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 investigated during autumn/early winter over three consecutive years (1992-1994). 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 larvae of the banded morwong (Cheilodactylus spectabilis) were also recorded. N. macropterus larvae were found within water masses derived from the East Australian Current (EAC) and Sub-tropical Convergence Zone (STCZ). Back-calculated spawning dates, significant differences in otolith microstructure, and inferred current patterns suggest that larvae from within each water mass originate from different spawning regions: a northern one (probably southern New South Wales and eastern Victoria) and a southern one (probably western and southern Tasmania). There was a significant positive relationship between larval age and distance offshore. Seasonal movements of the major water masses provide mechanisms that may facilitate regionally self-sustaining 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 studies and the multiple spawning population scenario suggested by otolith microchemistry studies.