LC50 Analysis of DDT Metabolites in Fish Species

Fish are the most commonly used vertebrates in pesticide toxicity testing. When regulators decide if a pesticide is safe to use near water, LC50 values from fish tests are a core part of that decision, as LC50 is the gold standard toxicity metric, i.e. the concentration that kills 50% of test organisms. DDT was chosen as the pesticide for this analysis as it has metabolites like DDE and DDD that can be compared to the parent compound, and it also has a lot of fish toxicity data. Channa striata, Heteropneustes fossilis, and Channa punctata are native to the Southeast Asian region (specifically Bangladesh, India, Thailand, and Myanmar), and DDT was widely used in those regions, which is why these species, particularly Channa striata, dominate the dataset. The dataset contains no parent DDT at all, just its breakdown products. 10 out of the eleven studies used DDE, the most persistent metabolite of DDT, and the last study used DDD, another metabolite. The data source was the USEPA ECOTOX database and was filtered by species (Fish), Effects (mortality), endpoints (LC50), test conditions, and toxicant (DDT and metabolites). From the data set, the mean LC50 value is 2.66, and the median LC50 value is 0.426. This means that the dataset is positively skewed (skewed to the right). There are a few positively high values, outliers, pulling the average up, while the middle value remains unaffected by these extreme numbers. The extreme numbers from this data set belong to Channa striata (1.346) and Heteropneustes (25), respectively. Higher LC50 values signal higher tolerance or resilience of the species to the DDT metabolite. The data analysis showed that Heteropneustes fossilis had the highest LC50 value amongst all the sampled species, and Lepomis macrochirus (bluegill) had the lowest LC50 value for the DDT metabolites. Lepomis macrochirus (bluegill) is the most sensitive species in the dataset, and Heteropneustes fossilis (catfish) is the most tolerant. This is a significant finding as Lepomis (bluegill) is a species the US EPA uses as a standard test organism for exactly this reason; its sensitivity makes it a good indicator species for pesticide risk assessment. Channa striata had 5 different LC50 values as different experiments were conducted, and the difference in the values could be due to differences in experimental conditions, water chemistry, life stages, bioavailability, or feeding habits of the fish. The limited sample size could also have an impact on the accuracy of the analysis. The tools used were tidyverse, which is a bundle of packages including dplyr and ggplot2. dplyr was used for data cleaning and summarising, while ggplot2 was used to make the plots. readxl was also used to import the Excel files.鱼类是农药毒性测试中最常用的脊椎动物模型。当监管机构判定某款农药可在水域附近安全使用时，鱼类毒性试验得到的半数致死浓度（LC50）是决策的核心依据之一——LC50作为毒性评价的金标准，指的是能杀死50%受试生物的毒物浓度。本次分析选取滴滴涕（DDT）作为研究农药，原因在于其代谢产物二氯二苯二氯乙烯（DDE）、二氯二苯二氯乙烷（DDD）可与母体化合物进行对比，且该类物质拥有大量鱼类毒性数据。 线鳢（Channa striata）、异口鲶（Heteropneustes fossilis）与斑鳢（Channa punctata）原产于东南亚地区（具体包括孟加拉国、印度、泰国与缅甸），且滴滴涕曾在该区域被广泛使用，这也是上述物种，尤其是线鳢，成为数据集主体物种的原因。本数据集未包含母体滴滴涕（DDT），仅包含其降解产物。11项研究中有10项采用了DDT最持久的代谢产物DDE，剩余1项采用了另一种代谢产物DDD。 数据集源自美国环境保护署（USEPA）ECOTOX生态毒性数据库，并通过物种（鱼类）、效应（死亡率）、终点（LC50）、试验条件与毒物（DDT及其代谢产物）进行了筛选。 该数据集的LC50平均值为2.66，中位数为0.426，说明数据集呈右偏（正偏）分布：存在少量极高值异常值拉高了平均值，而中位数不受这些极端数值的影响。本数据集的极端值分别来自线鳢（1.346）与异口鲶（25）。LC50数值越高，代表该物种对DDT代谢产物的耐受性或抗性越强。 数据分析显示，在所有采样物种中，异口鲶的LC50值最高，而蓝鳃太阳鱼（Lepomis macrochirus）的LC50值最低，是本数据集内最敏感的物种；异口鲶则是耐受性最强的物种。这一发现具有重要意义：蓝鳃太阳鱼正是美国环境保护署（USEPA）标准毒性试验所用的受试物种之一，其高敏感性使其成为农药风险评估的优质指示生物。 线鳢在不同试验中得到了5组不同的LC50值，数值差异可能源于试验条件、水体化学性质、鱼体发育阶段、生物利用度或摄食习性的不同。样本量有限也可能对分析的准确性造成影响。 本次分析使用的工具为tidyverse套件，其包含dplyr与ggplot2等多个R包：其中dplyr用于数据清洗与汇总，ggplot2用于绘图，此外还使用了readxl包以导入Excel格式文件。