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Strombus canarium
Research Journal of Fisheries and Hydrobiology, 3(2): 71-77, 2008
© 2008, INSInet Publication.
Growth, Mortality, Recruitment and Yield-per-recruit of Strombus canarium
Linnaeus, 1758 (Mesogastropoda: Strombidae) from the West Johor Straits, Malaysia.
1
1
Zaidi Che Cob, 2 Aziz Arshad, 3 Japar Sidik B., 3 S.M. Nurul Amin and 1 Mazlan Abd. Ghaffar.
Marine Ecosystem Research Center (EKOMAR), School of Environmental and Natural Resource
Science, Faculty of Science and Technology, National University of Malaysia,
43600 Bangi, Selangor, Malaysia.
2
Department of Aquaculture, Faculty of Agriculture II,Universiti Putra Malaysia,
43400 Upm Serdang, Selangor, Malaysia.
3
Department of Biology, Faculty of Science, Universiti Putra Malaysia,
43400 Upm Serdang, Selangor, Malaysia.
Abstract: Growth, mortality, recruitment and yield-per-recruit of Strombus canarium Linnaeus, 1758 were
estimated using length- frequency data collected from Sungai Pulai Estuary, W est Johor Straits, Peninsular
Malaysia from January to December 2005. The relative growth was isometric type with the exponent ‘b’
of the length-weight relationship was very close to 3 (3.05 ± 0.04 S.E.). The von Bertalanffy growth
function (V.B.G.F) estimates were: L 4 = 69.91 mm shell length; K = 1.30 year ! 1 . The growth performance
index (φ’) was estimated as 3.803. Total mortality (Z) was computed as 2.42 year !1 while the natural (M)
and fishing (F) mortalities were estimated at 0.93 year !1 and 1.49 year !1 respectively. The recruitment
pattern was continuous with one major peak within the months of June to August. The exploitation ratio
(E = F/Z) was 0.61 revealed over exploited stock conditions in the study area.
Key words: Growth, mortality, Strombus canarium, Peninsular M alaysia
INTRODUCTION
Although the species is widely distributed, conch
fishery in Peninsular Malaysia only limited within the
Johor Straits. The area has vast tidal flat and subtidal
shoals within the protected estuaries and channels,
which was easily accessible for conch collecting during
low tides. From only a subsistence fishery, conchfishing activity has now extended and the shells are
now available in local markets and sea-food restaurants
particularly during peak season. The objective of the
present study was to estimate the population parameters
and
exploitation
level of S. canarium, and to
assess the stock position of the species from west
Johor Straits, Malaysia.
The dog conch, Strombus canarium Linnaeus, 1758
is a mesogastropod from the family Strombidae,
commonly found in seagrass areas along the coasts and
sheltered Islands of Malaysian waters [2 4 , 8 ]. This species
is native to the coastal waters of Indo-Pacific region,
widely distributed from southern India to M elanesia,
and extended north to the Ryukus in Japan and south
to Queensland and New Caledonia, Australia [1 ]. In
many parts of Southeast Asia, such as Malaysia,
Indonesia, Philippines and Thailand, the species has
been traditionally fished and constitute important food
staples especially for those living along the seashore.
They largely collected for their meat, apart from the
shell
which
also
has considerable ornamental
value [2 3 , 2 4]. Though the fishing activity of this species
has long been reported, landing data are almost
non-existence, mainly because they only formed
subsistence or artisanal fishery and no specific fishing
gear involved [7 , 2 4 , 2 ] . Attempt has been made to
quantify the fishery in Bintan Island, Indonesia where
Amini [2 ] estimated about 10.4-15.6 tons of total
landings per year.
M ATERIALS AND M ETHODS
Study was conducted at Merambong Shoal, Sungai
Pulai estuary (01 o 19.778’N, 103 o 35.798’E), western
Johor Straits, Peninsular M alaysia (Fig. 1). It is
probably the most extensive, seagrass covered subtidal
shoal of the area. The dense seagrass meadows were
dominated by Enhalus acoroides and Halophila
spp. complex.
Corresponding Author: Zaidi Che Cob, Marine Ecosystem Research Center (EKOMAR), School of Environmental and
Natural Resource Science, Faculty of Science and Technology, National University of Malaysia,
43600 Bangi, Selangor, Malaysia.
Tel.: +603-89215238, Fax: +603-89253357, E-mail: [email protected]
71
Res. J. Fish. & Hydrobiol., 3(2): 71-77, 2008
Monthly samples of S. canarium were collected
using belt transaction. Then they were transported to
the laboratory. In laboratory the conch were cleaned
and all encrusting organisms were scrapped-off. Shell
length (from tip of spire to anterior end of siphonal
canal) was measured using a digital vernier caliper to
the nearest 0.01 mm and wet weight taken to the
nearest 0.01 g using an analytical balance. The data
were then grouped into shell length class of 2 mm
intervals and were analyzed using the FiSAT software
as explained by Gayanilo et al [1 2 ].
To establish the length-weight relationship, the
commonly used relationship W=aL b was applied [2 5 -2 6 ]
where W is the weight (g), L the total length (mm),
a the intercept (condition factor) and b is the slope
(growth coefficient, i.e. relative growth rate). The
parameter a and b were estimated using least squares
linear regression on log-log transformed data of
log 1 0 W =log 1 0 a + blog 1 0 SL. T he coefficient of
determination (r 2 ) was used as an indicator of the
quality of the linear regression [2 7 ]. The 95% confidence
limit of the parameters a and b and the statistical
significance level of r 2 were also estimated.
The asymptotic length (L 4 ) and growth coefficient
(K), of the von Bertalanffy growth function (V.B.G.F)
were estimated by means of ELEFAN-1 [1 9 ] incorporated
in the FiSAT software package. The L 4 value was
estimated using modified Powell-W etherall plot[3 0 , 1 7],
which was then used as seed value in ELEFAN-I
analysis to assess a reliable estimate of the growth
parameter K [1 1 ] . The estimates of L 4 and K were then
used to estimate the growth performance index (φ’) [2 0 ],
using the equation:
The weight-based von Bertalanffy growth equation
was also determined, by combining the von Bertalanffy
growth equation with the length-weight relationship [2 8 ].
W eight-at-age curve of S. canarium was then calculated
using the equation:
where W t is the mean weight (g) at age t, W 4 is
the asymptotic weight (g), K is the curvature of the
VBGF or growth coefficient (year -1 ), and to is the
hypothetical age (year) at which length equals to zero.
The asymptotic weight, corresponding to the asymptotic
length is determined by the equation W 4 =aL 4 b .
The annual instantaneous total mortality rate (Z)
was estimated using the ‘length converted catch
curve [1 6 , 2 0] . The natural mortality rate (M) was
estimated using the method as described by Froese and
Palomares. This method was based on Beverton who
pointed out that there is an intermediate age t o p t at
which the biomass (and egg production) of a year class
reaches a maximum: L o p t = L 4 *[3/(3 + M/K)],
where M is the natural mortality rate. Solving this
equation for M resulted in: M = K*[(3 L 4 /L o p t)-3]. To
obtain an estimate of L o p t the length data were grouped
into size classes of 2 mm shell length. The animal
weights in each size class were summed up, and the
L o p t was determined based on the length-class with
maximum weight.
Once Z and M values were obtained, fishing
mortality (F) was then estimated using the relationship
of: F = Z -M, where Z is the instantaneous total
mortality rate, F the fishing mortality rate and M is the
natural mortality rate. The exploitation level (E) then
could be determined using the equation of Gulland [1 3 ]:
E = F/Z.
The recruitment pattern of the stock was
determined by backward projection on the length axis
of the set of available length-frequency data as
described in FiSAT software package [1 1 ]. This routine
reconstructs the recruitment pulse from a time series of
length-frequency data to determine the number of
pulses per year and the relative strength of each pulse.
Input parameters were L 4 , K and to (t o = 0). Normal
distribution of the recruitment pattern was determined
by NORMSEP [1 8 ] in FiSAT program.
Analysis of yield per recruit was conducted based
on the Beverton and Holt [5 ] model as modified by
Pauly and Soriano [2 1 ] . The input parameters were L 4
and M, and length at first capture (L c ), which was set
at 40 mm shell length corresponding to the minimum
marketable size for the species. Levels of exploitations
were expressed as E 0 .1 , E 0.5 and E m ax . E 0 .1 is level of
exploitation at which the marginal increase in yield per
recruit reaches one-tenth of the marginal increase
The inverse von Bertalanffy growth equation [2 8 ] was
used to determine the lengths of the S. canarium at
various ages. Then VBGF was fitted to estimates the
length-at-age curve using non-linear squares estimation
procedures[2 2 ] . The VBGF is defined by the equation:
where L t is the mean length (mm) at age t, L 4 is the
asymptotic length (mm), K is the curvature of the
VBGF or growth coefficient (year -1 ), and to is the
hypothetical age (year) at which length equals to
zero [1 4 ]. The growth rate at any point in the lifespan
was calculated as:
72
Res. J. Fish. & Hydrobiol., 3(2): 71-77, 2008
computed at a very low value of E; E 0 .5 is exploitation
level which results in a reduction of the unexploited
biomass by 50%; and E m ax is sustainable exploitation
level that produced maximum yield. These parameters
were compared with the current rate of exploitation (E).
The state of the stock was evaluated as: in equilibrium
(E = E m ax ), overexploited (E > E m ax ), or underexploited
(E < E m ax ).
RESULTS AND DISCUSSION
Results: A total of 2095 individuals have been
collected throughout the 12 months study period, with
shell length ranged from 18 to 68 mm. Environmental
parameters (salinity and temperature) recorded
throughout the sampling period is presented in Fig. 2.
The salinity remains somewhat constant; ranged from
29.15 -30.75 ppt, with mean value of 29.85 ± 0.16 ppt.
The mean annual temperature was 29.36 ± 0.14 o C,
ranged from 28.65- 30.05 o C. There was slight increase
in seawater temperature after the monsoon during the
months of March to August.
Fig. 3: Length-weight relationship of S. canarium
from 0.57 g to 33.64 g. The calculated length-weight
equation was log W = -4.1943 + 3.0463 log SL, which
in exponential form was
W
=
0.00006
SL 3 .0 5 (r 2 = 0.88, p <0.01) (Fig. 3). The
growth
co-efficient (b) was 3.05 (± 0.04) with 95% confidence
limit between 2.905 to 3.187.
Length-weight relationship: A total of 749 individuals
were used for length-weight analysis. The lengths range
from 17.56 mm to 67.68 mm, while total weight range
Growth parameters: The Asymptotic length (L 4 ) of
the VBGF was 69.91 mm and the growth coefficient
(K) was 1.3 year -1 for S. canarium. The computed
growth curve using these parameters is shown over the
restructured length frequency distribution in Fig. 4.
The observed maximum length was 68.00 mm and
the predicted maximum length was 69.20 with 95%
confidence interval between 68.51 - 69.89 mm. The
best estimated value of K was 1.3 year -1 , at goodness
of fit (Rn) value of 0.195. The growth performance
index (φ’) was 3.803.
The weight based von Bertalanffy growth curve is
presented in Fig. 5. The estimated asymptotic weight
(W 4 ) value was 25.35 g, much inferior compared with
the observed maximum weight (W m a x ) value of 33.64 g.
Fig. 1: Sampling site (encircled) at Merambong Shoal,
west Johor Straits, Peninsular Malaysia.
Age and growth: By using the growth parameters
described above, growth rate (dL/dt) and shell length
at specific age were then calculated with the
assumption that to equals to zero [1 9 ]. The growth rates
and the absolute increase in length are presented in
Fig. 6. From the monthly shell length increment,
a ve ra ge growth ra te fo r S . ca n a riu m w a s
thencalculated, resulted in mean growth rate of
6.25 ± 0.2 mm month -1 for the first 6 months and
4.75 ± 0.16 mm month -1 in the following 6 months.
M ortality and exploitation: Fig. 7 presents the
summed weight at specific length classes for
S. canarium. The population showed maximum weight
in the 55 to 59 mm length-classes, thus suggesting an
optimum length (L o p t) of 56.51 ± 0.52 mm (from
Gaussian plot in ORIGIN ® software). Solving the
Fig. 2: Salinity and temperature fluctuations at the
study site.
73
Res. J. Fish. & Hydrobiol., 3(2): 71-77, 2008
Fig. 4: Restructured length-frequency distribution with growth curves superimposed using ELEFAN-1 (L 4 = 69.91,
K = 1.3 year -1 ).
Fig. 5: A weight-based growth curve of S. canarium
using the von Bertalanffy growth function,
based on computed growth parameters (L 4 =
69.91, K = 1.3 year -1 , to = 0)..
Fig. 7: Optimum length (i.e. the length-class with
maximum weight, L o p t ) of S. canarium
population.
Fig. 6: Plot of age and growth rate of S. canarium
using the von Bertalanffy growth function,
based on computed growth parameters (L 4 =
69.91, K = 1.3 year -1 , to = 0).
Fig. 8: Length converted catch curve of S. canarium.
presented in Fig. 8. Total instantaneous mortality
rate (Z) was at 2.39 year -1 (95% C.I. between 1.97
and 2.81 year -1 ). The fishing mortality rate (F = Z - M)
was therefore at 1.46 year -1 . The current exploitation
level (E = F/Z) for male S. canarium was therefore at
0.61. At this exploitation rate, the population was
considered slightly overexploited [1 3 ].
equation of M = K*[(3 L 4 /L o p t)-3], with L o p t = 56.51
mm, L 4 = 69.91 mm, and K = 1.30 year -1 resulted in
natural mortality rate (M) of 0.93 year -1 .
The length converted catch curve analysis is
74
Res. J. Fish. & Hydrobiol., 3(2): 71-77, 2008
Fig. 9: Probability of capture analysis.
Fig. 11:
Based on the length converted catch curve values,
the probability of capture was then analyzed and
presented in Fig. 9. Using the natural mortality rate
(M) = 0.93 as seed value, the length at first capture
(L c ) was estimated at 18.22 mm shell-length.
Yield per recruit (Y’/R) and biomass per
recruit (B’/R) analyses for S.canarium
population (Lc/ L 4 = 0.261 and M/K =
0.715).
where thickening and development of shell ornaments
took place in adults. S. canarium showed deterministic
type of growth. They grow in length until the onset of
sexual maturity at which time it starts building the
flaring lip, with minimal length increment[1 , 2 9]. Growth
is then more on shell lip (labial lip) thickness. There
have been suggestions of incorporating / using lip
thickness as reference for growth parameters
estimation, which was however only practical among
adult group [4 ]. Moreover, shell thickness can be highly
varied according to stress levels (pers. observ.), which
might interfere growth parameter estimation. The wide
variation of animal weight within the adult group also
resulted in much inferior value of W 4 compared with
the observed weight (W m a x ). Therefore the use of
weight-based von B ertalanffy equation for S. canarium
should be treated with cautions.
The growth parameters (L 4 , K) obtained in this
study was inferior compared with previous finding on
the same species by Amini and Pralampita [3 ] at Bintan
Island, Indonesia, where the L 4 was 8.25 and
K 1.656 year -1 . In their study the shell length range
from 37 to 78 mm, and the maximum length was
much higher than the population currently studied.
Erlambang [9 ] also recorded higher shell length range,
from 12 mm to 82 mm length for population around
Riau Archipelago, Indonesia. S. canarium in general
showed wide variation in size distribution among
locations. According to Abbott[1 ] the lengths of adult
shells varied from as low as 31 mm to the maximum
97 mm length. The current study was conducted at the
main conch collecting grounds where the animals were
harvested, which might contribute to the low frequency
of large sized conch sampled.
The overall growth rate for the first year conch
was about 5.5 mm month -1 (± 0.26). At this growth
rate, the conch could reach marketable size within
Recruitment: The recruitment pattern of S. canarium
was continuous throughout the year, but showed peak
between the months of June to August (Fig. 10), which
account to 53.77% of total recruitment throughout
the year.
Relative yield per recruit model: The relative yield
per recruit analysis is presented in Fig. 11. Using
length at first capture (L c ) of 18.22 mm as derived
from the probability of capture analysis, the E 0 .1 was
0.453, E 0 .5 was 0.329, and the maximum exploitation
level (E m ax ) was 0.519 (Fig.11).
Discussion: S. canarium considered to have an
isometric growth where the growth coefficient
parameter b was found very close to 3. The population
showed wide variation in weight among older
individuals, which is common among gastropods
Fig. 10: Recruitment pattern of S. canarium.
75
Res. J. Fish. & Hydrobiol., 3(2): 71-77, 2008
8 months. The growth rate was quite similar with other
reported studies [3 ] , thus suggests that culture or rather
sea ranching activity could potentially be a successful
industry. Growth to marketable size was superior
compare to the commercially important S. gigas, which
only reached marketable size at age 2.5 years, at 190
mm shell length [6 ].
The recruitment pattern suggests that annual
recruitment consists of one seasonal pulse (Fig.10),
which occurs between months of June to August. This
period of recruitment referred to migration of new
group of juvenile into the adult population, and not the
actual spawning. Very high percentages (> 50%) of
new recruits within this period suggest a highly
synchronized reproductive pattern. This was in
agreement with previous findings where S. canarium
reported to congregate in large numbers during
spawning season [1 ]. Field observation also found high
frequency of copulation and spawning activities during
the months of November to March. The major
recruitment peak detected in March (Fig. 4) could be
traced back to this period of active reproductive
activity and spawning.
To maintain this valuable resource, the exploitation
rate should be reduced below the optimum value as
well as increasing the length at first capture to increase
chances of new recruitments. The maximum Y’/R was
obtained at E m ax of 0.52. As the exploitation rate
increases beyond this value, relative yield per recruit
decreases. The results indicated that the present levels
of exploitation rate (E = 0.62) and fishing mortality
(F = 1.49) were higher than those which give the
maximum Y’/R values (E m ax ). For management
purposes, the exploitation rate of S. canarium should
therefore be reduced from the current E (0.62) to
E 0 .5 = 0.32, which maintained 50% biomass of the
stock (Fig. 11).
ACKNOW LEDGM ENTS
The authors would like to thanks the deanery and
staffs of Biology Department, Faculty of Science, UPM
and School of Environmental and Natural Resource
Science, UKM for technical support and laboratory
facilities. First author would also like to thanks the
Ministry of Science, Technology and Environment,
Malaysia for the scholarship awards, which make this
study possible.
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