A California Fish and Wildlife pathologist has identified the microbial pathogen he thinks is responsible for killing thousands of sharks and rays in the San Francisco Bay between February and July.

Early in the spring, volunteers with the Pelagic Shark Research Foundation, a conservation group based in Santa Cruz, CA, collected several stranded sharks and mailed them to Okihiro, who works in Southern California as a white seabass hatchery inspector. Okihiro cut the sharks open and found lesions all around the sharks’ brains.

Something was entering the sharks’ noses, climbing into their brains, and eating away, causing the sharks to become disoriented and ultimately strand or die. But the cause remained elusive until Okihiro extracted cerebrospinal fluid from several sharks