Stock Enhancement in Greenlip Abalone Part III: Bioeconomic Evaluation

This study presents a bioeconomic evaluation of the effect of stock enhancement on biomass, net present value, profitability, and gross value of product of the Australian greenlip abalone (Haliotis laevigata) fishery. Enhancement targets were defined as a function of natural recruitment (Nr) and compared with current harvest strategies. The model was conditioned on a Western Australian fishery, then applied to greenlip stocks throughout Australia. Two levels of releases (50% Nr and 100% Nr) at varying fishing mortality (F), size at harvest, and size at release were evaluated in detail. Model validation was also undertaken by comparing the model-derived spawning biomass (SSb) with an alternative estimate (SSbf) obtained using in-water surveys and a different growth model. Economic profitability and increased spawning biomass were achieved for most stock enhancement scenarios, and optimal profitability occurred with a 10-20% decrease in F from current levels, a 10% decrease in minimum legal length, and an annual enhancement of Nr juveniles to match natural recruitment. More radical scenarios, such as an annual release of 150% Nr combined with a 30% decrease in size at harvest resulted in greater profitability (+175%) but presented a higher risk of wild stocks being replaced with hatchery genotypes. Sensitivity analysis revealed that mortality, size at release, and harvest price were the critical parameters, while costs of production and fishing were less important. At the national scale, an enhancement scenario involving an annual release of 6.1 million 4-cm juveniles (approximate to age 2) resulted in a 60% increase in gross value of product ($25 to $40 million), a 120% increase in profitability ($12 to $26 million), and net present value ($190 to $420 million; 6% discount), and a 25% increase in SSb.