Nitrate (NO₃⁻), a potential toxic nitrogenous compound to aquatic animals, is distributed in aquatic ecosystems worldwide. The aim of this study was to investigate the effects of different NO₃⁻ levels on growth performance, health status, and endocrine function of juvenile turbot (Scophthalmus maximus) in recirculating aquaculture systems (RAS). Fish were exposed to 0 mg/L (control, CK), 50 mg/L (low nitrate, LN), 200 mg/L (medium nitrate, MN), and 400 mg/L (high nitrate, HN) NO₃-N for 60 d in experimental RAS. Cumulative survival (CS) was significantly decreased with increasing NO₃⁻ levels in LN, MN, and HN. The lowest CS was 35% in the HN group. Growth parameters, including absolute growth rate, specific growth rate, and feed conversion rate, were significantly different in HN compared with that in the CK. Histological survey of gills and liver revealed dose-dependent histopathological damage induced by NO₃⁻ exposure and significant differences in glutamate pyruvate transaminase and glutamate oxalate transaminase in MN and HN compared with that in the CK. The hepatosomatic index in HN was significantly higher than that in the CK. Additionally, NO₃⁻ significantly increased bioaccumulation in plasma in LN, MN, and HN compared to that in the CK. Significant decreases in hemoglobin and increases in methemoglobin levels indicated reduced oxygen-carrying capacity in HN. Additionally, qRT-PCR and enzyme-linked immunosorbent assay (ELISA) were developed to investigate key biomarkers involved in the GH/IGF-1, HPT, and HPI axes. Compared with that in the CK, the abundance of GH, GHRb, and IGF-1 was significantly lower in HN, whereas GHRa did not differ between treatments. The plasma T₃ level significantly decreased in LN, MN, and HN and T₄ significantly decreased in HN. The CRH, ACTH, and plasma cortisol levels were significantly upregulated in HN compared with that in the CK. We conclude that elevated NO₃⁻ exposure leads to growth retardation, impaired health status, and endocrine disorders in turbot and the NO₃⁻ level for juvenile turbot culture should not exceed 50 mg/L NO₃-N in RAS. Our findings indicate that endocrine dysfunction of the GH/IGF-1, HPT, and HPI axes might be responsible for growth inhibition induced by NO₃⁻ exposure.

硝酸盐（NO ₃ ^- ），一个潜在有毒的含氮化合物对水生动物，分布在水生生态系统全世界。这项研究的目的是探讨不同NO的影响₃ ^-生长性能，健康状况和大菱鲆幼鱼（内分泌功能水平鲆在循环水养殖系统（RAS））。鱼暴露于0毫克/升（对照，CK），50毫克/升（低硝酸盐，LN），200毫克/升（培养基中的硝酸盐，MN），和400mg / L（高硝酸盐，HN）NO ₃ - N在实验RAS中持续60 d。累积生存率（CS），没有增加而显著下跌₃ ^-LN，MN和HN中的级别。HN组的最低CS为35％。与CK相比，HN的生长参数（包括绝对生长速度，比生长速度和饲料转化率）显着不同。鳃和肝的组织学调查表明诱导NO剂量依赖组织病理学损伤₃ ^-与在对照相比在MN和HN在谷氨酸丙酮酸转氨酶和谷氨酸草酸转氨酶曝光和显著差异。HN的肝体指数显着高于CK。另外，NO ₃ ^-与CK相比，LN，MN和HN血浆中的生物蓄积显着增加。血红蛋白的显着减少和高铁血红蛋白水平的增加表明HN中携氧能力降低。此外，开发了qRT-PCR和酶联免疫吸附测定（ELISA），以研究涉及GH / IGF-1，HPT和HPI轴的关键生物标志物。与CK相比，HN中GH，GHRb和IGF-1的丰度明显降低，而GHRa在不同处理之间无差异。LN，MN和HN中的血浆T ₃水平显着下降，HN中的T ₄水平显着下降。与CK相比，HN中的CRH，ACTH和血浆皮质醇水平显着上调。我们的结论是升高NO ₃ ^-曝光导致生长迟缓，受损的健康状况，和内分泌失调在大菱鲆和NO ₃ ^-为大菱鲆幼鱼培养含量不应超过50毫克/升NO ₃ -N于RAS。我们的研究结果表明GH / IGF-1，HPT和HPI轴，用于实现内分泌功能紊乱可能负责由NO诱导的生长抑制₃ ^-曝光。