Prawn hatchery modifications and adaptions for temperate marine fish culture in northern NSW, Australia

•We evaluated two existing prawn hatchery facilities for conversion to marine fish culture.•Both hatcheries were easily adapted with minimal cost and modification.•The Australian design was the simplest and cheapest to convert with a theoretical fingerling output of 630,000×40mm (1g) per batch.•The Taiwanese design required more work due to the built-in nature of the concrete tanks with a fingerling output of 320,000×40mm (1g) per batch.•Use of these facilities would resolve the current poor availability and high cost of juveniles for grow-out. Marine fish culture is a new farming opportunity for NSW prawn farmers. To address current seed-stock supply issues two Palmers Island brackish-water prawn hatcheries (of Australian and Taiwanese design) were examined for conversion to mulloway (Argyrosomus japonicus) production. Both hatcheries were easily adapted with minimal cost and modification; the Australian design (1062m2) was the simplest and cheapest to convert. The Taiwanese design (695m2), required more work due to the permanent built-in nature of the concrete tanks, their rectangular shape and drainage. Fingerling output from the Australian hatchery was calculated at 630,000×40mm (1g) fingerlings or 150,000 larger 100mm (12g) fingerlings using a single annual hatchery run of 3 or 5 months, respectively, at a water temperature of 20–25°C. The smaller Taiwanese hatchery had a theoretical maximum production of 320,000×40mm (1g) per batch or 50,000×100mm (12g); if pure oxygen was used in the nursery area this could be increased to 100,000×100mm. Both hatcheries could operate with 3 to 4 staff and use of these facilities, in conjunction with staff training, would resolve the current poor availability and high cost of juveniles for grow-out.