Urchin barrens along the California coast disrupt the natural, productive kelp forests, displacing important habitats and native species. Uni ranching, which involves collecting and fattening the gonads (uni) of starved urchins from barrens, offers a potentially profitable, new opportunity to produce uni. We measured uni growth and quality in purple sea urchins (Strongylocentrotus purpuratus, PSU) over 12 weeks using four different diets: (1) an experimental formulated diet (EFD), (2) nutrient-enriched Ulva australis and (3) Devaleraea mollis, and (4) wild collected Macrocystis pyrifera. U. australis and D. mollis were considered enriched because they were cultured in the effluent stream of white seabass (Atractoscion nobilis). PSUs were fed 2% bodyweight/ind/day, and feces were collected weekly to calculate ingestion rate (IR), fecal rate (FR), and absorption efficiency (AE). PSUs were sacrificed every three weeks to measure gonadosomatic index (GSI) and quality. Quality was graded by color (CIELAB color space, where L* (lightness), a* (green-red), and b* (blue-yellow) values were used to calculate ΔE* for quantifying color difference), texture (granulation size), and firmness.
GSI from PSUs fed the EFD (15.7 ± 3.9%) and M. pyrifera diet (13.7 ± 2.7%) were not significantly different, but were significantly higher than U. australis (8.7 ± 3.1%,) and D. mollis (6.3 ± 2.1%) (Figure 1). Quality improved from grade ‘D’ to grade ‘B’ after 12 weeks in all treatments. Granulation size was significantly smaller for gonads fed D. mollis (0.64 ± 0.2mm) compared to the EFD, M. pyrifera, and U. australis (0.82 ± 0.24, 0.76 ± 0.26, 0.93 ± 0.26 mm). There was no significant difference in firmness of gonads between any diets. Based on values of L, a and b, ΔE was highest in gonads from EFD (17.54), followed by U. australis (16.75), M. pyrifera (15.87), and D. mollis (8.21). The EFD, which had the highest lipid content (6.42%) resulted in the greatest PSU IR (2.55 ± 1.14 DWg/ind/day) and AE (97.71 ± 1.52%). In contrast, PSUs fed M. pyrifera exhibited lower AE (68.78 ± 19.45%). D. mollis and U. australis had highest protein content (35.4 and 27.6%), but yielded the lowest PSU IR (0.17 ± 0.04 and 0.25 ±0.12 DWg/ind/day). Gonads of PSUs reached market size (GSI >10%) when fed with EFD and M. pyrifera for 12 weeks. D. mollis and U. australis require more time or feed supplementation to reach market size. Future research will extend the feeding period when using nutrient-enriched U. australis as the sole diet, and will also focus on mixed diets (EFD and seaweeds) in enhancing PSU gonad production and quality.