The reproductive biology of Ellochelon vaigiensis from the Vietnamese Mekong Delta.

The Squaretail mullet, Ellochelon vaigiensis, is a commercial fish species distributed throughout the Indo-Pacific region. This species tolerates wide variations in salinity, having been reported from both freshwater and marine habitats. Although economically significant, there is little information on its reproduction, especially in the Vietnamese Mekong Delta (VMD), where it is heavily extracted. Our study elucidates the breeding pattern, seasonality, first length at maturity, as well as potential and relative fecundity of this species. Fish specimens were collected by trawl nets from November 2020 to October 2021 at four estuarine sites within the VMD. We found this species to be a synchronous spawner, with peaks in reproductive activity from June to September. In the populations we surveyed, there was significant dominance of males (1.26:1.00) based on 942 fish samples (526 males and 416 females). The length at 50% maturity for females was significantly higher than for males at all sites and ranged from 10.6 to 19.3 cm. The diameter of the eggs examined for this species exhibited slight variation, from 0.43 to 0.54 mm. The potential fecundity ranged from 246,254 ± 35,878 to 411,970 ± 54,370 eggs, corresponding to female length and weight of 26.7-32.6 cm and 102.97-234.19 g, respectively. Relative fecundity values were highest at Thanh Phu, Ben Tre (6388 ± 605 eggs), and Dong Hai, Bac Lieu (6461 ± 637 eggs), followed by Tran De, Soc Trang (4729 ± 408), and were found to be lowest at Duyen Hai, Tra Vinh (3117 ± 223 eggs). Information on reproductive behavior in this species has far-reaching impacts on sustainable extractions, stock conservation, and aquaculture.

The use of mtCOI gene sequences in identifying Butis species in the Southwest of Vietnam.

Butis genus is characterised by their small body size and morphological variability, allowing them to adapt to different habitats. This paper analyses the mitochondrial cytochrome C oxidase subunit I gene sequences and morphology of Butis for identification purposes and to understand genetic relationships. The morphological characteristics of Butis koilomatodon differed obviously from Butis humeralis and Butis butis. After classification based on morphology, the total deoxyribonucleic acid of fish samples was isolated, and the mitochondrial cytochrome C oxidase subunit I genes were successfully amplified using the polymerase chain reaction method. At approximately 617bp, the obtained mitochondrial cytochrome C oxidase subunit I gene sequences were highly similar to the reference sequences on Genbank (85.90-100%). The phylogenetic graphic was divided into five distinct groups, where B. koilomatodon was grouped in one group; and B. humeralis and B. butis were grouped together. The results suggest that B. humeralis was an entirely different species from B. butis, with a mean genetic divergence of up to 14%. However, further studies using a combination of other types of deoxyribonucleic acid barcoding together with morphological features should be undertaken to confirm these findings.

Life-history traits of a tropical bagrid catfish, Mystus mysticetus Roberts, 1992, caught from the Mekong Delta, Vietnam.

Population's biological parameters, including length at first capture, mortalities, exploitation rates, growth coefficient, longevity, and recruitment times, are essential in assessing fishing status, but there is no data on Mystus mysticetus. Therefore, the study was conducted to provide these parameters to assess the fishing status of this species at Cai Rang, Can Tho (CRCT) and Long Phu, Soc Trang (LPST). A collection of 741 individual fish was used for analysis and showed that most fish size groups ranged from 9.0 cm to 12.0 cm, and the asymptotic length was 16.8 cm for both CRCT and LPST populations. The fish population von Bertalanffy curve was L t = 16.80(1 - e-0.51(t + 0.38)) at CRCT and L t = 16.80(1 - e-0.48(t + 0.40)) at LPST. The fish growth coefficient at CRCT (2.16) was higher than at LPST (2.13), whereas the reverse case was true for longevity ranging from 5.88 years (at CRCT) to 6.25 years (at LPST). At CRCT, fishing mortality, natural mortality, total mortality, and exploitation rate were 0.69/year, 1.40/year, 2.09/year, and 0.33, respectively; at LPST, these values were 0.75/year, 1.33/year, 2.08/year, and 0.36, respectively. Although the population parameter of this fish species exhibited a spatial variation, both CRCT and LPST fish resources have not been subjected to overexploit because E (0.33 at CRCT and 0.36 at LPST) is lower than E 0.1 (0.707 at CRCT and 0.616 at LPST).

Phylogenics of the genus Glossogobius in the Mekong Delta based on the mitochondrial cytochrome b (cytb) gene.

The Glossogobius species play an essential role in food supply and are distributed widely from the marine to freshwater, especially in the Mekong Delta, Vietnam (VMD). Some of their morphometrics and meristics are observed to vary with species and sampling sites. Therefore, the present study aims to verify if their mitochondrial Cytochrome b (Cytb) gene, one of the popular gene sequences used in fish phylogenic variation assessment, varies with species and sampling sites in the VMD. The Cytb gene size was 1300 bp for GcytbH/GcytbL primer pair and 1045 bp for GluMuq1-F/Mixcyto937-2R. The genetic distances within and among these three fish species groups were 0-11%. The Cytb gene sequences' similarity between this study and the NCBI database was 85.84-100%. The Glossogobius specimens were observed to disperse in small branches of the phylogenetic tree with a low K2P value, suggesting that the Cytb genetic diversity may be low among species.

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

Ellochelon vaigiensis is widely distributed and plays a vital role in the fisheries in the Mekong Delta (MD), but data on its population biological traits have not been known. Consequently, this study was conducted to provide data on its population biology used for fishing status assessment and fish resources management. Fish specimens were collected using trawl nets in two regions of the Hau River mouth, including northern in Ben Tre and Tra Vinh (BTTV) and southern in Soc Trang and Bac Lieu (STBL). Fish population biological parameters were estimated using FiSAT II software based on the fish length-frequency data. The male and female length-frequency data in each ecoregion were pooled together. Data analysis of 1,383 individual fish showed the sex ratio of the species was 1.00:1.30 at BTTV (309 females and 402 males) and 1.00:1.25 STBL (299 females and 373 males). Most fish specimens were caught at 12-22 cm total length (914 individuals), accounting for 66.09% of the fish collection. The difference in salinity between these two regions could affect the population biological parameters of E. vaigiensis. There were five cohorts (e.g., five growth curves) in the BTTV and STBL. The von Bertalanffy curves of fish populations at BTTV and STBL were L = 33.6 (1 -e -0.46(t + 0.34)) L = 31.5 (1 -e -0.56(t + 0.29)), respectively. The growth index (Φ') of this species at STBL (2.74) was higher than that at BTTV (2.72), whereas its longevity at BTTV (6.52 yrs) was higher than at STBL (5.36 yrs). The biomass and relative yield parameters, including E 0.1, E 0.5 and E max were 0.358, 0.265 and 0.436 at BTTV; and 0.418, 0.293, and 0.513 at STBL, respectively. The fishing (F), natural (M), and total (Z) mortalities were 0.35/yr, 1.06/yr, and 1.41/yr at BTTV; and 0.55/yr, 1.24/yr, and 1.78/yr at STBL, respectively. The BTTV and STBL population was not overexploited because the exploitation rate (E BTTV = 0.25, E STBL = 0.31) was lower than E 0.1 (0.358 in BTTV and 0.418 in STBL).

Burrow behaviour, structure and utilization of the amphibious mudskipper Periophthalmus chrysospilos Bleeker, 1853 in the Mekong Delta.

Burrows of fishes are important to their creators, but knowledge of burrowing behaviour, structure and utilization are poorly understood. We report the burrowing behaviour of the amphibious mudskipper, Periophthalmus chysospilos, from estuarine and coastal sites within the Mekong Delta (Vietnam). Activities at and around the burrow were observed over 12 months (April 2020 to March 2021). Burrow casts were recovered monthly to determine burrow structure and utilization in this species. Observations revealed that males excavate burrows with their mouths during the ebb tide. Burrows were J-, Y- and U-shaped, with 1-2 openings to the surface and a bulbous egg chamber. The burrow depth (BD) and total length (BL) varied with shape and site, but not season. The BD and BL ranged from 15.1 ± 0.9 to 18.6 ± 0.80 SE cm and from 22.1 ± 1.2 to 25.7 ± 1.0 SE (standard error of mean) cm, respectively. These results provide insights into the burrowing ability and better understand the ecology of these fishes in the Mekong Delta. Running head: Burrow behaviour, structure and utilization of Periophthalmus chrysospilos.

Variations in length-weight relationship, growth and body condition of the commercial mullet Ellochelon vaigiensis in the Vietnamese Mekong Delta.

The Squaretail mullet Ellochelon vaigiensis is widely found from marine to brackish and freshwater, including the Vietnamese Mekong Delta (VMD). Data on the length-weight relationships (LWRs), growth pattern, and condition factor (CF) play an essential role in fisheries assessment but are limited to E. vaigiensis - a commercial fish in VMD. To better understand the growth pattern of this fish, the study collected samples at four sites, including Thanh Phu, Ben Tre (BT); Duyen Hai, Tra Vinh (TV); Tran De, Soc Trang (ST); and Dong Hai, Bac Lieu (BL). Each month, fish samples were collected using bottom nets at each sampling site during high tide in both the dry season (January to May 2021) and the wet season (November to December 2020 and June to October 2021). This study qualified whether LWRs, growth pattern, and CF of E. vaigiensis vary with sexual, intraspecific, and spatiotemporal variables. The analysis results of 942 specimens showed that fish total length (TL = 17.86 ± 0.13 cm) and weight (W = 54.29 ± 1.05 g) changed with sex, season, month and site. The W could be appraised from a given TL due to high coefficients (r 2 ) of LWRs that were regulated by fish size and season but not sex. The species showed negative allometry as its slope value of LWRs (b = 2.34 ± 0.30) was <3. The growth pattern did not change with sex and site but varied fish size, season, and months ranging from negative allometry to isometry. The CF of females (1.08 ± 0.01) was higher than males (0.99 ± 0.01). Albeit this fish showed spatiotemporal variation in CF, ranging from 0.81 ± 0.01 to 1.25 ± 0.02, the CF (1.02 ± 0.01) was significantly higher than the one threshold, indicating that E. vaigiensis lived a favorable condition. Research results show that this fish species has a dominant growth pattern in terms of length (b < 3) and is well adapted to the environment (CF ≈ 1). These data have an important role in suggesting the future aquaculture of this fish.

Population dynamics of Acentrogobius viridipunctatus (Actinopteri: Gobiidae) in the Vietnamese Mekong Delta.

Spotted green goby Acentrogobius viridipunctatus is one of the commercial gobies living from brackish to freshwater globally, including the Southwest of Viet Nam, but there is no data on population biological traits used for assessing status of stock. Consequently, the research aimed to provide life history parameters of this fish like asymptotic length (L ∞ ), growth coefficient indicator (K), exploitation rate (E), length at first capture (L 50 ), the maximum age (t max ), growth coefficient (Φ'), and mortalities. A total 960 individuals (528 males and 432 females) were caught using trawls at two ecoregions, including Tra Vinh-Soc Trang (TVST) and Bac Lieu-Ca Mau (BLCM), from January to December 2020. Results showed that average fish length was 8.0-11.0 cm, and L ∞ was 14.70 cm at TVST and BLCM. The von Bertalanffy growth curve was L t = 14.70 (1 - e-0.73(t+0.27)) at TVST and L t = 14.70 (1 - e-0.84 (t+0.23)) at BLCM, respectively. The Φ' at BLCM (2.26) was higher than that at TVST (2.20), but t max at BLCM (3.57 yrs) was lower than that at TVST (4.11 yrs). The total mortality (Z), fishing mortality (F), and natural mortality (M) at TVST were 2.09 yr-1, 0.29 yr-1, and 1.80 yr-1, respectively. These parameters at BLCM were larger than those at TVST, with values of 3.47 yr-1, 0.34 yr-1, and 3.14 yr-1, respectively. The fishing status at TVST and BLCM was not subjected to overfishing as E in these regions (0.14 and 0.10) were lower than E 0.1 (0.76 and 0.72). Local authorities should ask fishers to increase fishing mesh size for sustainable exploitation.

Feeding ecology of Ellochelon vaigiensis (Quoy & Gaimard, 1825) living in the Mekong Delta, Vietnam.

Ellochelon vaigiensis (squaretail mullet) adapts to a wide salt spectrum, grows quickly and is easy to raise with other species, so it is the object of attention in aquaculture. Information on the biology and ecology of the species, diet, in particular, is still scattered. Here, we explore the feeding habit, feeding intensity, and food composition of the squaretail mullet. A total of 942 fish (526 males and 416 females) were collected from November 2020 to October 2021 at four coastal sites in the Mekong Delta, Vietnam. The squaretail mullet is an algi-omnivorous fish, as their relative gut length (1.81) falls into the range 1-3, and the diet is mostly based on algae. The feeding intensity is high due to the high value of the fullness index (4.39 ± 0.08%). The fullness index did not vary by sampling site and month, while RGL and FI changed depending on sex. Bacillariophyta (49.13%), detritus derived from organic matter (30.37%), and Cyanophyta (18.39%) are the dominant food items in the diet composition of E. vaigiensis, in which detritus is the most important food with the highest IRI index. Besides, Euglenophyta (1.00%), Chlorophyta (0.95%), Paramecium (0.06%), Copepoda (0.04%), Rotatoria (0.03%), Polychaeta (0.02%), and Cladocera (0.01%) are also recorded and ranked based on their biovolume. Some differences in diet composition are observed between immature and mature at different seasons and their interactions. Our results increase the knowledge about the feeding ecology of squaretail mullet and can help the sustainable management of this commercially important fish species.

Testicular development, reproducing references and length at first maturity of Acentrogobius viridipunctatus (Actinopteri: Gobiiformes) in the southwest Viet Nam.

This study provided knowledge on testicular development, spawning reference and length at first maturity (L m ) of Acentrogobius viridipunctatus-a target catching fish for food supply in the southwest Viet Nam. A total of 528 individuals were collected using trawl nets from January to December 2020 at Long Huu-Tra Vinh (LHTV), Trung Binh-Soc Trang (TBST), Dien Hai-Bac Lieu (DHBL) and Tan Thuan-Ca Mau (TTCM). Five developmental stages of testis were found, and testis diameter increased from ∼1.07 (stage I) to ∼4.82 mm (stage V). Testis stage I consisted of spermatogonia (S) which shifted to primary spermatocytes (SC1) in stage II. In testis stage III, SC1 was divided into secondary spermatocytes (SC2). At this stage, testis consisted of S, SC1, SC2 and some spermatid (ST). At stages IV and V, testis comprised mainly spermatozoa (SZ) besides S, SC1, SC2 and ST, showing that this goby was a multi-spawner. Testis frequency and gonadosomatic index analyses showed that this species released sperms yearly, with a prominent peak in the wet season. The L m decreased from 9.3 ± 0.3 cm at TBST to 9.0 ± 0.3 cm at LHTV, 8.9 ± 0.2 cm at TTCM and 6.5 ± 0.5 cm at DHBL. Findings provided fundamental data to set up suitable fishing and conserving strategies for this species.

Ovarian development, spawning season, size at maturity and fecundity of Acentrogobius viridipunctatus (Valenciennes, 1837) in the Vietnamese Mekong Delta.

This present study provides an overview of the reproductive traits, including ovarian development, spawning season, size at maturity (Lm ), and fecundity of Acentrogobius viridipunctatus - a high economic value fish in the Vietnamese Mekong Delta (VMD). A total of 432 females were collected using trawl nets from January to December 2020 at four sites, including Long Huu-Tra Vinh (LHTV), Trung Binh-Soc Trang (TBST), Dien Hai-Bac Lieu (DHBL), and Tan Thuan-Ca Mau (TTCM). The ovarian diameter was seen to increase from 2.09 (stage I) to 6.57 mm (stage V). Histological analysis showed that the goby was a multi-spawner due to the different oocyte stages found in stages IV and V of ovaries. It can release eggs all year-round due to the monthly appearance of ovarian stages IV and V during the study period, with the main peak in the 2-4 months of the wet season noted with the highest value of gonadosomatic index at that time. The Lm increased from 6.6 cm at TBST to 9.4 cm at TTCM. This species displayed relatively high fecundity, ranging from 5,481 to 130,683 eggs/female. The average fecundity of this fish was 27,698 ± 7,983 eggs/female at LHTV, 46,592 ± 7,264 eggs/female at TBST, 23,271 ± 4,985 eggs/female at DHBL, and 31,408 ± 2,515 eggs/female at TTCM. Egg diameter ranged from 0.45 ± 0.01 at DHBL to 0.50 ± 0.01 at TBST. For sustainable exploitation, local governments should ask the fishers to avoid catching fish during the main spawning period, and the fish length at first capture should be >Lm .

Monthly variations in growth pattern and condition factor of Periophthalmodon septemradiatus (Gobiiformes: Periophthalminae) living along the Bassac River in Viet Nam.

Periophthalmodon septemradiatus (Hamilton, 1822) is a unique mudskipper living in the mudbanks from estuarine to riverine regions in the Vietnamese Mekong Delta (VMD), but there is no data on its growth patterns and condition factors that are helpful for fish resource assessment and adaptation understanding. This study was conducted at five sites, from the lower (Soc Trang province) to middle (Can Tho city) and upper (An Giang province) reaches of Bassac River in VMD, to provide knowledge on growth patterns and condition factors to this mudskippers. Fish samples were caught using traps and hands for 24 months, from July 2017 to June 2019, at these five sites. The total length and weight of 3,417 individuals (1,340 females and 2,077 males) varied by sex, season and site (p < 0.001 for all cases). This species exhibited a positive allometric type as the slope (b = 3.06 ± 0.01) of the length-weight relationship (LWR) was >3 (p < 0.001) for both sexes. The growth pattern changed with sex as mudskipper showed isometry in females but positive allometry in males. Maturity also affected fish growth type since it shifted from negative allometry in immature groups to positive allometry in mature groups. The growth pattern of fish changed from isometry in the dry season to positive allometry in the wet season. The mudskipper showed isometry at the lower reaches in Soc Trang but positive allometry at the middle and upper reaches in Can Tho and An Giang. The condition factor (CF) did not change according to sex, size and season, but with month and site variables. The present environmental condition in these studied sites tended to be not good enough for this fish as CF (0.95 ± 0.01) was less than 1 (p < 0.001). The findings provided basic information on the growth and adaptation of P. septemradiatus being helpful in fish adaptation understanding and resource conservation in VMD.

Population biological traits of Periophthalmus chrysospilos Bleeker, 1853 in the Vietnamese Mekong Delta.

Periophthalmus chrysospilos is an amphibious fish living in mudflats from eastern India to Indonesia, including the Vietnamese Mekong Delta. Population biological traits play an important role in fishery assessment, but understanding is limited for this species. In total 1,031 specimens were caught in two regions covering four provinces, including the TVST (Duyen Hai, Tra Vinh and Tran De, Soc Trang) and BLCM (Dong Hai, Bac Lieu and Dam Doi, Ca Mau). Results found that the sex ratio was close to 1:1. The parameters of the von Bertalanffy in TVST were L∞ = 12.8 cm, K = 0.41 yr-1, t0 = -0.10 yr and in BLCM were 12.7 cm, 0.38 yr-1 and -0.08 yr, respectively. Although the growth coefficient (Φ') in BLCM (1.79), was lower than that in TVST (1.83), the species shared a similar size at first capture (7.9 cm in TVST and 7.9 cm in BLCM). The species suffered from heavy pressure of fishing in TVST as fishing mortality in TVST (2.32 yr-1) was higher than that in BLCM (1.38 yr-1), leading to the higher total mortality (Z = 3.60 yr-1) in TVST compared to BLCM (Z = 2.59 yr-1). By contrast, the species showed similar natural mortality over both sites (1.20 yr-1 in TVST and 1.22 yr-1 in BLCM). The Periophthalmus chrysospilos population was reasonably exploited because E values (0.64 in TVST and 0.53 in BLCM) were lower than E10 (0.706 in BTTV and 0.705 in STBL). Nonetheless, to avoid the consequences of overfishing, some sustainable fisheries practices should be implemented, such as protecting mangrove forests, restricting fishing during the recruitment period, using appropriate fishing tools and increasing mesh size.

Factors regulating growth pattern and condition factor of an amphibious fish Periophthalmus gracilis living in the Mekong Delta.

Growth pattern and condition factor (CF) are essential to fish resource assessment but limited to Periophthalmus gracilis-an amphibious fish living in the mudflats along the Indo-Pacific regions, including the Mekong Delta (MD), Vietnam. This study lasted from April 2020 to March 2021 to verify if their growth pattern and CF change with sex, size, season, month and site. The total length and weight of 486 individuals (236 females and 250 males) were 2.9-5.9 cm and 0.13-1.66 g, respectively. The mudskipper displayed negative allometry as the slope value (b = 2.69 ± 0.06) obtained from length and weight (LWR) was significantly less than 3 (p < 0.01), indicating that most fish specimens were caught in the immature stage. The fish growth pattern did not change with sex as both males and females displayed negative allometry but varied by size since the mudskipper showed negative allometry in the immature group and isometry in the mature group. Likewise, growth type changed with season since fish showed negative allometry in the dry season but isometry in the wet season. As the slope value (b) varied by site and month, the mudskipper displayed spatiotemporal growth patterns, ranging from negative to positive allometry. The CF was impacted by sex as this value of females (1.09 ± 0.02) was higher than that of males (0.96 ± 0.01, p < 0.01). Besides, CF was regulated by fish length since this value was higher in the mature group (1.12 ± 0.03) than in the immature group (1.01 ± 0.01, p < 0.01). Likewise, CF was affected by season as this value was higher in the wet season (1.05 ± 0.02) than in the dry season (0.99 ± 0.01, p < 0.01). Although the CF varied with site and month variables (p < 0.01), this value (1.02 ± 0.01) was generally higher than 1, showing fish adapted well to their habitat. The fish length at first capture should be increased to exploit this species sustainably.

Reproduction ecology of an emerging fishery resource, the amphibious mudskipper Periophthalmus chrysospilos, in the Mekong Delta.

Populations of Periophthalmus chrysospilos, the Gold-spotted mudskipper, within the Mekong Delta are facing extirpation risks due to indiscriminate harvesting for the growing aquarium and food-fish trade. This study provides some of the first information on reproductive ecology-such as spawning type and season, length at first maturity, and batch fecundity-of this species, to be used in their management. The sex ratio of wild populations, based on 1031 individuals (523 males and 508 females) is 1:1. The gonadosomatic index (GSI) values are exhibit a non-normal distribution and changed with gender, season, and site. A combination of GSIs and the monthly appearance of mature gonads suggest that this species reproduces throughout the year, with peak from July to October. This species exhibits sexual and spatial variation in size at first maturity (Lm ) as Lm is 6.2-8.6 cm in males and 6.4-7.3 cm in females. The batch fecundity (F = 2614 to 23,465 eggs/female) exhibits non-normal distribution and varies with site, with the highest values at Dam Doi, Ca Mau (13,336 ± 1,279 SE) and the lowest at Tran De, Soc Trang (6654 ± 851 SE). In addition, batch fecundity is directly proportional to body size due to high determination relationships between batch fecundity and fish size (r 2 > 0.64 for all cases). Information derived on the reproductive biology of this species can inform its conservation, sustainable exploitation, and ex situ propagation.

Spatiotemporal variations in length-weight relationship, growth pattern and condition factor of Periophthalmus variabilis Eggert, 1935 in Vietnamese Mekong Delta.

Length-weight relationship (LWR), growth pattern and condition factor (CF) play a vital role in fish resource evaluation and management but data on this is limited for Periophthalmus variabilis. This is an amphibious fish that lives in the mudflats of the mangrove forests in the Western Pacific regions, including the Vietnamese Mekong Delta (VMD). This paper contributes to the understanding of the LWR, growth pattern and CF of P. variabilis. Fish specimens were collected by hand at four sites in the VMD from April 2020 to March 2021. The analysis of 495 individual fish (259 females and 236 males) showed that fish weight could be estimated from a given fish length due to high determination values (r 2 = 0.70 - 0.97). Dusky-gilled mudskippers of the VMD exhibited positive allometry, as indicated by their larger than 3 b value (b = 3.094 ± 0.045, p = 0.04). However, the growth pattern of the mudskipper between the sex and maturation stage (immature versus mature) showed isometry. These fish displayed positive allometry in the dry season (b = 3.138 ± 0.065) (>3, p = 0.04) but isometry in the wet season (b = 3.058 ± 0.061) (≈3, p = 0.34). Fish growth ranged from isometry to positive allometry and varied by site (b = 2.850 ± 0.103-3.370 ± 0.114) and month (b = 2.668 ± 0.184-3.588 ± 0.299) based on the b value. The CF of P. variabilis was not affected by sex (p = 0.29), body size (p = 0.64) or season (p = 0.43), but was affected by site (p = 0.01) and month (p = 0.01). The CF of this species (1.05 ± 0.02) was higher than 1 (p < 0.001), indicating that the fish adapted well to their habitat.

Foraging ecology of the amphibious mudskipper Periophthalmus chrysospilos (Gobiiformes: Gobiidae).

The food composition and feeding ecology of fishes living in the intertidal zone play an essential role in understanding the energetic connectivity between terrestrial and aquatic systems. Periophthalmus chrysospilos is an amphibious fish species occurring in the intertidal zone, but data on its diet and foraging ecology is still poorly known. This study on Ps. chrysospilos was carried out from April 2020 to March 2021 at four sites within the Mekong Delta estuary to define the influence of spatio-temporal factors on the diet of this species. The diet composition and relative gut lengths (RGLs) of Ps. chrysospilos were analysed in relation to four parameters-sex, size, site, and season. A total of 1,031 individuals were collected, and their digestive tract lengths were used to calculate the RGL. The digestive tracts of only 546 individuals were with food items (approximately 1:1 of empty vs full digestive tract) and were subsequently used for further analyses. The ranges in total length and weight in both adult and juvenile individuals were 3.4-10.6 cm and 0.38-14.13 g, respectively. The RGL values varied with season, fish size and site, but was always lower than 1, indicating a predominantly carnivorous diet. The variability of food items found within the digestive tracts demonstrated its adaptability in pursuing prey items within the limits of the littoral zone, and its importance as a conduit of terrestrial-marine connectivity. This species is characterised as an opportunistic mesopredator feeding primarily on Acetes spp., Uca spp., Dolichoderus sp., and rarely on Polychaeta and Actinopterygii. Other items found within the digestive tract are Mollusca, and detritus. The diet composition of Ps. chrysospilos did not vary with season and size, but changed with sex and site parameters. Uca spp. contributed to the sexual variation in dietary component, whereas Mollusca, Uca spp., Dolichoderus sp. and detritus, were drivers for spatial variation in the dietary component. The research provides fundamental information on diet composition and feeding strategy, as well as contributes towards knowledge on foraging ecology and resource use by intertidal animal communities.

Land Invasion by the Mudskipper, Periophthalmodon septemradiatus, in Fresh and Saline Waters of the Mekong River.

There has been a long-standing controversy about whether vertebrates emerged in the Paleozoic from marine or freshwater environments. Several hypotheses have proposed coastal, estuarine and riparian areas as sites of the transition. Here, we report the ecology of an amphibious fish Periophthalmodon septemradiatus, which we presume is in the process of niche expansion into terrestrial habitats from estuarine to freshwater environments along the Mekong River, Vietnam. Adult fish are highly terrestrial and have not been observed to venture into water during our survey. Courtship behaviour was observed, and fertilised eggs were recovered from burrows in both brackish and freshwater environments. The smallest fish collected at 12, 96, and 148 km from the river mouth were juveniles shortly after starting an amphibious life. These findings suggest reproduction in both brackish and freshwater environments. In contrast, otolith Sr:Ca ratio indicates larval hatching only in brackish water. Analysis of a 940-base pair (bp) segment of the mitochondrial cytochrome c oxidase subunit II and a 934-bp segment of the mitochondrial D-loop demonstrated no genetic segregation between populations. The fish may provide a unique opportunity to study how ambient salinity affects the biology and ecology of a living vertebrate during transition from water to land.

Reproductive Traits of the Duckbill Sleeper Butis butis (Hamilton, 1822).

Butis butis is a commercially important goby that is widely distributed in Indo-Pacific regions; however, its reproductive biology is poorly characterized. We condcted the present study in estuarine and coastline areas in the Mekong Delta from June 2015 to May 2016 to generate reproductive parameters for this species. The results of data analysis based on a collection of 173 individuals (74 females and 99 males) indicate that the sex ratio of this species is close to 1:1. Mature (stage IV) and ripe (stage V) gonads were found monthly during the study period, suggesting that Butis butis is an iteroparous fish that spawns in a year-round cycle. Male B. butis matures first at 8.97 cm total length (TL), which is longer than females (6.82 cm TL). Butis butis is well adapted to tropical monsoon regions due to its high batch fecundity (46,017 ± 6941 SE), ranging from 15,000 (8.6 cm TL and 5.71 g W) to 78,500 (15.8 cm TL and 42.24 g W) eggs. Batch fecundity increases with fish size due to a strong positive relationship between batch fecundity and fish size (TL and W). These results help fill the knowledge gap on the reproductive biology of Butis butis, and contribute crucial information relevant to sustainable management of fish populations in the study region.