Growth patterns and mortality rates of the short-necked clam Paratapes undulatus (Born, 1778) in Ba Lua Archipelago, An Giang Province, Southwestern Sea of Viet Nam

ABSTRACT

This study investigated the growth patterns, mortality rates, and exploitation rate of the short-necked clam Paratapes undulatus (Born, 1778) in Ba Lua Archipelago, An Giang Province, Southwestern Sea of Viet Nam, using biological data. It was conducted from November 2022 to September 2023. A total of 1,140 individuals were randomly sampled from clam dredge landings for biological analysis. The length-weight relationship showed a negative allometric growth (b < 3), indicating that shell length increased proportionally faster than the body weight. The estimated von Bertalanffy Growth parameters were L∞ = 53.6 mm, K = 0.91 year^-1, t₀ = -0.149, and tmax = 4.2 year and the growth performance index Φ′ = 3.417. The estimated mortality rates were Z = 3.80 year^-1, M = 1.47 year^-1, F = 2.34 year^-1. The exploitation rate (E = 0.61) indicates that the stock is currently fully exploited. These findings emphasize the need for management measures to ensure the sustainable exploitation of this commercially valuable species.

Keywords:
Length-weight relationships; VBGF; Mortality rates; Exploitation rate; Short-necked clam

The short-necked clam Paratapes undulatus (Born, 1778), originally described as Paphia undulata (locally known as nghêu lụa or sò lụa), is a bivalve species of the family Veneridae. It is widely distributed across the tropical Indo-West Pacific region (Carpenter and Niem, 1998), typically inhabiting shallow sandy substrates in intertidal and shallow subtidal zones (Huber, 2015; Minh-Thu et al., 2023) to the depths of about 30m (Carpenter and Niem, 1998). In Viet Nam, they appear along the entire coastal zone (Minh-Thu et al., 2023; Tu et al., 2019; Tuyen et al., 2006), with its highest abundance in An Giang province.

The Ba Lua Archipelago (104°31′17.6″E; 10°09′04.8″N), in Kien Luong Ward, An Giang Province (Figure 1), to the Northwest of the Ecological Production Unit VI of Viet Nam (Bell et al., 2021), is one of the major fishing grounds for clam fishery. The archipelago consists of 45 small islands forming a closely distributed cluster along their coastal waters. This site is affected by a tropical monsoon with two seasons: a wet (April-October) and a dry one (November-March), with strong seasonal variability (Dang et al., 2020). The wet season is affected by the high rainfall: 1,860mm on average (ranging from 1,439 mm to 2,247 mm). In the dry season, although the rainfall is lower (156 mm on average), the coastal waters are heavily influenced by Mekong River discharge, which drives seasonal salinity fluctuations (Minh et al., 2022; Voss et al., 2014).

Figure 1
Location of the sampling site (diamond symbol) in which short-necked clam samples were collected in this study. The map was generated using Google Maps.

The short-necked clam resources play an important role in supporting small-scale fisheries and livelihoods, substantially contributing to local household income and food supply in coastal communities in An Giang. Fishermen use the small fishing boats equipped with dredges (locally known as “cào nghêu” or “cào lụa”) to harvest clams at a depth of 6-12m (per.com.). The dredge is normally made of steel and has a frame dimension of approximately 3.20 m × 0.13 m × 0.40 m (width × height × depth), with a bar spacing of 1.5 cm.

In recent years, clam dredge fishery has intensified due to increasing market demand. The dredge fishing fleet consist of about 300-400 fishing vessels catching 30-50 thousand tons annually (from interviews with fishermen), which has raised concerns about overfishing and resource depletion.

Despite the economic and commercial importance of this fishery in An Giang, studies on this species under natural conditions are limited, and the information required for fisheries management are either scarce or absent. However, several studies on biology and population parameters have been carried out for Paphia species in other tropical regions (Argente and Estacion, 2014; Mohite and Mohite, 2009; Nagvenkar et al., 2014; Thomas and Nasser, 2009; Vakily, 1992). However, the application of parameters derived from distant populations may lead to biased assessments and inappropriate management strategies as life-history traits are species- and region-specific (King, 2007) and may vary with fishing intensity. Therefore, region-specific study is required to provide baseline biological information to support sustainable exploitation and effective management of the short-necked clam in Viet Nam.

Biological data of the short-necked clam were collected monthly in the Binh An landing site (Figure 1), in which the fishing boats unloaded the harvested catches from Ba Lua Archipelago fishing grounds. Each month, at least 90 individuals were randomly sampled from the landings upon the return of the fishing boats to the port. Shell length (SL) was measured using a vernier caliper (±0.1 mm), and body weight (BW) was recorded with an electronic balance (±0.01 g).

The length-weight relationship (LWR) of short-necked clam was calculated using the equation $BW=a*S{L}^b$ by logarithmically transforming it into $\log \left(BW\right)=\log \left(a\right)+b*\log \left(SL\right)$, in which BW is the total body weight (g), SL is the shell length (mm), a is the intercept of the regression curve, and b is the regression coefficient. The growth of the clam is considered negative allometric if b < 3, positive allometric if b > 3, and isometric when b = 3 (Bayer, 1987). The relative condition factor (Kn) was used to assess the growth. It was calculated by $Kn=BW/\left(a\times S{L}^b\right)$ (Le Cren, 1951) . A higher Kn value indicates a faster increase in weight relative to the average weight of the species. Student’s t-tests were performed to determine whether the monthly, seasonal, and annual b values obtained from the LWRs were significantly different from the cubic value (b = 3). Welch’s t-test was used to compare the size of the clam and Kn values between the dry and wet seasons. All data were analyzed and plotted on R Studio (R Core Team, 2021), following the guidelines of Ogle (2016). Plots of Welch’s t-test were conducted using the ggstatsplot package (Patil, 2021).

The growth patterns of short-necked clam were assumed to be described by the von Bertalanffy Growth Function (VBGF) and be estimated using the ELEFAN I routine (Pauly and Morgan, 1987) on FAO-ICLARM Stock Assessment Tools (FiSAT II) (Gayanilo Jr and Pauly, 1997). Growth was modeled by fitting $S{L}_t=S{L}_{\infty }\left[1-e^{-K\left(t-t_o\right)}\right]$, in which SL∞ is the asymptotic SL, K is the growth curvature parameter (year^-1), SLt is the shell length at age t, and t0 is the theoretical size when SLt is equal to zero, estimated by the formula: $\log \left(-t_o\right)=-0.3922-0.2752\log \left(S{L}_{\infty }\right)-1.038\log \left(K\right)$. The growth performance index (Φ’) was estimated by the equation $\Phi \text{'}=\log K+2\log L_{\infty }$ (Pauly and Munro, 1984) and longevity (t_max), by the equation in Michaelson and Neves (1995) : $t_{max}=\left(\ln S{L}_{\infty }+K{t}_o\right)/K$.

The length-converted catch curve method on FiSAT II (Gayanilo Jr and Pauly, 1997; Gayanilo and Sparre, 2005) was used to estimate the total instantaneous mortality (Z) using the length data collected monthly from 2022 November to 2023 September, VBGF parameters, and a mean annual bottom temperature of 29.6 °C. Fishing mortality (F) was derived from the difference between Z and M$\left(F=Z-M\right)$. Mortality estimates (F and Z) were used to determine the exploitation rate $E=F/Z$.

A total of 1,140 specimens were collected from harvested catches in the Binh An landing site (Table 1). The SL ranged totaled 30 - 52 mm (mean ± SE: 41.2 ± 0.10 mm) and the BW, 2.9 - 14.4 g (mean ± SE: 6.9 ± 0.05 g). The mean SLs fluctuated by 38.8 ± 0.25 mm (5.6 ± 0.11 g) in August 2023 and by 45.5 ± 0.25 mm (8.7 ± 0.15 g) in April 2023. The length frequency distributions of the short-necked clam showed temporal variation from November 2022 to September 2023 (Figure S1, supplementary data). Most monthly distributions were unimodal, indicating relative synchronous size cohorts during the November 2022 - May 2023. Large size clams were dominant in February and April 2023, coinciding with the highest recorded mean SLs. In contrast, the SL distributions were left -skewed, with a higher proportion of smaller clams. One-way ANOVA found significant differences in SL (F = 84.88, p < 0.001) and BW (F = 71.12, p < 0.001) across months, reflecting strong temporal variation in clam size. Welch’s t-test indicated significant differences in SL (p = 9.16×10⁻¹⁵) and BW (p = 9.16×10⁻¹⁹) between the dry and wet seasons (Figure S2). The mean SL and BW in dry season were smaller (40.4 ± 0.15 mm and 6.5 ± 0.07 g) than those in wet season (42.00 ± 0.14 mm and 7.4 ± 0.07 g).

Table 1
Number of specimens and descriptive statistics of shell length (SL) and body weight (BW) of the short-necked clam (Paratapes undulatus) collected from clam dredge fishing catches in An Giang Province, Viet Nam, from November 2022 to September 2023. N = sample size; SE = standard error.

Year/Month; Season	SL (mm)			BW (g)			N
	Range	Mean	SE	Range	Mean	SE
2022 Nov.	36 - 49	41.0	0.25	4.1 - 10.8	6.7	0.13	149
2022 Dec.	30 - 42	36.7	0.22	2.9 - 7.5	5.2	0.09	120
2023 Jan.	32 - 49	40.6	0.35	3.4 - 13.1	7.3	0.18	90
2023 Feb.	37 - 48	43.2	0.29	4.5 - 11.5	7.8	0.16	91
2023 Mar.	35 - 47	41.3	0.22	3.3 - 9.6	6.0	0.10	120
2023 Apr.	39 - 52	45.5	0.25	5.1 - 14.4	8.7	0.15	90
2023 May	36 - 47	41.2	0.24	5.7 - 12.2	7.9	0.12	90
2023 Jun.	36 - 50	40.9	0.23	5.0 - 13.9	7.2	0.14	90
2023 Jul.	36 - 49	41.3	0.30	4.1 - 9.8	6.6	0.13	90
2023 Aug.	33 - 44	38.8	0.25	3.4 - 8.6	5.6	0.11	90
2023 Sep.	37 - 50	43.8	0.23	5.4 - 11.9	8.1	0.12	120
All Data	30 - 52	41.2	0.10	2.9 - 14.4	6.9	0.05	1,140
Dry season (November-March)	30 - 49	40.4	0.15	2.9 - 13.1	6.5	0.07	570
Wet season (April-October)	33 - 52	42.0	0.14	3.4 - 14.4	7.4	0.07	570

Monthly LWR parameters of the short-necked clam are shown in Table 2. The coefficients of determination (r ² ) were in acceptable range, indicating a good fit of LWRs. Negative allometric growth (b < 3, Student’s t-test p < 0.001) was observed, with b values ranging by 2.223 ± 0.135 in May 2023 to 2.913 ± 0.083 in December 2022 and with a pooled estimate of b = 2.616 ± 0.037 (N=1,140; r ² = 0.81). Seasonally, the b values totaled 2.560 ± 0.052 (N = 570; r ² = 0.81) and 2.595 ± 0.056 (N = 570; r ² = 0.79) for the dry and wet seasons, respectively (Figure S3, supplementary data).

Table 2
Parameters of the length-weight relationship (LWR) equation and relative condition factor (Kn) of short-necked clams (Paratapes undulatus) in An Giang waters, Southwestern Sea of Viet Nam. N = sample size; a = intercept; b = slope; r² = coefficient of determination; SE(b) = standard error of b; CI(b) = 95% confidence interval of b; Kn = relative condition factor; SE(Kn) = standard error of Kn.

Year/Month; Season	N	a	b	r2	CI (b)	SE (b)	Mean Kn	SE (Kn)
2022 Nov.	149	0.00013	2.913	0.89	2.75 - 3.08	0.083	1.0026	0.006
2022 Dec.	120	0.00029	2.714	0.85	2.5 - 2.93	0.107	1.0030	0.007
2023 Jan.	90	0.00030	2.722	0.89	2.52 - 2.92	0.101	1.0030	0.008
2023 Feb.	91	0.00017	2.846	0.89	2.64 - 3.05	0.104	1.0020	0.007
2023 Mar.	120	0.00016	2.824	0.78	2.55 - 3.09	0.136	1.0036	0.008
2023 Apr.	90	0.00044	2.590	0.74	2.27 - 2.91	0.162	1.0035	0.009
2023 May	90	0.00201	2.223	0.75	1.95 - 2.49	0.135	1.0024	0.007
2023 Jun.	90	0.00022	2.801	0.79	2.49 - 3.11	0.156	1.0029	0.008
2023 Jul.	90	0.00062	2.490	0.85	2.27 - 2.71	0.112	1.0026	0.008
2023 Aug.	90	0.00014	2.900	0.86	2.66 - 3.15	0.124	1.0024	0.007
2023 Sep.	120	0.00047	2.579	0.81	2.35 - 2.81	0.117	1.0026	0.007
All Data	1,140	0.00040	2.616	0.81	2.54 - 2.69	0.037	1.0028	0.002
Dry season (November-March)	570	0.00049	2.560	0.81	2.45 - 2.66	0.052	1.0028	0.003
Wet season (April-October)	570	0.00045	2.595	0.79	2.49 - 2.70	0.056	1.0027	0.003

The growth of the short-necked clam in the Ba Lua Archipelago showed negative allometry as b values were consistently < 3, indicating that shell length increased faster than body weight. This finding differs from results for other Vietnamese populations of short-necked clam. For example, Tuyen et al. (2006) observed positive allometric growth (b > 3) in specimens from Binh Thuan Province in central Viet Nam. This difference suggests that the growth patterns of P. undulatus may vary geographically and are likely influenced by local environmental conditions as well as fishing pressure.

Those authors (Tuyen et al., 2006) also recorded Fulton’s condition factor values ranging from 0.89 to 1.13, suggesting that the physiological condition of individuals varied temporally, and was occasionally affected by less favorable environmental conditions. In this study, although the Ba Lua Archipelago is influenced by two monsoon seasons, the growth pattern of short-necked clams seem to show no influence from them as Kn values remained highly stable (≈ 1.0) across months and seasons, indicating that the short-necked clam population maintained consistently good physiological condition throughout the study period. The consistently stable Kn values in this study indicate that the population remained in good physiological condition, unlike several previous studies reporting seasonal fluctuations in condition in other coastal regions. These results suggest that the environmental conditions in the Ba Lua Archipelago are favorable for sustaining this species.

The negative allometric growth pattern of the short-necked clams in Ba Lua Archipelago differed from that of other clam species in the genus Paphia. Farghaly et al. (2022) studied the growth and population aspects of carpet clams (P. textile) in Timsah Lake and Great Bitter Lake along the Suez Canal (Egypt) and concluded that the carpet clam is a positive allometry species with b = 3.284 and 3.250 for Timsah Lake and Great Bitter Lake, respectively.

Negative allometry has also been observed in the genus Paphia. Nagvenkar et al. (2014) reported b values of 2.44 for the length-wet weight relationship and 3.06 for the length-total weight relationship in P. malabarica from the estuarine region of Goa on the west coast of India. Similar variability in growth patterns and exploitation parameters has been widely reported for other species within the family Veneridae across tropical regions, reflecting strong influences of local environmental conditions and fishing intensity (Mohite and Mohite, 2009; Vakily, 1992).

Kn values were relatively stable (Table 2), with mean values of 1.0028 ± 0.0022 for the pooled dataset, 1.0028 ± 0.0032 for the dry season, and 1.0027 ± 0.0031 for the wet season. One-way ANOVA indicated no significant differences in Kn between months (F = 0.004, p = 1.000) and seasons (F = 0.001, p = 0.97), evincing that the relative condition of clams was consistently maintained throughout the study period.

The von Bertalanffy growth parameters estimated for short-necked clams were as follows: asymptotic shell length (SL∞) = 53.6 mm, growth coefficient (K) = 0.91 year^-1, the theoretical age at zero length (t₀) = -0.149 year^-1, and longevity (t_max) = 4.2 year. The growth performance index (Φ′) was estimated at 3.417, indicating a relatively rapid growth. Restructured length frequency, the growth curves derived using the VBGF, and the length converted catch curve for the short-necked clams from the Ba Lua Archipelago, An Giang Province, Southwestern Sea of Viet Nam, are shown in Figure S4 (supplementary data).

The estimated population parameters of the short-necked clam in this study were generally lower than those reported by Del Norte-Campos and Villarta (2010) for the same species in the Philippines (L∞ = 79.0 mm; K = 1.00 year^-1; Φ′ = 3.80). Similar trends were also observed when comparing the present estimates with those reported for P. textile and P. malabarica in several localities, in which higher asymptotic lengths and growth performance indices were recorded (Argente and Estacion, 2014; Mohite and Mohite, 2009; Thomas and Nasser, 2009). These differences may be explained by the fact that growth patterns are species-specific when comparing different species within a genus. In contrast, the variability within species across study areas may be related to local environmental conditions (e.g., food availability, productivity, water temperature) and differing levels of fishing pressure.

The total mortality rate (Z) of the short-necked clam was estimated to be 3.80 year^-1 based on length-converted catch curve analysis, with a 95% confidence interval from 3.36 to 4.23 year^-1. The natural mortality rate (M) was estimated at 29.6^oC is 1.47 year^-1, whereas the fishing mortality rate (F) was estimated at 2.34 year^-1. Thus, the exploitation rate (E) was determined to be 0.61, substantially exceeding the threshold commonly associated with sustainable exploitation (E ≤ 0.5). This high exploitation level suggests that the clam population is fully exploited and subjected to intense fishing pressure, which may lead to recruitment overfishing if inadequately managed.

The exploitation rate of the short-necked clam in Ba Lua Archipelago was relatively lower than that reported for the same species in southern Negros Occidental, Philippines (E=0.75) (Del Norte-Campos and Villarta, 2010) but slightly higher than that estimated for P. malabarica in Dharmadom, India (Thomas and Nasser, 2009). The results in this study indicate that the population of short-necked clams in Ba Lua Archipelago is fully exploited (E > 0.5, F > M) and currently under considerable fishing pressure with a high risk of depletion. Nevertheless, an M/K value of approximately 1.62 suggests that this species shows a fast-growing, short-lived life-history strategy and thus a relatively high potential for population recovery if effective management measures are put in place. Comparisons with studies in the Philippines (Del Norte-Campos and Villarta, 2010) and India (Thomas and Nasser, 2009) indicate that the growth and exploitation parameters of P. undulatus in the Ba Lua Archipelago reflect relatively high fishing pressure, emphasizing the need for population-based management measures to ensure sustainable exploitation of this local resource.

In short, the short-necked clam P. undulatus in the Ba Lua Archipelago (An Giang Province) showed a negative allometric growth pattern. Although seasonal variations in size were detected, no significant differences in growth dynamics were observed between months or seasons. This species is characterized by rapid growth and a short life span. Nevertheless, its population is currently under intensive fishing pressure, with exploitation levels exceeding the sustainable limit. These findings highlight the urgent need for effective management interventions to secure the long-term viability of this valuable fishery resource.

DATA AVAILABILITY STATEMENT

All data are available from the corresponding author upon reasonable request.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available via hyperlinks to the Zenodo service.

Supplementary Material 1

Supplementary Material 1

Supplementary Material 2

Supplementary Material 2

Supplementary Material 3

Supplementary Material 3

Supplementary Material 4

Supplementary Material 4

ACKNOWLEDGMENTS

We are grateful to the local fishers for their assistance in specimen collection. We also thank two anonymous reviewers for their valuable comments on the manuscript.

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• AI USE STATEMENT
  Artificial intelligence tools (ChatGPT, OpenAI) were used exclusively to refine the English language of this manuscript. The content was carefully reviewed by the authors to ensure consistency and correctness, and the authors take full responsibility for the final version of the manuscript.
• FUNDING
  This study received financial support from the People’s Committee of An Giang Province under contract no. 06/2022/HD-CCTSKG-VHS.

Edited by

• Associate Editor:
  Maikon Di Domenico

Publication Dates

• Publication in this collection
  22 June 2026
• Date of issue
  2026

History

• Received
  06 Sept 2025
• Accepted
  02 Mar 2026