Boom-Bust dynamics in native and non-native mussel Mytilus spp. and their hybrid crosses in Irish near shore environments over two decades.

Mussels of the genus Mytilus , including the blue mussel Mytilus edulis , the Mediterranean mussel Mytilus galloprovincialis , and their hybrids, play key ecological roles in nearshore habitats across Europe. Over recent decades, Mytilus populations have undergone pronounced fluctuations, with localised declines and subsequent recovery reported in multiple regions. Here, mussels were surveyed across sixteen sites around the Irish coastline between 2003 and 2023, integrating contemporary sampling with historical datasets to assess long-term changes in species composition and population structure. Species identity was determined using molecular markers, and biometric data were analysed in relation to environmental variables, including sea surface temperature (SST), salinity, chlorophyll a as a proxy for food availability, and the Atlantic Multidecadal Oscillation (AMO). M. galloprovincialis was the dominant species at most sites, particularly at locations characterised by higher and more variable SSTs, whereas M. edulis and hybrids were more prevalent at cooler, thermally stable sites. Salinity and chlorophyll a varied markedly among sites but showed no consistent temporal trends, with mixed species assemblages, including persistent hybrid populations, more commonly associated with intermediate environmental conditions. M. galloprovincialis was significantly overrepresented in the smallest weight cohort (<5 g), indicating recent recruitment. Mussels were absent from two sites where they had been recorded historically, suggesting site-specific population collapse. Overall, the results reveal dynamic “boom–bust” patterns in Irish Mytilus populations possibly associated with long-term environmental variability, highlighting the importance of sustained, regionally focused monitoring to understand population resilience and recovery in a changing marine environment. • Long-term (2003–2024) survey shows climate-driven shifts in Mytilus species; • M. galloprovincialis expanded with rising sea surface temperature and variability; • M. edulis and hybrids persisted in cooler, thermally stable coastal habitats; • Salinity variability and wave exposure shaped hybrid and species distributions; • Boom–bust mussel population cycles linked to Atlantic Multidecadal Oscillation.