Finishing feeds are widely used in terrestrial livestock and aquaculture industries to enhance final product quality before harvesting. These feeds are often suboptimal in terms of nutrition or cost during different phases of an animal’s lifecycle necessitating their use for short periods of time. This study explored whether similar strategies could improve gonad quality in purple urchin (Strongylocentrotus purpuratus) roe enhancement aquaculture, focusing on optimal timing for use of an extruded diet in combination with kelp (Macrocystis pyrifera) as a finishing feed. Scientific literature as well as industry perceptions have typically found that formulated diets produce roe that, while superior in gonad growth, are often inferior for roe quality compared to natural diets of macroalgae. The objective of this research was to determine if providing a pelleted feed followed by varying durations of kelp can enhance gonad size, texture, and color compared to single-feed controls.
Wild-caught purple urchins from a barren in Fort Bragg, CA, were used in a 10-week feeding trial. Five feeding treatments were tested: (1) 4 weeks of pellets + 6 weeks kelp, (2) 6 weeks pellets + 4 weeks kelp, (3) 8 weeks pellets + 2 weeks kelp, (4) control (kelp only, 10 weeks), and (5) control (pellets only, 10 weeks). Each treatment was fed to triplicate tanks with n= 25 individuals per tank. A subset of urchins were sampled at 4, 6, and 8 weeks just prior to switching to feeding the kelp diet. The remaining urchins, including controls, were sampled at the conclusion of the trial at week 10. Urchins were sampled for gonad somatic index (GSI), texture profile analysis (TPA), feed intake, and objective color via colorimeter implementing the CIE system (L*a*b*).
Results showed that gonad growth rate decreased in all treatments once kelp replaced the pelleted diet, with the kelp-only control showing the lowest GSI and the pellet-only control the highest GSI. Color analysis revealed that pellet-heavy treatments exhibited higher a* and b* values, indicating enhanced red and yellow hues in the roe compared to the kelp-only control. Texture profile analysis indicated that the pellet-only control had the highest chewiness, with values significantly higher than those in kelp-only or shorter pellet-phase treatments. While the difference in GSI was predicted between pellets and kelp, it was unexpected that results showed that the formulated diet used in this study may produce comparable or higher quality roe than kelp based on color and texture. It should be noted that a quantitative taste test was not performed on the roe from this study. The application of a kelp finishing feed paired with the formulated diet from this study for purple urchins was shown to be unnecessary, indicating that specific extruded diet formulations can achieve roe quality comparable or greater to kelp and in a significantly shorter timeframe improving farm production efficiency.