Effect of humic acid on survival, ionoregulation and hematology of the silver catfish, Rhamdia quelen (Siluriformes: Heptapteridae), exposed to different pHs

This study evaluates whether humic acid (HA; Aldrich) protects the silver catfish, Rhamdia quelen (Quoy & Gaimard, 1824), against exposure to acidic pH. Survival, levels of Na+, Cl- and K+ plasma, hematocrit, hemoglobin and erythrocyte morphometry were measured. Fish were exposed to 0, 10, 25 and 50 mg L 1 HA at four pH levels: 3.8, 4.0, 4.2 and 7.0 up to 96 hours. None of the fish exposed to pH 3.8 survived for 96 hours into the experiment, and survival of fish subjected to pH 4.0 decreased when HA concentration increased. Plasma Na+ levels decreased when pH was acidic, with no influence of HA, while Cl- levels declined at low pH with increased HA concentration. The levels of K+ at pH 4.0 and 4.2 increased without HA. Hematocrit and hemoglobin augmented under the effect of HA. At pH 4.0 and 4.2, erythrocytes of fish not exposed to HA were smaller, an effect that was partially offset by the presence of HA, since the values at pH 7.0 were higher. Although HA showed some positive effects changes in hematological and plasma K+ª in silver catfish caused by exposure to acidic pH, the overall findings suggest that HA does not protect this species against acidic pH because it increased mortality and Cl- loss at pH 4.0.

本研究评估了腐殖酸（humic acid, HA；奥德里奇（Aldrich））对银猫鱼（学名*Rhamdia quelen*，Quoy & Gaimard, 1824）抵御酸性pH暴露的保护效应。实验测定指标包括鱼体存活率、血浆钠离子（Na⁺）、氯离子（Cl⁻）与钾离子（K⁺）水平、红细胞比容、血红蛋白含量及红细胞形态计量参数。实验设置4个HA浓度梯度（0、10、25、50 mg·L⁻¹）与4个pH梯度（3.8、4.0、4.2、7.0），鱼体暴露时长最长达96小时。

所有暴露于pH3.8环境的鱼在实验96小时内全部死亡；pH4.0组的鱼存活率随HA浓度升高而降低。血浆Na⁺水平仅随酸性pH降低而下降，不受HA浓度的影响；而Cl⁻水平在低pH条件下会随HA浓度升高进一步下降。在pH4.0与4.2环境中，未添加HA组的鱼血浆K⁺水平出现升高。红细胞比容与血红蛋白含量在HA作用下显著升高。在pH4.0和4.2条件下，未暴露于HA的鱼红细胞体积更小，该效应可被HA部分抵消，因pH7.0组的相关指标数值更高。

尽管HA对酸性pH暴露下银猫鱼的血液学指标及血浆K⁺水平存在一定积极调控作用，但综合实验结果表明，HA无法保护该物种抵御酸性pH胁迫，反而在pH4.0条件下会提升鱼体死亡率并加剧氯离子流失。