Pilot studies for stock enhancement of purple sea urchins (Paracentrotus lividus, Lamarck, 1816): usefulness of refuges and calcein marking for the monitoring of juveniles released into the natural environment

In the framework of stock enhancement, field experiments were conducted between March and July 2021 in rocky shores of Central Portugal to test the usefulness of refuges and calcein marking for juvenile sea urchins releases. Individuals with 10–20 mm in test diameter were captured in nature and tagged through immersion in a calcein bath with a concentration of 150 mg L−1, during 48 hr. Artificial shelters were used to provide refuge and an acclimatization structure for the released sea urchins, and in situ monitoring was carried out by counting the marked specimens over three months. Results point out to the importance of using shelters to provide protection to sea urchins, and validated the efficiency of the calcein tagging protocol for in situ monitoring. Sea urchins' test diameter growth during the experiment was estimated to be 0.470 mm month−1 (SD = 0.181).