Key biological parameters and commercial fishery for ocean leatherjackets 'Nelusetta ayraudi' (Monacanthidae) off the coast of New South Wales, Australia

Ocean leatherjackets (Nelusetta ayraudi) are distributed along the entire coast of New South Wales (NSW) contributing significantly to the total catch of fish taken from the NSW Ocean Trap and Line Fishery (~340 tonnes in 2005/2006), in addition to other NSW fisheries, such as the Ocean Fish, Ocean Prawn and Haul-net Fisheries. In spite of their significance in commercial fisheries world-wide, only few studies have aged monacanthids. Of those, researchers have mainly used bony structures, such as vertebrae and anterior dorsal spines. In this study, commercially captured ocean leatherjackets were aged by counting growth increments in thin-sectioned otoliths. The periodicity of increment formation was validated using a vital stain oxy-tetracycline (OTC) injected into young of the year fish (0+). The von Bertalanffy growth function parameters provided good estimates of growth, k = 0.163 yr-1, t0 = -0.565 yrs and L∞ = 886 mm. The fishery was found to be dominated by two and three year old fish making up 83% of the ocean leatherjackets captured. The oldest fish (5+) and largest fish from both sexes (male – 605 mm, female – 656 mm) were found in northern NSW. All fish displayed a rapid rate of growth especially as juveniles, with no significant differences in growth between sexes and locations (southern and northern NSW). Ocean leatherjackets showed a geographical and temporal ‘preference’ for spawning. Gonado-somatic indices (GSI) revealed a peak in spawning during August in northern NSW. During this period adult fish were observed, via depth soundings, in large aggregations in depths ranging from 45 to 80 fathoms. Ocean leatherjackets are pelagic egg broadcasters with no parental egg care. Oocytes from ovulated ocean leatherjackets had a mean diameter of 0.66 mm (± S.E. 0.002). The estimated size and age at maturity (L50) for each sex was 352 mm and 2.5+ years old. Mature fish of each sex displayed clear differences in dimorphic and dichromatic features. This research has provided the biological information necessary for future stock assessments of ocean leatherjackets in NSW. In addition, it has contributed to management strategies designed to enhance the sustainability of the ocean leatherjackets fishery.