Population biology of Ellochelon vaigiensis (Quoy & Gaimard, 1825) in the Mekong Delta, Vietnam

Introduction

The Squaretail mullet Ellochelon vaigiensis (Quoy & Gaimard, 1825) is the monotypic species of the genus Ellochelon belonging to the family Mugilidae. This species is found from India to the Pacific Ocean (Crosetti & Blaber, 2015; Froese & Pauly, 2022). This fish is described with standard features such as 16 pectoral-fin rays; 25–29 longitudinal scales; 16 conjunctival scales; four hard dorsal fin rays, 9–10 soft dorsal-fin rays, three hard anal rays, 7–9 soft anal rays, pectoral fin black, caudal fin quite flat (Harrison & Senou, 1997; Tran et al., 2013). Most mullets are classified as saltwater fish because they often gather in schools and feed along coastal regions; however, E. vaigiensis can be found in brackish and freshwater areas about 10 km from the sea (Froese, 2006). This fish is distributed mainly in the surface layer with depths ranging from 0 to 5 m (Bacchet, Zysman & Lefèvre, 2006). Ellochelon vaigiensis belongs to the multiple spawning fish, and the fry, after hatching, can be found in mangroves (Breder & Rosen, 1966). Although E. vaigiensis plays a vital role in the food supply, especially in the estuarine and coastal region of the Mekong Delta (MD), its knowledge is limited to morphology, distribution (Penrith & Penrith, 1967; Kwun et al., 2013; Teimori & Hesni, 2020; Nguyen, Nguyen & Dinh, in press; Nguyen, Nguyen & Dinh, 2022), growth pattern and feeding habit (Dinh et al., 2022a; Dinh et al., 2022b). Besides, in the MD, especially downstream of Hau River—one of the tributes of the Mekong River, the survival and development of E. vaigiensis population face several reasons: exploitation pressure for food demand, pollution, and climate change (Tan & Thanh, 2013). Our preliminary observation shows that the number of fish individuals per catch tends to be decreasing trend, yet there have been no studies investigating this issue in the study area in order to verify whether this fish is being over-exploited or not. Therefore, this study was carried out to have a more general view of the impact of fishing on the population of this fish.

According to Al-Husaini et al. (2002), biological parameters of the fish population are helpful for fishery assessment. In addition, the population biological parameters help to understand the growth and mortality of the fish population (Amezcua, Soto-Avila & Green-Ruiz, 2006). Length-frequency data are input data for population biological parameters analysis (Tran, 2010). The basic principle of the length-frequency method is the frequency distribution of fish lengths in a given age group. The length frequency describes the growth and abundance of the population at different times (Pham & Tran, 2004). The fish growth and asymptotic length relationship regulate fish growth between locations (Pauly & Munro, 1984). The first capture length helps determine the maximum size the fish can reach (Pauly, 1987). Longevity provides information about the maximum lifespan of fish (Taylor, 1958). The growth rate and mortality show plasticity, changing according to location and species, the first capture length is common in fish and this is due to the trade-off between reproductive investment, growth, and mortality (Pauly, 1984).

Several studies on mullet populations have been carried out worldwide, e.g., Mehanna (2004) details some parameters in Liza carinata and Liza aurata populations, such as the initial asymptotic length, growth parameter, longitude, and growth performance. These parameters have also been documented for Liza saliens in Beymelek Lagoon (Balik et al., 2011), Rhinomugil corsula in Bangladesh (Ara et al., 2019), and Mugil cephalus in India (Waters, 2014). However, population biological traits of E. vaigiensis, a target catching fish in the MD, has not been known. Moreover, the salinity gradually increased from the northern and southern Hau River mouth (Dinh et al., 2021), resulting in the variation of some biological parameters of two Glossogobius sparsipapillus populations between two regions (Nguyen et al., 2021). Therefore, this study aims to provide new data on the population parameters of E. vaigiensis, e.g., first capture length, longevity, growth, exploitation rates, and mortalities. Moreover, the present study also presented the variations of these parameters between the northern and southern parts of the Hau River mouth. The findings will help to sustain this fish in MD.

Materials & Methods

Study sites and fish collection

This study lasted in two ecoregions: northern and southern Hau River mouth. Fish specimens were caught at two sites per ecoregion, including Ben Tre and Tra Vinh (BTTV) in the northern part and Soc Trang and Bac Lieu (STBL) in the southern part (Fig. 1). These two ecoregions were different salinity values, ranging from 12.9–13.7‰at BTTV to 16.9–29.6‰at STBL. At each site, fish specimens were caught monthly by trawl nets (mesh size of the codend: 2a = 1.5 cm) from November 2020 to March 2022. After collection, fish specimens were distinguished from other species by morphological features (Tran et al., 2013) before being anaesthetized with MS222 and transported to the laboratory. Fish specimens were then measured in the total length (L). Total length was determined from the mouth to the end of the fish’s tail. The use of fish in the present study was assessed and approved by the Scientific Committee of the School of Education, Can Tho University, under the Animal Welfare Assessment number BQ2020-05/KSP.

Results

A total of 1,383 individuals (608 females and 775 males) were collected at BTTV and STBL. In both of these sampling sites, there were more males than females. At BTTV, the number of males and females was 402 and 309, respectively. Similarly, at STBL, this number was 299 for females and 373 for males.

The initial L_∞ of BTTV and STBL populations obtained from Powell-Wetherall plot was presented in Fig. 2. These graphs gave L_∞ of the two populations (33.6 at BTTV and 31.5 at STBL) and Z/K (1.46 at BTTV and 2.12 at STBL). The length-frequency analysis (raw data can find: Raw data_Ellochelon vaigiensis) showed that the initial K was 0.46 at BTTV and 0.56 at STBL. The total length of E. vaigiensis at BTTV ranged from 6.3 to 32.6 cm L. The ELEFAN I analysis results showed that the fish population at BTTV had five distinct growth curves (Fig. 3A). Meanwhile, at STBL, the fish population had a total length ranging from 6.2 to 30.2 cm L and displayed five growth curves (Fig. 3B). The parameters of the growth curves were L_∞ = 33.6 cm, K = 0.46/yr, t₀ = −0.34 at BTTV and L_∞ = 31.5 cm, K = 0.56/yr, t₀ = −0.29 at STBL, respectively. The BTTV population exhibited the von Bertalanffy growth curve as L = 33.6(1 − e^{−0.46(t+0.34)}), and this growth curve of the STBL population was L = 31.5(1 − e^{−0.56(t+0.29)}).

The Z, M, F, and E determined from fish length-frequency data were 1.41/yr, 1.06/yr, 0.35/yr, and 0.25 at BTTV (Fig. 4A), and 1.78/yr, 1.24/yr, 0.55/yr and 0.31 at STBL (Fig. 4B), respectively. The study results also showed that the first length at first capture STBL (6.12 cm) was shorter than the BTTV (8.63 cm).

The Y′/B analysis result showed that the E_max, E_0.1, and E_0.5 of the BTTV population were 0.436, 0.358, and 0.265, respectively (Fig. 5A). Meanwhile, the values of these parameters in the STBL population were 0.513, 0.418 and 0.293, respectively (Fig. 5B). The t_max of this fish was 6.52 yrs at BTTV and 5.36 yrs at STBL. Its growth performance index was 2.72 at BTTV and 2.74 at STBL (Table 1). The isopleth ratio (L_c/L_∞) of E. vaigiensis was 0.18 at BTTV (Fig. 6A) and 0.27 at STBL (Fig. 6B).

Discussion

The maximum length of E. vaigiensis tended to vary with the salinity changes, as it reached 31.5 cm at STBL with a salinity of 12.9 to 13.7‰and 33.6 cm at BTTV with a salinity of 16.9–29.6‰in the present study. Indeed, according to Nikolsky (1963), differences in environmental conditions such as water quality, food sources, and extraction processes can affect the maximum length of fish. In the previous study, this species reached the maximum length of 63 cm in the Red Sea (Lieske & Myers, 1994), where salinity ranged from 36 to 40‰(Sofianos & Johns, 2003), suggesting that this fish could adapt well to higher salinity regions. This change is not only recorded in mullet but also in some other fish species. For example, the maximum length of G. sparsipapillus was inversely proportional to salinity as L_∞ in G. sparsipapillus is lower than in high salinity region than in low salinity one (Nguyen et al., 2021), seeming that the L_∞ tended to change differently for these species when changing salinity. On the other hand, in Periophthalmodon septemradiatus, L_∞ remained unchanged between freshwater and brackish water regions (Tran & Dinh, 2020). The maximum size of E. vaigiensis was relatively higher than some other mullets. For example, according to Mohamed & Abood (2020), in Iraq, the size of Planiliza abu (6.4 to 19.7 cm), P. klunzengeri (6.0–19.0 cm), P. subviridis (9.8–26.5 cm) was smaller in E. vaigiensis. Meanwhile, some other species of mullet, which were widely distributed in Australia but had a longer L_∞ than E. vaigiensis, such as Liza argentea (5.9–34.6 cm) and Myxus elongatus (5.7–39.3 cm) (Kendall, Gray & Bucher, 2009). Variations in K at each location affect L_∞ and longevity, as there was an inverse relationship between L_∞ and K. At BTTV, the L_∞ was higher, but the K was lower; thus, the lower growth rate leads to a higher L_∞, with the same number of curves, but with a higher t_max than that observed for STBL. The values between the locations were close, but their variation consequently generated variations with related parameters, such as higher K, lower L_∞, and lower longevity. The plasticity of K between locations for the same species was evident. These results showed that salinity may regulate the change of K, L_∞ , and t_max of E. vaigiensis and other mullets. Besides salinity, other biotic and abiotic factors could also influence this change, but these factors in the two ecoregions have not been studied yet, suggesting that there is a need to clarify if other environmental factors influence the variation of these population paremeters of E. vaigiensis and other mullets.

The Φ′ at BTTV (2.72) was lower than in the STBL(2.74), showing that the southern region of Hau River mouth was suitable for developing this species, or this fish tended to grow well in the higher salinity areas. However, the difference between these two regions was not too great. The Φ′ of this species was higher than that of some other mullets. Indeed, Φ′ of males, females, and whole populations of L. abu in the Euphrates River, AlNajaf, Iraq, were 2.27, 2.29, and 2.23, respectively (Tulkani, 2017), showing that L. abu displayed a lower growth rate than E. vaigiensis. The other three fish species, including P. abu, P. klunzengeri, P. subviridis in Al-Arab River, Iraq, exhibited different growth rates of 2.30, 2.62, and 2.54, respectively (Mohamed & Abood, 2020), and these values were generally smaller than E. vaigiensis (Table 1). At the same time, Chelon labrosus in North West Wales (Tulkani, 2017) and M. elongatus in Australia (Kendall, Gray & Bucher, 2009) displayed a relatively high growth rate with values of 2.75 and 2.78, respectively (Table 1). These results showed that although the same group of mullets, each species exhibited a different growth rate. This difference may be due to the fact that this was characteristic of each species. However, from the above examples, it can be seen that marine mullet species such as E. vaigiensis in MD, C. labrosus in North West Wales, and M. elongatus in Australia have relatively larger growth rates compared to other species. species distributed in river basins in Iraq. Thereby, it can be seen that the growth rate of fish depends not only on the characteristics of the species but also on the ecological conditions, habitat, food source, metabolic activity, reproductive activity, size of fish, sampling method, and fishing pressure in each area (Nikolsky, 1963; Panda et al., 2018).

The longevity (t_max) of E. vaigiensis varied with ecoregion, as this value at STBL was lower than at BTTV (Table 1). The possible reason is that variation in salinity between these regions regulated t_max. The overexploitation at STBL could also lead to a lower t_max in the STBL population than in the BTTV population. Besides, the fish population at STBL grew faster than BTTV, leading to faster fish maturation and the end of the life cycle. The spatial variations in t_max and Φ′ were also found in L. argentea and M. elongatus distributed in Australian estuaries (Kendall, Gray & Bucher, 2009). Compared to other mullets, E. vaigiensis exhibited shorter longevity. Kendall, Gray & Bucher (2009) reported that L. argentea and M. elongatus showed a longer t_max, reaching 9.09 yrs and 6.25 yrs, respectively. According to these authors, these two fish displayed a long lifespan due to being less affected by human exploitation. The t_max of three other mullets, e.g., P. abu, P. klunzengeri, and P. subviridis in Al-Arab River, Iraq, was 6.82, 6.12, and 7.50 yrs, respectively (Mohamed & Abood, 2020). Meanwhile, the t_max of L. abu in Central Iraq and C. labrosus in Northwest Wales reached 12.5 and 35.5 yrs, respectively (Tulkani, 2017) (Table 1). From these results, it is shown that the life expectancy of fish is not only affected by environmental conditions but also by human activities.

Total mortality (M) and natural mortality (Z) of the STBL population were higher than those in the BTTV population, suggesting that the environmental condition at STBL was less favorable than at BTTV. Fishing gears were increasingly developed in extermination (use electricity, small mesh), leading to higher mortality in this species. So, it is necessary to re-regulate the appropriate fishing gear when exploiting this species to protect them from being overexploited. Compared with two Australian mullets, e.g., L. argentea and M. elongatus (Kendall, Gray & Bucher, 2009), M and Z of E. vaigiensis were higher. However, in three fish species in Iraq (Tulkani, 2017), M was higher than those of the BTTV population but lower than those of the STBL. Meanwhile, the Z value of these three fish species was almost higher than that of E. vaigiensis (Table 1). These three fish in Irad seemed more vulnerable to exploitation and responsible for higher mortality. It could be because fishes in Iraq were caught in the riverine region (freshwater), whereas E. vaigiensis in the present study was caught in the estuarine one (brackish water). Besides, environmental conditions in MD were relatively stable between seasons compared to Australia and Iraq, leading to a lower natural mortality rate of this fish than some other fish species.

Both populations in BTTV and STBL have not been overexploited as E was <E_0.1. It showed that although this fish had high economic value and was intensely exploited, the fish population still met the fishing demand. Thus, the current exploitation intensity and mesh size were suitable for developing E. vaigiensis. Depending on the method used to estimate M, the estimate of E may be slightly smaller or larger (Pauly, 1980). But the frequency of this fish in two ecoregions showed the occurrence of many groups of fish of mature size. However, measures should be taken, such as limiting the fishing of these fish during the spawning period, to manage this resource sustainably in the MD. This activity could avoid overexploited as in some fish species in the MD, e.g., G. giuris (Dinh, Phan & Tran, 2017), G. aureus (Dinh, Tran & Tran, 2021), and P. schlosseri (Tran & Dinh, 2021).

The L_c/L_∞ of E. vaigiensis (0.18 at BTTV and 0.27 at STBL) were lower than that of other mullets, e.g., P. abu (0.44), P. subviridis (0.44) and P. klunzengeri (0.52) in Iraq (Tulkani, 2017) (Table 1). This suggested that in MD, E. vaigiensis was caught at an earlier stage than other mullets in Iraq. Compared to some fish species belonging to other families living in MD, E. vaigiensis was found to be caught in an earlier stage as its L_c/L_∞ was lower than that of them, e.g., Parapocryptes serperaster (0.57) (Dinh, Qin & Tran, 2015), Boleophthalmus boddarti (0.77) (Dinh, 2017), and Trypauchen vagina (0.57) (Dinh, 2018) (Table 1). This could be caused by the higher economic values of E. vaigiensis than these fish species in MD. On the other hand, the distribution area of this fish was mainly the estuary, and along the coast, so the fish is easily exploited (McDowall, 1997). Meanwhile, Parapocryptes serperaster or Boleophthalmus boddarti were mainly distributed in the alluvial zone, so it was more difficult to exploit than the species in this study. To conserve and sustain the fish resources, the local government should ask fishers to increase the size at first capture >L_m (19.61 cm at BTTV and 18.50 cm at STBL), which was the length in which 50% of fish reached maturity firstly and obtained from the equation Log₁₀ L_m = 0.898 × log₁₀(L_∞) − 0.0782 (Froese & Binohlan, 2000).

The results of this study show that the biological parameters of fish populations may be regulated by environmental conditions in two ecoregions, particularly salinity. The fish size limitation of 6.3–32.6 cm at BTTV and 6.2–30.2 cm at STBL could be a bias influencing the variation of biological parameters of this fish population. Moreover, other environmental conditions of these two ecoregions, e.g., flora and flow rate, could potentially affect the population’s biological parameters but have not yet been investigated. Therefore, there is a need to study the population relationship of this fish with other biological factors and try to expand the fish size range from <6.2 cm to >32.6 cm to take appropriate measures to protect and develop this resource in the future.

Conclusion

This study provided data on the population biology of E. vaigiensis in two ecoregions, BTTV and STBL. This fish population grows better in areas with low salinity. The maximum length and longevity of E. vaigiensis were higher in the BTTV, but the growth performance index was lower in this area than in the STBL. The mortalities of this species at STBL were higher than those at BTTV. The STBL fish population is not overexploited. However, we need to suggest that locals should avoid catching fish with a length <L₅₀ in order to conserve and sustain the fish resources.

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