Toxicity identification evaluations for the investigation of fish kills: a case study.

A large fish-kill was observed in the river Tajo during the Spring-Summer of 1991. The mortality was first detected between Aranjuez and Toledo, affecting several fish species. Then it was slowly going downstream, affecting only carp (Cyprinus carpio), reaching the Spanish-Portuguese border several months later. Short-term toxicity tests on Daphnia magna and in vitro cytotoxicity tests on RTG-2 cells were used as toxicity monitoring systems in water samples and different water fractions. The fish kill was associated to the outbreak of infectious diseases, spring viremia of carp and saprolegniosis, related to an increase in the fish's susceptibility due to the presence of a toxic chemical. Bioassay-directed sample fractionations allowed to detect a toxic chemical. HPLC-MS identified the compound as dehydroabietic acid, a resin acid previously described immunotoxic.