ASSESSMENT OF HUMAN IMPACT ON GIANT CLAMS, TRIDACNA MAXIMA

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2/ OF HUMAN IMPACT ON GIANT CLAMS

ASSESS~ENT

1

(TKIDACNA

MAXI~lA)

NEAR JEDDAh, SAUDI ARABIA

ALAIN BODOY* Faculty of

~Iarine

Sciences, King Abdulaziz University,

• Jeddah, Saudi Arabia AbSTRACT

1 A reprint from

PROCEEDINGS OF THE SYMPOSIUM ON COR1\.L REEF ENVIRONMENT OF THE RED SE 1\.

Edited by MASSOUD A. H. SAAD

1

1984

z1 OF HUMAN IMPACT ON GIANT CLAMS

ASSESS~ENT

1

(TKIDACNA MAXIMA) NEAR JEDDAH, SAUDI ARABIA ALAIN BODOY* Faculty of

~Iarine

Sciences, King Abdulaziz University,

• Jeddah, Saudi Arabia AbSTRACT

1 A reprint from

PROCEEDINGS OF THE SYMPOSIUM ON CORllL REEF ENVIRONMENT OF THE RED SEll

Edited by MASSOUD A. H. SAAD

1

1984

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 1ength,

134

lTIil

and the

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

of

parameters

shore,

B8

g.m- 1 .

were

19

individua1s,

113

in

subjecteà

respective1y,

while,

an

In

se1àorn

area

uti 1ized mm

and to

area,

those

18.6

~.1,,-1

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 1 82 mm and a biomass for one metre of shore of 1.5 g.m- . The effect of such human impact is aiscussed, accorainy to . the eco10gy ana the bio10gy of this species.

(*) Permanent b.P. 19u,17 39U

address: Laboratoi re Cultures La Tremblade, FRANCE.

l'iari nes IFKL~IEk

473

INTRODUCTION Among coral

the different species of

invertebrates

co10nizing

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). bivalves which

have

ever

They are probab1y the 1argest

existed,

and

1. giyas, can reach one metre in 1ength. of

view,

their

association

with

the

Pacific

species,

From a bio10gica1 point

symbiotic

a1gae,

which

was

studied by severa1 authors (Yonge, 1936; Jaffrey and Haxo, 1968; Goreau et al.

--

of

the

'.

genus

1973 and others), enab1es us to consiaer species 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

re1ative1y few works have been carried out on population

size

and biomass

(Sa1vat,

1971;

inhabitants, their growth,

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

Mastaller, 19BO), the first one being the most abundant. species are submitted to

and

These

heavy exploitation in the vicinity of

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

flat, both for food and for decorative purposes . The present study aims to assess on quantitative basis the

li zi ng

human impact on T. maxima.

f the

Therefore, abundance and biomass were

estimated for several populations of this species living in areas

were

differing for their accessibility. Similar studies, following the

rgest

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

Cl es,

and

on fishes (Oakley, 1984).

point



was ~IATERIALS

1968;

AND I1ETHOD

The analysis of different populations of T. maxima has been

ecies

carried out using a transect technique .

cers.

Four different transects

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

thei r

affected by human impact (Fig. 1).

The site located North of the

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

=aten

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

high

unai sturbed a r ea .

J81 ) •

of Jeddah.

il ity

The second site studied i s about 180 km South

80th the distance and the state of the road leading

to it make its access difficult.

:y of

A third site was on the reef,

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

976 ;

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

.nts,

desalination plant.

wth,

northern

Corni che,

3

was

carried out,

km North

of the

The latter area, turned into an artificial

beach, is very popular, due ta its easy access.

974;

a development scheme on

Since this work

the reef flat

has

destroyed the studied area.

ally

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

and

reef flat,

he se 1

studi ea on Jeadah

from

the shore-line to a depth

beginniny of the outer slope.

of

of 2 m at the

Along each transect,

all

the

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

reef

-- .

475

Tuwwal



• 20 Km

N

t Shoiba

RED

Fi~re

SEA

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

of 2 calcul ated for each unit area of 50 m . regression

techniques.

The

weight were values

calculated using

biomass

were

then

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

50 Depth

100

150

300 m

(m)

rid ge

----------~a 1 gal

2 reel

Inn",

undance an

1 gure

liaI

lomass

0

rl acna

transect across the reef flat in the undisturbed area (Tuwwa 1) . indiyiduals /50 m 2

Number of

~O db JJ~

': t,-------,-----L--L..LO--'..LU...LL-O 1m Biomass 3 -

--LL-.WJ..LjO

dry wei ght (g / m 2 )

2

lf-

• 1

1

1

.+

50

100

150

200

n

n-J1

250

1

300 m

Deplh (m) 1

-------

1-

a lg al

21-inn e r

r eet

fi at

:id9~;

1

, out er : reef 'fiat

Figure 3. Abundance and biomass of Tridacna maxima along a transect across the reef flat in a slig htly 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. maxima were found there.

However, at the ena of the outer reef

f1at (Fig. 2), individua1s were sett1ed. stud ied

here,

was

(1975), and no T.

--

,

sometimes

The externa1 slope, not

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 The highest biomass was 3.U6 9.01 -2

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

of the population of 1. maxima was 88.12 g.m -1 of shore . The fringing reef located Sou th of Sho iba aesa1ination plant for Hakkah city), ways.

( South of the

is different in va rious

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

N~:Lber 01 ,ind, ividuils l

t .

,~-L

______- L_ _ _ _ _ _

~

______

1 rD-O

rill 0

rD-O

0

~~~WU~U--LWUL

dry weight (g 1m 2 )

Biomass 32

lLr__

~~I~I

______LI____

, , 250

300

~ lu~n-~lli~hun~L~UiL ______

350

400

450

500 m

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

algal~idge j .ou t er :, reef

12

inner

Figure 4.Abundance

reef

Ilat

and biomass

,flat

of Tridacna

maxima

along

a

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

..

Number or individuals 1 50 m 2 t

':t Bicmass

3

c-

2

-

l

DI

0

>

DO

dry weight (g/m 2 )

l t (

1-



Depth 1 1

(m)

1'---

50

,

,

1-

1

,

150

200

.'

, 250 m

,

,,,./i~

2

>

l

1 outer : reef

2 in n e r

,1

, -'

"lgal

1 1

,

' 100

1

r e e 1 lia t

'fla t

Figure 5. Abundance and biomass of Tridacna maxima along a transect across the reef flat in a heavily disturbed area

k

(Jeddah).

"

480 (y = ax + b)

Table 1. Coefficients for the linear relations, 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

corre lation

coeffi ci ent .

a

r

b

• lm

a Jr) •

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. has not been coloni se d by seaweeds. green filamentous alyae (cf.

This fine substratum

At 300 m, a population of

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

transect, and only 19 inaividuals were found. zone had particularly few clams.

100

m of

the

The Enteromorpha

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

In -

kilorlleters North of Abhur creek. Wd S

275 \..

The width of th e fringing reef

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

the shore.

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 "~ asured

The

CIl1.

largest

individual

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 al gal ridge on the Du tar reef fIat. The highest biomass was 0.16 g.m -2 . The last transect,

carried out on

Jedddh, was 2H5 ,. long (Fig. 5).

the

North Corniche in

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 al~ae.

blue-green

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. of valves

fro~

The absence

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.

lell~th,

On l y 6 individuals were fuuno, the

lar~est

averaging

~.2

cm in

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

- -482 .he Enteromorpha

sp.,

which

usually

indicates

either

natural

or

om

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

se

The average size of the populations from Abhur and Jeddah varies

d.

only within 5%.

t, ,

The analysis of those different

pop ul ations

carried out 2 (number of individuals per 50 iii ),

.7

using criteria of density

th

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

al

stock (dry weight) enables a classification of these populations

ng

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

re

their accessibility.

he

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

standing



0f

Since T. maxima is a sessile species, it is

to excessive collection. in Jm

ld

The analysis of length-frequency distributions for the four transects

(Fig.

7)

leads

to

confirm

these

individuals, larger than 15 cm, were found

on the undisturbed

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

)f

sites, the distributions are polymodal.

big

results.

In botll

In the two other areas,

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

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 :e

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

n r

~Iuch

attention has

been paid to the symbiotic relation-

ships between the zooxanthellae and tridacnids.

But fe w studies

1

have been carried out on the ecology of related species.

The

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

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

be scarce in other atolls (Salvat, 1971).

t'Icl-iichael (1974) found

483

0 <.D

0

N

N

0

0 0

'"

~ ~

::J 1--

0

~

0

0

ci

0

111

'"

.0

N

L

:6

::J

tf)

.0

.r: .r: .r: ~

Number of individuals

Average size mm

Avera 9 e biomass glm 2

'"

u

U

Biomass c4 shore

g lm

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.

-~

.

484

:r: <{

l

0 0

..

w

rdb p

1

,

1

1

20rr~

-



15 f-

r

-

~

10 f-

a:

::J

r

m

<{

-

5 1-

-

1

0'-<{

m .-

-r

n

~

1

5-

0

r

Vl

0-

1

ri

riln

1111

1

p

n

q

10 ...J <{

-

3: 3: 5 -

-

::J

f-

--

~

r i oUl'e 7.

L e n " th -fr~ q u ~ "cy

5

--

-

-

-

-

0-

.

-

10

-

25

~

-

20

m

Gi s tri iJ ut io li of Triu ac "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, hi~hest

densi ty recoraed was 0.22 i ndivi dua 1 s per square metre, In the Pacific Ocean, 1. maxima

in Tuwwa1. shallow

wdters

(kichard,

of

1~77).

1ayoons,

e.g.

in

often sett1es in the

the

atoll

of

This

transects.

ana

was not recordea in any of the the

10w average

density

exp1ainea by the pecu1iarities of the Red Sea coasts. winter,

co1d

Takapoto

In the Rea sea,sett1ement in she1tered, shallow

waters close to the shore1ine fuur

since the

winds

can

10wer

the

water

can

be

During the

temperature

on

the

sha110w, inner reef, sometirues down ta Il'C (Hughes, 1977), which cou1d be a 1eta1 temperdture for many tropical species. other

hana,

sett1e"~nt

100se

sediments,

acteo

Dy

waves,

On the

might

1imit

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 other,

she lterea,

1 eeward

si de.

Islands,

In

but not on the

our case,

for

all

the

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

the

Therefùre,

prevdi1ill~

these

winds

ecu1o~icd1

From

fdctors

the

saine

shou1d

not

direction. affect

the

results. Un1y very few e',ipty shell s were founn, ana since these were a1ways

1ar~e,

nldin1y nue to

one Cdn

dssun,e that lIiOSt of the ':Jorta1ity is

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 areas.

~erta1anffy,

for

different

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 in New Gu i nea (['Iunro dno Gwyther, h. ~) . estÎlllatea asyrdptotic 1ength are 24.3 Clil

30.~

CIII

Uther values of this

( ~,unro

and Gwyther,19ts1),

-

---.--

~

486

27.5 cm (~ICr'lichae1, (l'ICKOy,

1980,

in

in Munro and Hes1inga,

op.cit.)

For

all

10ngevity was more than 20 years.

populations (unpub1. aata).

these

populations,

the

Assessments on such 1ongevity ,

agr~e

based on she11s rings counts,

1983) and 3U.5 cm

with such resu1ts for Red Sea

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

~quations

The growth

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 Mother prou1elll, men tionea.

collectin~

re1atea

T. illax ima is

il

ta

stops .

th~

reproduction,

protanoric

Ilas

her;ilaphrodite.

to

b~

Richard

(1982)

gave the following scherlle for a Po1ynesian population:

sexua1

IIlaturity is reach ed for

phase ,

up

to

7

Clll,

those

l ar~e

~onads.

animal s

size of 0 cm;

sitnultaneous

population between 7 and 15 Clil. have on1y feilla1e

ct

~ut,

after a ",ale

hermaphruaites

fon,l

th e

The 1arger inaividua1s wûu1d

for Illost of the other populations,

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 specime ns have often a

1 ar~e

size, and

such big inaividua1s were Observed in the four 10c a1it ies, a fter the end of the transects,

dee~er

than 2 r,i . It seelils that very few

bivalves are collected deeper then the f1at, were

ed~e

of the outer reef

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

scientific

like

to

conments

thank

and

Dr.

for

J.L.

l'iunro,

suyyesting

both

for

his

of

the

ililprovements

English lanyuage.

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(Serran i aae l po pu 1 a t i ons in Eastern Rea Sea. Proc. SYII'p. Coral keef lnviron. Rea Sea. Jeadah, 19H4: Ricard ri., Salvat e., 1977. Faeces of Tridacna maxima (l',ollusca, bivalvial, composition ana coral Third int. Coral Reef Syllip. Richard

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