Journal of Marine Biology & Oceanography ISSN: 2324-8661

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Research Article, J Mar Biol Oceanogr Vol: 3 Issue: 3

Population Dynamics of Bigeye Scad, Selar crumenophthalmus in Bangaa Faru, Maldives

Shaza Adeeb 1, Nik Fadzly1,2*, and Amir Shah Ruddin Md Sah 1
1School of Biological Sciences, Universiti Sains Malaysia, 11800, USM, Penang, Malaysia
2Centre for Marine & Coastal Studies (CEMACS), Universiti Sains Malaysia, 11800 Penang, Malaysia
Corresponding author : Nik Fadzly
School of Biological Sciences, Universiti Sains Malaysia, 11800, USM, Penang Malaysia
Tel: +9603327302; E-mail: [email protected]
Received: May 06, 2014 Accepted: July 08, 2014 Published: July 15, 2014
Citation: Adeeb S, Fadzly N, Md Sah ASR (2014) Population Dynamics of Bigeye Scad, Selar crumenophthalmus in Bangaa Faru, Maldives. J Mar Biol Oceanogr 3:3. doi:10.4172/2324-8661.1000133


 Population Dynamics of Bigeye Scad, Selar crumenophthalmus in Bangaa Faru, Maldives

Selar crumenophthalmus (bigeye scad) locally known as mushimas, is a widely distributed fish in Maldives. For the biological study, random samples of Selar crumenophthalmus (bigeye scad) were collected from September 2012 to February 2013. The samples were caught from Bangaa faru, in Male’ atoll, Maldives. A total of 1648 samples were collected, with their fork length ranging from 7.7 cm to 24.5 cm (16.85 ± 2.82 cm) and body weight ranging between 8g to 255.6 g (87.76 ± 40.41g). FiSAT software was used in estimating the population parameters.

Keywords: Population parameters; Selar crumenophthalmus; Length frequency analysis; Maldives


Population parameters; Selar crumenophthalmus; Length frequency analysis; Maldives


Studies on population dynamics of fishes are an important tool for effective management practices [1]. Population dynamics provides a significant input in decision making on sustainable management of the stocks. Furthermore, population dynamics results also provide indications on the exploitation level of stocks [2] and functions as biological indicators for defining the stock status [3]. The fundamentals of fishery management outcomes are favorably dependent on the estimation of growth, mortality and recruitment patterns [4]. According to Bagenal and Tesch [5] the growth parameters and the exploitation rate data for many tropical fishes are under- reported.
A study on Selar crumenophthalmus in Maldivian waters was first carried out in 1985 and 1986 [6]. Afterwards there is a lack of biological information reported on the fish in Maldivian waters. However, extensive growth, mortality and recruitment pattern studies on the species have been done in Hawaii [7], Philippines [8], Sri lanka [9], Indonesia [10] and the coast of Peninsular Malaysia [11].
The fishery of bigeye scad or mushimas (local name) is widespread throughout the Maldives and it is mainly caught by pole and line. However some fishermen use nets and this is not appreciated by majority of the fishermen, especially the older and experienced ones as they believe that use of nets would harm the stocks. In this study, all the samples were caught by pole and line.
Except the study of Hafiz in 1985 and 1986 [6], very little is known about the exploitation levels, recruitment pattern, mortality and growth parameters of S. crumenophthalmus in Maldivian waters as catch rates or number of vessels going for bigeye scad fishing are not recorded. Furthermore, the previous study [6] does not mention the exact location where the samples were caught [6]. However, the paper states that the samples were taken from the landings in Male’ fish market, and based on practice there is a possibility that the samples could be from Bangaa faru or the harbor area, inner lagoons or the shallow inner reef of the Male’ island as S. crumenophthalmus was reported to be abundant in those areas at the time. Hence, this study is the first reported on bigeye scad analysis in Bangaa faru. The study will make a comparison on the existing estimations and will also provide a detailed explanation on the present stock status of S. crumenophthalmus in Bangaa faru.
This paper presents the study of growth parameters, mortality rate, recruitment patterns and exploitation rate of S. crumenophthalmus in Bangaa faru, Maldives. The study was conducted for six months from September 2012 to February 2013 and analysis was performed on length frequency data set.

Materials and Methods

The samples for this study were from the commercial catch to be sold in Male’ fish market and before selecting the samples, it was made sure that the samples were caught from Bangaa faru as there were S. crumenophthalmus caught in various other parts of the country. Afterwards, the measurements (Fork length, Body weight) were recorded down. Fork length was measured to the nearest 0.1 cm and body weight to the nearest 0.1g. The data was taken for six months from September 2012 to February 2013. A total of 1648 bigeye scad data were recorded during the study period. The data was then transferred in to an Excel sheet and later in to FiSAT II software [12] for further analysis. FiSAT is a free program package developed for fisheries management that been used widely in the world. This program package is mainly based on the analysis of length-frequency data which also enables related analyses such as of size-at-age, catchat- age, selection and other analyses. The work was carried out with support from FAO and the European Union through the project “Fisheries Information and Analysis System (FIAS). All of the population parameters were computed using the incorporated FiSAT software package [13] and the routines of the FiSAT were followed thoroughly as recommended by the FiSAT user’s manual [14] and reference’s manual [13]. Monthly length-frequency distributions of S. crumenophthalmus were analyzed using FiSAT II. The parameters of the von Bertalanffy growth function (vBGF), asymptotic length (L) and growth coefficient (K) were estimated using ELEFAN – I routine incorporated in the software. K scan routine was conducted to assess a reliable estimate of the K value. The estimated L and K were used to calculate the growth performance index (Ø’) using the equation:
Ø’= 2log10 L + log10K
Total mortality (Z) was estimated using a length converted catch curve and natural mortality (M) was estimated by Pauly’s empirical equation,
M = -0.0066-0.279 log L + 0.6543 log K + 0.4634 log T,
Where T is the mean sea surface temperature which was 28.5°C [15]. Fishing mortality was obtained by F = Z-M, which is, total mortality subtracted from natural mortality. The exploitation rate (E) was computed using Gulland’s expression, E= F/Z [16].
The value for length at first capture was estimated by using catch curve analysis [17] and it was determined using the equation: Lc = (L-K (L - L’)/Z) [18], where Lc is the length at first capture, L’ is the mean length of the sample, K and L are parameters of von Bertalanffy growth equation and Z is the instantaneous mortality rate.
The recruitment pattern of the stock was determined by backward projection of the length axis of the available frequency in FiSAT. It reconstructs the recruitment pulses from a time series of length frequency data to determine the number of pulses per year. Normal distribution of the pattern was done by NORMSEP.
Relative yield per recruit (Y’/R) and relative biomass per recruit (R’/B) were estimated according to the Berverton and Holt model, by using the knife-edge selection [19]. Emax, E0.1 and E0.5 were estimated using the first derivative of the Length at first capture (Lc) function. Emax is the exploitation rate at Maximum Sustainable Yield (MSY) and E0.5 is the optimum exploitation rate.

Results and Discussion

A total of 1648 specimens were analyzed for the study of growth parameters, mortality parameters and recruitment pattern of Selar crumenophthalmus in Bangaa faru of Maldives. The size of the samples ranged from 7.7 to 24.5 cm (16.85 ± 2.82 cm) in their fork length.
Growth parameter estimation
The growth parameters estimated in ELEFAN by using K-scan gave the value of L as 26.54 cm and K as 0.930 with the starting sample of 1 and starting length of 14.25. L estimated in this study is quitelow compared to 31.0 cm estimated earlier in Maldives [6]. However relatively similar values to the present study for L were observed in Moro Gulf sea of Philippines (27.0 cm) [8], Hawaii (27.0 cm) [7] and the most recently reported study estimated L as 26.5 cm in Reunion island [20]. The lowest reported value is from Indonesia which is 25.90 cm and the largest reported value is 34.75 cm from Sri lanka [9]. The growth coefficient, K of 0.930 per year is considered as a fast growing fish. In the previously recorded studies the value for K ranged from 0.5 to 2.57 per year. Some of the previously reported values for K are moderately dissimilar between the studies. The largest reported value of 2.57 per year was in Hawaii [7], and smallest (0.5 year-1) in Sri lanka [9]. The first reported value for K in Maldives was 1.04 per year and L was 31.0 cm. A study done in Manila bay reported a K value of 0.89 per year and an L of 36.5 cm [21]. The variance of the parameters among the same species can be hugely subjective to the environmental conditions in which the species is living in [22,23].
Growth performance index
The growth performance index in this study is estimated to be 2.818. The Ø’ values estimated by other authors are quitesimilar to the current study. The highest value was observed in Philippines, which was 3.16 [8] and lowest in East coat of Peninsular Malaysia, which was 2.98 [11]. The former reported values for growth performance index (Ø’) ranges in between 2.98 to 3.16. In the former study in Maldives, Ø’ was reported as 3.01 [6]. High annual growth rate of fish leads to high turnover rates and higher dissimilarities in Ø’ values for the same species in various stock could also indicate inaccuracy of the estimated growth parameters [24].
Length-converted catch curve
Length-converted catch curve was used to get fishing mortality (F), natural mortality (M) and exploitation rate (E). The values for total mortality (Z), fishing mortality (F) and natural mortality (M) are 4.01, 2.23 and 1.78 year-1 respectively (Figure 1). There is a slight similarity for natural mortality with the present study and the previously reported ones, as stated in Hafiz’s study [6], the natural mortality value obtained in 1985-1986 study in Maldives was 1.82 year-1. Though it is difficult to estimate natural mortality directly, as natural deaths are rarely observed in fishes, these values can also be accepted [25]. Hence, to verify the estimates of M and K, M/K ratio is used to see if the ratio falls between the normal range of 1.12-2.50 as for most of the fishes it falls in between the range [26]. In this study the ratio is 1.913. Therefore, the estimated M value in this study could be considered as a reliable value. However, apart from this, the other mortality values (F and Z) are comparatively low in the present study. In Hafiz’s study in Maldives, the values for fishing mortality and total mortality are 4.97 and 8.79 year-1 respectively [6]. However, this could not be completely comparable as the sample location for his study was not mentioned and for the present study, the samples are from Bangaa faru area in Maldives and no prior information on the biology of the species in the area has been reported. In addition to the values from Maldives, most of the former reported studies also show a lower value for F and Z than the present study in Bangaa faru. The fishing mortality coefficient (F) in the present study is 2.23 year-1. According to Gulland [27], in an optimally exploited stock, fishing mortality should be equal to natural mortality, resulting in an exploitation rate of 0.50 year-1.
Figure 1: Length–converted catch curve with regression coefficient (R2).
Length at first capture
The value estimated for S. crumenophthalmus in Bangaa faru is 16.72 cm (Figure 2). Closer values to this were reported in the previous studies also. In ingles and Pauly’s (19) study the length at first capture was 17.9 cm in Manila Bay and in Java sea Indonesia, the length at first capture was recorded to be 17.80 cm [10]. Furthermore, lengths at first capture values arean important input in the computation of relative yield per recruit and relative biomass per recruit.
Figure 2: The probability capture curve of Selar crumenophthalmus.
Recruitment pattern
Two main peaks are observed (Figure 3) in annual recruitment of S. crumenophthalmus in Bangaa faru, Maldives. The major group is observed in September and minor group in April.
Figure 3: Recruitment pattern of Selar crumenophthalmus in Bangaa faru, Maldives.
Relative yield per recruit, relative biomass per recruit and exploitation rate
The 2D diagram in Figure 4 shows the relative yield per recruit and relative biomass per recruit prediction for S. crumenophthalmus in Bangaa faru, Maldives. The value for M/K is 1.91 and Lc/L is 0.63. The estimated optimum exploitation rates (Y’/R) and related coefficient rates (B’/R) from Knife-edge method for the species is presented in Table 1. The maximum allowable limit of exploitation rate (Emax) giving maximum relative yield per recruit was estimated as 1.000. E0.1, the level of exploitation at which the marginal increase in Y’/R reaches 1/10 of the marginal increase computed at a very low level of E is 1.000, and the exploitation level (E0.5) which corresponds to 50% of the relative B’/R of the unexploited stock is estimated as 0.405.
Figure 4: 2D yield per recruit graph for Selar crumenophthalmus for Bangaa faru, Maldives.
Table 1: Estimated optimum exploitation rates and related coefficient rates of Selar crumenophthalmus in Bangaa faru, Maldives.
In the yield per recruit and biomass per recruit model, the biomass per recruit (B’/R) curve always decreases with increasing exploitation ratio (E) and yield per recruit (Y’/R) curve shows an increase in Y’/R with increase in exploitation until it reaches Emax. Emax gives the highest Y’/R with a given value of Lc. Results from the analysis from mortality estimates indicates that the stock of S. crumenophthalmus in Bangaa faru is overexploited. In addition to this, the exploitation rate (Emax) that gives maximum yield per recruit was estimated as 1.000 by Knifeedge selection method and greatly differs from the exploitation rate (E=0.56) which further suggests that the stock is overexploited.
In Hafiz’s study in Maldives, the exploitation rate was estimated as 0.73 year-1 [6], which indicates a severely overexploited stock. Since his samples were also from the commercial catch in Male’ fish market, the possibility of it being from Bangaa faru stock is high. If it is so, over the last twenty-seven years the stock has made a commendable improvement. Most of the reported exploitation values for stocks in various parts of the world also show overexploited stocks of S. crumenophthalmus.


The current study is an extremely important study as the population parameters for S. crumenophthalmus in Bangaa faru, which is one of the most fished areas of the species in central Maldives, has not been reported previously. Though the findings of this study may have limitations, it provides novel and essential information on population dynamics of S. crumenophthalmus in Bangaa faru. The results of this study can be used as a reference for comparison of different stocks in future studies among different areas of S. crumenophthalmus.
The exploitation rate (E) is estimated as 0.56, which is higher than the optimum value of 0.5. Hence, the stock can be considered as overexploited. According to Froese [28] overfishing can be prevented by following certain rules, such as by catching fishes that have reached their optimum length, which is usually a bit larger than the length at first maturity [28]. However the spawning fishes must be avoided. Moreover, large females must be avoided as they are more fecund and a minimum catch length can be set such that more than 90% of the individuals get at least one chance to reproduce before being caught [28], as overfishing can be stopped if all the fishes gets a chance to reproduce before capture [29]. In this study it was observed that majority of the sample contained immature juvenile fishes and this could immensely effect on the healthy survival of the stock. If any population is affected by overfishing, the stock can easily collapse if proper attention is not given [30].
For further understanding and analysis it is important to record the catch and effort data for S. crumenophthalmus regularly, as with it, a virtual population analysis and stock assessment of the species would be possible.


The authors thank Islamic Development Bank (IDB) for providing financial assistance for this research and Universiti Sains Malaysia for providing the technical assistance. The cooperation of the local bigeye scad fishermen is very much appreciated. We are grateful to Dr. Mansor Mat Isa for his assistance in data analysis and for overall guidance in conducting the research. Shaza Adeeb collected data. Dr. Mansor assisted in data analysis. Dr. Nik Fadzly, Shaza Adeeb and Dr. Amir Shah Ruddin Md Sah were involved in writing the paper.


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