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In the pond culture of Chinese mitten crabs (Eriocheir sinensis), high stocking densities usually increase the frequency of agonistic behavior, which can directly impact crab integrity, survival, and growth and result in economic losses. We hoped to reduce agonistic behavior by adding additives to feed. In this study, we first observed the agonistic behavior of E. sinensisin 5 molting periods (from first molting to puberty molting) during the adult grow-out stage. Then, we assessed the effects of L-TRP as a feed additive on the agonistic behavior of juvenile, precocious, subadult, and adult crabs. Finally, we performed 106 days of cage culture to assess the effects of L-TRP on feeding behavior, animal vitality, growth performance, and nutritional composition. The results showed that the intensity of agonistic behavior was significantly higher after puberty molting than in other molting periods during the adult grow-out stage. Moreover, L-TRP supplementation inhibited the agonistic behavior of juvenile, subadult, and adult crabs, but it did not affect the precocious crabs. During cage culture, L-TRP significantly improved crab vitality after 3 weeks but not after 6 and 9 weeks, but it reduced the ingestion rate after 9 weeks. Moreover, the L-TRP group showed a significant increase in the weight gain rate and specific growth rate after the fourth molting. After 106 days of culture, the content of proline and fatty acids in the muscle decreased significantly after L-TRP diet supplementation. Some long-chain polyunsaturated fatty acids also decreased significantly in the hepatopancreas. However, we must focus on the adverse effects of L-TRP on Pro and fatty acid composition. Further research is required on the balance between inhibition of agonistic behavior and effects of L-TRP on the nutritional quality of E. sinensis.