INTRODUCTION
Indo-west Pacific mud crabs of the genus Scylla are large commercially important portunids that are typically associated with mangroves and have been overexploited in many countries of South-east Asia for the past three decades (MacNae, Reference MacNae1968; Angell, Reference Angell and Angell1992; Keenan, Reference Keenan, Keenan and Blackshaw1999; Le Vay et al., Reference Le Vay, Ut and Walton2007; Mirera, Reference Mirera2011). In addition to high fishing pressure, there are other factors determining the present status of mud crab stocks in South-east Asia, such as conversion of mangrove areas for large-scale shrimp farming, fishing of gravid females for high market value, capture of undersized crabs, collection of wild seeds for pond culture and, finally, not all the countries involved have any rules and regulations for mud crab fishing. Recently, the genus Scylla was recognized with four species (S. serrata, S. tranquebarica, S. olivacea and S. paramamosain); previously the genus was alleged to have only one species (Keenan et al., Reference Keenan, Davie and Mann1998). Previous literature on Scylla spp. is uncertain because of the identity of the species except in South Africa where S. serrata is the only known species (Le Vay et al., Reference Le Vay, Ut and Walton2007).
Length-weight relationships and growth parameters have great importance for estimating the population size of a stock for exploitation purposes. In addition, they play a vital role in implementing effective management strategies for the sustainable development and judicious exploitation of the resources (Bal & Rao, Reference Bal and Rao1984). Growth in crustaceans is a discontinuous process in which body size increases only at the time of moulting. Therefore, age determination in decapods often relies on the associated growth models. Methods of age determination in crustaceans involve the observation of specimens in captivity, tagging and recapture, length-frequency distribution analyses in wild populations, analysis of data on moult increment and inter-moult duration and assessment of the age pigment ‘lipofuscin’ in the brain (Hartnoll, Reference Hartnoll2001; Maxwell et al., Reference Maxwell, Matthews, Sheehy, Bertelsen and Derby2007; Vogt, Reference Vogt2011). However, length-frequency data have been widely used in studying ageing and growth parameters of crustaceans because it is a quick and reliable method.
The present study deals with the CWRs, age and growth parameters of S. olivacea found in the Pichavaram mangroves on the south-east coast of India based on length-frequency data where this species is dominant in catch over other mud crabs.
MATERIALS AND METHODS
Sampling
Specimens of mud crab Scylla olivacea were collected from the commercial fish landing centres around the Pichavaram mangroves located on the south-east coast of India (11°29′N 79°46′E), during June 2010 to May 2012. The species was confirmed by both molecular and morphometric level (Viswanathan et al., Reference Viswanathan, Elumalai, Pravinkumar and Raffi2012). A total of 2895 specimens of S. olivacea, comprising 1468 males and 1427 females were analysed for CWR, while for age and growth a total of 6094 crabs comprising 3372 males and 2722 females were analysed. Collected crabs were measured individually for carapace width (space between the tips of the 9th antero-lateral spines across the body) and weight to the nearest 1 mm and 1 g respectively. The length-frequency data for males and females of each sampling day were classified into 8 mm size intervals, then these data were pooled on a monthly basis (Sekharan, Reference Sekharan1962). A total of 13 and 12 size groups or class sizes (with 8 mm intervals) were obtained for males and females respectively.
Statistical analysis
CWR was determined by employing the Le Cren's (Reference Le Cren1951) non-linear equation (W = aLb), where W = body weight in g, L = length in mm and a, b = parameters that were estimated by employing linear regression analysis. Regression analysis was used to determine the constants, ‘a’ and ‘b’ values and relationships between carapace width (length) and weight with the aid of Microsoft Excel (MS-Excel 2007). Further, analysis of variance (ANOVA) was performed to test the significance of the difference between regression coefficient values (by value) at the 5% level for both sexes.
The von Bertalanffy growth function (VBGF) was used for describing age and growth of mud crab S. olivacea. The simplest version of this function is:
$$L_t = L_{\infty} (1 - {\rm e}^{- K(t-t_0)})$$
As crabs grow width wise, their length is termed the carapace width (CW), so hereafter ‘L’ will be replaced by ‘CW’, then the VBGF function can be expressed as:
$${\rm CW}_t = {\rm CW}_{\infty} (1 - {\rm e}^{- K(t-t_0)})$$
where, CW∞, asymptotic carapace width attained by the crab; K, growth constant (coefficient of catabolism); t 0, age the crab would have at length 0, if they had grown as per the equation; CW t , predicted carapace width at age ‘t’.
Age and growth studies were carried out in the FiSAT II software package, introduced by Pauly & David (Reference Pauly and David1981).
RESULTS
Carapace width-weight relationship
The exponent (b) values obtained for males and females are 3.035 (r 2 = 0.962) and 2.925 (r 2 = 0.933), respectively. This indicates that there exists a linear increase in weight per unit increase in carapace width of 3.035 and 2.925 units for males and females, respectively. The CWRs curves fitted separately for males and females were derived by plotting the observed values of carapace width (length) against weight and are shown in Figures 1 and 2, respectively. The regression line plotted from the data showed a straight-line (linear) relationship between the observed and calculated values of the two variables carapace width (length) and weight. The CWRs results revealed that significant difference was observed in the exponent values (slope ‘b’) for both sexes (P < 0.05).
Logarithmic relationship between carapace width (length) and weight of male Scylla olivacea.
Logarithmic relationship between carapace width (length) and weight of female Scylla olivacea.
Age and growth
The estimated growth parameters such as CW∞ and K derived through various methods are summarized in Table 1 and are presented in Figures 3–14. No significant difference was observed in the asymptotic carapace width (CW∞) obtained through various methods in both sexes. In the case of the growth coefficient (K), the values ranged between 0.762–2.310 year−1. The t 0 values acquired through von Bertalanffy plot for male and female were found to be −0.637 and −0.681, respectively. The monthly distributions of mean modal lengths (the Gaussian components) derived by Bhattacharya and NORMSEP analysis are presented in Tables 2 and 3. Around 46 components were resolved in Bhattacharya analysis for males, whereas in females 50 components are resolved. There exists a slight variation in Gaussian components between Bhattacharya and NORMSEP analysis. The results of decomposition of normal distribution by NORMSEP were plotted by taking sampling dates in the x-axis and mean lengths in the y-axis. The mean lengths which were believed to belong to the same cohorts were linked and as a result growth increment and carapace width at age (CW t ) files were created (Figures 15 and 16). The estimated growth performance index for male and females were found to be 4.222 and 3.911 respectively. The estimated longevity for male and female S. olivacea using different K values derived by different methods is given in Table 4.
ELEFAN I plot for male crabs.
ELEFAN I plot for female crabs.
Powell-Wetherall plot of male crabs.
Powell-Wetherall plot of female crabs.
Ford-Walford plot for male crabs.
Ford-Walford plot for female crabs.
von Bertalanffy plot for male crabs.
von Bertalanffy plot for female crabs.
Gulland and Holt plot for male crabs.
Gulland and Holt plot for female crabs.
Munro's plot for male crabs.
Munro's plot for female crabs.
Linking of means for male crabs.
Linking of means for female crabs.
Summary of estimated growth parameters (CW∞ and K) of S. olivacea.
Gaussian components as mean modal lengths (mm) derived through Bhattacharya analysis for S. olivacea.
Gaussian components as mean modal lengths (mm) derived through NORMSEP analysis for S. olivacea.
Estimated longevity for males and females of S. olivacea.
DISCUSSION
There is little knowledge on carapace width-weight relationship and age and growth of Scylla olivacea because the available literature is very scanty. The results of the CWRs showed that S. olivacea collected from the Pichavaram mangroves revealed a significant deviation from isometric growth in both males and females. The estimated exponential values for males and females were 3.035 and 2.903, respectively. These findings fall in agreement with the results of Jirapunpipat (Reference Jirapunpipat2008), who found values of 3.45 and 2.866 in males and females, respectively from the Klong Ngao mangrove swamp, Thailand. But Ikhwanuddin et al. (Reference Ikhwanuddin, Bachok, Hilmi, Azmie and Zakaria2010) observed low ‘b’ values such as 0.374 in males and 0.338 in females from Terengganu coastal waters, Malaysia. The results of the CWRs analysis in S. olivacea inferred that males are slightly heavier than females. The tendency of males to acquire heavier weight than females in portunids is in accordance with the observations of many authors (Prasad et al., Reference Prasad, Reeby, Kusuma and Neelakantan1989; Sukumaran & Neelakantan, Reference Sukumaran and Neelakantan1997; Razek et al., Reference Razek, Taha and Ameran2006; Babu et al., Reference Babu, Sreedhara and Muniyappa2007; Josileen, Reference Josileen2011).
In the present study, the growth parameters CW∞ and K values raised through different methods are comparable and these were above the maximum length (L max) values of 145 mm for males and 137 mm for females. There are very few investigations of the age and growth of Scylla spp. from Indian waters and hence the present results attain greater significance. The results of the present study found that females grew faster compared with males and it does not mean that females attain the CW∞ length faster than males. This trend was equally evinced by Lalitha Devi (Reference Lalitha Devi1985) who found females growing faster (10 mm month−1) than males (7 mm month−1) in S. serrata from Kakinada region, east coast of India. Mark-recapture studies suggested that the growth rates of S. olivacea are much slower and were around 10 mm CW month−1 in Thailand (Moser et al., Reference Moser, Macintosh, Pripanapong and Tongdee2002). A similar type of result was observed by Walton et al. (Reference Walton, Le Vay, Lebata, Binas and Primavera2006) from Vietnam and Philippines. Moser et al. (Reference Moser, Macintosh, Pripanapong and Tongdee2002) noticed different growth rates in individuals of cohorts where some exhibit fast growth while others show slow growth. In general, the increment in carapace width of crustaceans varied between individuals undergoing the same moult (Hartnoll & Abele, Reference Hartnoll and Abele1982). The inter-moult duration increases as the age increases, particularly after sexual maturity, thus the rate of moult increment was higher in smaller crabs (Marichamy et al., Reference Marichamy, Shanmugan and Rajapackiam1980; Kodama et al., Reference Kodama, Yamakawa, Shimizu and Aoki2005).
The nature of crustacean growth by moulting has led to much difficulty in determining the true growth rate under natural conditions (Zafar et al., Reference Zafar, Amin and Rahman2006). Growth parameters vary in the same species, and might depend on habitat, environmental factors such as availability of food, temperature, salinity and feeding nature of the species and also disease caused by parasites; these play a crucial role in the growth rate. As age and growth studies in crabs are chiefly done indirectly, employing mathematical and statistical methods, a single method cannot be relied upon for conclusions because each method has its own merits, demerits and limitations. In view of these, the present investigation of the age and growth of S. olivacea was assessed through various methods in such a way that the outcome of one method acts as a rider, check and control over the other.
The present study assumes greater significance in view of the fact that no attempt has so far been carried out to explain CWRs and age and growth of S. olivacea from Indian waters. The outcome of the study will stand as a steady reference for future works aiming at determining the growth, according to sex, spawning season, estimating the population size of stock for exploitation purposes and implementation of responsible fishing for sustainable utilization of mud crabs.
ACKNOWLEDGEMENTS
We are grateful to the authorities of Annamalai University for the necessary facilities provided to do this work.
FINANCIAL SUPPORT
We thank Ministry of Environment and Forest, Government of India for financial support (No. 22-9/2008-CS-I).
