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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