REPRODUCTIVE FACTORS AFFECTING THE CULTURING AND RESTOCKING OF THE WESTERN SCHOOL PRAWN Metapenaeus dalli

Many prawn species found throughout the Indo-West Pacific are the target of commercial and recreational fishers. One such species, the Western School Prawn Metapenaeus dalli was the focus of iconic commercial and recreational fisheries in the Swan-Canning Estuary in Perth, Western Australia, until its stocks declined dramatically due to over-fishing and anthropogenic influences. While, due to their smaller size and lower market value, there is limited interest in aquaculture of metapenaeids globally, a restocking project has been initiated to increase numbers of this species in the Swan-Canning Estuary. As a result, wild-caught brood stock has been selected for spawning and subsequent restocking of hatchery-reared post larvae.

Critical to maximizing the outcomes of the hatchery culture of M. dalli has been an understanding of the reproductive biology of wild-caught brood stock. Monthly observations, over two years, of the M. dalli in the Swan-Canning Estuary demonstrated a single breeding season each year between Nov-March, peaking in Dec/Jan when water temperatures were above 22 ^oC and salinity > 26. High numbers of females present with a spermatophore, while also containing ovigerous gonads, indicated that female brood stock condition was critical to natural spawning success of this fishery. Histological and morphological analysis of ovarian maturation indicated five stages of development, with ovigerous gonads defined by the presence of acidophilic oocytes with cortical bodies. Gonad Somatic Index peaked at 6.9 + 0.6, with carapace length at fifty percent maturity obtained at 20.01 mm. Additionally, a strong positive curvilinear relationship was detected between gonad weight and oocyte number (r² = 0.797), with individual fecundity ranging from 1.14 x 10⁴ - 1.26 x 10⁵ oocytes.

Overall, it was determined that, due to the natural population being at the extreme end of its thermal range and having comparatively low spawning potential to tropical metapenaeids as a result, lower brood stock conditioning has contributed to the poor recovery of this fishery. Therefore, it is likely that to maximize the chances of long-term recovery of stock numbers, hatchery reared post larvae should be cultured and released to reduce natural mortality during the critical phase of larval development. Selecting wild-caught brood stock of the best possible spawning condition will greatly aide in increasing survival of larvae in the hatchery.