Effect of dietary supplementation of l-tryptophan on thermal tolerance and oxygen consumption rate in Cirrhinus mrigala fingerlings under varied stocking density

A 60 day feeding trial was conducted to study the effect of dietary l-tryptophan on thermal tolerance and oxygen consumption rate of freshwater fish, mrigala, Cirrhinus mrigala reared under ambient temperature at low and high stocking density. Four hundred eighty fingerlings were distributed into eight experimental groups. Four groups each of low density group (10 fishes/75 L water) and higher density group (30 fishes/75 L water) were fed a diet containing 0, 0.68, 1.36 or 2.72% l-tryptophan in the diet, thus forming eight experimental groups namely, Low density control (LC) (basal feed +0% l-tryptophan); LT₁ (basal feed+0.68% l-tryptophan); LT₂ (basal feed+1.36% l-tryptophan); LT₃ (basal feed+2.72% l-tryptophan); high density control (HC) (basal feed+0% l-tryptophan); HT₁ (basal feed+0.68% l-tryptophan); HT₂ (basal feed+1.36% l-tryptophan); and HT₃ (basal feed+2.72% l-tryptophan) were fed at 3% of the body weight. The test diets having crude protein 34.33±0.23 to 35.81±0.18% and lipid 423.49±1.76 to 425.85±0.31 K Cal/100 g were prepared using purified ingredients. The possible role of dietary l-tryptophan on thermal tolerance and oxygen consumption rate was assessed in terms of critical thermal maxima (CT_Max), critical thermal minima (CT_Min), lethal thermal maxima (LT_Max) and lethal thermal minima (LT_Min). The CT_Max, CT_Min, LT_Max and LT_Min values were found to be significantly higher (p<0.05) in the treatment groups with CT_Max 42.94±0.037 (LT₂); LT _Max 43.18±0.070 (LT₂); CT_Min 10.47±0.088 (LT₂) and LT_Min 9.42±0.062 (LT₃), whereas the control group showed a lower tolerance level. The same trend was observed in the high density group (CT_Max 42.09±0.066 (LT₃); LT_Max 4₃ 23±0.067 (HT₃); CT_Min 10.98±0.040 (HT₃) and LT_Min 9.74±0.037 (HT₃). However, gradual supplementation of dietary l-tryptophan in the diet significantly reduced the oxygen consumption rate in both the low density group (Y=−26.74x+222.4, r²=0.915) and the high density group (Y=−32.96x+296.5, r²=0.8923). Dietary supplementation of l-tryptophan at a level of 1.36% improved the thermal tolerance level and reduced the oxygen consumption rate in C. mrigala fingerlings.