Provision of early mussel life stages via macroalgae enhances recruitment and uncovers a novel restoration technique

Mussel reefs have been decimated globally as a result of overharvesting, climate change, invasive species, and disease leading to the proliferation of conservation and restoration initiatives. Achieving and maintaining high recruitment rates is vital to the success of these efforts. Without adequate recruitment degraded areas cannot recover and reefs restored through mussel transplantation will dwindle and eventually disappear. Efforts to enhance recruitment for restoration projects typically prioritize the provision of conspecifics to attract juveniles, relying on the concept that transplanting adult mussels into a degraded environment will improve recruitment. However, the mussel aquaculture industry often pursues a different path to facilitate recruitment to their mussel farms, specifically the provision of wild-caught or hatchery-reared settlement-stage mussels (plantigrades) with the understanding that these plantigrades will then recruit onto substrates in the mussel farm. This technique has clear potential for mussel restoration practitioners seeking to enhance recruitment to a degraded area, however, it is largely untrialled in this context. In this study, a fully-factorial small-scale experiment was conducted to test the provision of adult mussels and plantigrades as techniques to enhance juvenile mussel (Perna canaliculus and Mytilus galloprovincialis) recruitment and growth in Kenepuru Sound, New Zealand, an area with a history of wild mussel overharvesting. After two-months, the provision of plantigrades via settlement-dense macroalgae resulted in a 10-fold increase in recruitment compared to the provision of adult mussels alone, as well as an increase in average juvenile recruit size. Notably, the provision of adult mussels did not enhance overall juvenile recruitment rates, however it did alter recruitment patterns as juveniles preferentially recruited directly onto conspecific adult mussels rather than rocks when provided with both options of substrate in close proximity. These results emphasize the value of healthy macroalgae populations to mussel reef functioning and shed light on a potential bottleneck to recruitment as both macroalgae populations and mussel recruitment rates have declined locally. Ultimately this study highlights the possibility of plantigrade-provision through macroalgae as an efficient recruitment enhancement technique for mussel restoration and emphasizes the importance of both cross-species and life cycle informed restoration.