Effect of stocking density on the expression of glucose transporter protein 1 and other physiological factors in the Lake Victoria Nile tilapia, Oreochromis niloticus (L.)

Abstract

Fish farmers have a tendency of employing high stocking density (HSD) as a means of increasing productivity. However, HSD is a chronic stressor that is likely to lower profitability of fish farming if not implemented properly. HSD induces stress which in turn elevates sequentially the levels of plasma cortisol and glucose. The resultant glucose is distributed to various tissues by glucose transporter protein (GLUTs) to restore normalcy. GLUT 1, transmembrane protein found in erythrocytes, is responsible for import and export of glucose in red blood cells. However, knowledge on how chronic stress impact glucose and GLUT 1 protein in fish subjected to HSD is still unclear. In this study, effect of HSD on the expression of GLUT 1 in Nile tilapia was investigated in an attempt to elucidate the role of GLUT 1 in glucose metabolism during chronic stress. Fish were reared for 4 weeks at 1.5 and 4.5 kg/m3 for low stocking density (LSD) and HSD, respectively. Four physiological parameters were determined from the blood samples obtained from fish at the end of experiment. At p < 0.05, there were significant differences between fish reared at HSD and LSD in plasma cortisol level (72.1 ± 5.9 ng/ml and 37.5 ± 4.6 ng/ml); blood glucose level (136.00 ± 1.3 mg/dL and 70.2 ± 1.0 mg/dL); erythrocytes count (7.2 ± 0.5 × 106 mm−3 and 2.1 ± 0.4 × 106 mm−3); and plasma GLUT 1 level (1.40 ± 0.17 rbi and 0.81 ± 0.07 rbi), respectively. HSD induced elevation of plasma cortisol level, blood glucose level, erythrocytes count and GLUT 1 level. These elevated physiological factors and particularly GLUT 1 can be used as a cellular stress biomarker in fish farming and aquaculture.