Forecasting the economic impacts of two biofouling invaders on aquaculture production of green-lipped mussels Perna canaliculus in New Zealand

Resource managers must weigh the costs of preventing biological invasions against the harm that may eventuate from inaction. The costs of intervention are assured, but impacts are typically uncertain. Quantifying the expected economic impacts of invaders before they occur is a pivotal element in justifying expenditure on intervention. We forecast the cumulative economic impacts of 2 invasive biofouling species (Styela clava and Sabella spallanzanii) on New Zealand green-lipped mussel Perna canaliculus aquaculture by combining outputs from an infestation model and ecosystem energy budget model with partial budgeting and equilibrium models. Simulations considered the direct and combined economic impacts of each species on producers and on export markets for the shellfish. Direct impacts on producers were estimated at NZ$23.9 million (Styela clava), $14 million (Sabella spallanzanii) and $26.4 million (both species combined), over a 24 yr period. Societal impacts at the market level were $10.2, $8 and $10.7 million, respectively. The societal impacts reflect changes in producer and consumer surplus after adjustment to altered market prices. Uncertainty boundaries of the estimates were $7.4-91.9, $2.5-56.7 and $7.4-99.7 million, respectively. We assumed that there are few strong alternatives to the New Zealand product on the world market. Producers therefore benefit from any increase in export price by partially shifting production losses caused by the invaders to foreign consumers. Relaxing this assumption produced greater societal impacts ($13.3 million). Slowing the spread of the pests, reducing densities and enhancing the premium market position of green-lipped mussels could significantly mitigate the potential impacts.