TOXICITY OF MICROCYSTIS-AERUGINOSA PEPTIDE TOXIN TO YEARLING RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The effects of the blue-green algae (cyanobacterial) toxin microcystin -LR on yearling rainbow trout were studied in a series of intraperiton eal injection tests, gavage trials and exposures to waterborne algae. Concentrations of the microcystin-producing algae Microcystis aerugino sa known to occur during algae blooms (8-16 mg freeze-dried algae/L, i .e. approximately 1-2 x 10(11) cells/L) were shown to be non-toxic to trout when present in aquarium water. On the other hand, trout died wi thin 96 h when gavaged with an amount of algae equivalent to that whic h passed through the gills within 18 h in the aqueous exposure tests ( 1440 mg freeze-dried algae/kg body weight, i.e. 6600 mug microcystin/k g body weight). Oral uptake of approximately one tenth of this dose (1 10 mg freeze-dried algae/kg body weight, i.e. 550 mug microcystin/kg b ody weight) proved to be non-toxic in single gavaging studies, but tox ic within 96 h when orally administered 8 times at 12-h intervals. Thi s study shows that the main uptake route of microcystin in trout is th e gastrointestinal tract and that toxicity is manifested as massive he patic necrosis. These results indicate that massive fish deaths report ed at cyanobacterial bloom sites can be explained by the ingestion of toxic blue-green algae.