A Predatory Impact Model and Targeted Stock Enhancement Approach for Optimal Release of Mulloway (Argyrosomus japonicus)

Abstract

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.

Keywords:

• stocking density
• key habitat
• Argyrosomus japonicus
• targeted stocking

Notes

^# These parameters were excluded from the sensitivity analysis.

* Daily production estimates for the temperate estuarine mysid Rhopalophthalmus terranatalis.

^# These values are totals for the entire post-stocking period of estuarine residency (∼3.5 y). Commercial catch is the total reported commercial catch in the Botany Bay/Georges River Recreational Fishing Haven for 3.5 years before the establishment of the recreational fishing haven 1997–2001 (NSW DPI Commercial Catch Statistics Database).

* Note these are maximum values for the period of estuarine residency only, as peaks in C may not be attained during the period of mulloway estuarine residency for these species and may occur after fish leave the estuary.