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This study supplements the gender determination methodology based on the relationship of some morphological indicators in Mystus albolineatus. A total of 229 specimens were caught by trawl nets at Cai Rang – Can Tho and Long Phu – Soc Trang in four months from April to July 2023. After collection, the samples were transferred to the laboratory to determine the total length (TL), standard length (SL), eye diameter (ED), eye distance (DE), body height (BH), head length (HL), and mouth diameter (MD) before surgery to determine the sex based on the gonad. The results showed that the TL-SL regression relationship could be used to determine the sex of this fish. Besides, TL-BH and TL-MD growth relations were relatively balanced in females versus males. In addition, based on SL, DE, and MD, it was also possible to determine whether the sampling time was the wet or dry seasons. These findings form the basis for the initial study of the biological characteristics of this fish in the Mekong Delta.

Caragobius urolepis has a wide range of distribution, from brackish to freshwater water, ranging from India to the Philippines and Fiji, including the Vietnamese Mekong Delta (VMD), where it lives mainly in the coastal regions and plays a vital role in food supply. Our preliminary observation showed that its population tended to decline, but little is known about this species. This study, therefore, was conducted to provide data on the ovarian structure, spawning pattern and season, length at first maturity, and batch fecundity of this fish. The study of 306 specimens caught with trawl nets from April 2022 to March 2023 in two coastal areas of VMD—Dong Hai, Bac Lieu (DHBL), and Dam Doi, Ca Mau (DDCM)—showed that this goby spawned more than once because the oocytes in stage IV ovary showed different stages of development. It could release eggs monthly during a 12-month study, with a peak in July–September since stage IV ovaries were found monthly during the study period and the GSI exhibited a high value in July–September. The fish length at first maturity and batch fecundity varied with the site, as they were 18.8 cm and 3,757–9,187 (6,142 ± 707 SE eggs/female) in DDCM and 20.5 cm and 3,760–11,118 (5,634 ± 750 SE eggs/female) in DHBL. The findings provide valuable foundations for exploiting their resources and serve as a basis for studying artificial reproduction. It is suggested to avoid catching these fish species during the breeding season to ensure sustainable exploitation in the future.

The present study reported an alternative way of sex-determining for scaleless worm goby Caragobius urolepis based on the regression relationship of some morphological parameters. A total of 328 samples were collected at the trawl nets in two coastal regions in Bac Lieu and Ca Mau from April to August 2022. After collection, the samples were transported to the laboratory in order to measure the total length (TL), body height (BD), head length (HL), the distance of eye (DE), and mouth width (MD) before surgery for accurate sex determination based on gonads. Regression analysis results of TL-BD (growth pattern A+ in females and I in males), TL-HL (growth pattern A+ in females and A- in males), and TL- MD (growth pattern A- in females and A+ in males) could be used to determine the sex of this fish. Moreover, TL-DE and TL-BD could be used to estimate when the samples were collected, e.g , in the dry or wet season, because DE and BD displayed A+ in the wet season but A- in the dry one. The findings could be used as an alternative way to determine fish sex and catching season for this species and others in the Mekong Delta.