Cultivating Microalgae With Pickled Cabbage Wastewater: A Sustainable Alternative to Replacing Fishmeal in Sea Urchin ( <i>Tripneustes gratilla</i> ) Diets

Recent studies have suggested that plant‐based proteins, such as algal powder, can partially replace fishmeal used in formulated aqua feeds while maintaining the growth performance of aquaculture species. In Taiwan, the traditional pickled cabbage industry produces high‐salinity wastewater rich in nutrients and microelements essential for plant growth. Reusing this nutrient‐rich brine as a medium for microalgae cultivation offers a cost‐effective and sustainable solution. This study aimed to evaluate the feasibility of using Taiwanese pickled cabbage wastewater (TPCW) to cultivate the halotolerant microalgae ( Dunaliella salina ), and of partially replacing fishmeal in Tripneustes gratilla (TG) feed with the resulting algal powder. The experiment consisted of two phases: (1) cultivation of D. salina using seawater partially substituted with varying proportions of TPCW and (2) assessment of the effects of replacing fishmeal with different levels of algal powder on the growth, survival, and gonad index of T. gratilla (0%: TG0, 30%: TG30, 35%: TG35, 40%: TG40, 45%: TG45, and 50%: TG50). Results showed that TPCW significantly enhanced the growth of D. salina , with the TPCW100 group (100% wastewater) displaying the highest growth rate, yielding 10.07 g of algal powder with 59.6% protein. Besides, a notable CO 2 decline in late culture stages indicated strong carbon uptake. For T. gratilla , the TG0 group exhibited the lowest weight gain and gonad index among all treatments, while TG40 showed the highest, significantly exceeding TG0 and TG30 ( p &lt; 0.05). Moreover, higher replacement levels of fishmeal with algal powder also increased gonad redness and yellowness, suggesting greater accumulation of pigments such as astaxanthin and carotenoids. Overall, this study confirms that agricultural waste can be effectively recycled for algal cultivation, contributing to carbon reduction, and that algal protein can successfully replace fishmeal in sea urchin feed, enhancing both growth and gonad coloration.