Improving the efficiency of mussel reef restoration

Abstract

The translocation of shellfish from aquaculture to sites on the seafloor is a popular method used in the restoration of shellfish reefs. Globally, there is interest in developing the practice to restore function to degraded nearshore systems such as benthic habitat, water quality regulation, and sediment stabilisation. In New Zealand, the translocation of adult green-lipped mussels (Perna canaliculus) to restore lost subtidal reefs has so far been a slow and expensive process, demonstrating a need to make the practice more efficient to achieve large scale restoration goals (e.g., >1000 km2 of restored reef habitat). Through multiple field and lab experiments, this body of research investigated the efficacy of translocating sub-adult and juvenile mussels to increase the number of mussels translocated per kg harvested (Chapter 2, 3), the importance of mussel source to improve translocated mussel survival (Chapter 3), and timing deployments to limit predation and improve translocated mussel survival (Chapter 4). Experimentally translocated sub-adult and juvenile mussels consistently exhibited high mortality (60 – 100% loss) in the hours-to-days following transfer to the seafloor. These losses were often attributed to removals by mobile predator species, like Australasian snapper (Pagrus auratus) and New Zealand eagle rays (Myliobatis tenuicaudatus), who visited experimentally translocated mussels within 24 hours after placement at experimental sites. High mussel mortalities quickly demonstrated that restoration practice will need to account for mussel losses before more efficient practice can be scaled up. This study provides evidence that this may eventually rely on the selection or culture of sub-adult and juvenile mussels from resistant stock (e.g., with crush-resistant shells and/or numerous attachment threads) to improve mussel resilience against translocation stresses from harvest, transport, and transfer to the seafloor. Further improvement to mussel survival following transfer to the seafloor will depend on planning translocations for times of year, or at sites, where predator abundance is low. Overall, the results of this thesis suggest that efficient mussel reef restoration practice is attainable but will require further study of the nuanced relationships among mussel physiology, reef-building behaviour following transfer to the seafloor, and the community of organisms likely to be attracted to restored reefs.