Assisted Juvenile Recruitment Faces Barriers as a Mussel Restoration Technique

Coastal mussel populations have declined worldwide, largely due to overharvesting, water pollution, climate change, invasive species and other anthropogenic stressors. Mussel restoration projects have emerged to combat these declines, with restoration methods primarily prioritizing mussel transplantations from healthy populations into degraded areas. While this technique can be effective, it is also costly and often infeasible to upscale. One alternative restoration technique is assisted juvenile recruitment through plantigrade or spat provision, which has facilitated the creation of mussel reefs on small scales. This study assessed the effectiveness of upscaled assisted recruitment of juvenile mussels, specifically using plantigrades attached to macroalgae experimentally placed into restored mussel reefs and adjacent bare cobble habitat. Ultimately, low natural settlement of plantigrades to macroalgae (eight plantigrades per gram macroalgae) and high post-settlement losses of plantigrades (> 99%) resulted in assisted recruitment being unsuccessful as very few juvenile mussels small enough to have originated from the plantigrade provision (0.6 juveniles/m²) were found in either habitat. These results demonstrate two key barriers limiting the success of assisted recruitment as a restoration technique: highly variable natural settlement processes to macroalgae and low successful progression from post-settlement plantigrades to established juveniles due to mortality or emigration. Addressing these limitations will require assessing more reliable sources of plantigrades, such as hatcheries, and identifying the mechanisms currently impeding the transition of plantigrades to juvenile recruits.