The red sea urchin (RSU, Mesocentrotus franciscanus) is the largest echinoid in kelp forest communities along the west coast of North America, and is a well-established commercial fishery in California. Over-grazing by sea urchins is contributing to the decimation of kelp forests, creating “urchin barrens”. Urchins on barrens have little to no commercial value because of their low gonadosomatic index (GSI). Harvesting urchins from barrens and enhancing their gonad through aquaculture has the potential to transform these destructive grazers into high-quality seafood that also benefits the ecological restoration of kelp forests. In addition to developing formulated diets, seaweeds cultivated with nutrient-enriched effluents in integrated multi-trophic aquaculture (IMTA) systems are considered as good food resources for urchins.
We integrated green macroalgae sea lettuce Ulva lactuca and red macroalgae dulse Devaleraea mollis cultures in the effluent stream of white seabass Atractoscion nobilis tanks in a land-based integrated multi-trophic aquaculture (IMTA) system. Harvested U. lactuca and D. mollis were then fed three times per week to RSUs for eight weeks. Biochemical components of U. lactuca and D. mollis were measured. Sub-samples of RSUs were dissected to assess the GSI (Gonad wet weight/whole urchin wet weight × 100), and gonad quality at the middle and end of the experiment.
The results showed that GSI of RSUs fed with IMTA-harvested U. lactuca and D. mollis was 4.64±0.66 % and 6.35±1.30 % at the end of trial, respectively, with an average weekly GSI increase of 0.18 ± 0.07 %/week and 0.37 ± 0.07 %/week. Gonad color changed from dark brown at the beginning to bright yellow/orange in the end. RSUs fed with either U. lactuca or D. mollis all reached Grade B at the end compared to Grade D at the beginning (Grade A: premium; Grade B: high; Grade C: mediocre; Grade D: unacceptable). The taste of gonads was categorized as “normal” when assessed by a local distributor. The results indicated that U. lactuca and D. mollis cultured in IMTA systems can be used as a sole food source for gonad enhancement of RSUs. Biochemical composition of the seaweeds is pending.