Reproductive biology of fourfinger threadfin, Eleutheronema tetradactylum (Polynemidae) from north-eastern Arabian Sea

The fourfinger threadfin, Eleutheronema tetradactylum (Family: Polynemidae) is a commercially important marine finfish species in India. Information on the reproductive biology of this species is limited, which poses challenges for management of its fisheries and exploring its aquaculture potential. Therefore, reproductive biology of E. tetradactylum was studied from the northeastern Arabian Sea based on 824 samples from the coast of Navsari, Gujarat, India during September 2021 to August 2022. E. tetradactylum males and females were classified into five reproductive stages: immature, developing, mature, ripe and spent based on macroscopic and histological investigations. Intersex individuals were categorised into three stages viz., initiation of transition, mid-transition and late transition phases. The overall sex ratio of males to females was 1:0.89 (p<0.05), indicating a male-dominant population in the region. Length at first sex change in transition fish was 36.2 cm in total length (TL), length at first maturity (Lm50) of males and females were 24 and 41 cm TL respectively. The absolute fecundity ranged from 1,58,080 -12,53,571 eggs with mean value 5,85,886±53,315.1. Both males and females showed seasonal variations in the GSI values, with the highest values observed from May to October with a peak in August. The presence of all stages of oocytes in mature fish indicated that E. tetradactylum is a multiple spawner with asynchronous ovary having prolonged spawning season from March to November with a peak during August to September that coincides with the monsoon season in north-eastern Arabian Sea.