Over the last few decades, the demand-driven growth in aquaculture has been tremendous due to development and adoption of cutting-edge technologies and production systems such as intensification and high input aquaculture. This growth, however, has witnessed an insurgence of various environmental and husbandry-related stressors posing major challenges to the aquaculture sector limiting its further expansion worldwide. These stressful conditions impair the health of cultured animals and predispose them to diseases resulting in economic losses. Hence, optimizing the stress resilience of the cultured species is paramount to ensure sustainable development of aquaculture sector. Addressing the concerns of stress mitigation in aquaculture, a plethora of research investigations have been intensively carried out all over the world targeting the management of cultured environment as well as cultured species. Among various mitigation strategies, dietary interventions appear to be pliable and a sustainable approach to develop immunocompetence and stress resistance in fish as requirement of several nutrients may increase under stressful conditions. The suboptimal and/or excessive levels of several essential and non-essential nutrients modulate the immune response and stress/disease resistance. Hence, the present review is an inclusive and informative compilation of dietary interventions for management of stress in aquaculture. This review highlights the stress-mitigating roles of different nutritive and non-nutritive compounds/additives such as amino acids, essential fatty acids, phospholipids, vitamins, minerals, carotenoids, different synthetic chemicals and biological derivatives from bacteria, yeast, fungi, algae, plants and animals as a promising stress management strategy to enhance the overall performance of the cultured species.

Two experiments were conducted to elucidate the possible effects of dietary L-tryptophan (TRP) in Labeo rohita based on growth performance and physio-biochemical responses. In the experiment I, a 60-day feeding trial was carried out to elucidate the effects of dietary TRP enrichment on growth performance and physio-biochemical responses. In the experiment II, the TRP pre-fed L. rohita, from experiment I, was exposed to temperature and salinity stress to evaluate stress-mitigating efficacy of TRP. In L. rohita, dietary supplementation of TRP showed significant effect on weight gain percentage and feed conversion ratio but not on blood glucose. A significant increase in RNA content and RNA/DNA ratio upon TRP supplementation was observed and was positively correlated with growth performance. The results of experiment II indicated that weight gain percentage, serum T3 and T4 levels were significantly reduced in groups that were exposed to temperature and salinity stress and fed diets without TRP supplementation. However, dietary supplementation of TRP significantly augmented weight gain percentage in stress-exposed groups. Tryptophan supplementation helped in bringing back T3 and T4 levels comparable with control. A significant increase in superoxide dismutase, catalase, Adenosine triphosphatase, blood glucose and serum cortisol was observed in temperature- and salinity-exposed groups fed without TRP-supplemented diets. However, TRP supplementation was found to be effective in restoring the above parameters. The results of these experiments suggest that dietary TRP supplementation augments growth, lowers energy demand and helps in mitigating thermal and salinity stress in L. rohita.

A feeding trial of 70-days was carried out to study the haemato-immunological and stress responses of Labeo rohita fingerlings reared at two water temperatures [ambient (Amb) - 27 °C and 32 °C] fed with graded levels of gelatinized corn carbohydrate (GC). Two hundred and sixteen fingerlings were randomly distributed into six treatment groups in triplicate. Three semi-purified diets were prepared containing 30% crude protein with graded levels of GC 40%, 50% and 58%. The six treatment groups were T(1) (40% GC × Amb), T(2) (40% GC × 32 °C), T(3) (50% GC × Amb), T(4) (50% GC× 32 °C), T(5) (58% GC × Amb) and T(6) (58% GC × 32 °C). The blood glucose level was significantly (p < 0.05) lowered in groups fed with 58% GC level. Neither dietary GC levels nor temperature had a significant (p > 0.05) effect on serum cortisol and superoxide dismutase activity. Lysozyme activity was significantly higher (p < 0.05) in T(1) during pre- and post-challenge period while temperature alone had a significant (p < 0.05) effect on post-challenge Nitroblue Tetrazolium and found higher at 32 °C. A significant effect of GC levels and rearing temperature was recorded on WBC in the pre- and post-challenge period. Highest pre-challenge WBC was observed in T(4) group and in the post-challenge period T(1) group recorded maximum. Water temperature had significant effect on pre-challenge haemoglobin content, highest being at 32 °C (T(2) ). A significant (p < 0.05) effect of rearing temperature and dietary GC level on total serum protein and albumin was also observed. Highest total serum protein and albumin was recorded in T(1) and globulin in T(2) . Percentage survival after challenging with Aeromonas hydrophila was highest in T(1) followed by T(3) group and lowest in T(6) . The results obtained in the present study suggest that L. rohita fingerlings may utilize higher levels of dietary GC at higher temperature (32 °C) but may affect its immunity status.

Effect of nitrite exposure on growth and immunometabolic responses of Labeo rohita fed L-tryptophan (TRP) was studied. Fish previously fed normal and elevated levels of tryptophan for 60 days were exposed to nitrite (2.0 mg L À1 ) for another 45 days with same feeding regime. There were four treatment groups, viz., TRP 0 -N (control), TRP 0 +N, TRP 0.75 +N (0.75% supplemental tryptophan in the diet) and TRP 1.5 +N (1.5% supplemental tryptophan in the diet). Highest weight gain% and SGR were observed in control and lowest in TRP 0 +N. Dietary supplementation of elevated levels of tryptophan augmented weight gain% and SGR. Nitrite exposed groups recorded higher catalase, SOD, LDH, AST and ALT activities compared with control. However, activities reduced with additional levels of tryptophan supplementation. Nitrite exposure reduced WBC count, total protein, albumin, globulin and lysozyme activity compared with unexposed group but groups which were fed additional amounts of tryptophan restored total protein, albumin and globulin similar to TRP 0 -N. In conclusion, nitrite exposure had adversely affected growth, increased activities of LDH, AST, ALT, catalase, but decreased WBC, serum protein, lysozyme and acetylcholine esterase activity of L. rohita. Normal requirement of tryptophan was unable to combat nitrite stress. However, dietary fortification with tryptophan (minimum 0.75% of diet + normal requirement) found effective in combating nitrite induced stress.