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.