Influence of mesoscale oceanographic processes on larval distribution and stock structure in jackass morwong (Nemadactylus macropterus : Cheilodactylidae)

The distribution of morwong larvae in the southwestern Tasman Sea was inves tigated during autumn/early winter over three consecutive years (1991-1994) . Larvae were confined to surface tows and large numbers (6-30 mm in length ) were captured up to 250 km east of Tasmania (the limit of sampling). The jackass morwong (Nemadactylus macropterus) dominated samples, although larv ae of the handed morwong (Cheilodactylus spectabilis) were also recorded. N , macropterus larvae were found within water masses derived from the East A ustralian Current (EAC) and Sub-tropical Convergence Zone (STCZ). Back-calc ulated spawning dates, significant differences in otolith microstructure, a nd inferred current patterns suggest that larvae from within each water mas s originate from different spawning regions: a northern one (probably south ern New South Wales and eastern Victoria) and a southern one (probably west ern and southern Tasmania). There was a significant positive relationship b etween larval age and distance offshore. Seasonal movements of the major wa ter masses provide mechanisms that may facilitate regionally self-sustainin g populations in northern and southern regions with an area of recruitment derived from both regions covering eastern Tasmania and Bass Strait. Larval distribution and advection processes suggest spatially variable levels of mixing between spawning regions. These data provide an explanation for both the lack of previously detected population sub-structuring from genetics s tudies and the multiple spawning population scenario suggested by otolith m icrochemistry studies.