A predatory impact model and targeted stock enhancement approach for optimal release of mulloway (Argyrosomus japonicus)

Habitat, diet, and life history information were used to estimate appropriate stocking density and the potential predatory impact of a stocked finfish. Our Predatory Impact Model uses data from the literature for fish in freshwater or estuarine habitats. Model simulations were run for the Georges River Recreational Fishing Haven (RFH), Sydney, to evaluate appropriate stocking density and associated predatory impact. The estuary contained about 1,760,000 m2 of key nursery habitat for mulloway (Argyrosomus japonicus), and 10% of mysid shrimp production within this habitat was assigned to support stocked fish, as mysids represent the immediate forage requirements of stocked mulloway. Given these values, this section of river could support 17,500 stocked mulloway of 8 cm TL. During the first 3.5 years post stocking, when mulloway are predominantly estuarine residents, predatory impact includes 1 t mysid shrimp, 80 t forage fish, 45 t prawns, 3 t miscellaneous invertebrates and 5 t cephalopods. For comparison, this predatory impact represents 107%, 154%, and 24% of the commercial fishery in Botany Bay/Georges River for forage fish, prawns, and cephalopods, respectively, for 3.5 years before the declaration of the RFH. To maximize the benefit of the approach, a targeted approach to stocking should be taken. Stocked fish should be stocked directly into key habitats, as opposed to being released from a few shore-based sites within the estuarine system.