Significant impact from blue mussel Mytilus galloprovincialis biofouling on aquaculture production of green-lipped mussels in New Zealand

Biofouling by blue mussels Mytilus galloprovincialis is a persistent regional-scale problem for green-lipped mussel Perna canaliculus aquaculture in New Zealand's Marlborough Sounds. M. galloprovincialis impacts on P. canaliculus crop production were assessed using a 4 yr dataset representing 243 different mussel farms. Together with information on other production impacts from M. galloprovincialis and associated management costs, we estimated the regionalscale economic consequences of mussel biofouling. Mean ( 95% CI) M. galloprovincialis cover on mussel crops across the study region was 9.17 0.5%, with the worst-affected crops experiencing up to 99% cover. P. canaliculus yield per crop depended on the spat type used for grow-out, and declined with increasing grow-out duration at a rate of ca. 1 kg yr -1 m -1 of cultivation line, due to M. galloprovincialis biofouling and a range of other possible factors. For M. galloprovincialis alone, regression models predicted a decrease in annualised P. canaliculus yield of ca. 5 to 10% at mean M. galloprovincialis cover, depending on spat type. This decline represented an average loss of regional economic value of $11.4 million yr -1 USD (range $8.0 to 15.4 million). When impacts on seed-stock supply and costs incurred for mitigation were also accounted for, the economic loss from M. galloprovincialis biofouling was estimated at $16.4 million yr -1 , which represents 10% of the regional value of the mussel industry. The discussion highlights a dearth of comparable quantitative studies of the economic consequences of biofouling. There is a need for consistent approaches to reporting such consequences to enable comparisons among different locations, biofouling species and types of aquaculture.