Brown macroalgal forests of the complex Cystoseira sensu lato have undergone a severe decline along Mediterranean coastlines, making active restoration strategies, based on the ex-situ cultivation of juveniles from fertile apices, increasingly necessary.

Under the Nature Restoration Regulation, the recovery of Cystoseira s.l. species, including Cystoseira compressa, is a priority action for achieving restoration targets, given their role in shaping macroalgal forests and supporting biodiversity.

This study aims to support restoration efforts by assessing the reproductive performance of wild C. compressa individuals and comparing them with individuals subjected to higher temperature and extended photoperiod (Treatment), as well as with individuals maintained under laboratory conditions that mimic those found in the field (Control).

Ex-situ outplanting was carried out for all the conditions by cultivating the fertile apices on clay tiles and ropes. After three weeks of laboratory cultivation, the density of juveniles (individuals cm⁻²) and their length were measured. Juvenile density was used as a direct indicator of establishment success, defined as their ability to settle and persist on the cultivation substrate.

Cultures that achieved suitable growth were subsequently transferred to the sea and deployed at the Aqua De Mâ finfish aquaculture facility (Lavagna, Liguria, Italy). Juvenile density and length were since then monitored monthly.

The experimental design allows the implementation of C. compressa ex-situ outplanting protocols, while determining how rearing conditions and substrate type influence juvenile establishment and persistence under natural environmental conditions. Comparisons among treated, control, and wild populations are conducted in both laboratory and field settings.

The findings will provide insights into factors influencing both laboratory growth and post-transplanting performance, contributing to the optimization of active restoration and aquaculture techniques for Cystoseira s.l. forests in the north-western Mediterranean Sea.