STOCKING DENSITY ON WATER QUALITY AND NUTRITIONAL QUALITY OF ANGUILLA JAPONICA IN SUMMER AND AUTUMN

This research aimed to investigate the influence of stocking density on the nutrient and water quality parameters of cultured Anguilla japonica in summer and autumn. The stocking densities of the low-density group in summer were 1.51 (pond 1), 1.50 (pond 2) and 1.49 kg/m² (pond 3), while those of the high-density group were 2.17 (pond 4), 2.14 (pond 5) and 2.13 kg/m² (pond 6), respectively. The stocking densities were 1.74 (pond Ⅰ), 1.72 (pond Ⅱ) and 1.75 kg/m² (pond Ⅲ) for the low-density group and 2.36 (pond Ⅳ), 2.34 (pond Ⅴ) and 2.36 kg/m² (pond Ⅵ) for the high-density group in autumn. Each time, five water samples and four eels were collected from each pond and each density groups, respectively. The results showed that high-density ponds exhibited lower water transparency and dissolved oxygen levels compared to that of low-density ponds (P<0.05). Moreover, high-density ponds had higher levels of nitrogen, phosphorus, ammonia, nitrite, chemical oxygen demand, and Chlorophyll a than that of their low-density counterparts (P<0.05). Bacillariophyte, Chlorophyll, and Cyanophyte were found to be ten times more abundant in high-density ponds. The Shannon index of bacteria decreased in high-density ponds while the Simpson index was the opposite. Actinobacteria in high-density group was significantly lower than that of low-density group in both seasons (P<0.05). Proteobacteria was significantly higher than that of low-density group in the summer (P<0.01), while Bacteroides was significantly higher than that of low-density group in autumn (P<0.05). In terms of nutritional quality, muscle firmness, shear force, chewiness, reparability parameters and crude fat and crude protein contents of Japanese eels in high-density group were significantly lower than those in low-density ponds (P<0.05), while the moisture content was the opposite (P<0.05). In conclusion, compared to low-density ponds, parameters like water quality index, algal composition, bacterial diversity, fish body composition and nutrient content in high-density group showed negative effects, which together constituted the sensitive parameters of Japanese eel quality and its culture environment. Moreover, 2.13—2.17 kg/m² in summer seems to be an inappropriate density, which caused eutrophication and mild pollution in this study.