), and no T. , --maxima were found there. However, at the ena of the outer reef

f1at (Fig. 2), individua1s were sett1ed. The externa1 slope, not

stud ied here, was sometimes inhabited by scattered , large

inaiviaua1s.

The total number of individua1s found a1on~ this transect

was 63, the majority of which were on the reef f1at. The 1ar gest

one measured 23 cm, and the smallest, 6 cm. The average size for

the who1e of the transect was 13 . 4 cm, whereas for inuividua1s

from the inner reef f1at (Fig. 2), betwen 195 and 25U m, it was

13 .6 cm.

11 CTiI.

The average size of those 1 iving beyond the rioge was

-2 The highest biomass was 3.U6 9.01

If une considers a band of one metre width, perpendicu1ar to

the shore, the biomass can be expressed in grams per 1 inear Tiletre

of shore, which allows a comparison between fringing reefs which

are not of the same extension. For the Tuwwa1 site, the biolilass

-1 of the population of 1. maxima was 88.12 g.m of shore .

The fringing reef located Sou th of Sho iba (South of the

aesa1ination plant for Hakkah city), is different in various

ways. Its width is greater (505 m) and for the first 2UO 01 from

N~:Lber 01 ,ind, ividuils

. 1 rD-O rill 0 0 rD-O ,~-L ______ -L ______ ~ ______ ~~~WU~U--LWUL

Biomass dry weight (g 1m2 ) 3-

2

lLr __ ~~I~I ______ LI ____ ~ ______ lu~n-~lli~hun~L~UiL , , 250 300 350 400 450 500 m

Depth Cm) ~--~ L(-r------,-------,-------.-------~------. ~ ~I __ ~__ 1 1 1

1- algal~idge j t .ou er 2 : reef ,

inner reef Ilat ,flat

Figure 4.Abundance and biomass of Tridacna maxima along a

transect across the reef flat in a visited area (Abhur).

Number or individuals 1 50 m2

':t DI 0 DO

Bicmass dry weight (g/m 2 ) 3 c-

2 -

1-

• 1 ' , , , ,

50 100 150 200 250 m Depth (m)

1 1'---, , -' .' ,

1

,,,./i~ 1 1-

"lgal

1 2 - 1 outer

: reef

1

in n e r r e e 1 lia t 'fla t

Figure 5. Abundance and biomass of Tridacna maxima along a tran-1 , sect across the reef flat in a heavily disturbed area

(Jeddah).

l

t

..

t

l -> l

t

(

2

> l

k

"

lm

a

Jr) •

In -

480

Table 1. Coefficients for the linear relations, (y = ax + b)

between the logarithm of the length (y) in ITITI and the logarithm

of the different weights (x) in g . Number of samples = 20. a =

slope of the straight line. b = intercept. r = correlation

coeffi ci ent .

a b r

• Total weight (live animal) 3.8043 - l .5834 O. 9914

Fresh weight (organic tissues) 3 .0739 - l .7238 0.9639

Ory weight (organic tissues) 3 .2237 -2.5890 0 . 9715

the shore, there is a zone of intense sedimentary accumulation,

with a large proportion of fine particles. This fine substratum

has not been coloni sed by seaweeds. At 300 m, a population of

green filamentous alyae (cf. Enteromorpha) appears. The alyal

belt of Saryassum is absent ana is replaced by a population of

Turbinaria cf. triquetra . The outer fringing reef consisted of

patches growing from a step 15 m deep, whereas, in the other

loc ations, the reef flat ended with a precipitous slope .

The distribution of the number of inoividuals and the

biomass along the transect are shown in Fig. 3. The population

of T. r,laxima was concentrated over the l ast 100 m of the

transect, and only 19 inaividuals were found. The Enteromorpha

zone had particularly few clams. The average length of this was

population was 11 .3 cm, but some individuals were up to 25 cm

long. The average biomass per linear metre of shore was 18.6 -1 -2 g .I,\ , and the highest biomass (2 y.m ) was found on

the outer fl at of the fringins reef.

The thiro transect was carried out on a popular beach, a few

kilorlleters North of Abhur creek. The width of the fringing reef

Wd S 275 \.. After a zone of back reef, composed of a seaiment of

481

a fine sand covering a flagstone, there were populations of thE

algae Padini. pavona and C. racemosa (Fig. 4).

The first individuals of T. maxima were found at 120 m from

the shore. On the top of the al ga 1 ri dge, where a very dense

belt of Sargassun, sp. was located, only one individual was found.

The total number of individuals was 46 over the whole transect,

while the biomass, expressed in grams per linear metre, was 5.7

g.m- 1 Ali the i ndividua ls however, were very small, with

dn averaye 1 ength of only 6.9 CIl1. The largest individual

"~asured 14 cm. Apparent ly, the distribution of the anima I s along

the transect was not size-related. The highest densities were

fuund on the outer edge, and Just after the

Du tar reef fIat. The highest biomass was 0.16

al gal ridge -2 g.m .

on the

The last transect, carried out on the North Corniche in

Jedddh, was 2H5 ,. long (Fig. 5). For the first 50 m, the bottom

was IIIade of a fi agstone covered wi th a very fi ne coat of sil t and

blue-green al~ae. Further on, the algal cover cunsisted in the

",ain of P. pavona, T. cf. triquetra and C. racemosa. The belt of

Sargassum sp. was weIl developed. The coral populations were

recoveriny slowly after having been heavily choked up with mud,

when the Corniche Road was built two years ago (Antonius, pers .

comn,.) .

The population of T. ["axima was very scattered. The absence

of valves fro~ dead individuals l eads one to think that the small

size of the popul .. tion was not due to d r"ortality resulting from

the ,~uu. On l y 6 individuals were fuuno, averaging ~.2 cm in

lell~th, the lar~est one ,nedsuring 11 c,,;. The biomass per linear

",etre of shore was the lowest of the four trdnsects: 1.5 9.01-1

The hi,jhest biornass was u.14 g.nr- 2 .

Fi~ure 6 shows the quantitative uata for the population of

the fuur stuoied areas. The average biùnrass indicates that these

four dreas can be c la ssified in the sa,lle way. The low number of

indiviouals fou na in Shoiba can be relateo to the presence of

.he

om

se

d.

t, ,

.7

th

al

ng

re

he

in

Jm

ld

)f

J,

:e

n

r

1

f

e

f

f

- - -482

Enteromorpha sp., which usually indicates either natural or

man-induced alterations in the quality of the marine environment.

The average size of the populations from Abhur and Jeddah varies

only within 5%.

The analysis of those different pop ul ations carried out 2 using criteria of density (number of individuals per 50 iii ),

of longevity (average size of the population), and of standing •

stock (dry weight) enables a classification of these populations

to be obta i ned, correspond i ng to the order es tab 1 i shed in terln 0 f

their accessibility. Since T. maxima is a sessile species, it is

likely that its scarcity in the most visited areas is mainly due

to excessive collection.

The analysis of length-frequency distributions for the four

transects (Fig. 7) leads to confirm these results. big

individuals, larger than 15 cm, were found on the undisturbed

reefs of Tuwwal and to a smaller degre2, of Shoiba. In botll

sites, the distributions are polymodal. In the two other areas,

only individuals smaller than 15 cm were founo, ano the shape of

the distribution is much sil11pler. In Jeddah, the number of T.

maxima found along the transect is too low ta allow any cOIl."ents

on a preferential exploitation size, but the large nurilber of

young individuals founo in Abhur transect clearly indicates a

size-dependant collection upon the largest animal s.

OI5L USSlUN

~Iuch attention has been paid to the symbiotic relation-

ships between the zooxanthellae and tridacnids. But fe w studies

have been carried out on the ecology of related species. The

density of T. maxilila can reach up to 224 individuals per square

metre, in

be scarce

the atoll of Reao (Salvat, 1972), but this species iOay

in other atolls (Salvat, 1971). t'Icl-iichael (1974) found

0 <.D

0 ~

0 N

Number of individuals

0 N

0

Average size mm

483

N

0

ci

0 0

0 111

Avera 9 e biomass glm2

'" ~ ~ ::J

1--

'" .0 L :6 ::J

.r: .r: .r: tf) .0 '" ~ u

U <l>

Biomass g lm c4 shore

Figure 6. Abundance of Tridacna maxima, average size, average

biomass in g.m-2, and average biomass for one metre

shore in g.m- 1 for the four sites.

-~ .

:r: <{

0 0 w ..

-a: ::J r m <{

<{ m .-0 r Vl

...J <{

3: 3:

- ::J f-

484

l 1 rdb p 1 1 ,

20r-r-~

-•

15 f-

r ~

10 f-

5 1- - -

-

1 -r n 0'-- ~ 1

5-

riln ri 1111 n p q 0- 1 1

10 -

-5 - -- - -- ~

-- - - -

~ - m . - -

0-5 10 25 20

r i oUl'e 7. L en " th -fr~ q u ~ "cy Gi s tri iJ ut io li of Triuac "d L,ax i 'dd fo r

til e f our s t uu i eo s ites .

25 cm

an dverage density of 0.8 T. maxin,a per square metre, at a site

on the Great barrier Reef.

These oensities are higher than in our study, since the

hi~hest densi ty recoraed was 0.22 i ndivi dua 1 s per square metre,

in Tuwwa1. In the Pacific Ocean, 1. maxima often sett1es in the

shallow wdters of 1ayoons, e.g. in the atoll of Takapoto

(kichard, 1~77). In the Rea sea,sett1ement in she1tered, shallow

waters close to the shore1ine was not recordea in any of the

fuur transects. This ana the 10w average density can be

exp1ainea by the pecu1iarities of the Red Sea coasts. During the

winter, co1d winds can 10wer the water temperature on the

sha110w, inner reef, sometirues down ta Il'C (Hughes, 1977), which

cou1d be a 1eta1 temperdture for many tropical species. On the

other hana, 100se sediments, acteo Dy waves, might 1imit

sett1e"~nt uf Red Sea populations.

The exposure can dffect the quantitative nistriDution of

T. n~xi ••. ~a1vat (1~70) encountereo this species on the windwaru

sioe of the barrier reef of Gambier Islands, but not on the

other, she lterea, 1 eeward si de. In our case, for all the

trdnsects, the reefs showea sÎlnilar, North-,outll orientation, ana

receiveo the prevdi1ill~ winds From the saine direction.

Therefùre, these ecu1o~icd1 fdctors shou1d not affect the

results.

Un1y very few e',ipty shell s were founn, ana since these were

a1ways 1ar~e, one Cdn dssun,e that lIiOSt of the ':Jorta1ity is

nldin1y nue to collectin~. This collecting rate seems ta vary ,

with the difficulty of access. Severa1 authors have estilllatea

the asy"ptotic 1enyth (1",,), corresponding to the averaye nlaxima1

size in the growth equation of Von ~erta1anffy, for different

areas. Tnese oata dre sUlillilarized by 11unro and Hes1 ing (1983).

Trie values are rankea frolil 12.4 CIII froll: a population coming frolli

an dtoll of French Po1ynesia (kichara, 1977 and 1%1), to 30.~ CIII

in New Gu i nea (['Iunro dno Gwyther, h. ~) . Uther values of this

estÎlllatea asyrdptotic 1ength are 24.3 Clil ( ~,unro and Gwyther,19ts1),

- ---.-- ~

486

27.5 cm (~ICr'lichae1, in Munro and Hes1inga, 1983) and 3U.5 cm

(l'ICKOy, 1980, in op.cit.) For all these populations, the

10ngevity was more than 20 years. Assessments on such 1 ongevity ,

based on she11s rings counts, agr~e with such resu1ts for Red Sea

populations (unpub1. aata). In these 10ca1ities, the asymptotic

1ength was ca1cu1 ated to be within 20.23 cm, if the transect of

Shoiba was excl uded, were individua1 s up to 25 Cln were observed.

Accordin~ to Rosewater (1965), the maximal recorded 1ength is 35

The growth ~quations given by McMichae1 (1974), Mu nro ana

Gwyther (l9U1) ana Ricllard (19é!1), indicate thât T. maxitlla wou1a

reach a 1ength of 12 c,,, in 6-13 years. Therefore, to regenerate

an overco ll ected popu1atioll to a norlola1 1eve1 (e.g. in Tuwwa1),

will take Iolany years, once collectin~ stops .

Mother prou1elll, re1atea ta th~ reproduction, Ilas to b~

men tionea. T. illax ima is il protanoric her;ilaphrodite. Richard

(1982) gave the following scherlle for a Po1ynesian population:

sexua1 IIlaturity is reach ed for ct size of 0 cm; after a ",ale

phase , up to 7 Clll, sitnultaneous hermaphruaites fon,l th e

population between 7 and 15 Clil. The 1arger inaividua1s wûu1d

have on1y feilla1e ~onads. ~ut, for Illost of the other populations,

those l ar~e animal s relnain s imu1taneous hermaphrodites.

According to Jaubert (lY77), T. maxima can reach a depth of 10 m,

after whicn tnere is not enouyh 1ight for the zooxanthe11.e to

photosynthesize. These specimens have often a 1 ar~e size, and

such big inaividua1s were Observed in the four 10ca1it ies, after

the end of the transects, dee~er than 2 r,i . It seelils that very few

bivalves are collected deeper then the ed~e of the outer reef

f1at, were they are on1y accessibl e by divilty.

Thi s preserved stock of 1 arye tri dacni ds shou1 d a110w tû

--------- -

487

avoid any long-term disappearance of the species by over­

collectiny, as long as it remdins untouched.

ACKNOWLEDGE~IENTS

1 would like to thank Dr. J.L. l'iunro, both for his

scientific conments and for suyyesting ililprovements of the

English lanyuage.

REFŒlNCES

Alcala A.C., 1981 . Standing stock anu yrowth of four species of

mo11uscs (family Triaacnidae) in Sumilon Island, central

Visayas, Philippines. Proc . Fourth int. Coral Reef Synlp.

l'Ianilla, lYB1, 2: 757.

Antonius A.,1984. Hunlan impact on Corals in fringiny reefs near

Jeddah, Rea Sea. Proc. Symp. Cordl Reef Environ. Rea Sea.

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~attistini R., BourrouiHI F., Chevalier J .-P ., Coudray J.,

Denizot ~I., Faure G., Fisher J.-C., Guilcher A.,

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~ ., Thoillass in B., Vasseur P., "eydert P., 1975. El éments de

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cla~s, Tridacna yigas, Tridacna aerdsa, ana Triaacna

squamosa in Paldu, Caroline Islanas. Aquaculture, 24: 1-2.

Goreau T.F., Goreau N.I., Yonye C.h., 1973. On the utilization

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488

dissolved amino-acid on Tridacna maxima f. elongata

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Hughes R.N. ,1977. The biota of reef flats and linlestone cliffs

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Jameson S.C. ,1976. Early life history of the giant clams,

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hippopus (Linnaeus). Pacif. Sci., 30: 219-233 .

Jaubert J./ 1977. Light, nletabo lism and the distribution of

Tridacna maxirlla in a South Pacifie atoll: Takapoto (French

Polynesia). Proc. Third int. Coral Reef Syolp. l'dami, 1977:

489-494.

Jeffrey S.w. and Haxo F.l., 1968. Photosynthetic pigments of

symbiotic ainoflagellates (zooxanthellae) from corals and

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