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This study provided new knowledge on reproductive traits of the unique mudskipper Periophthalmodon septemradiatus that is also a potential aquarium pet. Fish specimens were obtained by using fishing rods at the muddy flat in tributaries of the Hau River from August 2017 to July 2018. Data analysis of a collection of 1,504 individuals (930 males and 674 females) showed that the sex ratio of this species was significantly different from the ratio 1:1. As mature gonads were found monthly during the study period, it is suggested that P. septemradiatus is an iteroparous species releasing eggs for a year‐round cycle. The fish length at first mature (Lm) showed an increase from estuary to upstream areas. Male P. septemradiatus had higher Lm values than that of females. Likely, the batch fecundity increased from estuary region (5,916 ± 312 SEM eggs) towards upstream area (11,451 ± 953 SEM eggs). The fish total length and weight values had strong positive relationships with batch fecundity, as shown by high determination values (r2), suggesting that batch fecundity increased with fish size. This study provided more information on reproductive biology of P. septemradiatus, which can be utilized for fish population conservation and artificial reproductive study.

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Several studies indicate that burrows play an essential role in burrowing fish, especially mudskippers, but little is known on burrow morphology and utilization of Periophthalmodon septemradiatus. This study was therefore conducted at five sampling sites from the estuary to upper reaches of Hau River to contribute to the knowledge of the behavior of Pn. septemradiatus, via in situ and ex situ observations of the utilization of burrow structures. Burrows made by polyester resin were used to determine their morphology. The male mudskippers Pn. septemradiatus were observed to excavate burrows using their mouths during the low tide. The burrows' shapes were found to be J, U and W, with 1-3 openings and one bulbous chamber. The chamber was used as egg container where the mudskipper deposits their fertilized eggs. W-shaped burrows were found to be interconnected chambers where the mudskipper used it to change direction of movements inside the burrow. For the J-and U-shaped burrows, the commonality at all five sites was identified. The W-shaped burrows were common and were only found at two sites (Long Duc and An Lac Tay). For the burrows' structure, the openings to the surface were oval, circular and bulbous in shape. Yet, there were variations between sites, and the burrows' size was similar during the dry and wet seasons. At different sites, the depth of the burrows seasonally changed. The burrows used by mudskipper provide shelter, spawning sites, and access to feeding grounds for other mudskipper species. 'Fish Use for conducting Research' singed by Vice Head of Department of Research Affairs on 8 th November 2018 for the study from April 2017 to April 2019. Burrow structureThe burrow structures of Pn. septemradiatus were studied in situ, by creating casts of burrows at field sites when the fish relocated out of burrows, to avoid polyester resin's effect on fish. The constructed burrows by Pn. septemradiatus were

Background The key to fishery management is knowing the appropriate reproductive strategies of the targeted fish. For most gobiid species, the iteroparous pattern is dominant compared to semelparity. Albeit Butis koilomatodon plays an important role in the Mekong Delta’s food supply, its reproductive biological data have not been known. Hence, this study was conducted to provide new fundamental knowledge of reproductive traits of Butis koilomatodon in the Mekong Delta. Results A total of 1314 individuals (903 males and 411 females) were monthly collected by bottom gill nets from July 2019 to June 2020 at six sampling sites along estuarial and coastal regions, from Tra Vinh to Ca Mau provinces, southern of Vietnam. pH and salinity of these six sampling sites are 7.72–7.93 pH and 11.17–26.17‰, respectively. The pH varies with sites, but not seasons; whereas a reverse case is found in salinity. Different types of oocytes are found in histological specimens of ovaries prove that B. koilomatodon is a multi-spawner. The gonadosomatic index value, together with the monthly presence of mature ovaries reveal that this species spawns throughout the year. The length at first mature male Butis koilomatodon (5.1–8.6 cm) is higher than that of females (4.8–6.7 cm), except in Hoa Binh and Dong Hai. Batch fecundity (3085 to 32,087 eggs/female) increases with fish weight (1.48–12.30 g) and length (4.8–9.0 cm) due to high determination values (r2 > 0.6). Conclusion Knowledge of reproductive traits gained from this study was a reference source for future studies and helped manage this species’ resources.