FISH EMBRYO TESTS AND ACUTE FISH TOXICITY TESTS ARE INTERCHANGEABLE IN THE APPLICATION OF THE THRESHOLD APPROACH.

A database was compiled for algal (OECD 201 Test Guideline [TG]), Daphnia magna (OECD 202 TG), acute fish toxicity (AFT, OECD 203 TG), and fish embryo toxicity (FET, OECD 236 TG) to assess the suitability and applicability of the FET in a threshold approach context. In the threshold approach, algal and Daphnia toxicity are assessed first after which a limit test is conducted at the lower of the two toxicity values using fish. If potential fish toxicity is indicated, a full LC50 assay is performed. This tiered testing strategy can significantly reduce the number of fish used in toxicity testing as algae or Daphnia are typically more sensitive than fish. A total of 165 compounds had AFT and FET available and of these 82 had algal and Daphnia acute toxicity data available. Algae and Daphnia were more sensitive 75-80% of the time. Fish or FET were most sensitive 20 and 16% of the time when considered as the sole fish toxicity indicator and 27% of the time when both were considered simultaneously. When fish were the most sensitive trophic level, different compounds were identified as most toxic to FET and to AFT; however, differences were not so large that they resulted in substantially different outcomes when binning potency using the United Nations categories of aquatic toxicity under the Globally Harmonized System for classification and labelling. It is recommended that the FET could be used to directly replace the AFT in the threshold approach or be used as the definitive test if a AFT limit test indicated toxicity potential for a chemical.