First characterization of a nursery ground for the commercial sea cucumber Cucumaria frondosa

This study characterised the nursery ground of the cold-water sea cucumber Cucumaria frondosa for the first time. New recruits and early juveniles 0.9–40 mm in length were discovered at depths of 1.8–2.5 m in Qikiqtait (traditional name of the Belcher Islands, Nunavut, Canada) at a site with salinities ≥27 psu. They were primarily attached to live individuals and empty shells of the blue mussel Mytilus edulis and to stones. Based on laboratory rearing and known spawning times, the smallest individuals (0.9–1.4 mm) found in October 2021 and June 2022 were estimated to be 3–4 and 11–12 months old, respectively (year class 0-I). Other juveniles found at the same time were mostly ≤4-year-old, likely representing subsequent anual recruitment pulses. Densities of juveniles measuring 0.9–40 mm varied between 4 and 104 ind. m⁻². Recruits <2 mm occurred in complex substrata, mostly mussel byssal threads, while larger juveniles, especially those >9 mm, were on exposed surfaces of shells and stones. No adults occupied the shallow nursery site. This study draws attention to ontogenetic migration allowing the occurrence of cryptic nursery sites that may occur in shallower environments than the typical adult habitats, of particular significance for the management of commercial species.