We know what caused Florida fish to spin in circles until they died

At least 54 sawfish died after exhibiting unusual spinning behavior. Exposure to multiple toxins from dinoflagellates (a type of microscopic algae) likely caused the behavior.

Testing of water and fish samples indicated elevated levels of Gambierdiscus, a genus of these toxic algae. Normally, dinoflagellates inhabit seagrass and larger algae on the seafloor, but an unidentified factor — possibly a heat wave or storm — caused them to become free-floating in the water column.