Data from: Sub-lethal effects on fish provide insight into a biologically-relevant threshold of hypoxia

Hypoxia (low dissolved oxygen) is a mounting concern for aquatic ecosystems as its prevalence increases with rising anthropogenic nutrient inputs. Hypoxia is most commonly defined as 2.0 mg l–1 of dissolved oxygen, although this level varies widely across studies and agency regulations. Such definitions may be too conservative, as ecologically-relevant non-lethal effects (e.g. consumption and growth) of hypoxia on important aquatic species, such as fish, often occur at oxygen levels much higher than 2.0 mg l–1. In addition, many mechanisms that regulate hypoxia tolerance in fish have been proposed, including temperature, habitat, location in the water column, and body size, but there is ongoing debate over which mechanisms are most important. Using a structured meta-analysis of published studies, we showed consistent, significant negative effects on fish growth and consumption below 4.5 mg l–1. While the total amount of variation explained was generally low, below 4.5 mg l–1 of dissolved oxygen, phylogenetic relationships accounted for most of the explained variation in fish growth. Ecological factors including body size, location in the water column (pelagic, demersal, or benthopelagic), habitat (freshwater, marine, or diadromous), and temperature explained very little of the effect of hypoxia on fish growth and explained only a moderate level of variation in consumption. Our results suggest a dramatically higher threshold for sub-lethal effects of hypoxia on fish than oxygen levels generally set for regulation purposes, and provide little support for accepted ecological mechanisms thought to influence hypoxia tolerance.随着人为活动引发的营养盐输入不断增加，低氧（Hypoxia，即溶解氧浓度过低）现象的发生频率逐年上升，对水生生态系统构成的威胁也日益严峻。目前学界对低氧的通用定义为溶解氧浓度低于2.0 mg·l⁻¹，但不同研究与监管机构所采用的阈值差异极大。此类定义可能过于保守——低氧对鱼类等重要水生生物产生的、具备生态学意义的非致死效应（如摄食能力下降与生长抑制），通常在溶解氧浓度远高于2.0 mg·l⁻¹时就已显现。 此外，学界已提出多种调控鱼类低氧耐受能力的机制，包括温度、生境、水层位置以及体型大小，但关于哪种机制发挥核心作用的争论仍在持续。本研究通过对已发表文献开展结构化元分析（meta-analysis），发现当溶解氧浓度低于4.5 mg·l⁻¹时，会对鱼类生长与摄食产生一致且显著的抑制作用。尽管整体上模型所能解释的变异量普遍较低，但在溶解氧浓度低于4.5 mg·l⁻¹的情况下，系统发育关系是解释鱼类生长相关变异的最主要因素。包括体型大小、水层位置（中上层、底栖或底中水层）、生境（淡水、海水或洄游性）以及温度在内的生态因子，对低氧引发的鱼类生长抑制的解释度极低，仅能中等程度解释摄食相关的变异。 本研究结果表明，低氧对鱼类产生亚致死效应的阈值远高于监管机构通常设定的溶解氧浓度标准，且未能为学界公认的、影响鱼类低氧耐受能力的生态机制提供有效支撑。