Exposure to ambient UVB light influences the welfare, energy requirements, metabolism, and relevant molecular biology of the Himalayan fish Tor putitora: Investigating the hidden costs of sunlight in upland aquaculture

Abstract

The present study used a mid-to-high altitude dwelling migratory fish, Tor putitora, as a model for studying the impact of ambient UVB on flourishing fish culture in the high-altitudinal areas of the Indian Himalayas. For this, two sets of the juveniles of T. putitora (22 ± 6.3 g) were exposed for 60 days to two ambient UVB levels- 1.0 and 1.5 W/m² (T1 and T2). The two levels corresponded to about 24.0 % and 36.0 % of the daily average UVB incidence of the peak noon hours (1100 to 1300) of the summer. A third, unexposed set served as control (CT). After the experimental trial, the fish were evaluated for fitness indices, including thermal tolerance and respiratory metabolic rates. Important physiological, genotoxic, molecular and histological changes were also evaluated in the blood plasma and body tissues. Among the welfare indices, routine and maximum metabolic rates and lower critical temperature increased, whereas thermal scope decreased in the UVB-exposed fish. The exposure caused a significant increase in the total antioxidative capacity (TAC), nitric oxide (NO), cholesterol, cortisol, and aminotransferases in the plasma. However, the antioxidative enzyme levels significantly decreased, whereas malonaldehyde levels increased in the kidney and liver of the UVB-exposed fish, dose-dependently. In addition, the dehydrogenase enzyme (LDH and MDH) activities significantly increased in the liver, kidney, and muscle, dose-dependently. Among the non-specific immune parameters, plasma lysozyme, neutrophil respiratory burst activity, and total blood leucocyte count also increase dose-dependently. Molecular analysis revealed that the antioxidative pathway genes (gst, cat1, sod1, and cyp1a1) and immune gene (nf-κb2) were differentially expressed in the liver, kidney, and muscle tissues of the UVB-exposed fish, showing an overall downregulatory trend. The cortisol-related gene, hsd11b1a, was upregulated in the kidney and downregulated in the liver of the UVB-exposed fish. Moreover, UVB exposure resulted in structural, non-pathogenic changes in the gill, liver, and kidney. Our study showed that although prolonged exposure to ambient UVB radiations did not affect the physiology of the fish beyond their threshold, it did increase the energetic costs and affected the antioxidative mechanisms. The results indicate that any further increase in the environmental UVB can harm the welfare of the fish, both in the wild and captivity, in high-altitude regions.