71 Indian J. Fish., 66(2): 71-77, 2019 DOI: 10.21077/ijf.2019.66.2.78397-10 Biochemical and haematological responses to thermal stress in Labeo rohita (Hamilton, 1822) fngerlings PREM KUMAR 1 , ASIM K. PAL 2 , NAROTTAM P. SAHU 3 , L. CHRISTINA 1 AND ASHISH KUMAR JHA 3 1 Kakdwip Research Centre of ICAR-Central Institute of Brackishwater Aquaculture, South 24 Parganas - 743 347 West Bengal, India 2 ICAR-Central Institute of Fisheries Education, Yari Road, Versova, Mumbai - 400 061, Maharashtra, India 3 APC Nutrients (A unit of APC Global), 507, Minerva House, S. D. Road, Secunderabad - 500 003, Telangana, India e-mail: prem.cife@gmail.com ABSTRACT An experiment was conducted to understand the efect of thermal shock on stress and recovery in Labeo rohita (Hamilton, 1822) fngerlings. L. rohita fngerlings were exposed to a temperature of 38 0 C for 2 h and permitted to recuperate afterwards for 48 h. Primary and secondary stress parameters were analysed after 2, 4, 8, 12, 24 and 48 h of recovery. The activity of glucose-6-phosphate dehydrogenase and acetylcholine esterase reduced signifcantly (p<0.05). Erythrocytes, haemoglobin, respiratory burst activity and lysozyme levels increased signifcantly (p<0.05) by 7.31, 23.91, 109.09 and 70.13% after thermal shock, whereas leucocytes, serum protein, albumin and globulin showed a reverse trend. During recovery, biochemical parameters normalised in 48 h. However, immunological variables such as serum total protein, albumin and globulin, and histopathological changes in gill and kidney tissues did not return to normal condition. Kewwords: Enzymes, Haematology, Histopathology, Labeo rohita, Stress recovery, Thermal stress Introduction Rohu Labeo rohita (Hamilton, 1822) is an economically important aquaculture species in India. The water temperature in freshwater bodies in India goes up to 34-37°C during summer months, which is beyond the optimum temperature for growth of this species (Das et al., 2005). During farming, fsh are often exposed to several kinds of stressors, among which temperature is a major one, owing to the poikilothermic nature of fsh. Therefore, any change in water temperature directly afects physiology of the fsh (Jonassen et al., 1999). Stress responses activate the neuro-endocrine system that alters the body metabolism (Chatterjee et al., 2006). Most commonly studied stress response in fsh and other vertebrates are primary responses which are measured as the level of cortisol and catecholamine (Barton and Iwama, 1991). Commonly measured parameters for secondary stress responses are blood glucose and lactate levels (Chatterjee et al., 2006). There is ample literature available on thermal stress responses in fshes; however, there is very limited information on the time required for normalisation of the stress responses and on the histopathological changes due to thermal stress. Therefore, the present study was carried out to understand the level of stress responses against heat shock and duration for recouping to normal state in L. rohita. Materials and methods Experimental animal Fingerlings L. rohita (mean weight of 6.5±0.05 g) were transported from a private fsh hatchery to the laboratory in an open container (500 l) under oxygenation. The stock was acclimatised to laboratory conditions for a month and fed with a commercial diet. Experimental setup Ideal water temperature for Indian major carps (Catla catla, L. rohita and Cirrhinus mrigiala) is 28 0 C (Akhtar et al., 2012). However, carps can grow well between the temperature range of 18.3 and 37.8 0 C (Jhingran, 1975). Therefore, in this study L. rohita fngerlings were given heat shock at 38 0 C. Six fshes maintained at 28 0 C were used as control. Forty-two fshes were randomly selected and exposed to thermal shock at 38 0 C for 2 h in a 70 l glass aquarium, using a digital water heater to bring the water temperature to the desired experimental temperature of 38 0 C. Immediately after 2 h of thermal shock (0 h of recovery), six fshes were sampled. The remaining fshes were transferred back to the acclimatisation tank (28 0 C) and sampled (n=6) at an interval of 2, 4, 8, 12, 24 and 48 h, of recovery. Six fsh from each samplings were anesthetised with clove oil (50 µl l -1 ) and blood was collected by