Described as “almost the size of a small state,” a 400-square-mile bloom of red tide is making its presence felt off Florida’s southwest coast. Commercial fishermen are reporting fish kills, including grouper, plus dead crabs and porpoises. “I’d have to say that this is the largest red tide bloom that I have ever seen,” said a Florida Keys National Marine Sanctuary official.

Red tides are caused by high concentrations of Karenia brevis algae, which produce toxins that affect the central nervous system in fish, and can afflict humans who eat shellfish from affected areas. National Oceanic and Atmospheric Administration and FWC scientists are planning water tests this weekend to track the bloom. Scientists presently believe the bloom is moving southward away from the Fort Myers coastline.