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类群。当监管机构判定某农药在水体周边使用的安全性时，鱼类毒性试验获取的半数致死浓度（LC50, lethal concentration 50）是决策的核心依据之一——LC50是公认的金标准毒性评价指标，指可导致50%受试生物死亡的化学物质浓度。本研究选取滴滴涕（DDT）作为分析对象，因其代谢产物包括二氯二苯二氯乙烯（DDE）与二氯二苯二氯乙烷（DDD），可与母体化合物开展对比分析，且该农药拥有大量鱼类毒性相关数据。斑鳢（Channa striata）、印度异唇鲶（Heteropneustes fossilis）与点鳢（Channa punctata）均为东南亚本土物种（分布覆盖孟加拉国、印度、泰国及缅甸），且滴滴涕曾在上述区域广泛施用，因此这三类物种（尤以斑鳢为主）构成了本数据集的主体。本数据集未包含母体滴滴涕，仅收录其降解产物。11项相关研究中，10项采用了滴滴涕最持久的代谢产物DDE，剩余1项采用了另一代谢产物DDD。数据集数据源自美国环境保护署生态毒性数据库（USEPA ECOTOX database），并按物种（鱼类）、效应（死亡率）、测试终点（LC50）、试验条件及毒物类型（滴滴涕及其代谢产物）进行了筛选。 数据集的平均LC50值为2.66，中位LC50值为0.426，表明数据集呈右偏（正偏）分布。少数极高异常值拉高了整体平均值，而中位数值不受此类极端数值的影响。本数据集的两个极端值分别来自斑鳢（1.346）与印度异唇鲶（25）。LC50值越高，意味着该物种对滴滴涕代谢产物的耐受性或抗逆性越强。数据分析显示，在所有采样物种中，印度异唇鲶的LC50值最高，而蓝鳃太阳鱼（Lepomis macrochirus）的LC50值最低，是本数据集内敏感性最强的物种；印度异唇鲶则为耐受性最强的物种。这一发现具有重要意义：蓝鳃太阳鱼正是美国环境保护署用于此类标准毒性测试的模式生物，其高敏感性使其成为农药风险评估的优良指示物种。斑鳢在5组不同试验中得到了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、鱼类生命阶段、生物利用度或摄食习性的不同。样本量有限也可能对分析结果的准确性造成影响。 本研究采用的分析工具为tidyverse包集，该集成包包含dplyr与ggplot2等工具。其中dplyr用于数据清洗与汇总，ggplot2用于绘制可视化图表，此外还使用了readxl包以导入Excel格式的数据文件。