Abstract
The orange‐spotted therapon, Mesopristes kneri , is an ecologically and economically important riverine fish endemic to Fiji, yet its life history and reproductive biology remain poorly understood. This study examined the reproductive biology, life history and sexual development of M. kneri to support conservation and fisheries management. From April 2022 to May 2023, field‐based histological analyses were conducted on 118 individuals sampled from the Navua River, comprising 86 females, 24 males, five transitional individuals and three sexually undifferentiated individuals. The findings support protandrous hermaphroditism, potentially through a monogynic pathway, as indicated by the absence of functional females in smaller size classes, size‐specific sex ratios and histological evidence of transitional gonads. Sexual dimorphism was evident, with females attaining larger body sizes than males. The length predicted to comprise 50% mature individuals was estimated at 20.39 cm SL, while the fitted transition model estimated X50, the length predicted to comprise 50% males, at 20.14 cm SL. Histological analyses confirmed asynchronous oocyte development and the presence of transitional gonads containing regressing testicular tissue and developing ovarian structures. The transitional gonads of M. kneri exhibited a delimited configuration, with a connective tissue barrier separating testicular and ovarian regions. The sampled population also showed a strongly female‐biased sex ratio, particularly in larger size classes, and a positive relationship between batch fecundity and female size. These findings indicate that management strategies should consider the protection of larger, highly fecund females while also maintaining sufficient smaller and intermediate‐sized functional males to support reproductive success. This study provides critical baseline information on the reproductive biology of M. kneri and contributes to the development of conservation and fisheries management strategies for Fiji's riverine ecosystems.