Effects of background color and rearing density on stress-related hormones in the juvenile Japanese eel Anguilla japonica

Sustainable aquaculture of the Japanese eel Anguilla japonica requires an understanding of the physiological conditions of the fish under culture conditions. Therefore, we examined the effects of tentative stressors such as background color and rearing density on stress-related hormones in juvenile Japanese eel. In Experiment 1, fish were divided into white- or black-coated tanks and reared for 35 days. Plasma cortisol levels were significantly higher in the white-acclimated fish on day 35. No significant differences were observed between the groups in hypothalamic CRH mRNA levels. In Experiment 2, fish were divided into three rearing density groups (0.5 kg/m2, 1.2 kg/m2, and 2.4 kg/m2) and reared for 28 days. Plasma cortisol levels were significantly lower in the low-density-acclimated fish than in the medium- and high-density-acclimated fish. However, no significant differences were observed in hypothalamic CRH mRNA levels. Evaluation of plasma cortisol levels indicates that a white background and high rearing density induce more stress for juvenile Japanese eel.