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Water temperature alone can affect the growth, metabolic rates and physiological responses of aquatic organisms. Our earlier study reported that higher temperature affects cellular and hemato-biochemical responses in rohu, Labeo rohita. In this backdrop, the present study assessed the effect of higher acclimation temperature on the regulatory mechanisms of growth and stress responses of juvenile L. rohita acclimatized in three temperature conditions (30 °C, 33 °C, and 36 °C) for a period of 30 days. The relative expression of genes for growth hormone (GH), insulin-like growth factors (IGF-1 and IGF-2) and heat shock proteins (hsp70 and hsp90) were measured by real-time quantitative PCR. The results revealed that the highest acclimation temperature (36 °C) significantly decreased the weight gain (WG) and specific growth rate (SGR), and increased the feed conversion ratio (FCR) compared to 30 °C (control), while increased WG, SGR and lowered FCR were observed in fish reared at the intermediate temperature (33 °C) compared to 30 °C. Similarly, the GH gene expression in the pituitary was significantly decreased and increased at 36 °C and 33 °C, respectively as compared to 30 °C. A significantly lower expression of IGF-1 and IGF-2, and higher expression of hsp70 and hsp90 were observed in the liver of fish at 36 °C. The results of the present study indicate that although slightly elevated temperature promotes the growth of juvenile L. rohita , the higher acclimation temperature may induce stress response and impair growth performance by suppressing GH/IGF system. [Display omitted] • The rohu were acclimatized in three temperatures (30 °C, 33 °C and 36 °C) for 30 days. • Growth performance of rohu decreased at the highest temperature (36 °C), while increased at 33 °C. • The gh expression in the pituitary decreased at 36 °C and increased at 33 °C. • Lower expression of igf-1 and igf-2 , and higher expression of hsp70 and hsp90 were observed in the liver of fish at 36 °C. • The higher acclimation temperature may induce stress response and impact the growth performance by suppressing GH/IGF system. [ABSTRACT FROM AUTHOR]

Increase in water temperature due to anthropogenic and climatic changes is expected to affect physiological functions of fish. In this study, we determined high temperature tolerance (CTmax) of a common aquacultured Indian major carp, rohu, Labeo rohita fingerlings (15.96 ± 0.72 g BW, 11.56 ± 0.42 cm TL) followed by acclimatization at three temperatures (30, 33, 36 °C). To determine the CTmax, we analyzed the major hemato-biochemical indices - hemoglobin (Hb), red blood cell (RBC), white blood cell (WBC), blood glucose levels, and erythrocytic nuclear abnormalities (ENAs) and erythrocytic cellular abnormalities (ECAs) of peripheral erythrocytes in the fish sampled at the start and end point at each acclimated temperature. Significantly decreased CTmax of the fish was found at 36 °C compared to 30 °C and 33 °C. The fish in the highest (36 °C) temperature were found with significantly lower Hb and RBC content and significantly higher WBC and blood glucose levels than that of the fishes in the lowest (30 °C) temperature both at the start and end points. The highest frequencies of ENAs and ECAs were found in the highest (36 °C) temperature group compared to the lowest (30 °C) temperature group at both the points. We also evaluated growth performance of the rohu fingerlings reared in the three temperatures for 60 days. The growth parameters - final weight gain, percent weight gain and specific growth rate were the highest at 33 °C and the lowest at 36 °C. The present study revealed that the highest temperature (36 °C) tested here may be hazardous to rohu and the temperature should be kept below 36 °C in the aquaculture setting to avoid physiological damage and growth and production loss to the fish. Image 1 • Upper thermal limit of Labeo rohita decreased in increased acclimation temperature. • Lowered Hb and RBC but higher WBC and blood glucose level were noticed at the CTmax in increased acclimation temperature. • Higher frequencies of erythrocytic nuclear abnormalities (ENAs) and erythrocytic cellular abnormalities (ECAs) of erythrocytes were in the increased acclimation temperature. • The growth performances were the highest at 33 °C and the lowest at 36 °C. • High temperature (36 °C) is stressful for L. rohita. [ABSTRACT FROM AUTHOR]