Fishing guides in the Florida Keys began reporting unusual sightings to Ross Boucek last fall. Small bait fish, especially at night, would start spinning in tight circles in the water, seemingly in distress.

As the months went by, more reports trickled in to Dr. Boucek, a biologist with the Bonefish & Tarpon Trust, a nonprofit conservation group. Bigger fish — jacks, snook — were swimming in spirals or upside down in the shallow waters of the Atlantic Ocean and Gulf of Mexico. So were stingrays and the occasional shark.