Possible cause of unnatural mass death of wild birds in a pond in Nishinomiya, Japan: sudden appearance of toxic cyanobacteria.

During the summer of 1995, about 20 spot-billed ducks died unnaturally in a pond (Shin-ike) in Nishinomiya, Hyogo Prefecture, Japan. The suspected cause was the sudden appearance of toxic freshwater bloom of cyanobacteria. However, no birds died in a nearby pond (Oo-ike) in which the cyanobacteria was also present. Morphological observation of these cyanobacteria by microscope revealed that they were almost unialgal and were both Microcystis aeruginosa. The lyophilized algal cell powder from Shin-ike contained large amounts of microcystins which showed acute toxicity for mouse, while that from Oo-ike had only a very small amount of microcystin-RR which did not show acute toxicity. Autopsy of one of the birds revealed that the liver was necrotic and severely jaundiced with a dark green color, suggesting the toxicity of the microcystins. These results point to the cause of the unnatural death of spot-billed ducks in Shin-ike as being the sudden appearance of toxic Microcystis aeruginosa. This was due to eutrophication of the pond, following the influx of untreated sewage related to damage from the Great Hanshinn Earthquake of January 1995. This is the first experimental report of toxic cyanobacteria being implicated in the mass death of wild birds in Japan.