Melatonin is a hormone that can modulate aggressive interactions in fish, stimulate antioxidant enzymes, and neutralize the biological damage caused by high free radical production. Fish, especially those kept in artificial environments, are subject to damage caused by free radicals resulting from stressful conditions, such as a social challenge. The matrinxã (Brycon amazonicus) arouses great commercial interest and has been widely used in farming systems. However, these animals have a high frequency of aggressive behavior and cannibalism, which is considered a limiting factor for the production system. Thus, we evaluated the influence of melatonin on aggressiveness and oxidative stress parameters in matrinxã juveniles, testing whether melatonin increases glutathione concentrations and reduces lipid peroxidation (thiobarbituric acid-reactive substances) during social challenge. For this, 24 fish were subjected to three concentrations of melatonin in water: control (0 μmol/L), low MEL (1 μmol/L), and high MEL (10 μmol/L), with eight replicates each. The fish were isolated for 96 h, subjected to the mirror test (social challenge), and then euthanized and dissected to remove the liver. Melatonin reduced total aggressive interactions in animals treated with melatonin (P = 0.036), increased glutathione concentrations (P = 0.002), and decreased levels of lipid peroxidation (P = 0.001). We concluded that melatonin reduces aggressiveness and acts as a potent antioxidant in matrinxã juveniles, contributing to the reduction in stress levels and thus improving the maintenance conditions of animals in production systems.

中文翻译：

---

褪黑素可降低遭受社会挑战的matrinxã（Brycon amazonicus）的侵略性并改善其氧化状态。

褪黑激素是一种激素，可以调节鱼类中的攻击性相互作用，刺激抗氧化酶并中和由高自由基产生所引起的生物损害。鱼类，尤其是在人工环境中饲养的鱼类，会受到诸如社会挑战等压力条件所导致的自由基的破坏。Matrinxã（Brycon amazonicus）引起了巨大的商业兴趣，并已广泛用于农业系统。但是，这些动物的攻击行为和同类相食的频率很高，这被认为是生产系统的限制因素。因此，我们评估了褪黑激素对Matrinxã幼虫的攻击性和氧化应激参数的影响，测试褪黑素是否在社交挑战中增加了谷胱甘肽的浓度并减少了脂质过氧化（硫代巴比妥酸反应性物质）。为此，对24条鱼在水中进行了三种浓度的褪黑激素处理：对照（​​0μmol/ L），低MEL（1μmol/ L）和高MEL（10μmol/ L），每个重复八次。将鱼分离96小时，进行镜检（社会挑战），然后安乐死并解剖以去除肝脏。褪黑素减少了用褪黑素治疗的动物的总侵略性相互作用（P = 0.036），谷胱甘肽浓度升高（P = 0.002）和脂质过氧化水平降低（P = 0.001）。我们得出的结论是，褪黑激素可降低Matrinxã幼虫的攻击性并充当有效的抗氧化剂，