Gonadal Production and Quality in the Red Sea Urchin Mesocentrotus franciscanus Fed with Seaweed Devaleraea mollis and Ulva australis from a Land-Based Integrated Multi-Trophic Aquaculture (IMTA) System

Harvesting sea urchins from barrens and enhancing their gonads through aquaculture offers a promising way to convert low-value individuals into high-quality seafood. This study evaluated whether red sea urchins (<i>Mesocentrotus franciscanus</i>, RSUs) fed nutrient-enriched seaweeds produced in a land-based integrated multi-trophic aquaculture (IMTA) system could significantly improve gonad size and quality. Two seaweed species, <i>Ulva australis</i> and <i>Devaleraea mollis</i>, were grown in effluent from white seabass (<i>Atractoscion nobilis</i>) tanks and used to feed RSUs over an 8-week period. RSUs readily consumed both seaweeds, with measurable ingestion, fecal output, and absorption efficiency. We hypothesized that RSUs fed IMTA seaweed would exhibit increased gonadosomatic index and improved gonad quality. GSI significantly increased in both groups, from an initial 3.00 ± 0.50% (9.02 ± 1.80 g) to 4.64 ± 0.66% (23.04 ± 10.20 g) in the <i>U. australis</i> group and to 6.35 ± 1.30% (31.20 ± 7.20 g) in the <i>D. mollis</i> group. Gonad quality improved from "D" (unmarketable) to average "B" (high-quality) grade, based on color, firmness, and texture. These results demonstrate that RSUs collected from barrens can be enhanced into premium seafood using nutrient-enriched seaweeds. Integrating RSUs into land-based IMTA systems may increase aquaculture efficiency, reduce waste, and diversify seafood production in a sustainable and economically viable way.