Long‐term chromium exposure and its residual impact on growth, biochemical changes and immune gene expression of climbing perch ( Anabas testudineus )

Accumulation of heavy metal in fish not only causes morphological, biochemical, haematological and genetic changes but also poses serious health risks to their consumers. The present study investigated the effects of chromium (Cr) exposure on the alteration of physiological parameters and the expression pattern of three candidate genes: metallothionein 2 (MT-2), interleukin 1 beta (IL-1β) and tumour necrosis family 1 (TNF) in a climbing perch (Koi fish). The potassium dichromate (KCr2O7) was used as a source of Cr for fish feed and fed at three experimental doses (1, 2 and 5 ppm of Cr/kg feed) along with a control (0 ppm) for 90 days under aquarium conditions. Cr concentration found in the organs followed the order: muscle > gill > liver. Cr deposition was above the recommended level (0.05 ppm) and was significantly higher in muscle than in gill and liver. The growth in fish length, weight, specific growth rate (SGR) and food conversion ratio (FCR) did not vary significantly with different Cr doses but showed a negative correlation of overall weight gain, SGR and FCR with Cr deposition. Blood haemoglobin, glucose levels and protease activity decreased with increasing chromium concentrations, showing a strong negative correlation between total chromium accumulation and protease activity (r = 0.64, p = 0.022). The overall expression of the MT-2 gene was downregulated in the Cr-exposed fish with time and dose but did not significantly affect its expression in the gills and muscles. However, genes encoding TNF and IL-1β showed an upregulation in the muscle, gill and liver tissues. The sensitivity of these candidate genes to heavy metals was also evident by its positive correlation with Cr concentration and few growth parameters. Therefore, the findings of the present study will help conduct routine health assessments of the fish populations in areas prone to Cr contamination.