Aseasonal and short life cycles of the protandrous hermaphrodite blue threadfin (Eleutheronema tetradactylum) in a near-equatorial tropical region

Abstract

Context. Eleutheronema tetradactylum is exploited throughout its distribution in the Indo-West Pacific region. However, there is a lack of data on its life cycle in near-equatorial regions and the northern hemisphere. Aim. To investigate the age, growth, sex change and reproduction of Eleutheronema tetradactylum in southern Thailand. Methods. Analysis of length, otolith and gonad data of 449 fish. Key results. Contrary to findings from regions at higher latitudes, otoliths did not show seasonal increment formation. Instead, with the exception of large individuals, daily increments were distinguishable. Within the age range of 69-341 days, growth was linear, with results indicating a total length of 430 mm (69% of maximum length) at 1 year. The youngest mature male was 137 days old. Moreover, the length at which half the individuals changed from male to female was 376 mm, corresponding to 282 days. The gonadosomatic index indicated no specific spawning season, and the hatch-date calculations indicated year-round hatching. Conclusions. Eleutheronema tetradactylum inhabiting near-equatorial regions is characterised by rapid population turnover compared with populations of higher latitudes. Implications. Our findings indicated life-history variability, including seasonality loss in near-equatorial habitats, of an important coastal fish. This information is important for species conservation.