1. Test No. 203: Fish, Acute Toxicity Test

This is the more common one, and it involves:

• Exposing fish (usually zebrafish or rainbow trout) to different concentrations of a test substance for 96 hours.
• Monitoring the fish for mortality and any abnormal behavior throughout the test.
• Calculating the LC50 (lethal concentration at which 50% of the fish die) based on the results.
• This test helps assess the short-term impact of a chemical on fish populations and is widely used in environmental hazard assessment.
  
  

2. OECD 203 Fish Embryo Acute Toxicity Test

• This version focuses on the effects of a chemical on fish embryos and larvae.
• Fish eggs are exposed to different concentrations of the test substance for a specific period (usually 48-96 hours).
• The development and survival of the embryos and larvae are monitored.
• The LC50 is calculated based on the mortality or developmental abnormalities observed.
• This test helps assess the potential of a chemical to harm fish populations at early life stages.

Both versions of OECD 203 are standardized protocols to ensure consistency and reproducibility of results. They are important tools for environmental regulators and chemical manufacturers to evaluate the potential risks of chemicals to aquatic ecosystems.