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.