Expansion of shellfish aquaculture has no impact on settlement rates

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.