L-tryptophan dietary supplementation is a useful tactic to increase aquatic animals’ energy stores and antioxidant capability, thereby enhancing their resistance to environmental stressors like ammonia stress. Therefore, the present study aims to investigate the influence of tryptophan supplementation on the growth in Nile tilapia as well as the protective efficiency of tryptophan on blood indices, immune response, and antioxidant status against ammonium chloride (NH ₄ Cl) stress. Fish were fed levels of tryptophan diets (0, 2 g, 4 g, and 8 g/kg diets) for 70 days after which fish were exposed to total ammonia (TA-N) (0 and 5 mg/L; 0.35 NH ₃ –N) for 10 days. The study found that higher dietary L-tryptophan levels led to a linear increase in final body weight ( P = 0.031), weight gain ( P = 0.045 ), and specific growth rate ( P = 0.032) in Nile tilapia fish. However, dietary supplementation with L-tryptophan at 4 and 8 g/kg significantly improved various physiological parameters in fish, irrespective of ammonia (NH ₄ Cl) exposure. Notably, the 8 g/kg dosage yielded the most pronounced benefits ( P < 0.05). L-tryptophan enhanced hematological health by elevating hemoglobin, hematocrit, red blood cells, white blood cells, and lymphocytes ( P < 0.05). It also alleviated NH ₃ –N stress by lowering liver enzymes (alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase), cortisol, and glucose levels, while improving total protein, albumin, globulin, and nitric oxide concentrations ( P < 0.05). Additionally, hepatic antioxidant status improved, as seen by increased superoxide dismutase and glutathione levels, with the highest levels recorded in fish fed on the 8 g L-tryptophan/kg diet and exposed to ammonia, and reduced malondialdehyde content ( P < 0.05). Moreover, L-tryptophan supplementation enhanced immune responses in fish exposed to NH ₃ -N, as indicated by increased levels of complement component (C3 and C4), and IgM ( P < 0.05). These results suggested that L-tryptophan (8 g/kg diet) plays a protective and enhanced role in the growth, immunity, and antioxidant status of Nile tilapia under stress conditions such as ammonia exposure.

Increasing concerns about environmental stability and lack of natural resources have emphasized the immediate need for an innovative approach to manage agricultural–industrial wastes, which has a noteworthy impact on ecosystem and human welfare. Previously seen as a settlement problem, these wastes are now recognized as valuable resources with vast capacity in various industries. Agro-industrial sectors are prioritizing safe and efficient recycling of waste materials to lower environmental effect and establish new economic chances. Progress in biotechnology and green chemistry has improved the transformation of agricultural–industrial waste into high-value products, which promotes permanent industrial practices. The circular bioeconomy promotes global sustainability by innovating waste management practices, supporting environmental goals, and concrete the way for a fresher future. This chapter highlights the amplest agricultural–industrial wastes and their diverse recyclable products, such as microbial enzymes, pigments, single-cell proteins, biofuels, bioplastics, and bioactive compounds. By adopting these permanent practices, agricultural–industrial byproducts can play a key role in creating a more sustainable and resilient world.

As the world’s population rises, the aquaculture sector has put a lot of money into creating ecologically benign, long‐lasting protein sources to replace fishmeal (FM) in aquatic diets. This study evaluated the potential of applying a 1:1 mixture of processed black soldier fly larvae (BSL, Hermetia illucens ) meal and poultry by‐product (PB) meal as a replacement for FM in the diets of Nile tilapia ( Oreochromis niloticus ). Both ingredients were treated with physical, chemical, and enzymatic hydrolysis treatments. A 70‐day feeding trial was conducted to investigate the effects of BSL–PB mixture on growth performance, digestive enzyme activities, feed utilization, nutrient digestibility, intestinal histomorphology, blood biochemical indicators, and hepatic antioxidant activities. Five experimental diets were created to replace FM with PB‐BSL at 0% (control), 25% (PB‐BSL _25% ), 50% (PB‐BSL _50% ), 75% (PB‐BSL _75% ), and 100% (PB‐BSL _100% ). A total of 450 Nile tilapia fry (3.78 ± 0.08 g) was stored in five groups, with three replicates per group. Each replicate was stocked in a 100 L fiberglass tank at a stock density of 30 fry/tank. During the 70‐day feeding period, fish were fed three times daily to ensure adequate nutrient intake. The fish were weighed once every 15 days throughout the experiment. Groundwater was used, and 10% of the water was replenished regularly. Results showed that groups of PB‐BSL _50% and PB‐BSL _75% (containing 50% and 75% PB‐BSL replacement) had significantly increased the values of growth performance, feed utilization, digestibility coefficients, digestive enzyme activity, intestinal histomorphometry, serum biochemical, and hepatic antioxidant activities compared to the control group. In contrast, groups receiving PB‐BSL _25% and PB‐BSL _100% [25% and 100% PB‐BSL replacement, respectively] replacement, respectively) had no negative effect on the overall health of Nile tilapia. Interestingly, the polynomial regression model for feed conversion ratio (FCR) and weight gain (WG) of O. niloticus indicated that replacing 50%–60% of FM with the processed PB‐BSL mixture provides an environmentally friendly and sustainable alternative to FM in Nile tilapia diets.