Stocking density and rearing environment affect external condition, gonad quantity and gonad grade in onshore sea urchin roe enhancement aquaculture

Abstract

Finding the ideal density to optimise growth, health and welfare of aquaculture species reared in cage or tank environments allows farmers to produce the best quality product per unit area. Appropriate stocking densities are well known for most major aquaculture species, but limited information exists for the developing sea urchin aquaculture industry. The temperate purple sea urchin Heliocidaris erythrogramma is a prime candidate for aquaculture, yet how stocking density and/or the rearing environment influence urchin health and roe production remain unknown. Here, we tested whether stocking density (low vs. high) and rearing environment (individual vs. group) influences urchin external condition, gonad index (GI) and roe grade in H. erythrogramma after 12-weeks of roe enhancement during autumn-winter (April–June) and winter-spring (July–September) periods. Across rearing environments, high density reduced the proportion of urchins with healthy external condition by 26–30% during autumn-winter and 7–20% in winter-spring compared to low density. During the autumn-winter period, while higher density resulted in lower GIs in the group reared treatments, higher density did not affect the GI of individually reared urchins. Rearing environment affected gonad grade in the autumn-winter period, with a higher proportion of quality (B-grade) roe produced in individually reared treatments compared to group reared treatments. Our results highlight the importance of stocking density and rearing environment for onshore urchin roe enhancement. Maximising production will require a trade-off between investing in greater infrastructure cost to individually rear urchins at high density to ensure higher GIs versus investing in less expensive infrastructure, but group rearing urchins at lower densities to ensure high GIs after enhancement.