Detection of sub-tidal mussel beds on rocky reefs using acoustics

Abstract

The collection of recently settled wild mussels from beach-cast seaweed provides the seed for a large and rapidly expanding mussel aquaculture industry in New Zealand, but the locations of adult populations producing this seed are unknown. Traditional methods of assessing sub-tidal habitats of rocky reefs (diver census, video camera drops) are neither cost-effective nor practical for the relatively large spatial scales involved. Remote sensing tools offer solutions to this problem. We tested the ability of the acoustic seafloor classification system, Quester Tangent Corporation (QTC) Impact, to detect the presence of green-lipped mussel (Perna canaliculus) populations on sub-tidal rocky reefs. Acoustic data were collected from a range of visual subtidal habitats, combined, and analysed using QTC Impact's principal component analysis and clustering software. Discriminant function analysis assessed whether visually distinctive habitats could be correctly classified using this acoustic data only. Consistent and reliable classifications were found for continuous mussel beds (100% correct, 0% misclassification from other habitat types), and sand, while the other various classes of rocky reef habitats (i.e. reef with varying levels of biological cover of kelps and encrusting invertebrates, including occasional mussel patches at one site) were identifiable to a general class of rocky reef, without significant mussel cover. These results demonstrate that QTC Impact can reliably discriminate subtidal mussel beds on rocky reefs from other habitats, making it feasible to proceed with large-scale mapping.