Stocking density affects survival and gonad indices of sea urchins during commercial scale roe-enhancement

Roe-enhancement aquaculture can transform overabundant, malnourished sea urchins from barren reefs into a valuable seafood product. Numerous experimental scale studies show roe-enhancement is viable, but there are few commercial scale experiments, making commercial viability uncertain. We tested the effects of two diets and three density levels on urchin survival, and roe quality and quantity, during a 13-week commercial scale roe-enhancement trial. 6750 adult purple sea urchins ( Heliocidaris erythrogramma ) were hand collected from an urchin barren by divers into crates on the seafloor at three different densities: 40, 65 and 100 urchins m −2 . The crates were transported in seawater to an onshore rearing facility and placed in 3000 L tanks for the 13-week roe enhancement trial. Urchin mortality was low at 40 and 65 urchins m −2 (3.3 and 5.4%, respectively), but significantly higher at 100 urchins m −2 (11.5%). Densities above 40 urchins m −2 reduced gonad index (GI) by 17%. The proportion of marketable urchins at 40 urchin m −2 (63.2%) was higher than at 65 urchins m −2 (46.5%), but not statistically different from 100 urchins m −2 (49.2%). Diet type had no significant effect on gonad index, gonad quality or the marketability of the urchins. GIs and roe quality in all treatments exceeded market standards but were low compared to laboratory trials. Through careful transport and culture techniques, commercial scale roe-enhancement while ensuring acceptable mortality rates is viable, even at high densities, but differences observed in the GIs achieved by laboratory and commercial scale trials indicate substantial room for improvement.