Restoration, recruitment, and recovery of green-lipped mussels in Kenepuru Sound, New Zealand

Abstract

Populations of reef-building shellfish have been decimated globally by overharvesting and habitat loss, leading to a corresponding degradation in the vital ecosystem services they provide like water filtration, denitrification, and sediment stabilization. Shellfish restoration initiatives are emerging worldwide to counteract these declines, including in New Zealand where the endemic green-lipped mussel (Perna canaliculus) is a taonga (treasured) species. Of particular interest to green-lipped mussel restoration efforts are the Marlborough Sounds, a complex drowned river valley system that anecdotal reports indicate once supported extensive green-lipped mussel reefs. The research presented in this thesis lays the groundwork for green-lipped mussel restoration in the Marlborough Sounds by systemically reviewing the existing evidence base for shellfish restoration and then narrowing down to establish comprehensive baseline data for mussel reefs in Kenepuru Sound, an inner branch of the Marlborough Sounds. This baseline data revealed a 97% decline in local mussel reefs as a result of overharvesting in the 1970s as well as a complete lack of recovery of mussel reefs following the closure of the fishery in the 1980s. The remainder of the thesis focuses on why these reefs have not recovered and what restoration initiatives can do to jumpstart natural recovery. Specifically, multidisciplinary methods including observational studies, field experiments, and computational analyses are used to investigate each stage of the mussel life cycle (i.e., adult survival, larval supply, early settlement, and recruitment) as possible bottlenecks to recovery. The results of this thesis suggest that neither adult survival nor larval supply are hindering natural recovery as restored adult mussels maintained high survival one-year post transplantation and larval supply was present over a forty-year period of monitoring. However, evidence suggests that early settlement stages may be a key factor preventing natural recovery in the area as almost no juvenile mussels were found to have settled on naturally occurring substrates despite clear larval supply, potentially due to high levels of suspended sediment. Additionally, recruitment was only recorded after the provision of post-settlement stage mussels, further emphasizing the importance of settlement to mussel reef recovery. On a local scale these results provide a clear direction for future restoration efforts by establishing concrete baselines, demonstrating the feasibility of local adult mussel restoration, and pinpointing potential bottlenecks to recovery. More broadly, the research presented in this thesis validates a life cycle informed approach to ecosystem recovery and demonstrates the value of novel restoration techniques that improve outcomes across multiple life stages of the target ecosystem.