Life-history characteristics and mortality of the protogynous hermaphroditic frostback rockcod (Epinephelus bilobatus) from the eastern Indian Ocean

Abstract

The frostback rockcod, Epinephelus bilobatus, is a small epinephelid (<50 cm total length) with a limited distribution in the tropical eastern Indian Ocean (i.e., north-western Australia and parts of Indonesia). Although its fishery harvest in Western Australia is low (<6 t year−1), its life history is not well known (i.e., reproductive biology, growth, and natural mortality) preventing assessment of its inherent vulnerability to overfishing. Samples of E. bilobatus were collected from ∼2000 km of the north-western Australian coast, comprising the Kimberley, Pilbara, and Gascoyne regions, between 2012 and 2022 to determine its life-history characteristics. Detailed histological analysis of gonads (n = 1460) identified that E. bilobatus is a monandric protogynous hermaphrodite. The length and age at which females matured was 278 mm LT and 2.5 years in the Pilbara region, and the lengths and ages at which they changed to males was 336 and 342 mm LT and 6.2 and 6.5 years in the Kimberley and Pilbara, respectively (i.e., less than 5% variation between regions). Based on histology, female E. bilobatus were in spawning condition in all months. However, the mean monthly gonadosomatic indices and proportions of developed/spawning females were highest (i.e., >0.95 and 40%, respectively) for 5 months of the year (i.e., January to April and September). Marginal increment analysis confirmed the annual formation of one opaque and one translucent zone in the sagittal otoliths of this species. Epinephelus bilobatus had a moderate longevity (∼20 years) and attained a larger mean length-at-age in the cooler southern Gascoyne region compared to the more northern Kimberley and Pilbara regions. Three values of natural mortality (M) were used to assess the sensitivity of fishing mortality (F) in the Kimberley and Pilbara commercial trap fisheries, with all point estimates of F < M, indicating that at the time of sampling, the harvest of this species over a long period (>20 years) was sustainable for these contemporary fisheries.