Hydrodynamic factors drive mesoscale salp distribution in the northwestern Mediterranean Sea

Salps are pelagic tunicates capable of rapid growth, impacting marine ecosystems. Salp blooms may result from the effect of environmental factors on its biology or physical aggregation caused by hydrodynamics. Understanding salp distributions requires a regional perspective, but most studies in the northwestern Mediterranean have been local. We (i) analyse how mesoscale spatial patterns are structured by local physical and biological conditions and (ii) characterize life-stage distributions of Salpa fusiformis and Thalia democratica. Using generalized additive models, we analysed spatial patterns across 80 stations along transects off the Catalan coast. Results showed that S. fusiformis concentrated along the shelf break and slope, following the Northern Current, with occurrences shaped by transport, passive accumulation at the shelf-slope front and intrusions around anticyclonic eddies. In stations with high S. fusiformis abundance, populations were characterized by a higher proportion of mature oozoids and blastozoid buds, suggesting active asexual reproduction. Thalia democratica was restricted to warm stratified coastal waters and absent from colder, less stratified northern areas influenced by slope water intrusions. Its population was dominated by blastozoids with a visible embryo, suggesting limited growth. This study shows that mesoscale hydrodynamics are important in shaping salp distribution, while indirectly influencing population demography in the northwestern Mediterranean.