Long-term assessment of recruitment, early stages and population dynamics of the endangered Mediterranean fan mussel Pinna nobilis in the Columbretes Islands (NW Mediterranean)

- Recruitment of Pinna nobilis in artificial collectors showed high annual variability.
- Recruitment in artificial collectors was correlated with water temperature at 25 m in June.
- Larval settlement concentrates mainly in September, showing two spawning events.
- Natural recruitment and mortality were very low, evidencing the slow dynamics of the species.
- Fast growth in early years and size distribution show a possible refuge size of 45 cm.

A long-term experimental approach was undertaken to assess viability and resilience of the endangered Mediterranean fan mussel Pinna nobilis. Artificial and natural recruitment, mortality, population traits and juvenile growth were assessed in seasonal and annual surveys. In the Columbretes Islands, P. nobilis thrives in differing substrate types, from coarse sand to boulders, in Cymodocea nodosa meadows and among rhodoliths, and is always found sharing habitat with the less abundant sibling species P. rudis. In artificial collectors larval settlement occurred over a several months period, concentrating its peak in September and resulting from two separated spawning events. Recruitment in the collectors showed high inter-annual variability and was independent of depth, but positively correlated with seasonal water temperature increase in June. Natural recruitment of P. nobilis was low and showed little variability, evidencing the existence of intense post-settlement processes. Adult mortality was also low, thus leading to slow population dynamics and to the species' vulnerability to catastrophic events. Population size structure suggests the existence of a refuge size above 45 cm shell length. The fast growth during the first years of life would help shortening this vulnerability period. Altogether, essential information and tools for the species' conservation are provided, which will be critical in the current context of mass mortalities affecting P. nobilis.