
Research Article


Age Structure, Growth, Mortality and YieldPerRecruit of Sergestid Shrimp, Acetes indicus (Decapoda: Sergestidae) From the Coastal Waters of Malacca, Peninsular Malaysia 

S.M.N. Amin,
A. Arshad,
J.S. Bujang
and
S.S. Siraj



ABSTRACT

Age structure, growth, mortality and yieldperrecruit
of Acetes indicus were examined in the coastal waters of Malacca,
Peninsular Malaysia between February 2005 and January 2006. Monthly length
frequency data were analyzed using FiSAT software for estimating population
parameters, including asymptotic length (L_{∝}), growth coefficient
(K) and exploitation rate (E) to assess the status of the stock. The L_{∝}
and K for males were estimated at 29.40 mm and 1.70 year^{1}
and for the females that were 42 mm and 1.20 years^{1}, respectively.
The growth performance index (φ’) was calculated as 3.16 and
3.33 for males and females. The growth pattern of males and females showed
positive allometric nature of growth (b>3, p<0.05). The maximum
life span (t_{max}) of males and females was 1.76 and 2.50 years,
respectively. Total mortality (Z) by length converted catch curve was
estimated at 4.15 year^{1} for males and 3.50 year^{1}
for females. The rate of natural mortality (M) for males and females was
calculated as 2.65 and 1.91 year^{1} and the fishing mortality
(F) was 1.50 and 1.59 year^{1} for males and females, respectively.
The recruitment pattern of A. indicus was continuous throughout
the year with two major peaks. The exploitation rate (E) of males was
0.36 and that of females was 0.45. The maximum allowable limit of exploitation
(E_{max}) of males and females was 0.71 and 0.57 for the highest
yield. The exploitation rates were less than the predicted E_{max}
values of males and females. Thus, the stock of A. indicus was
found to be under exploited in the investigated area.




How
to cite this article:
S.M.N. Amin, A. Arshad, J.S. Bujang and S.S. Siraj, 2009. Age Structure, Growth, Mortality and YieldPerRecruit of Sergestid Shrimp, Acetes indicus (Decapoda: Sergestidae) From the Coastal Waters of Malacca, Peninsular Malaysia. Journal of Applied Sciences, 9: 801814. DOI: 10.3923/jas.2009.801.814 URL: https://scialert.net/abstract/?doi=jas.2009.801.814





INTRODUCTION
The shrimp of the genus Acetes, is mainly used in subsistence fisheries
and therefore, commercially important in the Peninsular Malaysia (Amin
et al., 2008). Knowledge on the shrimp’s population characteristics
are important if proper management of those resources is to be effective. Age
in crustacean is difficult to estimate because exoskeletons are lost during
moulting process (Xiao and Greenwood, 1993). Thus, analysis
of lengthfrequency data has been used to identify year classes (Amin
and Zafar, 2003, 2004; Amin et
al., 2008; Paraconstantinou and Kapiris, 2001).
Lengthweight relationships are useful for a wide number of studies, such as
estimating population structure (Smith et al., 2008),
growth rates and other aspects of fish/shrimps population dynamics (Tsoumani
et al., 2006).
The geographical distribution of Acetes has been described by Xiao
and Greenwood (1993). This species A. indicus occurs in the central
part of the IndoWest Pacific. The distribution ranges from the South China
Sea (Dong Hoa) through the Gulf of Siam (Sanudpragarn), Java (Surabaya), Straits
of Malacca (Singapore and Malacca) and the Bay of Bengal to the Arabian Sea.
The shrimp plays a substantial role in the food webs of coastal waters, acting
as predators, feeding on a variety of foods ranging from diatoms, copepods and
larvae of decapods to detritus and in turn as prey for many fishes and other
predators (Xiao and Greenwood, 1993). It appears in very
large swarms in the shallow inshore coastal waters, which is brackish with a
salinity of 30 ppt or less, during certain seasons of the year. In West coast
of Malay Peninsula, A. indicus, along with A. japonicus, is commercially
exploited from March to November, using the estuarine push net (Arshad
et al., 2007).
Spectacular school or swarms of Acetes, particularly in coastal waters
of Asia, are the bases of important commercial fishes for consumption by humans
and domestic animals (Omori, 1975, 1978).
The commercial importance also derives from the use and potential of Acetes
as a food organism for aquaculture industry (Kungvankij et
al., 1986). These combined features make Acetes excellent candidates
for population dynamics studies. In spite of greater abundance and importance
of Acetes in the fishery of Asian countries, very little information
is available on the population dynamics so far except the studies carried out
by Zafar et al. (1997, 1998),
Zafar and Amin (2002) and Oh and Jeong (2003).
Knowledge of various population parameters like as asymptotic length (L_{∝}),
growth coefficient (K), motilities (natural and fishing) rate and exploitation
level (E) are necessary for planning and management of Acetes resources.
There are many tools for assessing exploitation level and status of stock. Of
these, FiSAT (FAOICLARM Stock Assessment Tools) has been most frequently used
for estimating population parameters of shrimps (Jayawardane
et al., 2002, 2003; Paraconstantinou
and Kapiris, 2001; Etim and Sankare, 1998;
Enin et al., 1996) primarily because it requires only lengthfrequency
data. The objective of the present study was to estimate the population parameters
and exploitation level of A. indicus in order to assess the stock status
of the species around the coast waters of Malacca, Peninsular Malaysia and to
provide data that could be useful for management.
MATERIALS AND METHODS
Collection of data: Monthly fresh samples of A. indicus were
collected between February 2005 and January 2006 from commercial push net catches
landed at Klebang Besar (N 02° 13.009’ and E 102° 11.921’), Malacca, Peninsular
Malaysia (Fig. 1). Acetes shrimps were caught by the
push net (triangular shape) known locally as Sungkor, as described by Omori
(1975) in the coastal waters of Klebang Besar, Malacca, Peninsular Malaysia.
Dimensions of the net were 56 m in length, 4.04.5 m in wide and 3.0 3.5 m
in height. The mean mesh sizes were 3.2 ± 0.27 cm at the anterior section,
0.75 ± 0.0 cm at the middle and 0.5 ± 0.08 at cod end (stretched). After collection,
samples were fixed in 10% formalin solution in the field and analyzed after
23 days of preservation. In the laboratory, specimens were identified using
a Nikon dissecting microscope (Nikon122764, Japan). Sexes were determined by
the presence or absence of petasma on the first pleopods and clasping spine
on the lower antennular flagellum (Omori, 1975). The
studies of Omori (1975) were followed during the identification
of different species of Acetes. Total Length (TL) of 4900 individuals
(males 1628 and females 3272) was measured from the tip of the rostrum to the
tip of the telson to the nearest 0.1 mm and total weight was taken by an electronic
balance of 0.001 g accuracy. The lengthfrequency distributions for males and
females A. indicus samples are shown in Table 1 and
2.
Data analysis: To estimate the age structure, the lengthfrequency data
of A. indicus were analyzed by using the MINITAB Version 14 and SPSS
Version 11.5. The differences in the sizefrequency distributions of population
between sexes were determined by the KolmogorovSmirnov twosample test (Sokal
and Rohlf, 1995). Student’s ttest was used for comparison of the mean total
length of males and females (Zar, 1996). Sizefrequency distributions of A.
indicus were plotted for each month from February 2005 to January 2006.
Bhattacharya’s method, implemented from the package FiSAT (Gayanilo
et al., 1996), was used to identify the modes in the polymodal lengthfrequency
distributions of A. indicus. All the identified size/age groups were
derived from at least three consecutive points and selection of the best results
was based on the following criteria: (a) the values of Separation Index (SI)
for the different age groups; (b) No. of the identified age groups and (c) Standard
Deviation (SD) (Gayanilo et al., 1989).
To establish the lengthweight relationship, W = a L^{b} was applied
(Quinn II and Deriso, 1989), where, W is the weight (mg),
L is the total length (mm), a is the intercept (condition factor) and b is the
slope (relative growth rate). The parameters a and b were estimated by least
squares linear regression on loglog transformed data: Log_{10} W = Log_{10} a+b Log_{10} L.
Table 1: 
Lengthfrequency data of male A. indicus from
the coastal waters of Malacca, Peninsular Malaysia 

Table 2: 
Lengthfrequency data of female A. indicus from
the coastal waters of Malacca, Peninsular Malaysia 


Fig. 1: 
Sampling station (dot) and location of Klebang Besar,
Malacca, Peninsular Malaysia 
The coefficient
of determination (r^{2}) was used as an indicator of the quality of
the linear regression (Scherrer, 1984). Additionally, 95%
confidence limits of the parameter b and the statistical significance level
of r^{2} were estimated.
Monthly lengthfrequency distributions of male and female A. indicus
were analyzed using the FiSAT computer programme (Gayanilo
et al., 1996). An estimate of maximum length (L_{max}) was
obtained using the data described earlier and the extreme value theory (Formacion
et al., 1991) as implemented in the FiSAT software. The parameters
of the von Bertalanffy Growth Function (VBGF), asymptotic length (L_{∝})
and growth coefficient (K) were estimated by means of ELEFANI (Pauly
and David, 1981). Estimated of L_{∝ }and K was used to calculate
the growth performance index (φ’) (Pauly and Munro, 1984)
using the equation:
φ’ = 2 log_{10}L∝ + log_{10}K 
Potential longevity (t_{max}) of the species was calculated from Pauly
(1984) formula: t_{max} = 3/K. The inverse von Bertalanffy growth
equation (Sparre and Venema, 1992) was used to determine
male and female of A. indicus lengths at various ages. The VBGF is defined
by the equation:
Where:
L_{t} 
= 
Mean length at age t 
L_{∝} 
= 
Asymptotic length 
K 
= 
Growth coefficient 
t 
= 
Age of the A. indicus 
t_{0} 
= 
The hypothetical age at which the length is zero (Dulcic
and Kraljevic, 1995) 
Once the growth parameters of VBGF were obtained, a linearized lengthconverted
catch curve was constructed using the following formula to estimate total
mortality (Z):
where, N is the No. of individuals of relative age (t) and Δ_{t
}is the time needed for the shrimp to grow through a length class.
The slope (b) of the curve with its sign changed gives Z.
Natural mortality (M) was estimated using the empirical relationship of Pauly
(1980):
Log_{10}M =  0.0066 – 0.279Log_{10}L_{∝
}+ 0.6543.Log_{10}K + 0.4634 Log_{10}T 
where, M is the natural mortality, L_{∝} the asymptotic
length, K the growth coefficient of the VBGF and T the mean annual habitat
water temperature °C (here it was 30.58 °C).
Once Z and M were obtained, fishing mortality (F) was estimated using
the relationship:
where, Z is the total mortality and M, natural mortality. The exploitation
level (E) was obtained from E = F/Z (Gulland, 1971).
Ascending left arm of the lengthconverted catch curve was used to estimate
the probability of capture. By plotting the cumulative probability of
capture against midlength of class interval, we obtain a resultant curve
from which the length at first capture L_{c }was taken as corresponding
to the cumulative probability at 50%.
The recruitment pattern was obtained by projecting the lengthfrequency data
backwards on the time axis using growth parameters (Moreau
and Cuende, 1991). Normal distribution of the recruitment pattern was determined
by NORMSEP (Pauly and Caddy, 1985) in FiSAT.
Relative yieldperrecruit (Y/R) and relative biomassperrecruit (B/R) were
estimated according to Beverton and Holt (1966) lengthbased
method as modified by Pauly and Soriano (1986) using the
knifeedge selection. From the analysis, E_{max} (maximum exploitation
rate giving maximum relative yieldperrecruit) and E_{0.5 }(the value
of exploitation rate at 50% of the unexploited relative biomassperrecruit)
were estimated.
RESULTS
Size frequency distribution: Between February 2005 and January
2006, total number of individuals collected for this study were 1628 (33.22%)
for males and 3272 (66.78%) for females, respectively. In males, the minimum
and maximum total lengths were 13 and 29 mm and in females, they were
13 and 41 mm, respectively. The mean total lengths were 21.37 ±
2.86 and 25.44 ± 4.66 mm for males and females, respectively (Fig.
2). According to the annual sizefrequency distributions, there were
significant difference between males and females (KolmogorovSmirnov test:
d_{max} = 0.396, p<0.001). The mean total length of females
was 4.08 mm taller than that of the males (Fig. 3) and
it was significantly different (ttest, p<0.001). Monthly size frequency
distribution over 12 months suggested that the population consisted of
a maximum of two age groups (Fig. 4) with mean values
being 21.01 and 30.98 mm of total length.
Table 3: 
Estimated lengthweight parameters of A. indicus
from the coastal water of Malacca, Peninsular Malaysia 

N: Sample size; TL range: Minimum and maximum total
length (mm); TW range: Minimum and maximum total weight (mg); a and
b: Parameters of the lengthweight relationship; CI: Confidence Interval;
r^{2}: Coefficient of determination 

Fig. 2: 
Annual lengthfrequency distribution of males and females
A. indicus collected from the coastal waters of Malacca, Peninsular
Malaysia 

Fig. 3: 
Box plots of male and female Acetes indicus from
the coastal waters of Malacca 
Lengthweight relationships: Length and weight of individuals
for determining the lengthweight relationships ranged from 1329 mm and
9.50174.10 mg for males, 13 to 41 mm and 13.30 to 430 mg for females,
respectively (Table 3). The regression between TL (total
length) and TW (total weight) for the males and females showed positive
relationship (Fig. 5). The parameters of total lengthbody
weight relationship for different groups (males, females and combined
sexes) are shown in Table 3 and the lengthweight relationship
equations were established as:
Table 4: 
Estimated population parameters of A. indicus
from the coastal waters of Malacca, Peninsular Malaysia 

Log TW = 3.307 Log TL – 2.7136, r^{2} = 0.85 for male A.
indicus
Log TW = 3.472 Log TL – 2.9437, r^{2} = 0.92 for female
A. indicus
Log TW = 3.411 Log TL – 2.8565, r^{2} = 0.92 for combined
sexes of A. indicus
Growth parameters: The observed length and predicted extreme length
(L_{max}) of males were 28.00 and 30.16 mm, respectively with
95% confidence interval for extreme length was 28.6031.73 mm. For females
the observed length and predicted extreme length (L_{max}) were
40 and 46.37 mm, respectively with 95% confidence interval for extreme
length was 42.02 to 50.71 mm (Table 4). The von Bertalanffy
Growth Function (VBGF) parameters L_{ ∝ }and K in males were
obtained for the best fit with L_{ ∝ } = 29.40 mm and K =
1.70 year^{1} (Fig. 6a). For females, these
parameters were estimated L_{ ∝ } = 42 mm and K = 1.20 year^{1}
(Fig. 6b). The response surface, R_{n} values
were calculated by ELEFANI as 0.205 for males and 0.284 for females.
The growth performance index (φ’) of male was 3.16 and that of females
was 3.33.
Age and growth: Based on the growth coefficients, the maximum life span
(t_{max }= 3/K) of males was 1.76 years and that of females was 2.50
years.

Fig. 4: 
Monthly length frequency distributions of males and
females A. indicus caught in the coastal waters of Malacca,
Peninsular Malaysia 
The application of modal progression analysis by Bhattacharya’s method
determined modal lengths for combined sexes of A. indicus between 17
mm in February and 35 mm in March, with satisfactory separation index for combined
sexes of A. indicus (Table 5). Maximum two welldiscriminated
modal groups were identified reflecting different annual cohorts (Table
5). Table 6 shows the total lengthage relationship estimated
from Bertalanffy’s model using L_{ ∝ } and K parameters of males and
females and assuming t_{0} = 0 (Pauly and David,
1981). Therefore, the sizes attained by males and females A. indicus were 23.89 and 28.99 mm at the end of one year age, respectively (Table
6).
Mortality and exploitation: Total mortality (Z) was estimated
using the length converted catch curve (Fig. 7).
The dark points of the curve were used by fitting a least square regression
line to them. Good fit to the descending right hand limits of the catch
curve was considered with mentioned regression line values:
Male 
: Y = 11.225 + (– 5.312) X (r = – 0.99) 
Female 
: Y = 10.398 + (– 4.778) X (r = – 0.99) 
The estimated total mortality, Z values were 4.15 and 3.50 year^{1}
for males (Fig. 7a) and females (Fig.
7b), respectively. The natural mortality rate, M values of males was
2.65 year^{1} and that of females was 1.99 year^{1}.
The fishing mortality rate (F) of males and females was 1.50 and 1.59
year^{1}, respectively (Table 4). The natural
mortality of males (2.65 year^{1}) was higher than the value
of females (1.99 year^{1}) indicating the unbalance position
in the stock.

Fig. 5: 
Length weight relationship of male (a) and female (b)
A. indicus from the coastal water of Malacca, Peninsular Malaysia 


Fig. 6: 
Von Bertalanffy growth curves of males (a) and females
(b) A. indicus superimposed on the restructured lengthfrequency
histograms. The black and white bars are positive and negative deviation
from the weighted moving average of three length classes and they
represent psedocohorts 
From these figures, exploitation rate, E values were obtained
0.36 and 0.45 for males and females in the coastal waters of Malacca,
which seemed to be below to the optimum level of exploitation (E = 0.50).
Table 5: 
Identified age groups from the lengthfrequency analysis
of male and female A. indicus during the monthly sampling,
using Bhattacharya’s method 

AG: Age Group, TL: Total Length, SD: Standard Deviation,
SI: Separation Index 
Table 6: 
Lengthatage key for A. indicus from coastal
waters of Malacca, estimated using the VBGF parameters (L_{ ∝
} = 29.40 mm, K = 1.70 year^{1} for males and L_{
∝ } = 42 mm, K = 1.20 year^{1}) 

Length at first capture: The length at first capture, L_{c}
(the length at which 50% of the shrimps becomes vulnerable to the gear)
of A. indicus in the coastal waters of Malacca are shown in Fig.
8. The mean L_{c} value of males and females A. indicus
was 14.44 and 14.79 mm, respectively during the study period. The L_{25%}
value of males and females was 12.35 and 12.04 mm. The mean length at
which 75% of the shrimps were retained in the gear was estimated as L_{75%}
= 16.31 mm for males and L_{75%} = 17.92 mm for females in the
investigated area.


Fig. 7: 
Length converted catch curve of males (a) and females
(b) A. indicus; the darkened full dots represent the points
used in calculating through least square linear regression and the
open dots represent the point either not fully recruited or nearing
to L_{ ∝ } 
Recruitment pattern: The recruitment pattern of males was continuous
throughout the year, with two major peaks (Fig. 9a).
The first spell of males was from February to May with peak in March (11.59%)
and the second spell was from July to October with peak in September (13.91%).
In case of females, the recruitment pattern was also two spells during
the 1year study (Fig. 9b). The first spell of females
was from January to March with peak in January (5.12%). The second spell
was August to October with peak in September (21.32%).
Relative yield per recruit (Y/R) and biomass per recruit (B/R):
The computed maximum allowable limit of exploitation (E_{max})
that give the maximum relative yieldperrecruit at maximum sustainable
yield (MSY) level was 0.71 for males (Fig. 10a) and
0.57 for females (Fig. 10b).

Fig. 8: 
Probability of capture of each length class of males
(a) and females (b) A. indicus (L_{25%} = 12.35 mm,
L_{50%} or L_{c} = 14.44 mm and L_{75%} =
16.31 mm for males and L_{25%} = 12.04 mm, L_{50%}
or Lc = 14.79 mm and L_{75%} = 17.92 mm for females) 

Fig. 9: 
Recruitment pattern of males (a) and females (b) A.
indius in the coastal waters of Malacca, Peninsular Malaysia 

Fig. 10: 
Yieldperrecruit and biomass per recruit models, showing
levels of yield index of males (a) and females (b) A. indicus:
MSY = Maximum Sustainable Yield 
The exploitation level
(E_{0.5}) which corresponds to 50% of the relative biomassperrecruit
of the unexploited stock was 0.36 for males that of females was 0.32,
respectively.
DISCUSSION
The size structure of the population in the investigated area consists of a
relatively higher percentage of the females than the males. The females attained
a greater size indicating a size dimorphism. The growth coefficient b generally
lies between 2.5 and 3.5 and the relation is said to be isometric when it is
equal to 3 reported for most aquatic organisms (Ecoutin et
al., 2005). Table 7 shows earlier published values
of the coefficient a and b for the genus Acetes. The values of b show
considerable variation, ranging from 2.155 (Lei, 1988)
to 3.472 (present study). Estimated b (3.307) of males lies between the values
mentioned by Ecoutin et al. (2005) and is significantly
higher than isometric value (3) at 5% level. In females, estimated b (3.472)
is also significantly higher from then the isometric value (3) at 5% level.
This indicates the positive allometric nature of growth for A. indicus
in the coastal waters of Malacca.
Estimated L_{ ∝ } and K are 29.40 mm and 1.70 year^{1} for
males. In females they are 42 mm and 1.20 year^{1}. Comparisons with
population parameters from other studies (Table 8) show that
differences exist for different species of the genus Acetes from different
areas in the world. The highest value of L_{ ∝ }(42 mm in females)
is observed in the present study; the lowest value (31 mm in combined sexes)
was in Bangladesh waters (Zafar et al., 1997).
The K values are more or less similar to the present value except the reported
by Oh and Jeong (2003) stated the lowest values (K =
0.84 year^{1} in males and K = 0.69 year^{1} in females for
A. chinensis. The index of phi prime by Munro and
Pauly (1983) is suitable for comparing and computing the overall growth
performance of different species of shrimp stocks. The phi prime for this species
with the present estimates of L_{ ∝ } and K were found to be excellent
and varied between 3.16 and 3.33 (Table 8). Though phi prime
is supported to be more or less constant for a family or for similar taxa, the
range here (Table 8) is low except the report by Oh
and Jeong (2003) stated the lowest values (φ’ = 1.97 in males and φ’ =
2.10 in females for A. chinensis).
Longevity of Acetes, as in other crustaceans, is difficult to estimate
accurately because of the obvious difficulties in ageing these animals and hence
estimates are unavailable for both natural and laboratory populations (Xiao
and Greenwood, 1993). The maximum age, t_{max} = 3/K (Pauly,
1984) is frequently using for estimating longevity of shrimp (BotterCarvalho
et al., 2007; Niamaimandi et al., 2007;
Cha et al., 2004). The estimated maximum age for
males and females are 1.76 and 2.50 years in the present study. The mean life
span of penaeids is often regarded as <2 year (Dall et
al., 1990). Estimated longevity of Acetes is less than that of
penaeids is well justified because of its smaller size. Yoshida
(1949) suggested that Acetes chinensis has a lifespan of about one
year in western Korea. Except this no reports are available for comparison of
longevity of Acetes so far researcher aware. Maximum of two age groups
of combined sexes of A. indicus with mean values being 21.01 and 30.98
mm of total length were estimated by Bhattacharya’s method. Similar studies
were reported on red shrimp Aristeus antennatus by Paraconstantinou
and Kapris (2001) and on C. madrasensis by Amin
et al. (2008), by the Bhattacharya’s method, which also followed
in the present study.
Table 7: 
Parameters of lengthweight relationship (a and b)
for the genus Acetes from various locations 

a: Condition factor; b: Growth coefficient; r^{2}:
Coefficient of determination 
Table 8: 
Growth parameters (L_{ ∝ } and K) and
computed growth performance index (φ^{/}) of the genus
Acetes from different tropical countries 

M: Male, F: Female, C: Combined sexes 
The estimated ages from the study are presented in Table
6. The overall average growth rate of A. indicus showed 1.83 ± 0.78
mm/month for male and 2.30 ± 0.72 mm/month for females which enable it to attain
a total length of around 21.96 mm for males and 27.6 mm for females in 12 months.
Similar reports are available on other shrimp by Amin and
Zafar (2003) and Cha et al. (2004) and on
fish by Amin and Zafar (2004), Isaac
and Ruffino (1996) and Blaber et al. (1998),
who used lengthconverted age method for their study.
The recruitment pattern suggests that continuous recruitment consists of two
peaks seasonal pulse (Fig. 9). This is in good agreement with
Zafar et al. (1997) for A. indicus and
Oh and Jeong (2003) for A. chinensis. There are no published reports
on Acetes recruitment in Malaysia. However, it has been reported that
the Acetes spawns throughout the year in the tropics and subtropics,
spawning peaks can be recognized and these almost always lie in the warmer months
(Nataraj, 1947). Spawning patterns in these areas (tropical
and subtropical) are probably related to monsoonal influences on precipitation
and wind direction (Omori, 1975). Major spawning was
observed in February to March in the coastal waters of Malacca, Peninsular Malaysia
(Fig. 6). It is expected that the major recruitment peak (SeptemberOctober)
detected in this study should correspond to the major spawning season.
On the basis of 175 independent data sets Pauly (1980)
has made a regression analysis of M (year^{1}) on K (year^{1}),
L_{ ∝ } and T (average annual temperatures). Sparre
and Venema (1992) warned against using Pauly’s formula for crustaceans,
mollusc or cephalopods or any other invertebrates, as the formula does not cover
these groups. But in the absence of any data this formula can be used for shrimp
(Niamaimandi et al., 2007). Pauly
et al. (1984) noted that the formula can be used for shrimp and any
other invertebrates, because these marine organisms generally share the same
habitats, resources and predators and that therefore, they are not likely to
differ widely in their vital parameters. Hence Pauly’s equation allows a rough
estimation of M in shrimp stock where L_{ ∝ }, K and T are known.
The estimated natural mortality rate of A. indicus for both sexes in
the present study (M = 2.65 year^{1} in males and M = 1.91 year^{1}
in females) is higher than the estimated values of fishing mortality (F = 1.50
year^{1}in males and F = 1.59 year^{1} in females). Higher
natural mortalities of A. indicus verses the fishing mortalities indicate
the unbalance position in the stock. This is based on the assumption that a
stock is optimally exploited when fishing mortality (F) equals natural mortality
(M), or E = (F/Z) = 0.5 (Gulland, 1971). Estimated exploitation
level (E) of females (0.45) is higher than males (0.36) in A. indicus
population. It is may be for the larger size of females they can not escape
from the gear than males. In Bangladesh waters M and F have been estimated at
4.76 and 1.31 year^{1} for the combined sexes of A. indicus
(Zafar et al., 1997). Higher natural mortality
than fishing mortality was also reported from other Acetes studies (Arshad
et al., 2007; Zafar et al., 1998; Zafar
and Amin, 2002; Oh and Jeong, 2003) in the Asian
countries (Table 9). Hass et al.
(2004) noted production mortality can be influenced by shrimp length, movement
and the habitat type. Mortality due to predation is influenced by the characteristics
of the predation population. Xu et al. (1995)
reported that the influences of abiotic factors are probably the most important
reasons affects of natural mortality.
In most shortlived species, it is difficult to identify the component age
groups and estimate the numbers at each age. Therefore, for estimation of Z,
lengthfrequency data are used and a lengthfrequency distribution can be converted
to an age frequency distribution by means of a lengthconverted catch curve.
Catch curve analysis is feasible, assuming that total mortality is constant
from length upwards. This assumption could be violated by migration out of the
fishing area of mature shrimp (Niamaimandi et al.,
2007). In lengthcatch curve analysis for estimation of total mortality,
the initial ascending data points are not included in the regression (Fig.
7a, b). These points represent younger age groups, which are subjected to
a lower fishing mortality because they are either not fully recruited or not
fully vulnerable to the fishing gear used (King, 1984).
Table 9: 
Mortality parameters (Z, F and M) and computed exploitation
rate (E) of the genus Acetes from different tropical countries 

M: Male, F: Female, C: Combined sexes 
The level of total mortality, Z = 6.07 year^{1} from Bangladesh waters
(Zafar et al., 1997) is higher than present study
(Z = 4.15 year^{1} for males and Z = 3.50 year^{1} for females).
The values of E = 0.36 for males and E = 0.45 for females are lower than optimum
level of exploitation (E = 0.50), suggesting underexploitation in this fishery
(underfishing occurs if M>F or E>0.50, Gulland, 1971).
Considering our relative Y/R and relative B/R analyses (Fig.
10a, b) our computed exploitation rate, E values (E = 0.36 for males
and E = 0.45 for females) are below the predicted maximum values of E_{max}
= 0.71 (males) and 0.57 (females). This reveals that the fishery is being
under exploited in the coastal waters of Malacca. Results from the analysis
of the exploitation rate (E) based on the fishing mortality estimates
and from the relative Y/R, indicate that the fishery is below the optimum
fishing pressure and probably the catch is below to the full exploitation
(MSY). Thus, the more yield could be obtained by a reasonable increase
in the effort. This implies that further increase in fishing effort could
be possible to get the maximum sustainable yield, thus driving the stock
sustain and economic benefit.
ACKNOWLEDGMENTS
This research is part of a Ph.D thesis funded by the Ministry of Science,
Technology and Innovation (MOSTI), Malaysia (Grant No. 050104SF0613).
The authors would like to thank to MOSTI for providing financial support
to carry out this research work. Special thanks to Universiti Putra Malaysia
for providing partial financial support through Graduate Research Fellowship
(GRF) during the study period. In addition, thanks go to Mr. Ibrahim,
fisherman and Mr. Perumal, Science Officer of Marine Science Laboratory
and Aquaculture, Institute of Bioscience, UPM for the assistance during
field sampling.

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