These fish were analyzed by the Northwest Biological Science Center in Seattle and the Vibrio Alginolyticus bacteria were found to exist in them. These bacteria are common in salt water species. Tunas, mahi mahi, bluefish, sardines, amberjack, and mackerel are often found with Vibrio bacteria. Vibrio kills the fish it infects. It is theorized that, as the Tilapia were dying because of the bacteria, their bodies became host to the botulism virus.
Botulism requires an anaerobic environment to survive.
Because of this, normal healthy fish cannot support the virus. As the fish weakened and, shortly before their death, their bodies offered conducive conditions for the virus. The theory continues to suggest that some pelicans caught and ingested the dying Tilapia that now contained the virus, and thus became ill themselves. Botulism outbreaks occur in late summer because water temperatures are greater. As the water cools, the virus is less active.