Abstract
<title>Abstract</title> <p> The Mediterranean mussel <italic>Mytilus galloprovincialis</italic> is one of the world’s most successful marine invaders and represents a growing threat to mussel aquaculture. Throughout 2024, we monitored nine mussel farms across Santa Catarina Island Bay, southern Brazil (27°37′10.66″S; 48°33′8.87″W), using commercial spat-collector and grow-out ropes to assess the species’ current distribution and recruitment. <italic>M. galloprovincialis</italic> recruitment occurred year-round across all monitored sites, exhibiting strong spatial heterogeneity. Recruitment rates in spat collectors revealed that <italic>M. galloprovincialis</italic> already recruits as successfully as the native mussel <italic>Perna perna</italic> . Furthermore, despite the site-specific variation during initial recruitment, <italic>M. galloprovincialis</italic> reached significantly higher overall densities on cultivation ropes. Individuals ranging from recently settled juveniles to adults were found on grow-out ropes across all sites, with extreme density events favoring the invasive species, indicating its successful establishment and persistence throughout Brazil’s principal mussel-producing region. These findings demonstrate that <italic>M. galloprovincialis</italic> is now widely established in southern Brazil and undergoing sustained recruitment, highlighting the need for continued monitoring and targeted management of this invasive species. </p>