The purple sea urchin Strongylocentrotus purpuratus is over abundant in California, turning many once-productive kelp forests into low productivity urchin barrens. Resource managers and stakeholders alike, are concerned with large reductions of kelp and are investigating ways to recovery kelp ecosystem including urchin removal. While a significant urchin fishery currently operates in California, increased fishing effort to remove these overabundant urchins is not currently commercially viable due to the unmarketable condition of the urchin gonads, being that they are primarily too small to justify commercial capture. However, capture and subsequent culture of wild urchins in an aquaculture setting can enhance gonad quality and offers a possible solution to economically incentivize their removal and restore kelp ecosystems . We investigated co-culture of S. purpuratus and Pacific dulse , Devaleraea mollis, in an integrated multi-troph ic aquaculture (IMTA) system to maximize efficiencies by allowing urchin metabolites to enhance seaweed growth which also serves as the urchin’ s feed. We predicted that we could cultivate a self-sustaining co-culture (constant seaweed biomass ) with urchins and dulse in the same tank using seaweed tumble culture, reducing infrastructure footprint, husbandry effort and feed associated costs. This study aimed to identify th e optimal urchin stocking density for such a system. We stocked 1350 urchins in 10 (986 L) conical bottom tumble culture tanks of increasing densities of urchins ( 0, 0.453, 0.599, 0.789, 1.046, 1.271, 1.354, 1.489, 1.802, 1.954 kg/m2) with a fixed starting density of 3.79kg/m2 dulse , for four consecutive two-week trials after which seaweed biomass was measured. Increasing urchin densities did not affect urchin mortalities for any of the trials and the mean number of mortalities per tank was low (0.94 +/- 1.60). Increasing urchin densities did affect mean total dulse growth (kg) and mean daily dulse growth rates (kg/day-1 ) in a negative, linear fashion (p<0.001). Our results demonstrate how urchin can be co-cultured with dulse, without increased mortality, for up to urchin densities of at least 1.954 kg/m2 , and positive dulse growth can occur in urchin densities up to 0.789 kg/m2 equating to 100 urchins per tank for the study. While the study shows the technical feasibility of this urchin fattening system it remains to be determined if the potential benefits of this IMTA design outperform a more conventional system reliant on exogenous supply of feed an likely higher urchin stocking densities.