Reproductive cycle, growth, mortality and exploitation rate of Acetes indicus were examined in the coastal waters of Malacca, Malaysia between February 2005 and March 2007. Monthly length frequency data were analyzed using FiSAT software for estimating population parameters of A. indicus, including asymptotic length (L∝), growth co-efficient (K) and exploitation rate (E) to assess the status of the stock. The annual mean L∝ and K for males were estimated at 29.93 mm and 1.60 yr−1, and for the females were 40.95 mm and 1.20 yr−1. Mean growth performance index (φ′) was calculated as 3.15 and 3.31 for males and females, respectively. Maximum life span (tmax) of males and females was 1.88 and 2.50 years. Total mortality (Z) by length converted catch curve was estimated at 4.30 yr−1 for males and 3.62 yr−1 for females. The rate of natural mortality (M) for males and females was calculated as 2.54 yr−1 and 1.93 yr−1, and the fishing mortality (F) was 1.77 yr−1 for males and 1.70 yr−1 for females. Recruitment pattern of A. indicus was continuous throughout the year with two major peaks. The exploitation rate (E) of males was 0.41 and that of females was 0.47, though the exploitation rates were slightly less than the optimum level of exploitation (E = 0.50). Thus, the stock of A. indicus is under pressure in the coastal waters of Malacca, Malaysia.
Planktonic shrimp of the genus Acetes, family Sergestidae, are locally known as Udang geragau and occur widely in the west coast of Malay Peninsula (Amin et al., 2009a; Arshad et al., 2007; Pathansali, 1966). The world-wide geographical distribution of Acetes has been summarized by Omori (1975). The shrimp Acetes play 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 serving 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 (Pathansali, 1966). Only a very small proportion of the catch is disposed of as fresh shrimp, but the greater part is sun dried and sold as dried shrimp or processed into a paste known locally as ‘belachan’ or pickled whole to give a product known as ‘chinchalok’ (Pathansali, 1966). The annual landing of Acetes in Malaysia was 13,797 tons during 2007 (DOF, 2007).
The commercial importance also derives from the use and potentiality of Acetes as a food organism for the aquaculture industry (Kungvankij et al., 1986). In spite of greater abundance and importance of the genus Acetes in the fishery of Asian countries, very little information is available on age, growth, mortality and recruitment, except for the studies carried out by Amin et al. (2008a, 2009b), Deshmukh (2002), and Oh and Jeong (2003). Crustacean age is difficult to estimate because exoskeletons are lost during the moulting process. Thus, analysis of length-frequency data has been used to identify year classes (Amin and Zafar, 2004; Amin et al., 2008b).
Knowledge of various population parameters such as asymptotic length (L∝) and growth coefficient (K), mortalities (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 (FAO-ICLARM Stock Assessment Tools) has been most frequently used for estimating population parameters of fish and shrimps (Amin et al., 2008a; Papaconstantinou and Kapiris, 2001; Etim and Sankare, 1998) because primarily it requires only length-frequency data. Therefore, the present study was undertaken to estimate the population parameters and exploitation level of A. indicus in order to assess the stock position of the species around the coastal waters of Malacca.
Materials and Methods
Monthly samples of Acetes indicus were collected between February 2005 and April 2007 from Klebang Besar (N 02°13.009/ & E 102°11.921/) in the coastal waters of Malacca (Figure 1). Acetes were caught by the push net (triangular shape) known locally as ‘Sungkor’, as described by Omori (1975). Dimensions of the net were 5–6 m in length, 4.0–4.5 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 (±0.05) cm at the middle and 0.5 (±0.08) at cod end (stretched). The push net was operated along the coast of the Klebang Besar between the depths of 0.5 and 1.5 m. Samples were immediately preserved in 10% formalin in the field and transported to the laboratory. The specimens of A. indicus were identified using a Nikon dissecting microscope (Nikon-122764, Japan) and Acetes taxonomy key by Omori (1975). Physical variables such as water temperature, salinity, dissolved oxygen and conductivity were measured monthly using a properly calibrated Hydrolab (Model: Surveyor 4A, Hydrolab Corporation, USA). Total suspended solids (TSS) were calculated as follows:
From the 20 g sub-sample, A. indicus were separated into males and females on the presence of clasping spine or petasma (Omori, 1975). Monthly sex ratio (females/total) was calculated and the result was tested by chi-square analysis (χ2 tests) for the differences from the hypothetical ratio 1:1.
About 30–32 females were dissected monthly under microscope in order to obtain the ovaries. Before dissection, the body weight was measured to the nearest millimeter 0.1 mg. The ovary weight was measured to the nearest 0.01 mg using an electronic digital balance. Then gonadosomatic index (GSI) was determined as follows (Oh and Jeong, 2003):
Growth and mortalities
Total length (TL) of 4900 individuals (males 1628 and females 3272) during 2005–2006 and 5926 individuals (males 1792 and females 4134) during 2006–2007 of A. indicus were measured from the tip of the rostrum to the tip of the telson to the nearest 0.1 mm and total weight was taken using an electronic balance with 0.001 g accuracy. The data were then grouped into total length classes by 2 mm intervals. Subsequently, the data were analyzed using the FiSAT software as explained in detail by Gayanilo et al. (1996). Asymptotic length (L∝) and growth co-efficient (K) of the von Bertalanffy growth function (VBGF) were estimated by means of ELEFAN-1 (Pauly and David, 1981). Potential longevity (tmax) of the species was calculated from the Pauly (1984) formula: tmax = 3/K. The estimates of L∝ and K were used to calculate the growth performance index (ϕ/) (Pauly and Munro, 1984) of A. indicus using the equation:
Total mortality (Z) was estimated by using the length converted catch curve. Natural mortality rate (M) was estimated using empirical relationship of Pauly (1980):
Log10M = −0.0066 − 0.279Log10L∞ + 0.6543. Log10K + 0.4634 Log10T, where M is the natural mortality, L∞ the asymptotic length, K the growth co-efficient of the VBGF and T the mean annual habitat water temperature °C. Once Z and M were obtained, fishing mortality (F) was estimated using the relationship:1971):
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. Input parameters were L∞, K and t0 (t0 = 0). Normal distribution of the recruitment pattern was determined by NORMSEP (Pauly and Caddy, 1985) in FiSAT. The relative yield-per-recruit (Y/R) and relative biomass-per-recruit (B/R) were estimated by using the model of Beverton and Holt (1966) as modified by Pauly and Soriano (1986) and incorporated in FiSAT software package. From the analysis, the maximum allowable limit of exploitation (Emax) giving maximum relative yield-per-recruit was estimated.
Total annual stock, average standing stock and maximum sustainable yield (MSY) of A. indicus were estimated. For these purposes, at first, exploitation rate (U) was estimated by the equation given by Ricker (1975) as U = F/Z (1−e−z). Then by using the values of U, F and estimated annual catch (Y), the total annual stock (Y/U) and average standing stock (Y/F) were determined. The approximate MSY was then calculated by the equation proposed by Cadima (Troadec, 1980) for exploited fish/shrimps:
Total number of 5,255 specimens of A. indicus was examined for sex ratio estimation with 3,570 females and 1,685 males. The result indicated that the overall sex ratio of the males to females was found to be 1:2.12. Total number of females was higher than that of males in the samples throughout the sampling period (Figure 2). The peak highest number of females in the samples was in June 2005, while the lowest number of females was in October 2006. Chi-square test revealed that the total number of females was significantly greater than that of males throughout the sampling period (χ2 = 85.61, df = 22, P < 0.05).
The peak values of mean GSI for females were recorded between February–July and November–December (Figure 3). The maximum value, GSI = 5.05, was observed in December and the minimum value, GSI = 0.41, was observed in August. The mean monthly GSI for females showed positive correlation with conductivity (r = 0.67; p < 0.05), salinity (r = 0.65; p < 0.05) and TSS (r = 0.59; p < 0.05). No significant correlation was found between the mean monthly GSI and the remaining two variables (temperature and dissolved oxygen). The analysis of the annual variation of GSI shows the existence of two spawning seasons in the coastal waters of Malacca: a longer one from February to July and shorter one within November and December.
The observed and predicted extreme lengths (Lmax) of male A. indicus were 28.00 mm and 30.16 mm with range 28.60–31.73 mm at 95% confidence interval in 2005–2006 and 29.00 mm and 31.66 mm with range 29.42–33.90 mm at 95% confidence interval in 2006–2007 (Table 1). The values of observed and predicted maximum length for female A. indicus were 40 mm and 46.37 mm with range 42.02–50.71 mm at 95% confident interval in 2005–2006 and 38 mm and 41.65 mm with range 38.45–44.86 at 95% confidence interval in 2006–2007 (Table 1). The asymptotic length, L∝ values estimated for male A. indicus were 29.40 mm in 2005–2006 and 30.45 mm in 2006–2007 and it was 42 mm in 2005–2006 and 39.90 mm in 2006–2007 for female (Table 1). The growth co-efficient, K values were 1.70 yr−1 in 2005–06 and 1.50 yr−1 in 2006–07 for male and 1.20 yr−1 in both 2005–06 and 2006–2007 for female. Optimized growth curves of female A. indicus for 2005–2006 and 2006–2007 were shown in Figure 4, superimposed over their restructured length-frequency data with calculated values of L∝ and K. The response surface, Rn values were calculated by ELEFAN-I as 0.205 and 0.211 for males in 2005–2006 and 2006–2007, respectively. The Rn values of females were 0.284 in 2005–2006, decreased to 0.208 in 2006–2007 (Table 1). The growth performance index, ϕ/ values of males were 3.22 in 2005–2006 and 3.20 in 2006–2007. The ϕ/ values of females were 3.44 in 2005–2006 and 3.45 in 2006–2007 (Table 1). Based on the mean growth parameters of the two years study, the maximum life span (tmax) of males was 1.88 years and that of females was 2.50 years (Table 1).
Mortality and exploitation
Estimated total mortality (Z) values of males were 4.15 yr−1 in 2005–2006 and 4.45 yr−1 in 2006–2007, that of females were 3.50 yr−1 in 2005–2006 and 3.74 yr−1 in 2006–2007 (Table 1). The natural mortality rate, M values of males were 2.65 yr−1 in 2005–2006 and 2.42 yr−1 in 2006–2007, and that of females were 1.91 yr−1 in 2005–2006 and 1.94 yr−1 in 2006–2007. Fishing mortality rates of males were 1.50 yr−1 in 2005–2006 and 2.03 yr−1 in 2006–2007, and those of females were 1.59 yr−1 in 2005–2006 and 1.80 yr−1 in 2006–2007 (Table 1). The average natural mortality (2.54 yr−1) was higher than the average value of fishing mortality (1.77 yr−1) for males, indicating their unbalanced position in the stock. Exploitation rates, E values of males were 0.36 in 2005–2006 and 0.46 in 2006–2007, and that of females were 0.45 in 2005–2006 and 0.48 in 2006–2007. The average E value of males and females was 0.41 and 0.47, respectively.
Annual recruitment pattern of males showed continuous recruitment with two major peaks in 2005–2006 (Figure 5a*). The first spell was from March to May with a peak in March (11.59%) and the second was from August to October with a peak in August (12.91%). In 2006–2007 the male recruitment pattern was also observed to be continuous with two spells (Figure 5b*). The first long and high spell was from January to May with the highest peak in April (20.50%). The second short and low spell was from September to October (Figure 5b*) with a low peak in September (6.78%). In the case of females, annual recruitment patterns showed two spells in both the years (Figure 5c–d*). In 2005–2006, the first spell of females was from January to March with a peak in January (5.12%). The second spell was from August to October (Figure 5c*) with a peak in September (21.32%). In 2006–2007 (Figure 5d*), the first short spell was in February to March with a peak in March (4.44%). The second long spell was during July to November with the highest peak in August (25.16%).
Relative yield-per-recruit (Y/R) and biomass-per-recruit (B/R)
The maximum allowable limit of exploitation level (Emax) that give the maximum relative yield-per-recruit at MSY level were estimated at 0.71 in 2005–2006 and 0.65 in 2006–2007 for males; and that of females were 0.57 in 2005–2006 (Figure 6a*) and 0.52 in 2006–2007 (Figure 6b*). The exploitation level (E-0.5) which corresponds to 50% of the relative biomass-per-recruit of the unexploited stock were 0.36 in 2005–2006 and 0.35 in 2006–2007 for males; and that of females were 0.32 in 2005–2006 and 0.31 in 2006–2007, respectively.
The mean E and corresponding Emax values of males were 0.41 and 0.68; and that of females were 0.47 and 0.55, respectively (Table 1). The estimated E values when compared with hypothetical ‘ideal E’ value of 0.50 for desirable fishing effort and Emax both indicated the stock of A. indicus was under-exploited in the investigated areas, although the exploitation level of female A. indicus was very close to the optimum. The values of total annual catches, standing stock size and MSY of A. indicus were 37.24 mt, 24.10 mt and 46.09 mt in 2005–2006 and 88.65 mt, 46.29 mt and 94.78 mt in 2006–2007, respectively.
The annual sex ratio in A. indicus population of the coastal waters of Malacca was found to be 1:2.12 (males:females). In general, the sex ratio is known to be close to 1:1 (males:females) in nature (Fisher, 1958), although the population of A. indicus in the coastal waters of Malacca was in favour of females in most months of the year. Similar results were observed in other Acetes species (Henry, 1977; Oh and Jeong, 2003). A. indicus showed continuous breeding throughout the year with a peak in June, August, October, February and April, as reflected by gonadosomatic index of females in the study area. This result generally is consistent with observations on the coastal waters of Bombay (Deshmukh, 2002) in which A. indicus breeds continuously throughout the year with the peak during September-January. In this study, no female with spent ovaries was found in natural conditions. Similar observations were made on A. chinensis in the coastal areas of Korea (Oh and Jeong, 2003) and A. japonicus in the Ariake Sea (Ikematsu, 1953).
Comparisons with population parameters obtained in other studies (Table 2) show that differences exist for different species of the genus Acetes from different areas in the world. The L∝ value for the combined sex of A. indicus reported by Zafar et al. (1997) from Bangladesh waters was much lower than the value of the present study but K value is more or less similar to the present value. The highest value of L∝ (41.48 mm) for female A. indicus is observed in the present study. The lowest value of K (0.69 yr−1) is observed in Korean waters (Oh and Jeong, 2003) for A. chinensis. The index of phi prime by Pauly and Munro (1984) is suitable for comparing and computing the overall growth performance of different species of shrimp stocks. The phi prime (φ/) for this species with the present estimates of L∝ and K were found to be excellent and varied between 2.83 and 3.44 (Table 2). Though phi prime is supported to be more or less constant for a family or for similar taxa, the present estimated φ/ value is very near to the previous studies of Acetes (Table 2) except the report of Oh and Jeong (2003).
The Z value obtained by Oh and Jeong (2003) for combined sex of A. chinensis was far less than the present values (Table 3). While Zafar et al. (1997) reported more or less similar Z (6.07 yr−1) value for A. indicus in the Bangladesh waters. Only the total mortality (Z) of A. indicus female (3.62 yr−1) in the coastal waters of Malacca is close to the value (3.93 yr−1) obtained by Oh and Jeong (2003) for A. chinensis in the western coast of Korea. Higher natural mortality of males A. indicus verses the fishing mortality observed from the present study (Table 1) indicates the unbalanced position in the stock of male population in A. indicus. Exploitation level (E) of females is higher than males in A. indicus population. It may be that for the larger size of females they cannot escape from the gear-like males. The estimated E value of females is very near to the optimum level of exploitation (E = 0.50), which indicates the fishery of female populations in the coastal waters of Malacca is under pressure. This is based on the assumption that a stock is optimally exploited when fishing mortality (F) equals natural mortality (M), or E = (F/Z) = 0.50 (Gulland, 1971).
This present study indicated two major recruitment events per year for A. indicus. Zafar et al. (1997) reported two recruitment peaks per year for A. indicus in the coastal waters of Bangladesh. This was in good agreement with both Oh and Jeong (2003) and Zafar et al. (1997). There is no published report on recruitment of A. indicus 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, 1974). In the present study, it was observed that the major spawning of A. indicus occured in the months of February–June in the coastal waters of Malacca. The highest recruitment peak in August–November detected in this study should correspond to the major spawning season. Estimated annual catches of A. indicus were 37.24 tons in 2005–2006 and 88.65 tons in 2006–2007, while MSY were 46.09 in 2005–2006 and 94.78 tonnes in 2006–2007, respectively. It is revealed that the annual catches were slightly lower than the estimated their MSY in both the years for A. indicus. This implies that any further unrestrained increase in fishing effort might overshoot the level giving maximum sustainable yield, thus driving the stock down and leading to economic losses.
The analysis of the annual variation of GSI showed the existence of two spawning seasons in the coastal waters of Malacca: a longer one from February to July and a shorter one within November and December. Overall yearly sex ratio was found to be 1:2.12 (males:females). This study indicated two major recruitment events per year; i.e. two cohorts were produced per year. Higher natural mortality (2.54 yr−1) verses the fishing mortality (1.93 yr−1) observed for A. indicus in the present study indicated their unbalanced position in the stock. Exploitation levels (E) were computed as 0.41 (male) and 0.47 (female) indicating the fishery of A. indicus in the coastal waters of Malacca is very near to optimum exploitation.
This work is part of a PhD thesis funded by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia (Grant No. 5450247). The authors would like to thank MOSTI for providing financial support to carry out this research work. Special thanks to University of Chittagong, Bangladesh for providing partial financial support through a Fellowship during the study period.
*Figures 5 and 6 for this article are available online athttp://www.aehms.org/Journal_15_3_Amin_Appendix.html