Introduction Brown shrimp, Crangon crangon, present the major coastal fishery in the southern North Sea with annual landings of up to 40,000 t (ICES, 2022). While only the abdominal muscle fraction is used for human consumption, the unused processing remains account for up to 60 % of the shrimp biomass. Incorporation of the remains into aquafeeds for high-valued species like L. vannamei would create by-product value, reduce waste, and promote circular economy approaches. This study aims to assess the suitability of brown shrimp processing remains (BSPR) as a sustainable feedstuff for L. vannamei to replace fishmeal. A detailed biochemical characterization was made and key apparent nutrient digestibility coefficients of BPSR examined. Controlled feeding trials with juvenile L. vannamei were conducted to determine effects on growth performance, feed utilization, digestive enzyme activities, and immunological parameters. Materials and Methods Frozen remains of mechanically peeled brown shrimp were obtained from a shrimp trader in northern Germany, dried at 60 °C, and ground to a homogenous meal. A detailed biochemical characterization of the meal was made using established methods (AOAC, 2010; Folch et al, 1957; Percot, Viton and Domrad, 2003) and by certified laboratories. To measure apparent nutrient digestibility coefficients, a reference and test diet was prepared using yttrium as inert marker. A 40-day feeding trial with 192 juvenile shrimp (6.5 ± 1.3 g) was conducted using pelleted diets replacing 0, 50 and 100 % of the fishmeal content with BSPR. Survival, molting frequency, growth, digestive enzyme activities, and muscle proximate composition of L. vannamei was determined. Based on the first trial, a second feeding trial with 720 shrimp (4.31 ± 1.0 g) was performed with six extruded feeds replacing the fishmeal content in steps of 20 % from 0 to 100 % to obtain an estimate for the optimal substitution level. The high water stability of the extruded pellets allowed for accurate measurements of feed intake and assimilation efficiency. Hemolymph was analyzed for phenoloxidase and prophenoloxidase activities and protein content. Results and Discussion Brown shrimp processing remains contain substantial amounts of key nutrients (521 g·kg-1 crude protein, 74 g·kg-1 total lipid, 15 MJ·kg-1 gross energy), minerals (phosphorus, magnesium, copper, manganese, selenium, zinc, and valuable functional ingredients (cholesterol, astaxanthin, chitin). Apparent digestibility coefficients of energy, protein, methionine, lysine, and copper exceeding 80 %, demonstrated excellent nutrient bioavailability of BSPR. In the first feeding trial, shrimp showed best growth with diets containing a mixture of 50 % fishmeal and 50 % BSPR. No negative effects on muscle quality (moisture, protein, lipid) appeared and digestive enzyme activities were not affected by diet. The daily molting ratio increased with higher BSPR inclusion levels and a higher molting synchronicity was observed. The second feeding trial showed significantly enhanced growth when the BSPR substitution level exceeded 60 %. Based on the polynomial regression of the BSPR substitution level and shrimp weight gain, best growth is expected at 85 % inclusion level (Figure 1.). Different crustacean meals have been shown to stimulate growth in penaeid shrimp (Cordova-Murueta & Garcia-Carreno 2002; Williams et al. 2005; Nunes et al., 2019 ). This has been attributed to well-balanced nutrient profiles, increased palatability and even unknown growth promoters. Yet it remains unclear what the underlying mechanisms are. The shrimp hemolymph parameters (protein, phenoloxidase, prophenoloxidase) and digestive enzyme activities were not affected by the diets. Therefore, the enhanced growth performance might primarily be related to superior nutritional value and not immunological or digestive enzymatic improvements. We observed a reduced feed intake at concomitantly improved growth with diets containing 80 – 100 % BSPR. This supports the hypothesis that inherent component(s) of BSPR may promote growth in L. vannamei. Since the molting behavior of shrimp was also affected by the dietary treatment, it is possible that chitin-oligomers or chitin-protein derivatives in the BSPR are beneficial for shrimp growth. Good bioavailability and supply of key dietary nutrients further underline the suitability of BSPR as excellent feedstuff for L. vannamei. References Aline Percot, C. V., and Alain Domard. (2003). Optimization of Chitin Extraction from Shrimp Shells. 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