Wild shellfish reefs have been decimated in many parts of the world over the last century, diminishing their vital ecological roles as habitat generators and the ecosystem services they provide, such as water filtration. Over this same timescale, shellfish aquaculture has rapidly expanded to become an impressive global industry with an annual worldwide production worth US$35.4 billion in 2020. Both wild reefs and aquaculture operations typically rely on abundant shellfish settlement levels to maintain their respective populations. At the same time, shellfish aquaculture has the potential to influence settlement, as the addition of cultured shellfish to an ecosystem increases the quantity of reproductive adults and may therefore increase settlement rates. Alternatively, shellfish aquaculture may lead to an overall reduction in settlement in an ecosystem, either directly through cannibalistic consumption of larvae or indirectly by straining carrying capacity. We assessed the role of marine shellfish aquaculture on settlement by comparing changes in the abundance of settling green-lipped mussels Perna canaliculus with the expansion of mussel farms at the north end of New Zealand’s South Island over a 47 yr timespan. Overall, mussel settlement did not increase over this period despite an estimated 16000-fold increase in the number of mussels living in the region as mussel aquaculture proliferated. The disconnect between the extent of mussel settlement and mussel aquaculture was consistent across 3 separate areas within the region, suggesting that aquaculture mussels may be unable to produce larvae capable of settlement and emphasizing the importance of wild mussel populations for ecosystem resilience.

Restoration of mussels typically focuses on either subtidal or intertidal habitats, although it is important to consider the full historical range of a species. However, it remains unclear how environmental changes can impact the ability of mussels to survive in tidal heights where they occurred historically. Additionally, there is limited research on the viability of reducing mussel stock size for restoration purposes. In this study, green-lipped mussels Perna canaliculus of 2 size classes (80 and 60 mm) were assessed when transplanted as a single size class or as mixed cohorts in 9 m2 plots at 3 shore heights (i.e. neap low tide, spring low tide, and subtidal). The mussels were sampled over a 1 yr period to understand the effect that shore height and size class had on mussel metrics, such as survival, growth, and condition. The results revealed that shore height had a greater effect than size class on mussel survival, with a total loss of mussels transplanted into areas that were exposed at neap tides in contrast to 39% mussel survival transplanted into areas that were only exposed on spring low tides. Further, mussels transplanted in the adjacent subtidal had higher overall survival (74%). This suggests that aerial exposure time determines the upper vertical limit for restoration by transplantation of mussels sourced from aquaculture, despite their historical distribution. The results of this study also support the use of smaller mussels (~60 mm) for transplantation for mussel reef restoration, as a 25% reduction in size resulted in 50% more mussels being deployed.