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.

低氧（Hypoxia，即溶解氧含量过低）对水生生态系统的威胁日益加剧，其发生频次随人为营养盐输入的增加而不断升高。目前低氧最通用的定义为溶解氧浓度≤2.0 mg·l⁻¹，不过该阈值在不同研究与管理机构的规范中存在显著差异。此类定义或过于保守，因为低氧对鱼类等重要水生生物产生的生态学相关非致死效应（如摄食与生长受抑），往往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，涵盖温度、生境、水层分布位置以及体型大小，但关于哪些机制最为关键，目前仍存在持续争论。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，会对鱼类的生长与摄食产生一致且显著的负面效应。尽管整体可解释的变异占比普遍较低，但在溶解氧浓度低于4.5 mg·l⁻¹的条件下，系统发育关系是解释鱼类生长变异的最主要因素。包括体型大小、水层分布位置（中上层、底栖或近底中水层）、生境（淡水、海水或洄游性）以及温度在内的生态因子，对低氧影响鱼类生长的解释度极低，且仅能中等程度解释摄食相关的变异量。本研究结果表明，低氧对鱼类产生非致死效应的阈值远高于常规监管设定的溶解氧标准，且并未为学界公认的、影响鱼类低氧耐受能力的生态机制提供足够支持。