Reproductive biology and estimates of length and age at maturity of longtail tuna (Thunnus tonggol) in Australian waters based on histological assessment

The annual global catch of longtail tuna (Thunnus tonggol) has almost tripled in the past 20 years. In the Indian Ocean tuna fishery, longtail tuna catches now exceed those of some principle target species (e.g. bigeye and albacore tuna), resulting in numerous stock assessments concluding the species has been subject to overfishing and overfished for several years. Stock assessment is currently hindered by unreliable biological and catch data, which provided the impetus to study and describe the reproductive dynamics of longtail tuna in Australian waters using histology. Mean (95% confidence interval) length and age at 50% maturity were 535 mm (463–570 mm) and 2.51 years (2.14–2.79 years). Spawning occurred between October and February, when females produced a mean (±s.d.) of 1 516 680 ± 743 980 oocytes per spawning. Three spawning locations were confirmed based on the presence of postovulatory follicles in a small number of ovaries, although major spawning areas may be located offshore. The present study provides reliable maturity and fecundity ogives that may improve future assessment of longtail tuna stocks throughout the Indo-Pacific region.