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.

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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 (r2 = 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.

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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.